SURGICAL ANATOMY OF THE HAND AND UPPER EXTREMITY part 02

 

plexus and courses posteriorly through the quadrangular

space to reach the deep surface of the deltoid muscle. The

nerve then crosses from posterior to lateral along the deep

surface of the muscle approximately 5 cm distal to the

acromion. The axillary nerve gives off motor branches along

its course, and courses anteriorly as far as the anterior edge

of the deltoid muscle. Although the axillary nerve comprises mostly fibers from C5 and C6, it may contain a contribution from C4.

Actions and Biomechanics: Deltoid

Muscle

The deltoid is able to contract certain portions or parts

independently of others. Thus, parts of the muscle can act

separately as well as together. When the entire muscle contracts, the humerus can be abducted slightly beyond 90

degrees (3,4). (Additional humeral abduction actually is

produced in conjunction with scapular rotation.) The anterior (clavicular) fibers contracting independently assist the

pectoralis major in producing forward flexion and internal

rotation of the humerus. The posterior fibers contracting

independently can assist the latissimus dorsi and teres major

in producing extension (to approximately 45 degrees) and

external rotation of the humerus. The clavicular and spinous portions can contract simultaneously to assist with stabilization of the humerus. The central portion of the muscle is multipennate. This central (acromial) portion assists

with strong abduction of the humerus. Aided by the

supraspinatus, it can abduct the humerus until the joint

2 Muscle Anatomy 93

FIGURE 2.1. Anterior (A) and posterior (B) views of the scapula, showing muscle origins (red)

and insertions (blue).

(continued on next page)

A

capsule is tense inferiorly. The trapezius also assists the deltoid with humeral abduction. In general, the most effective

abduction takes place with the humerus in external rotation. When the abduction takes place in the plane of the

body of the scapula, scapular rotation can be fully effective

in assisting with humeral abduction and raising the arm

above the head. During humeral abduction, the central

(acromial) fibers of the deltoid contract strongly, aided by

the anterior (clavicular) and posterior fibers, both of which

help prevent departure of the humerus from the plane of

motion. In the early stages of abduction, there is an upward

traction force on the humeral head produced by the deltoid.

The humeral head is prevented from translating upward by

the synergistic downward pull of the subscapularis, infraspinatus, and teres minor (3,4). Electromyography suggests

that deltoid contributes little to internal or external rotation, but confirms that it does take part in most other

shoulder movements. When a weight or load produces a

downward drag on the upper extremity, the deltoid and the

supraspinatus contract to help resist the downward force.

Other common actions of the deltoid include assisting to

produce arm swinging during ambulation, and helping to

forward flex the arm to position the hand at various heights

during manual tasks (3,8,11,16).

Anomalies and Variations: Deltoid Muscle

Several variations of the muscle belly of the deltoid have

been noted (11). Each of the three parts may appear as a

separate muscle, so that there is a split in the muscle mass

or of the distal insertion tendon. A separate clavicular part

is the most common of these anomalies. The acromial and

spinous parts also may appear as a separate muscle.

The deep portion of the deltoid may be separated from

the major mass portion of the muscle, and this deep portion

may insert into the shoulder capsule or extend distally onto

the humerus (11).

Portions of the deltoid may be absent, especially those

originating from the clavicle or acromion.

Several accessory muscle or tendon slips may attach to

the deltoid. These muscle slips can connect to the fascia

covering the infraspinatus muscle or connect directly to the

trapezius muscle. Muscle slips also attach to the vertebral or

axillary borders of the scapula. An accessory tendon of

94 Systems Anatomy

FIGURE 2.1. (continued)

B

2 Muscle Anatomy 95

FIGURE 2.2. Anterior (A) and posterior (B) views of the humerus, showing muscle origins (red)

and insertions (blue).

A B

insertion has been noted to extend to the radial side of the

forearm.

The muscle belly of the deltoid may coalesce with adjacent muscles, and appear structurally joined to these muscles. Coalescing muscles include the pectoralis major,

trapezius, infraspinatus, brachialis, and brachioradialis (11).

Clinical Correlations: Deltoid Muscle

Axillary palsy can produce severe deltoid atrophy. This, in

turn, results in prominence of the acromion, which can

simulate dislocation of the shoulder joint. The distance

between the acromion and humeral head is increased to

the extent that a fingertip may be inserted between them

(3,4).

Deltoid paralysis from axillary nerve injury is a well recognized complication of shoulder dislocation, especially

anterior and inferior (luxatio erecta) dislocations (17).

Central nervous disorders such as stroke can result in

deltoid paralysis. The paralysis can result in inferior subluxation of the humeral head, which can secondarily result in

traction on the brachial plexus with associated pain or limb

paresthesias.

Thickening of the distal edge of the deltopectoral fascia

may produce compression of the median nerve (11,17,18).

CORACOBRACHIALIS MUSCLE

Derivation and Terminology. The coracobrachialis

derives its name from its origin from the coracoid process

and its insertion into the brachium. Coracoid is derived

from the Greek korakoeides, which means “crowlike” or

“like a crow’s beak” (korax = raven, and eidos = appearance),

and pertains to the coracoid process, which resembles a

bird’s beak. The word brachialis is derived from the Latin

and Greek brachialis and brachion, respectively, which designate or pertain to the arm (1,2). Note that brachi and

brachial pertain to arm, and should not be confused with

brachy (from Greek brachys), which refers to “short” (i.e.,

brachydactyly for short digits).

Origin. Apex of the coracoid process, along with the

conjoined tendon of the short head of the biceps. Muscle

fibers of the coracobrachialis also may originate from the

tendon of the short head of the biceps along the proximal

10 cm of the tendon.

Insertion. Medial humeral diaphysis, over a 3- to 5-cm

insertional impression on the cortex. The area of insertion

is roughly at the junction of the proximal and middle thirds

of the humerus, between the attachment of the triceps and

the brachialis.

Innervation. Musculocutaneous nerve (C5, C6, C7).

Vascular Supply. Muscular branches from the axillary

artery, the brachial artery, and the anterior circumflex

humeral artery (3,11).

Principal Action. Forward flexion and adduction of the

humerus.

Gross Anatomic Description:

Coracobrachialis

The coracobrachialis is a relatively long and slender muscle,

somewhat cylindrical in shape. Along with the biceps

brachii and brachialis, the coracobrachialis helps comprise

the anterior muscle compartment of the arm (Appendix

2.2). The coracobrachialis helps form the inconspicuous

rounded ridge on the upper medial side of the arm. The

pulse of the brachial artery often can be seen or palpated in

the depression posterior to the coracobrachialis. The muscle

fibers extend obliquely and in a parallel fashion. The muscle usually contains an aponeurotic band that continues

from the deep surface of the muscle to the insertion. The

muscle originates from the apex of the coracoid process,

along with the conjoined tendon of the short head of the

biceps (see Fig. 2.1). Muscle fibers of the coracobrachialis

also may originate from the tendon of the short head of the

biceps along the proximal 10 cm of the tendon. The muscle may be separated into two heads or parts, separated by

the musculocutaneous nerve, which passes in between.

When the superficial and deep parts are clearly defined, the

tendon of origin of the superficial part may be clearly separated from that of the deep part and may be closely associated with the tendon of the short head of the biceps brachii

(3,4,8,11,19–25). The muscle extends from the coracoid

process in a distal direction toward the medial diaphysis of

the humerus. The muscle is cylindrical or fusiform in shape.

The muscle inserts into the medial humeral diaphysis over

a 3- to 5-cm insertional impression on the cortex (see Fig.

2.2A). The area of insertion is roughly at the junction of the

proximal and middle thirds of the humerus, between the

attachment of the triceps and the brachialis. At the insertion point, there also may be two separate tendons from the

superficial and deep parts of the muscle (3,4,8,11).

The musculocutaneous nerve, derived mostly from C5,

C6, and C7, innervates the coracobrachialis. The nerve

exits the brachial plexus from the lateral cord near the level

of the acromion. The branch to the coracobrachialis usually

is the first (most proximal) motor branch from the musculocutaneous nerve, followed by motor branches to the

biceps and then the brachialis. After exiting from the lateral

cord, the musculocutaneous branch to the coracobrachialis

enters the proximal third of the medial aspect of the muscle

and crosses through the muscle from medial to lateral near

its midline.

Flatow and colleagues (26) and Eglseder and Goldman

(27) have quantified the anatomic aspects of coracobrachialis innervation in relation to the coracoid process.

The distance from the coracoid process to the point where

the musculocutaneous nerve enters the coracobrachialis

muscle averages between 46 and 56 mm (range, 31 to

96 Systems Anatomy

82 mm) (26,27). Small nerve twigs to the coracobrachialis

(proximal to the main nerve trunk) enter the muscle as

close as 17 mm distal to the coracoid process, with an average of 31 mm. The authors note that the frequently cited

range of 5 to 8 cm below the coracoid for the level of penetration cannot be relied on to describe a “safe zone”

because 29% of the nerves entered the muscle proximal to

50 mm below the coracoid (74% if the proximal twigs are

considered) (26). The musculocutaneous nerve exits the

coracobrachialis muscle at a mean of 75.5 mm distal to the

coracoid process (27).

Actions and Biomechanics:

Coracobrachialis

The coracobrachialis functions mainly to assist with flexion

and adduction of the humerus. With the humerus in extension, the coracobrachialis assists in returning the humerus

to a neutral position. In abduction, the coracobrachialis acts

with the anterior fibers of the deltoid to stabilize the

humerus in the plane of motion. The coracobrachialis also

helps stabilize and maintain the head of the humerus in the

glenoid fossa. Theoretically, the coracobrachialis can help

rotate the scapula if the humerus is stabilized (3,8,11).

Anomalies and Variations:

Coracobrachialis

There are several variations of the insertion of the coracobrachialis, including those more proximal and those more

distal on the humerus. Those more proximal than the proximal diaphysis include insertions into the surgical neck of

the humerus or capsule of the shoulder joint (28). The coracobrachialis brevis (or coracobrachialis superior, coracobrachialis rotator humeri) is an anomalous muscle that

arises from the coracoid process and inserts proximally, into

the bicipital ridge of the humerus in the proximal diaphysis,

approximately 1 cm distal to the lesser tuberosity (3,11).

This muscle may represent a remnant of a separate portion

of the muscle formed embryologically. Those inserting

more distally may include attachment sites along the medial

margin of the humerus, or a separate insertion in the medial

distal humerus or medial epicondyle. The distal insertion

may consist of an elongated tendinous extension. The coracobrachialis inferior or coracobrachialis longus denotes an

anomalous muscle that inserts much farther distally than

usual (3,11). These often insert into either the distal medial

aspect of the humerus, into the fibrous band of the medial

intermuscular septum, or into the ligament of Struthers.

The muscle also may extend distally into the medial supracondylar ridge, medial epicondyle, or an anomalous supracondylar process. The coracobrachialis inferior or coracobrachialis longus has been referred to as Wood’s muscle,

based on Wood’s descriptions of several muscle variations in

1870 (8,11,18).

Similar to the anomalous distal insertions, the coracobrachialis may have several accessory slips that attach to the

muscle distally. These include extensions to the medial epicondyle, the medial intermuscular septum, or the distal

medial aspect of the humerus (11,21).

Muscle or tendon slips have been noted to extend to various structures in the shoulder area, including the tendons

of the latissimus dorsi and teres major, or to the lesser

tuberosity of the humerus (22,23). Among these is the coracobrachialis minor (le court coracobrachialis of Cruveilhier), an accessory muscle that arises from the coracoid

process and crosses the radial nerve in the axilla and inserts

into the tendinous part of the latissimus dorsi (11). Complete absence of the coracobrachialis can also occur.

An anomalous muscle has been noted to arise from the

medial aspect of the distal half of the humerus, between the

coracobrachialis and brachialis, passing obliquely across the

front of the brachial artery and median nerve and attaching

with the common origin of the forearm flexor muscles

(22,23). It did not appear to be an additional head of the

coracobrachialis, biceps, or brachialis. The muscle appeared

to place the median nerve and brachial artery at risk for

compression; the authors suggest that the existence of this

muscle be kept in mind in a patient presenting with a high

median nerve palsy together with symptoms of brachial

artery compression (22,23).

Several variations in the musculocutaneous innervation

of the coracobrachialis have been noted, with most of the

differences involving the path of the nerve before muscle

innervation (11,21,26,27,29–33). Although the motor

branch from the musculocutaneous nerve usually pierces

the muscle and travels in its substance, the nerve may not

pierce the muscle in the proximal portion. Instead, the

nerve may continue along with or in the substance of the

median nerve, travel distally along the muscle, and then, as

a single trunk or as several branches, pass between the

biceps and brachialis, supplying these muscle as well as the

coracobrachialis from the more distal aspect. This variation

has been suggested to occur in approximately 20% of arms

(11). Alternatively, the motor nerve to the coracobrachialis

may split, with a branch entering and supplying the muscle,

and then a portion may rejoin the main musculocutaneous

nerve trunk. The nerve also may pass posterior to the coracobrachialis or between it and the short head of the biceps

muscle before innervating the coracobrachialis. Rarely, the

lateral cord may enter as a nerve into the coracobrachialis

and then divide into the musculocutaneous nerve and the

lateral head of the median nerve.

Clinical Correlations: Coracobrachialis

Muscle

Several operative procedures involve mobilization or exposure of the coracoid process. The musculocutaneous nerve

and motor branches to the coracobrachialis muscle are at

2 Muscle Anatomy 97

risk for injury. The points of innervation of the musculocutaneous nerve to the coracobrachialis have wide variability,

with muscular branches entering the coracobrachialis from

31 to 82 mm distal to the coracoid process (26). In the past,

there has been a frequently cited “safe zone” of 5 to 8 cm

distal to the coracoid for the level of penetration of these

nerve branches. This safe zone cannot be relied on, however, because of the established variability of the nerve. This

variability should be kept in mind when exposing or mobilizing the coracoid process, and the vicinity of the musculocutaneous nerve and its branches should be appreciated.

The coracobrachialis, including the axillary vessels, can

be used as a local muscle flap for coverage of exposed infraclavicular or postmastectomy defects (34).

Isolated musculocutaneous nerve palsy has been noted to

occur. Atraumatic palsy (35) as well as palsies associated

with heavy exercise or violent extension of the elbow have

been reported (36–39). It can occur bilaterally (40). The

coracobrachialis, however, usually is spared weakness, and

the area of compression is thought to be possibly within the

muscle itself (39). The syndrome usually produces weakness

of the biceps brachii and brachioradialis, with sensory

abnormalities along the lateral forearm. It usually resolves

with rest, but may take weeks or months (see later, under

Clinical Correlations: Biceps Brachii).

The wide variation in the course of the musculocutaneous nerve before and inside the coracobrachialis, and the

high percentage of anomalies, emphasize the complexities

and irregularities of this anatomic region with regard to surgical approaches (11,21,26,27,30–32).

BICEPS BRACHII MUSCLE

Derivation and Terminology. Biceps is derived from the

Latin and Greek bi, meaning “two,” and the Latin caput,

meaning “head.” Biceps thus refers to “two heads.”

Brachialis is derived from the Latin and Greek brachialis and

brachion, respectively, which designate or pertain to the arm

(1,2). Note that brachi and brachial pertain to the arm, and

should not be confused with brachy (from Greek brachys),

which refers to “short” (i.e., brachydactyly for short digits).

Origin. Short head: from the coracoid process, in the

conjoined tendon of the coracobrachialis. Long head: from

the supraglenoid tubercle of the scapula and from the posterior part of the glenoid labrum by a long tendon of the

origin approximately 9 cm long.

Insertion. The bicipital tuberosity of the radius and into

the bicipital aponeurosis, which inserts into the deep fascia

on the ulnar aspect of the forearm.

Innervation. Musculocutaneous nerve (C5, C6).

Vascular Supply. The brachial artery and the anterior circumflex humeral artery. The short head may receive a

branch from the axillary artery (3,8,11,19).

Principal Action. Flexion and supination of the forearm.

Gross Anatomic Description: Biceps

Brachii

The biceps brachii is a relatively large, thick, and roughly

fusiform muscle comprising a major portion of the anterior

muscle compartment of the arm (Appendix 2.2). The muscle has two heads, arising from two separate origins. The

muscle heads then partially coalesce into a single large muscle belly, although it still grossly retains some features of two

separate heads (41).

The short head arises from the tip of the coracoid

process, originating as a thick, flat tendon that is conjoined

with the origin of the coracobrachialis muscle (see Fig. 2.1).

The short head then separates, and the muscle belly

becomes more defined. The muscle fibers of the short head

descend from the dorsomedial surface of the tendon, in a

vertical fashion, and join the fibers of the long head. The

fibers increase in number from proximal to distal as the

muscle approaches the insertion.

The long head arises from a rough or raised point just

superior to the rim of the glenoid fossa, known as the supraglenoid tubercle of the scapula. It is intracapsular at its origin.

From the origin, there is a well defined, long, stout tendon

that is approximately 9 cm long. The tendon runs from the

apex of the glenoid cavity enclosed in a double tubular

sheath that is an extension of the synovial membrane of the

joint capsule. The tendon is intracapsular as it crosses and

then arches over the head of the humerus. It emerges from

the joint posterior to the transverse humeral ligament. The

tendon then descends in the intertubercular sulcus of the

humerus, where it is held in place by the transverse humeral

ligament and a fibrous expansion from the tendon of the

pectoralis major. At the myotendinous junction, the muscle

belly of the long head joins the belly of the short head. The

muscle fibers extend distally and obliquely. The two bellies

appear joined together, and form a single elongated belly.

The two heads, however, can be separated from each other

to within approximately 7 cm of the elbow joint. The muscle fibers then form a terminal tendon in the distal fourth

of the arm. The fibers coalesce and become tendinous, taking the shape of a flattened or oval tendon. As the tendon

approaches its insertion point, it spirals from proximal to

distal, so that the anterior surface turns to face laterally. The

tendon passes between the brachioradialis and the pronator

teres. It then inserts into a rough posterior attachment area

of the radial tuberosity (Fig. 2.3A). There is a bursa in the

vicinity of the tendon that separates the tendon from a

smooth anterior area of the tuberosity. Proximal to the

elbow joint, the tendon also has a broad medial fascial

expansion, the bicipital aponeurosis. This aponeurosis actually forms in the proximal part of the terminal tendon and

is first identifiable as a vertical septum between the two

heads of the biceps. More distally, it becomes a broadened

and flattened aponeurosis. Muscle fibers insert on the sides

of the septum and surfaces of the aponeurosis, the long

98 Systems Anatomy

head chiefly on the deep surface, and the short head primarily on the superficial surface. This fascial attachment

extends distally and medially superficial to the brachial

artery to coalesce with the deep fascia of the distal upper

arm and proximal forearm. This is in the vicinity of the origin of the flexor–pronator muscles of the forearm. The tendon often can be split as far distally as the radial tuberosity,

where the anterior and posterior layers can be traced back

to the separate bellies of the short and long head, respectively (3,4,10–13,42).

The biceps muscle is innervated by the musculocutaneous nerve (C5, C6). Although each head receives its own

nerve branch, the two branches may extend together as a

small common nerve trunk. Several separate smaller

2 Muscle Anatomy 99

FIGURE 2.3. Anterior (A) and posterior (B) views of the radius and ulna, showing muscle origins

(red) and insertions (blue).

A B

branches may enter the muscle on the deep surface in the

proximal portion of the middle third. A distinct intramuscular fissure in each head has been noted where the nerve

enters the muscle (11,43).

The path and variations of the musculocutaneous nerve,

including the branch patterns to the biceps and brachialis

muscles, have been studied by Yang et al. (43) and Chiarapattanakom and colleagues (44). In microdissections of 24

fresh-frozen cadaver specimens, Yang et al. found that the

motor branch to the biceps exited from the musculocutaneous nerve 119 mm distal to the coracoid process. Variations were seen in the innervation of the two heads of the

biceps. A common primary motor branch that bifurcated to

supply the two heads was seen in 20 specimens (type I).

Two specimens had two separate primary branches originating from the main musculocutaneous nerve trunk to

individually supply each head of the biceps (type II). The

third variation (type III), also seen in two specimens, was

similar to type I, but with an additional distal motor branch

innervating the common belly of the biceps muscle. The

motor branch to the brachialis muscle exited from the musculocutaneous nerve 170 mm distal to the coracoid process.

The motor branches to the biceps and brachialis muscles

may be dissected proximally from their points of exit from

the main trunk of the musculocutaneous nerve for mean

distances of 44 and 53 mm, respectively. These variations

have clinical application in the operative exposure of the

musculocutaneous nerve, especially in performing intercostal nerve to musculocutaneous motor branch transfer for

elbow flexion in patients with brachial plexus injuries (43).

In a subsequent study, Chiarapattanakom and colleagues

studied 112 musculocutaneous nerves from 56 cadavers

(44). There were three distinct types of branching patterns

for the biceps innervation: in 62%, there was one branch

only; in 33%, there were two branches; and in 5%, there

were three branches. The origin of the first branch averaged

130 mm from the acromion, regardless of branch type. The

maximum distance between the first and second branch was

53 mm. In 92%, there was only one branch to the brachialis

muscle (44).

Actions and Biomechanics: Biceps Brachii

The biceps is one of the primary flexors of the elbow. Flexion of the elbow is most effective with the forearm in

supination. The biceps is also the strongest supinator of the

forearm, especially with rapid or resisted movements. The

short head, from its origin on the coracoid process, can

assist with adduction and forward flexion of the humerus.

The long head, from its origin just above the glenoid,

assists in stabilizing the humeral head in the glenoid cavity.

The long head can specifically help to prevent superior

migration of the humeral head during contraction of the

deltoid.

Based on cross-sectional analysis of the major elbow

muscle flexors, the biceps brachii appears to contribute

34% of flexion torque, with the brachialis contributing

47% and the brachioradialis 19% (20).

Anomalies and Variations: Biceps Brachii

Several variations of the biceps have been noted (11,22,23,

41,42). Most of these consist of accessory heads or interconnecting anomalous muscles bellies. Accessory heads are

often associated with variations in the musculocutaneous

innervation or with abnormal courses of the axillary and

brachial arteries (41). There may be an absence of one or

both heads of the biceps brachii or both heads may be separate along their complete course from origin to insertion.

Both heads may also be coalesced along most of their

course.

Supernumerary heads are common, occurring in over

10% of specimens (11,45,46). An accessory head may arise

from the coracoid process, capsule of the shoulder joint,

tendon of the pectoralis major, or the region of the deltoid

insertion (21,22,42).

A third or fourth (humeral) head has been found in

approximately 12% to 14% of arms (11,45,47,48). It usually arises from the proximal humerus in the region of the

greater tuberosity. Less commonly, two accessory heads may

arise together from the neck of the humerus or posterior to

the tendon of the pectoralis. These two anomalous heads

may be joined to the pectoralis tendon. The lateral of the

two accessory slips usually joins the long head of the biceps

and the medial head usually joins the short head.

A third head often arises from the superomedial part of

the brachialis and attaches to the bicipital aponeurosis and

medial side of the tendon of insertion (3). This head often

is located deep to the brachial artery. It also may consist of

two slips that extend distally, one slip superficial and one

deep to the brachial artery.

Muscle or tendon slips may extend from the lateral

aspect of the humerus or intertubercular sulcus to join the

main muscle belly of the biceps. The most common anomalous slip arises from the humerus near the insertion of the

coracobrachialis and extends distally between the coracobrachialis and brachialis. This anomalous slip usually joins

the short head, but most of the fibers pass into the part of

the tendon that forms the bicipital aponeurosis. This slip

also may be completely separated and terminate entirely in

the bicipital aponeurosis (11).

An accessory slip may arise from the deltoid.

Several variations have been noted at the distal end of the

muscle, including various muscular or tendinous slips that

extend from the biceps to the distal humerus, ulna, radius,

forearm fascia, or neighboring muscles (11,49,50). Supernumerary heads may extend to or from the biceps to the

brachialis, brachioradialis, pronator teres, flexor carpi radi100 Systems Anatomy

alis (FCR), flexor digitorum profundus (FDP), intermuscular septum, or medial epicondyle (11,49,50).

Muscle coalitions from the biceps have been noted where

the muscle “fuses” with the belly of neighboring muscles,

including the pectoralis major and minor, coracobrachialis,

and brachialis (11).

Attachments from a muscular or tendinous extension

from the distal biceps to the palmaris longus have been

noted (11). Attachments from a muscular or tendinous

extension from the distal biceps to the extensor carpi radialis brevis (ECRB) have also been noted (51).

An anomalous muscle has been noted to arise from the

medial aspect of the distal half of the humerus, between

the coracobrachialis and brachialis, passing obliquely

across the front of the brachial artery and median nerve

and attaching with the common origin of the forearm

flexor muscles (22,23). The muscle did not appear to be an

additional head of the coracobrachialis, biceps, or

brachialis. The muscle, however, appeared to place the

median nerve and brachial artery at risk for compression.

The authors suggest that the existence of this muscle be

kept in mind in a patient presenting with a high median

nerve palsy together with symptoms of brachial artery

compression (22,23).

As noted previously in the descriptions of the coracobrachialis, several variations in the course and innervation of

the musculocutaneous nerve to the coracobrachialis, biceps,

and brachialis have been noted. Most variations involve the

path of the nerve before muscle innervation (11,26,27,

29–33,41). Although the motor branch from the musculocutaneous nerve usually pierces the coracobrachialis and

travels in its substance, the nerve may not pierce the muscle.

Instead, the nerve may continue along with or in the substance of the median nerve, travel distally along the coracobrachialis, and then, as a single trunk or as several branches,

pass between the biceps and brachialis, supplying these muscles as well as the coracobrachialis from the more distal

aspect. This variation has been suggested to occur in approximately 20% of arms (11,21,26,27,29–33,41).

The musculocutaneous nerve may be absent. The biceps

(and brachialis) can receive its innervation directly from the

median nerve (32).

Clinical Correlations: Biceps Brachii

Rupture of the biceps tendon is among the most common

of closed tendon ruptures. These occur either proximally in

the tendon of the long head (or short head) (52–56), or distally, at or near the insertion (57–68).

Bicipital tendinitis occurs in the tendon of the long

head, usually along the anterior shoulder in the intertubercular groove. Chronic tendinitis is associated with tendon

rupture, as well as a high incidence of associated related

shoulder problems, including impingement syndrome and

frozen shoulder (69–76).

The lacertus fibrosus is a structure known to cause or

contribute to median nerve compression in the forearm

(49,50).

Proximal paralysis of the brachial plexus involving the

C5 and C6 nerve roots (Erb-Duchenne palsy) results in

paralysis of the biceps. If C7 remains intact, the innervation

of the triceps remains intact. Functioning of the triceps in

conjunction with paralysis of the biceps results in the elbow

positioned in full extension. To restore elbow flexor power,

several operative flexorplasty procedures have been

described (77–80). These include proximal transfer of the

forearm flexor–pronator or wrist extensor mass (which

increases their moment arm across the elbow and enhances

their ability to act as secondary elbow flexors) (81–85),

transfer of part or all of the pectoralis major (with or without transfer of the pectoralis minor) (86–89), transfer of the

latissimus dorsi (90–94), anterior transfer of the triceps tendon (95–97), and transfer of the sternocleidomastoid (98).

Proximal transfer of the flexor pronator muscle origin is

known as the Steindler flexorplasty (81–85), described in

1918 (81).

Weakness of the biceps brachii and brachialis muscle due

to isolated palsy of the musculocutaneous nerve has been

reported. It can follow heavy exercise (36–39) or occur

atraumatically (35). Bilateral palsy also has been noted (40).

Violent extension of the forearm may be a factor. The syndrome features painless weakness of the biceps and

brachialis, sensory loss in the distal lateral forearm, and a

history of recent vigorous upper extremity resistive exercises. Loss of contour of the biceps has been noted (38).

The syndrome usually resolves with rest, but may take

weeks or months (37). The musculocutaneous nerve usually

is injured distal to the innervation of the coracobrachialis.

It has been postulated that nerve entrapment or stretching

occurs where the nerve passes through the coracobrachialis

(38). The condition should not be confused with C5 and

C6 radiculopathy, brachial plexopathy, or rupture of the

biceps brachii muscle belly or tendon.

With 6 weeks of heavy isometric strength training, the

strength of the elbow flexors can be increased by 14%, with

a mean increase in cross-sectional area of 5.4% (99). Male

and female percentage increases in strength and muscle size

are similar (no significant differences) (99).

The variations of the musculocutaneous innervation to

the biceps (described earlier under Gross Anatomic

Description: Biceps Brachii) should be appreciated when

planning intercostal to musculocutaneous nerve transfer to

restore elbow flexion in the patient with brachial plexus

palsy (43).

BRACHIALIS

Derivation and Terminology. Brachialis is derived from

the Latin and Greek brachialis and brachion, respectively,

2 Muscle Anatomy 101

which designate or pertain to the arm (1,2). Note that

brachi and brachial pertain to “arm,” and should not be

confused with brachy (from Greek brachys), which refers to

“short” (i.e., brachydactyly for short digits).

Origin. Distal two-thirds of the anterior humerus,

medial and lateral intermuscular septa.

Insertion. Proximal ulna, base of the coronoid process,

anterior capsule of the elbow.

Innervation. Musculocutaneous nerve (C5, C6). Additional innervation is from small branches from the radial

and median nerves.

Vascular Supply. Muscular branches from the brachial

artery, ulnar artery, superior and inferior ulnar collateral

arteries, anterior ulnar recurrent artery, radial collateral

branch of the profunda brachii, and radial recurrent artery

(3,4,11).

Principal Action. Flexion of the forearm.

Gross Anatomic Description: Brachialis

The brachialis is a relatively large, wide muscle, and along

with the biceps brachii and coracobrachialis, the brachialis

comprises the anterior muscle compartment of the arm

(Appendix 2.2). The brachialis originates on the distal twothirds of the anterior humerus (see Fig. 2.2). The attachment area of the origin is long and wide, commencing proximally along the anterior and posterior margins of the

insertional tendon of the deltoid and extending distally

along the anterior humerus to end in an inverted “V” at the

level just proximal to the elbow capsule. The origin may

extend to within 2.5 cm of the articular surface of the

elbow, ending proximal to the radial and coronoid fossae

(3,4,11). At the level of the humerus below the midshaft,

the muscle envelops the distal humerus on the anterior, lateral, and medial aspects to partially surround the shaft, covering approximately two-thirds of the bone circumference.

The muscle also arises from the medial intermuscular septum and from the lateral intermuscular septa proximal to

the origin of the brachioradialis and extensor carpi radialis

longus (ECRL), with more attachments from the medial

side. The muscle belly is somewhat flat, and is convex anteriorly and concave posteriorly as its extends distally. The

muscle fiber bundles descend in a specific pattern. The

middle bundles descend in a straight vertical direction. The

medial bundles descend in an oblique course, from medial

to lateral. The lateral bundles also descend in an oblique

course, from lateral to medial. In the distal fourth of the

muscle, the myotendinous junction begins. A portion of

the dorsal side of the lateral edge initially becomes tendinous. This tendinous portion enlarges as the muscle extends

distally, and an additional tendinous portion joins the

myotendinous junction on the anterior surface of the muscle proximal to the elbow joint. The tendon thickens and

converges as it extends distally. It passes along the anterior

capsule of the elbow joint and inserts onto a roughened area

on the anterior aspect of the base of the coronoid process

(see Fig. 2.3A). Cage and colleagues studied the anatomic

aspects of the brachialis in reference to the coronoid process

and associated fractures (100). The brachialis was found to

have a musculoaponeurotic insertion that included the

elbow capsule, coronoid, and proximal ulna. The bony

insertion averaged 26.3 mm in length, with its proximal

margin averaging 11 mm distal to the coronoid tip. The tip

of the coronoid process usually was not covered by capsule

or muscle attachments (in only 3 of 20 specimens did the

capsule actually insert onto the tip) (100). In general, it was

found that the brachialis insertion was more along the distal portion of the base of the coronoid, and only in Morrey

type III fractures (those through the base of the coronoid)

would the fracture fragment be large enough to include the

brachialis bony insertion (100).

The brachialis is innervated by the musculocutaneous

nerve (C5, C6). The nerve passes from medial to lateral

between the brachialis (located posterior to the nerve) and

the biceps (located anterior to the nerve). A motor branch

usually enters the brachialis on the anterior surface in the

proximal and medial portions of the muscle. The radial nerve

(C7) may supply a small branch to the distal lateral part of

the muscle (101). The median nerve also may supply a small

branch to the medial side of the brachioradialis (3,4,11).

As noted earlier in the discussion of the corocobrachialis

and biceps brachii, the path and variations of the musculocutaneous nerve, including the branch patterns to both the

biceps and brachialis muscles, were studied in detail by Yang

and colleagues (43). In 24 fresh-frozen cadaver specimens,

the motor branch to the brachialis muscle exited from the

musculocutaneous nerve a mean of 170 mm distal to the

coracoid process. A single primary motor branch (type I)

was seen in most specimens, and the rare specimen (type II)

showed two separate primary motor branches innervating

the muscle. The motor branches to the biceps and brachialis

muscles may be dissected proximally from their points of

exit from the main trunk of the musculocutaneous nerve for

mean distances of 44 and 53 mm, respectively. These variations have clinical significance for the operative exposure of

the musculocutaneous nerve, especially in performing intercostal nerve to musculocutaneous motor branch transfer for

elbow flexion in patients with brachial plexus injuries (43).

In a subsequent study, Chiarapattanakom and colleagues

dissected 112 musculocutaneous nerves in 56 cadavers (44).

In 92% of specimens, there was one motor branch to the

brachialis muscle. It always emerged from the main trunk

distal to the nerve to the biceps and averaged 170 mm from

the acromion (44).

Actions and Biomechanics: Brachialis

The brachialis provides strong flexion to the forearm, in

both pronation and supination. Based on cross-sectional

analysis of the major elbow muscle flexors, the brachialis

appears to contribute 47% of flexion torque, with the

biceps brachii contributing 34% and the brachioradialis

102 Systems Anatomy

19% (20). The brachialis also has a probable contribution

as a secondary stabilizer of the elbow joint (100).

Anomalies and Variations: Brachialis

The muscle belly of the brachialis may be divided into two

or more separate heads or bellies (11). When the brachialis

exists as two separate heads, each head commences on either

side of the deltoid tuberosity (one anterior and one posterior to the deltoid insertion).

If two or more muscle bellies exist, the distal insertion

becomes more variable or irregular, to include several additional anomalous insertional sites. These insertion sites

include (besides portions of the coronoid process) the radius

on or below the bicipital tuberosity (radial tuberosity), both

the proximal radius and ulna, the radius with a tendinous

band joining it to the coronoid process of the ulna, fascia of

the forearm, or muscles of the forearm arising from the

medial epicondyle and from the flexor muscle origin (11).

The brachiofascialis muscle of Wood denotes an anomalous insertion portion of the brachialis into the forearm

fascia (11,18).

A slip from the brachialis may insert into the bicipital

aponeurosis. A slip of the brachialis may also insert into the

capsule of the elbow joint, and is known as the capsularis

brachialis muscle.

The brachialis may coalesce with several muscles, including the brachioradialis, pronator teres, or biceps. The

brachialis may also be absent.

Variations in innervation may exist. The brachialis usually is innervated by the musculocutaneous nerve. The

radial nerve usually sends a small branch into the distal lateral portion of the muscle. The median nerve also may

innervate a small portion of the brachialis, sending a small

branch into the medial side of the distal muscle near the

elbow joint (11).

The musculocutaneous nerve may be absent. The

brachialis can receive its innervation directly from the

median nerve (32).

Clinical Correlations: Brachialis

Although rupture of the proximal or distal tendons of the

biceps is a relatively common injury, isolated rupture of the

brachialis has been noted only rarely (102).

It is well established that the median nerve can be compressed in the forearm by several structures, including the

lacertus fibrosus, pronator teres, and flexor digitorum

superficialis (FDS). In addition, an accessory slip of the

brachialis tendon distal in the forearm has been noted to

cause median nerve compression (49).

Weakness of the biceps brachii and brachialis due to isolated palsy of the musculocutaneous has been reported. It

can follow heavy exercise (36–39) or can occur atraumatically (35). Bilateral palsy also has been noted (40). Violent

extension of the forearm may be a factor. The syndrome features painless weakness of the biceps and brachialis, sensory

loss in the distal lateral forearm, and a history of recent vigorous upper extremity resistive exercise. Loss of contour of

the biceps may be noted (38). The syndrome usually

resolves with rest, but may take weeks or months (37). The

musculocutaneous nerve is injured distal to the innervation

of the coracobrachialis. It has been postulated that nerve

entrapment or stretching occurs where the nerve passes

through the coracobrachialis (38). The condition should

not be confused with C5 to C6 radiculopathy, brachial

plexopathy, or rupture of the biceps brachii muscle belly or

tendon.

With 6 weeks of heavy isometric strength training, the

strength of the elbow flexors can be increased by 14%, with

a mean increase in cross-sectional area of 5.4% (99). Male

and female percentage increases in strength and muscle size

are similar (no significant differences) (99).

TRICEPS BRACHII

Derivation and Terminology. Triceps is derived from the

Latin and Greek tri meaning “three,” and the Latin caput,

meaning “head.” Triceps thus refers to “three heads.” Brachii

is derived from the Latin and Greek brachialis and brachion,

respectively, which designate or pertain to the arm (1,2).

Note that brachi and brachial pertain to “arm,” and should

not be confused with brachy (from Greek brachys), which

refers to “short” (i.e., brachydactyly for short digits) (1,2).

Origin. From three heads. Long head: from the infraglenoid tubercle of the scapula. Lateral head: from a narrow,

linear or oblique ridge on the posterolateral surface of the

proximal humeral shaft and from the lateral intermuscular

septum. Medial head: from an extensive area including the

posterior surface of the humeral shaft, distal to the radial

groove from the insertion of the teres major to the distal

humerus (3,4,11).

Insertion. The olecranon process of the ulna.

Innervation. Radial nerve (C6, C7, C8), with separate

branches to each head.

Vascular Supply. The triceps is supplied by the axillary

artery through branches of the posterior humeral artery,

branches from the profunda brachial artery (including deltoid and middle collateral branches), and from the superior

and inferior ulnar collateral arteries and interosseous recurrent artery (3,4,11,103).

Principal Action. Extension of the forearm. The long

head may assist with adduction of the abducted humerus,

or extension of the forward-flexed humerus.

Gross Anatomic Description: Triceps

Brachii

The triceps is a wide, powerful muscle that comprises the

entire posterior muscle compartment of the arm (Appendix

2.2). The muscle is complex, with three heads and an exten2 Muscle Anatomy 103

sive, complex origin principally from the posterior humerus

(3,4,8,9,11,13).

The long head of the triceps originates from the infraglenoid tubercle of the scapula (see Fig. 2.1). It occasionally

may extend along the axillary border of the scapula to varying distances. The long head initially is a broad, flat tendon,

with attachments that blend with the inferior aspect of the

shoulder capsule (104). The long head extends distally and

somewhat laterally to join the lateral head. The long head

initially passes superficial to the medial head. In the midportion of the humerus, the long head joins with the muscle bellies of the lateral and medial head to from a large, single muscle belly. To some degree, the fibers and course of

the long head can be traced from the insertion to the origin.

From the origin of the long head, the tendon splits into two

layers, one located inferiorly and one superficially (11). The

muscle fibers from the two layers extend distally in a parallel fashion and then twist as they descend distally. At the

insertion level, the original anterior surface of the origin

becomes the dorsomedial portion of the tendon at the

insertion. The fibers of the long head of the muscle are

found on the medial side of the tendon, and terminate at

approximately the distal fourth of the arm as the myotendinous junction is formed.

The long head of the triceps contributes to the formation of the well known quadrangular space and the triangular space of the axillary region. From its origin, the long

head extends distally anterior to the teres minor and posterior to the teres major, dividing the wedge-shaped interval

between them into the triangular and quadrangular spaces

(3,4,11). The triangular space is bordered by the teres

minor (superiorly), the long head of the triceps (laterally),

and the teres major (inferiorly). Branches of the circumflex

scapular artery cross through the triangular space. The more

anatomically significant quadrangular space is bordered by

the teres minor and subscapularis (superiorly), the long

head of the triceps (medially), the teres major (inferiorly),

and the humeral neck (laterally). The axillary nerve and

posterior humeral circumflex artery pass through the quadrangular space (3,4,8,13,68).

The lateral head of the triceps originates as a flattened

tendon from a narrow, linear, oblique ridge on the posterior surface of the proximal humeral shaft, just distal to the

neck (see Fig. 2.2B). The origin is medial to the insertion

of the teres minor, and is anterior and lateral to the proximal portion of the radial groove. The distal portion of the

origin of the lateral head is located just posterior to the

insertion of the deltoid. In addition, part of the lateral

head originates from the lateral intermuscular septum. The

fibers of the lateral head extend distally to coalesce with the

fibers of the long and medial heads. The superior fibers of

the lateral head pass vertically and the inferior fibers pass

obliquely to insert into the dorsal and ventral surfaces of

the proximal lateral margin of the common insertional tendon (3,4).

The lateral head of the triceps is visible as a prominence

in the posterolateral aspect of the proximal arm, most

apparent in athletic individuals. The prominence is parallel

and medial to the posterior border of the deltoid. The muscle head becomes most prominent when the elbow is

actively extended. The mass that is located medial to the lateral head is the long head (3,4).

The medial head of the triceps has an extensive origin

from the distal half of the posterior humeral shaft (see Fig.

2.2B). It is located posterior and medial to the radial groove

of the humerus. The origin extends from the vicinity of the

insertion of the teres major (proximal on the humerus) to

the distal portion of the humerus, to within 2.5 cm of the

trochlea. A portion of the medial head also originates from

the medial intermuscular septum and the lower part of the

lateral intermuscular septum. The medial head lies deep to

the long head, and when the muscles coalesce, the medial

fibers remain in the deeper parts of the muscle. Some of the

fibers attach directly to the olecranon, although most first

coalesce with the other heads to form the common tendon

of insertion (3,4,11).

Once the long, medial, and lateral heads have coalesced,

the fibers continue distally to converge into a thick, stout

tendon. The myotendinous junction is relatively large and

begins in the middle third of the muscle. Operative exposure of the distal half of the muscle often exposes only a

large tendinous portion. The tendon has two layers, one

superficial and one deep. The layers unite to form the common tendon, which extends distally to attach to the olecranon (see Fig. 2.3B). Some of the muscle fibers or a portion

of the tendon on the lateral side form a band of fibers that

inserts into the articular capsule of the elbow or continues

distally over the anconeus to coalesce with the antebrachial

fascia. A part of muscle slip that inserts into the articular

capsule is referred to as the subanconeus muscle or articularis

cubiti (3,11).

The triceps muscle is innervated by the radial nerve (C6,

C7, C8). Each head receives a separate branch or branches.

The branch to the long head is the most proximal branch.

It arises in the axilla and enters the lateral margin of the

proximal muscle. The nerve may penetrate the muscle as

several small branches. The radial nerve continues distally

along the radial groove of the humerus, between the lateral

and medial heads. The radial head gives off two or three

small branches to supply the medial head, followed by separate branches to the lateral head.

Actions and Biomechanics: Triceps Brachii

The principal action of the triceps muscle is to extend the

forearm. The long head, which originates proximal to the

shoulder on the infraglenoid tubercle of the scapula, also

functions to assist with humeral adduction. When the

humerus is in a forward-flexed position, the long head can

assist with extending the humerus back to the neutral posi104 Systems Anatomy

tion. The lateral head is the strongest and contributes most

to elbow extension. The long head has more effect on the

shoulder joint then at the elbow (104).

Electromyographic studies indicate that the medial head

is active in all forms of extension of the forearm. The long

and lateral heads, however, are minimally active except in

extension of the forearm against resistance (105). This

occurs as in pushing or supporting body weight on the

hands with the elbows in mid-flexion. The long head

appears to give support to the lower part of the shoulder

capsule, especially when the arm is raised (104).

The triceps has an important function in stabilization of

the elbow during forceful supination of the forearm with

the elbow flexed. In forceful forearm supination, there is

strong contraction of both the supinator and biceps brachii.

The triceps contracts synergistically to maintain the flexed

or semiflexed position of the elbow. Otherwise, without this

triceps cocontraction, it would be difficult forcefully to

supinate the forearm without simultaneously flexing the

elbow (3,4,11,68).

Anomalies and Variations: Triceps Brachii

The three heads of the triceps may coalesce with the neighboring muscles (11). A fourth muscle head has been noted

to occur with the triceps (106). This head has been noted

to arise from the humerus, axillary margin of the scapula,

capsule of the shoulder joint, coracoid process, or tendon of

the latissimus dorsi (11,106).

The radial nerve is rarely noted to be absent. The triceps

is then innervated by the musculocutaneous or ulnar nerve

(11). The radial nerve rarely passes through the quadrangular space, along with the axillary nerve. The radial nerve still

innervates the three heads of the triceps (11).

The patella cubiti is a sesamoid bone in the triceps tendon, located near the insertion (107). It also is referred to as

the sesamum cubiti or elbow disc (11). Its presence has been

noted to be associated with a rupture of the distal triceps

tendon (see later) (107–116).

The latissimocondyloideus or dorsoepitrochlearis is an

anomalous muscle found in approximately 5% of individuals. The muscle extends from the tendon of the latissimus

dorsi to the brachial fascia, triceps brachii, shaft of the

humerus, lateral epicondyle, olecranon, or fascia of the forearm (11). When absent (95% of individuals), the muscle

normally is represented by a fascial slip from the tendon of

the latissimus dorsi to the long head of the triceps of from

the brachial fascia. The muscle is innervated by the radial

nerve (11).

Clinical Correlations: Triceps Brachii

Ulnar neuropathy or neuritis at the elbow in conjunction

with an abnormal triceps muscle slip or an aberrant muscle

belly is well documented (117–125). This type of cubital

tunnel syndrome has been related to either a separate or

prominent medial head of the triceps (119,123), an unstable, dislocating medial triceps tendon (117,118,124,125),

or an abnormal insertion or subanconeus muscle (an auxiliary extension of the medial portion of the medial triceps

that inserts into the joint capsule, fascia, or medial epicondyle) (120,122).

Radial nerve entrapment by the lateral head of the triceps has also been noted (126).

Complete avulsion or incomplete rupture of the triceps

tendon is well documented (107–116). It usually involves

rupture at the distal tendon, but may occur at the musculotendinous junction (110). Rupture has been associated

with patients on hemodialysis (112,113,115) and with

those with secondary hyperparathyroidism (114), seizure

disorders (115), hypertension (110), or diabetes mellitus

(110). Spontaneous rupture also has been reported in association with a patella cubiti, a sesamoid bone in the triceps

tendon (107). In a rare case, it also has occurred in association with radial neuropathy (111). Similar to rupture of the

biceps tendon, operative repair for complete rupture usually

is indicated (108–110,114,116). With incomplete rupture,

conservative management has been used successfully (112).

In arthrogryposis, the elbow is often in a fixed position

in varying degrees of extension. Triceps lengthening, in conjunction with capsulotomy or tendon transfer, often is performed to gain elbow motion (127,128).

ANCONEUS

Derivation and Terminology. The word anconeus is

derived from the Greek ankon, which means “elbow” (1,2).

Origin. The posterior surface of the distal aspect of the

lateral epicondyle.

Insertion. The lateral aspect of the olecranon and the

proximal fourth of the posterior surface of the shaft of the

ulna.

Innervation. Radial nerve (C6, C7, C8).

Vascular Supply. The interosseous recurrent artery, middle collateral (posterior descending) branch of the profunda

brachii (3,4,11).

Principal Action. Extension of the forearm. The

anconeus may have a secondary role in stabilizing the ulna,

especially during rotation of the forearm.

Gross Anatomic Description: Anconeus

The anconeus is a small, triangular or quadrangular muscle

of the posterolateral elbow. It is often partially blended with

the distal portion of the triceps, and is thought morphologically and physiologically to belong to the triceps. It has a

similar function of elbow extension and is supplied by the

same (radial) nerve. In some primates, the anconeus in not

distinguishable from the triceps (3).

2 Muscle Anatomy 105

The anconeus originates from the distal aspect of the

posterior lateral epicondyle of the humerus (see Fig. 2.2B).

The origin consists of a short tendon, often covered with

muscle. The tendon extends on the deep surface and lateral

margin of the muscle. A portion of the muscle also originates from the adjacent portion of the posterior elbow joint

capsule. The fibers of the anconeus diverge medially toward

the ulna, with the more proximal fibers extending transversely directly to the ulna, and the more distal and lateral

fibers extending more obliquely. The muscle covers the posterior aspect of the annular ligament. The anconeus inserts

onto the lateral aspect of the olecranon and on the adjacent

lateral aspect of the proximal ulna (see Fig. 2.3B). The superior part of the muscle usually is continuous with the

medial head of the triceps brachii. The insertional area

extends distally to stretch along the proximal quarter of the

ulna.

The anconeus is innervated by the radial nerve (C6, C7,

C8). The motor branch arises from the radial nerve trunk

in the radial groove of the humerus. This motor branch

passes through the medial head of the triceps, supplying the

triceps and continuing distally to enter the proximal border

of the anconeus (3,4,11).

Actions and Biomechanics: Anconeus

The anconeus assists the triceps with extension of the

elbow. The major function of the anconeus may not be fully

recognized. The anconeus may have a secondary role in stabilizing the ulna, especially during rotation of the forearm.

During pronation of the forearm, it has been postulated

that the anconeus moves the ulna laterally at the ulnohumeral joint. In this way, the anconeus allows the forearm

to turn over the hand without translating it medially

(3,4,11,13).

Anomalies and Variations: Anconeus

The anconeus may be coalesced to the medial head of the

triceps to varying degrees. It also may blend with the extensor carpi ulnaris (ECU) (11).

The subanconeus (articularis cubiti) is a small muscle

extension formed from fibers from the deep surface of the

distal part of the medial head of the triceps. It is a separate

muscle from the anconeus. The subanconeus crosses or covers a portion of the anconeus, attaching to the posterior

aspect of the elbow capsule or blending with the antebrachial fascia (3,11).

The epitrochleoolecranonis anconeus epitrochlearis

(epitrochleoanconeus, epitrochleocubital, or anconeus sextus) is a muscle distinct from the anconeus and the triceps

(129). It extends from the medial epicondyle of the

humerus, arches across the groove for the ulnar nerve, and

inserts onto the olecranon process of the ulna. It is thought

to occur in 25% of individuals and takes the place of a

fibrous arch that usually passes between the epicondylar and

ulnar heads of the flexor carpi ulnaris (FCU) (11). The

anconeus may coalesce with the epitrochleoolecranonis.

Clinical Correlations: Anconeus

The anomalous muscles associated with the anconeus (the

epitrochleoolecranonis anconeus epitrochlearis, epitrochleoanconeus, epitrochleocubital, or anconeus sextus) may

be associated with cubital tunnel syndrome (120,121,130,

131). The muscles extend from the medial epicondyle and

cross superficial to the cubital tunnel to reach the olecranon. There is thus a potential compression of the ulnar

nerve.

BRACHIORADIALIS

Derivation and Terminology. Brachioradialis is derived

from the Latin and Greek brachialis and brachion, respectively, which designate or pertain to the arm. Radialis is

from the Latin radii, which means “spoke” (used to describe

the radius of the forearm) (1,2). Note that brachi and

brachial pertain to “arm,” and should not be confused with

brachy (from Greek brachys), which refers to “short” (i.e.,

brachydactyly for short digits).

Origin. From the proximal two-thirds of the lateral ridge

of the humeral epicondyle and from the anterior surface of

the lateral intermuscular septum.

Insertion. The lateral aspect of the base of the styloid

process of the radius.

Innervation. Radial nerve (C5, C6).

Vascular Supply. The radial collateral branch of the profunda brachii, the radial artery, and the radial recurrent

artery from the radial artery (3,4,132,133).

Principal Action. Flexion of the forearm. It may assist in

rotating the forearm to the neutral rotation position from a

position of full pronation or full supination.

Gross Anatomic Description:

Brachioradialis

The brachioradialis consists of muscle fibers in its proximal

half and a long, strong tendon in its distal half. Positioned

on the lateral aspect of the forearm, it forms the lateral margin of the cubital fossa. The brachioradialis, along with the

ECRL and ECRB, occupies the muscle compartment

known as the mobile wad compartment of the forearm

(Appendix 2.2) (12). The muscle originates mostly from the

proximal two-thirds of the lateral epicondylar ridge of the

humerus (see Fig. 2.2). Additional fibers originate from the

anterior aspect of the lateral intermuscular septum. The

muscle fibers extend distally and volarly to terminate in a

penniform manner on the tendon. The muscle belly twists

slightly as it extends from proximal to distal. At the origin,

106 Systems Anatomy

its broad surface faces laterally; in the forearm, the broad

surface faces anteriorly; and in the distal forearm, the tendon

twists so that it again faces laterally. The muscle may have

extensive fascial attachments or attachments to the bellies of

the neighboring muscles. The muscle fibers usually end

proximal to the mid-forearm level, and appear to form a

short, abrupt myotendinous junction. The tendon, however,

usually extends quite proximally on the deep surface of the

muscle. The brachioradialis tendon is oval or flat, and

extends distally along the radial margin of the radius to reach

the insertion point just proximal to the styloid. Along its

course, the tendon tapers and becomes narrower, and winds

around the radius from the volar to the lateral surface. It

widens proximal to the insertion point. Near the insertion

point of the tendon, the brachioradialis is crossed by the

abductor pollicis longus (APL) and extensor pollicis brevis

(EPB). The tendon inserts into the lateral aspect of the base

of the styloid process of the radius (see Fig. 2.3A).

Vascular studies have been performed on the brachioradialis because of its potential use as a rotation musculocutaneous flap for local soft tissue reconstruction (132,133).

Sanger and colleagues found that the dominant perforator

to perfuse the muscle arose from the brachial artery in 27%,

from the radial recurrent artery in 33%, or from the radial

artery in 39% (132). Additional studies by Leversedge et al.

confirm the brachioradialis is perfused (partly) by the radial

recurrent artery [which perfuses an average of 41% (range,

20% to 60%) of the muscle length]. Injection studies of

combined radial artery and radial recurrent arteries show

that the two arteries combined account for perfusion of

80% (range, 59% to 100%) of the muscle length. This corresponds to 90% of the muscle volume (133).

Muscle function and design can be evaluated by the results

of tendon transfers from studies on muscle architecture

(15,134–142). Architectural features of a muscle include the

physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle

(angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural

studies by Lieber, Friden, and colleagues provide the data for

the brachioradialis (135–139) (Table 2.1 and Fig. 2.4). The

brachioradialis has relatively long fibers arranged at a small

2 Muscle Anatomy 107

TABLE 2.1. ARCHITECTURAL FEATURES OF SELECTED MUSCLES OF THE UPPER EXTREMITY

Muscle Muscle Fiber Pennation Cross-Sectional Fiber Length/

Mass Length Length Angle Area Muscle Length

Muscle (g) (mm) (mm) (Degrees) (cm2) Ratio

BR (n = 8) 17 ± 2.8 175 ± 8.3 121 ± 8.3 2 ± 0.6 1.33 ± 0.22 0.69 ± 0.062

PT (n = 8) 16 ± 1.7 130 ± 4.7 36 ± 1.3 10 ± 0.8 4.13 ± 0.52 0.28 ± 0.012

PQ (n = 8) 5 ± 1.0 39.3 ± 2.3 23 ± 2.0 10 ± 0.3 2.07 ± 0.33 0.58 ± 0.021

EDC I (n = 8) 3 ± .45 114 ± 3.4 57 ± 3.6 3 ± 0.5 0.52 ± 0.08 0.49 ± 0.024

EDC M (n = 5) 6 ± 1.2 112 ± 4.7 59 ± 3.5 3 ± 1.0 1.02 ± 0.20 0.50 ± 0.014

EDC R (n = 7) 5 ± .75 125 ± 10.7 51 ± 1.8 3 ± 0.5 0.86 ± 0.13 0.42 ± 0.023

EDC S (n = 6) 2 ± .32 121 ± 8.0 53 ± 5.2 2 ± 0.7 0.40 ± 0.06 0.43 ± 0.029

EDQ (n = 7) 4 ± .70 152 ± 9.2 55 ± 3.7 3 ± 0.6 0.64 ± 0.10 0.36 ± 0.012

EIP (n = 6) 3 ± .61 105 ± 6.6 48 ± 2.3 6 ± 0.8 0.56 ± 0.11 0.46 ± 0.023

EPL (n = 7) 5 ± .68 138 ± 7.2 44 ± 2.6 6 ± 1.3 0.98 ± 0.13 0.31 ± 0.020

PL (n = 6) 4 ± .82 134 ± 11.5 52 ± 3.1 4 ± 1.2 0.69 ± 0.17 0.40 ± 0.032

FDS I(P) (n = 6) 6 ± 1.1 93 ± 8.4 32 ± 3.0 5 ± 0.2 1.81 ± 0.83 0.34 ± 0.022

FDS I(D) (n = 9) 7 ± 0.8 119 ± 6.1 38 ± 3.0 7 ± 0.3 1.63 ± .22 0.32 ± 0.013

FDS I(C) (n = 6) 12 ± 2.1 207 ± 10.7 68 ± 2.8 6 ± 0.2 1.71 ± .28 0.33 ± 0.025

FDS M (n = 9) 16 ± 2.2 183 ± 11.5 61 ± 3.9 7 ± 0.7 2.53 ± .34 0.34 ± 0.014

FDS R (n = 9) 10 ± 1.1 155 ± 7.7 60 ± 2.7 4 ± 0.6 1.61 ± .18 0.39 ± 0.023

FDS S (n = 9) 2 ± 0.3 103 ± 6.3 42 ± 2.2 5 ± 0.7 0.40 ± .05 0.42 ± 0.014

FDP I (n = 9) 12 ± 1.2 149 ± 3.8 61 ± 2.4 7 ± 0.7 1.77 ± .16 0.41 ± 0.018

FDP M (n = 9) 16 ± 1.7 200 ± 8.2 68 ± 2.7 6 ± 0.3 2.23 ± .22 0.34 ± 0.011

FDP R (n = 9) 12 ± 1.4 194 ± 7.0 65 ± 2.6 7 ± 0.5 1.72 ± .18 0.33 ± 0.009

FDP S (n = 9) 14 ± 1.5 150 ± 4.7 61 ± 3.9 8 ± 0.9 2.20 ± .30 0.40 ± 0.015

FPL (n = 9) 10 ± 1.1 168 ± 10.0 45 ± 2.1 7 ± 0.2 2.08 ± .22 0.24 ± 0.010

BR, brachioradialis; PT, pronator teres; PQ, pronator quadratus; EDC I, extensor digitorum communis (index finger); EDC M, extensor digitorum

communis (middle finger); EDC R, extensor digitorum communis (ring finger); EDC S, extensor digitorum communis (small finger); EDQ,

extensor digiti quinti; EIP, extensor indicis proprius; EPL, extensor pollicis longus; PL, palmaris longus; FDS I (P), flexor digitorum superficialis of

index finger, proximal belly; FDS I (D), flexor digitorum superficialis of index finger, distal belly; FDS I (C), flexor digitorum superficialis of index

finger, combined properties of the proximal and distal bellies; FDS M, flexor digitorum superficialis (middle finger); FDS R, flexor digitorum

superficialis (ring finger); FDS S, flexor digitorum superficialis (small finger); FDP I, flexor digitorum profundus (index finger); FDP M, flexor

digitorum profundus (middle finger); FDP R, flexor digitorum profundus (ring finger); FDP S, flexor digitorum profundus (small finger); FPL,

flexor pollicis longus.

Reproduced from Lieber RL, Jacobson MD, Fazeli BM, et al. Architecture of selected muscles of the arm and forearm: anatomy and implications

for tendon transfer. J Hand Surg Am 17:787–798, 1992, with permission.

pennation angle, with a relatively small physiologic crosssectional area. This indicates that the brachioradialis is

designed more for excursion and velocity than for force generation (135). Its relative difference index values compare it

with other upper extremity muscles, based on architectural

features. These values are listed in Appendix 2.3.

The brachioradialis is innervated by the radial nerve

(C5, C6). This innervation is anatomically unusual because

the brachioradialis is a flexor of the elbow; the same radial

nerve also innervates the extensors (triceps) of the elbow.

The motor nerve branch to the brachioradialis exits from

the radial nerve trunk proximal to the level of the elbow, as

the radial nerve descends between the brachialis and brachioradialis. The nerve branch continues distally and enters

the muscle in its proximal third.

Actions and Biomechanics:

Brachioradialis

The primary function of the brachioradialis is elbow flexion. It has maximal mechanical advantage when the forearm is in 0 degrees of pronation or supination, or in slight

108 Systems Anatomy

FIGURE 2.4. Architectural features of selected

upper extremity muscles. A: Muscle fiber lengths

of selected upper extremity muscles: bar graph of

the fiber lengths from several studied muscles of

the upper extremity. Note that the flexors and

extensors are similar to one another and that the

brachioradialis differs significantly. B: Physiologic

cross-sectional areas of selected upper extremity

muscles: bar graph of the physiologic cross-sectional areas from several studied muscles of the

upper extremity. Note that the flexors and extensors are similar to one another and that the BR

and the PT differ significantly. C: Cross-sectional

area versus fiber length: scatterplot of fiber

lengths versus physiologic cross-sectional area of

selected upper extremity muscles. Fiber length

value (in millimeters) for the BR is listed in parentheses next to it on the chart because it would

actually place off the graph. Similarly, the physiologic cross-sectional area for the combined FDP

and FDS muscles also is shown in parentheses.

Muscles that cluster together in this graph are

architecturally similar. Because fiber length is proportional to muscle excursion (or velocity), and

physiologic cross-sectional area is proportional to

force generation, the location of each muscle indicates its design characteristics and specialization.

(Muscles with higher fiber lengths are designed

more for excursion or velocity; muscles with

higher physiologic cross-sectional areas are

designed more for force generation.) Each bar

represents mean ± standard deviation (SEM). FCR,

flexor carpi radialis; FCU, flexor carpi ulnaris; PL,

palmaris longus; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU,

extensor carpi ulnaris; FDS (I), flexor digitorum

superficialis (index finger); FDS (M), flexor digitorum superficialis (middle finger); FDS (R), flexor

digitorum superficialis (ring finger); FDS (S), flexor

digitorum superficialis (small finger); FDP (I),

flexor digitorum profundus (index finger); FDP

(M), flexor digitorum profundus (middle finger);

FDP (R), flexor digitorum profundus (ring finger);

FDP (S), flexor digitorum profundus (small finger);

FPL, flexor pollicis longus; EDC (I), extensor digitorum communis (index finger); EDC (M), extensor

digitorum communis (middle finger); EDC (R),

extensor digitorum communis (ring finger); EDC

(S), extensor digitorum communis (small finger);

EDQ, extensor digiti quinti; EIP, extensor indicis

proprius; EPL, extensor pollicis longus; PT, pronator teres; PQ, pronator quadratus; BR, brachioradialis. (A–C from Lieber RL, Jacobson MD, Fazeli

BM, et al. Architecture of selected muscles of the

arm and forearm: anatomy and implications for

tendon transfer. J Hand Surg [Am] 17:787–798,

1992, with permission.)

A

B

C

pronation. With the forearm in full pronation or full

supination, it may assist in bringing the forearm back to the

neutral position of 0 degrees of pronation or supination.

The brachioradialis can thus act as a supinator when the

forearm is extended and pronated (139). It can act as a forearm pronator when the forearm is extended and supinated.

Based on electromyographic studies, the brachioradialis is

minimally active with slow flexion movements of the elbow

or with the forearm supine. It does, however, generate

increased activity when movements are rapid (105). The

brachioradialis also may function to help stabilize the elbow

during forearm rotation (3,4).

Based on cross-sectional analysis of the major elbow

muscle flexors, the biceps brachii appears to contribute

34% of flexion torque, with the brachialis contributing

47% and the brachioradialis 19% (20).

Anomalies and Variations: Brachioradialis

The muscle belly of the brachioradialis may be divided,

doubled, or multiple. The tendon may be doubled along its

course (11) (Fig. 2.5). An accessory brachioradialis may

exist, and may cause proximal radial nerve compression at

the level of the elbow (143).

In approximately 7% of individuals, the tendon of the

brachioradialis may divide into two or three separate slips

that insert into the radial styloid (11). A slip may insert into

the forearm fascia. A second belly may attach distally to the

radius near the radial tuberosity, or to the ulna (11). When

two slips of the brachioradialis tendon are present, the

radial sensory nerve may pass between them. The nerve is at

risk for compression if it penetrates between the slips

(144–146) (see Fig. 2.5).

The supinator longus accessories or brachioradialis brevis is an accessory brachioradialis. It arises adjacent to the

brachioradialis and inserts onto the radial tuberosity or into

the supinator (see Fig. 2.5). It acts as a supinator of the forearm. The brachioradialis brevis also may insert into the

pronator teres or into the ulna (11).

The brachioradialis may be coalesced or tethered with

other muscles, most commonly the brachialis (near the origin of the brachioradialis) as well as the ECRL, pronator

2 Muscle Anatomy 109

FIGURE 2.5. The normal brachioradialis (left) and some of its clinically relevant variations. The

split or duplicated muscle may cause confusion during harvest for tendon transfer. The split tendon may be responsible for neuropathy of the superficial branch of the radial nerve, if the nerve

passes through the split tendon. The brachioradialis brevis is an anomalous muscle that inserts

into the radial tuberosity or the biceps tendon. It can function as a supinator of the forearm, as

well as an elbow flexor (11).

teres, and FCR (147). The brachioradialis may send slips to

the deltoid (see later), supinator, or APL (11).

The origin of the brachioradialis may extend proximally

as far as the mid-humerus, at the level of the deltoid insertion (11). The insertion point may be located more proximally or more distally than the styloid. The brachioradialis

may insert as far proximal as the middle third of the radial

shaft. It may insert as far distally as the scaphoid, trapezium,

or base of the index metacarpal (11,148). The brachioradialis muscle or tendon may be absent. If the tendon is

absent, the brachioradialis muscle may insert onto the

radius more proximally along the lateral diaphysis (11).

The brachioradialis usually is innervated by the radial

nerve. Anomalous innervation by the musculocutaneous

nerve has been reported as an unusual variation (149).

Clinical Implications: Brachioradialis

Sensory radial neuropathy may be caused by a split brachioradialis tendon or muscle, resulting in compression of

the superficial branch of the radial nerve passing through

the split tendon.

Because the brachioradialis is relatively expendable, it is

used as a donor muscle for several reconstructive procedures, including tendon transfer (134,139,150,151), as a

myocutaneous or rotation muscular flap for soft tissue

reconstruction (152,153), or for retinacular reconstruction

(154). Freehafer and associates studied the anatomy, properties, and value of the brachioradialis for tendon transfer in

the tetraplegic patient. The relatively large excursion and

adequate muscle force measurements of the brachioradialis

support its use as a donor for tendon transfer (134).

Friden et al. studied the architectural properties of the

brachioradialis and further emphasized the muscle’s value in

tendon transfers (139) (see Fig. 2.4). Its relatively high fiber

length indicates its design for excursion and velocity. The

brachioradialis does, however, have limitations as to excursion secondary to extrinsic soft tissue constraints and interconnections, which may limit its potential true excursion

when used in reconstructive procedures. These constraints

include presence of an internal tendon, as well as substantial fascial interconnections to the bellies of the neighboring

muscles and associated fascia (see earlier, under Anomalies

and Variations). Mobilization of the muscle and release of

these soft tissue constraints should increase the functional

range of excursion (135). When using the brachioradialis as

a donor for tendon transfer, it is optimal to mobilize and

free the muscle belly quite proximally in the forearm.

Awareness of the possible split muscle belly (and other

anomalies as described previously) avoids confusion if it is

encountered during harvest of the brachioradialis for tendon transfer (Fig. 2.5).

The brachioradialis may be a major participant in spastic flexion of the elbow in patients with acquired spasticity.

Selective denervation or recession (proximal release) of the

brachioradialis in selected patients can help relieve the flexion attitude of the elbow (155).

PRONATOR TERES

Derivation and Terminology. Pronator is derived from

the Latin pronus, meaning “inclined forward” (the Latin

pronatio denotes the act of assuming the prone position or

a state of being prone). Teres is derived from the Latin indicating “long and round” (1,2).

Origin. Two heads exist. The humeral (principal) head

originates from the anterior surface of the medial epicondyle (common flexor origin) and from the intermuscular septum. The ulnar (deep) head originates from the

medial border of the coronoid process (3,4).

Insertion. The middle third of the lateral surface of the

radius.

Innervation. Median nerve (C6, C7).

Vascular Supply. The ulnar artery, by direct muscular

arterial branches (3,4).

Principal Action. Pronation of the forearm, through

rotation of the radius on the ulna.

Gross Anatomic Description: Pronator

Teres

The pronator teres is the most radial muscle of the superficial flexors of the forearm (which also include the FCR, palmaris longus, FDS, and FCU). The pronator teres lies in

the superficial volar muscle compartment of the forearm

(Appendix 2.2). As the name implies, the pronator teres is

a long, round, and somewhat cylindrical muscle. The

pronator consists of two heads: a larger, more superficial

humeral head (often designated as the principal or primary

head ), and a smaller, deeper ulnar head (also referred to as

the accessory or deep head ). The humeral head has been

found to be consistently present. The ulnar head, however,

may be absent in approximately 22% of specimens

(156,157).

The humeral head arises from the common tendon of

the flexor–pronator muscles (see Fig. 2.2A). This tendon of

origin attaches to the medial epicondyle, arising from a

point of attachment on the proximal half of the anterior

surface of the epicondyle. The humeral head also arises

from the overlying antebrachial fascia, and from the intermuscular septum that separates the pronator teres from the

medial head of the triceps and the FCR.

The ulnar head is smaller, and positioned deeper. It arises

from an aponeurotic band attached to the medial border of

the coronoid process, located medial to the tendon of the

brachialis (see Fig. 2.3A). The origin is distal to the attachment of the FDS. The ulnar head joins the humeral head at

an acute angle. The morphology of the ulnar head is variable. In 11 of 60 limbs it was found to be muscular; in 6 of

110 Systems Anatomy

60 it was predominantly tendinous, and in 30 of 60, it was

found to be mixed (156,157). A fibrous arch is formed by

the humeral and ulnar heads. The arch is located within 3

to 7.5 cm of the arch created by the origin of the FDS muscle (158). In 83% of arms, the median nerve passes between

the pronator muscle heads. The median nerve is at risk for

compression as it passes through this arch (147,156–165).

The nerve is separated from the ulnar artery by the ulnar

head of the pronator (3,4,13).

The humeral and ulnar head join to form a common

muscle belly. The muscle passes obliquely across the proximal volar forearm in a medial-to-lateral direction. The muscle fibers converge to end in a flat tendon that attaches to a

rough area on the lateral surface of the radial shaft (see Fig.

2.3B). The point of insertion is roughly at the junction of

the proximal third and distal two-thirds of the radius, at the

“summit” of the lateral curve of the radius (3,4). The lateral

border of the muscle forms the medial border of the cubital

fossa. At the point of insertion, the tendon of the pronator

teres becomes broader and winds around the anterior surface of the radius, finally attaching to the cortex. Most of

the insertional tendon is continuous with muscle fibers

from the humeral head. The muscle fibers of the ulnar head

extend distally along the lateral border of the fibers from the

humeral head. Much of the ulnar head inserts or blends

into the radial side of the deep surface of the humeral head

(3,4,8,11).

Architectural features of the pronator teres include the

physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle

(angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural

studies by Lieber, Friden, and colleagues provide the data for

the pronator teres (135–139) (see Table 2.1 and Fig. 2.4).

The pronator teres has a relatively large physiologic cross-sectional area, indicating that its design is more optimal for force

generation. It has a relatively short muscle fiber length, indicating that it is not specifically designed for excursion or

velocity. Its relative difference index values compare it with

other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3.

The pronator teres is innervated by a branch or branches

from the median nerve (C6, C7). Each head receives a separate branch. The branches usually exit the median nerve

trunk before the median nerve passes between the two

heads of the pronator. The nerve branch to the humeral

head enters the proximal part of the middle third of the

belly of the muscle, on its deep surface near the radial border. The branch to the ulnar head usually enters the muscle

proximal to the point where the two bellies join (11).

Actions and Biomechanics: Pronator Teres

The pronator teres pronates the forearm and acts with

cocontraction of the pronator quadratus. With full flexion

of the elbow, the fibers of the muscle are short and unable

to produce maximal force. The pronator teres also functions

as a weak elbow flexor (3,4,11).

Anomalies and Variations: Pronator Teres

A supracondylar process is a small, curved, hook-shaped

process of the distal humerus, several centimeters proximal

to the elbow, and usually located on the medial side. It often

is associated with a ligament (or muscle) slip that extends

distally to the medial epicondyle. The ligament, known as

the ligament of Struthers, is thought to be an extension of

the pronator teres. The median nerve may pass deep to the

ligament, and may thus be at risk for compression

(166–168). The brachial artery also may pass deep to a ligament of Struthers, and brachial artery entrapment (presenting as ischemia during extension of the elbow) may

occur (169).

Accessory slips may attach from the pronator teres to the

biceps brachii, brachialis, or to the median intermuscular

septum. Nebot-Cegarra et al. studied 60 upper extremities

and found slips to the biceps brachii in 3.3%, to the

brachialis in 5.0%, to the FDS muscle in 1.6%, and to

Gantzer’s muscle in 1.6%. In all cases, the accessory slips

were connected to the deep (humeral) head, and were in the

vicinity of the median nerve, possibly producing a risk for

nerve encroachment (156).

Clinical Correlations: Pronator Teres

The median nerve may become compressed as it passes

between the humeral and ulnar heads of the pronator teres,

referred to as pronator syndrome (147,156–165).

The median nerve (and brachial artery) may become

compressed if passing deep to the anomalous ligament of

Struthers. The ligament of Struthers, which is thought to be

an extension of the pronator teres, originates from a supracondylar process of the humerus and attaches to the medial

epicondyle (166–169).

FLEXOR CARPI RADIALIS

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Carpi is from the Latin

carpalis and Greek karpos, both of which indicate “wrist”

(the carpus). Radialis is from the Latin radii, which means

“spoke” (used to describe the radius of the forearm) (1,2).

Origin. Medical epicondyle through the common flexor

origin.

Insertion. To the volar base of the index finger

metacarpal. An accessory slip may attach to the adjacent

volar base of the long finger metacarpal.

Innervation. Median nerve (C6, C7).

2 Muscle Anatomy 111

Vascular Supply. The ulnar aspect by direct intramuscular branches; superior and inferior ulnar collateral arteries;

to a variable degree, contributions from the anterior and

posterior ulnar recurrent arteries; in the distal aspect, superficial palmar branch of the radial artery; at the insertion, the

palmar metacarpal arteries and perforating branches from

the deep palmar arch. The anterior interosseous artery also

may supply the FCR (3,4,8,11).

Principal Action. Flexion of the wrist. Working with the

radial wrist extensor, the FCR can assist with wrist radial

deviation.

Gross Anatomic Description: Flexor Carpi

Radialis

The FCR comprises one of the more radially located muscles of the superficial flexors of the forearm (along with the

pronator teres, palmaris longus, FDS, and FCU). The FCR

lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The muscle is positioned between the

pronator teres (medially) and the palmaris longus (laterally)

(170). It originates from the common flexor origin of the

medial epicondyle (see Fig 2.2A). Additional sites of attachment include the adjacent intermuscular septum and the

adjacent fascia of neighboring muscles. The muscle belly is

relatively large and fusiform, and usually extends to at least

the mid-portion of the forearm halfway to the wrist. The

muscle fibers from the epicondyle extend distally in a vertical fashion to the anterior and sides of the tendon. The

fibers that originate from the intermuscular septa tend to

extend in an oblique fashion to the deep surface of the tendon. The mid-portion of the muscle belly lies in the central

portion of the proximal forearm. The myotendinous junction spans several centimeters and gives rise to a long ten112 Systems Anatomy

FIGURE 2.6. Anterior (A) and posterior (B) views of the skeleton hand, showing muscle origins

(red) and insertions (blue).

A

don. Studies by Bishop et al. have shown the myotendinous

portion of the muscle begins an average of 15 cm (range, 12

to 17 cm) proximal to the radiocarpal joint. The muscular

fibers end an average of 8 cm (range, 6 to 9 cm) proximal

to the wrist (171). The tendon is initially flat, but becomes

rounder as it continues distally. The tendon passes across

the distal half of the forearm, coursing distally and radially

to the wrist. There is a torsional component of the tendon

as it passes distally (172). The radial artery usually is located

radial to the tendon of the FCR, situated between it and the

brachioradialis. The tendon passes radial to the carpal tunnel, and travels through its own fibroosseous tunnel formed

in part by a groove in the trapezium and overlying fibrous

arch. The tendon occupies 90% of the space in the

fibroosseous tunnel and is in direct contact with the slightly

roughened surface of the trapezium (171). The tendon does

not pass through the carpal tunnel. In this distal portion of

its course, the tendon often has a synovial sheath. The tendon dives deep, deep to the oblique head of the adductor

pollicis, to reach the proximal aspect of the base of index

metacarpal (Fig. 2.6A). The tendon inserts into the proximovolar aspect of the index metacarpal, and also commonly

sends a slip to the adjacent base of the long finger

metacarpal (173). A small slip often attaches to the trapezial

crest or tuberosity (171). The insertion tendon of the FCR

extends out from the muscle mass a distance equivalent to

approximately 75% of the muscle length.

Architectural features of the FCR include the physiologic cross-sectional area of the muscle, the fiber bundle

length, muscle length, muscle mass, and pennation angle

(angle of the muscle fibers from the line representing the

longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the

data for the FCR (135–139,174) (Table 2.2; see Fig. 2.4).

2 Muscle Anatomy 113

FIGURE 2.6. (continued)

B

The FCR has a moderate fiber length and physiologic crosssectional area, indicating that its design is moderate for

both excursion and force generation. Its relative difference

index values compare it with other upper extremity muscles, based on architectural features. These values are listed

in Appendix 2.3. In comparing the architectural features of

the FCR with the FCU, the FCR muscle length is shorter

than the FCU, although the muscle fibers of the FCR are

longer (136,174). The relatively longer fiber length indicates that the FCR is designed more for excursion and

velocity of contraction (because excursion and velocity are

proportional to fiber length) compared with the FCU. The

FCU, in contrast, has a higher pennation angle with a larger

physiologic cross-sectional area. This indicates that the

FCU is designed more for force production and less for

excursion and velocity compared with the FCR (because

cross-sectional area is proportional to force production)

(174,175) (see Table 2.2 and Fig. 2.4).

The FCR is innervated by the median nerve (C6, C7,

C8). It usually is supplied by a direct branch that divides

into smaller branches before entering the muscle. The nerve

branches usually enter the muscle near the junction of its

proximal and middle third, and enter on the deep surface

(3,4,176).

Actions and Biomechanics: Flexor Carpi

Radialis

The FCR functions mainly to flex the wrist. It works with

the FCU and the digital flexors during strong wrist flexion.

In addition, in working with the ECRL (and ECRB), the

FCR may assist with radial deviation of the wrist. The FCR

also can assist with elbow flexion, and can act as a relatively

weak pronator of the forearm.

As noted previously, from an architectural standpoint in

comparison with the FCU, the relatively longer fiber length

of the FCR indicates that it is designed more for excursion

and velocity than for force production (135,174).

Anomalies and Variations: Flexor Carpi

Radialis

The FCR may be absent (11,177). The FCR may exist as a

double or split muscle (11,178,179). Several accessory slips

114 Systems Anatomy

TABLE 2.2. ARCHITECTURAL FEATURES OF WRIST EXTENSOR AND FLEXOR MUSCLES

One-Way

Measured Properties of Muscles and Tendonsa ANOVA

Significance

Parameter ECRB ECRL ECU FCR FCU Levelb

Muscle properties

Muscle length (mm) 186.4 ± 4.5 155.3 ± 6.9 209.9 ± 6.0 192.8 ± 4.8 220.6 ± 8.6 p < .01

Fiber length (mm) 70.8 ± 1.7 127.3 ± 5.6 58.8 ± 1.7 59.8 ± 1.5 41.9 ± 1.6 p < .0001

Physiological CSA (mm2) 240.1 ± 20.5 130.0 ± 11.1 210.0 ± 14.1 211.9 ± 15.4 363.6 ± 34.3 p < .0001

Predicted maximum tetanic 58.8 ± 5.0 31.9 ± 2.7 51.5 ± 3.4 51.9 ± 3.7 89.0 ± 8.4 p < .0001

tension (N)

Tendon properties

Aponeurosis length (mm) 101.3 ± 2.1 81.9 ± 15.2 153.7 ± 7.6 126.5 ± 5.8 160.6 ± 10.3 p < .0001

External tendon length (mm) 102.7 ± 4.6 182.1 ± 5.1 61.4 ± 8.7 103.8 ± 7.4 47.0 ± 4.7 p < .0001

Total tendon length (mm) 204.0 ± 4.4 264.1 ± 15.7 215.1 ± 4.9 230.3 ± 5.6 207.6 ± 9.1 p < .0001

Tendon length: fiber length 2.89 ± 0.11 2.10 ± 0.18 3.67 ± 0.13 3.86 ± 0.12 4.96 ± 0.18 p < .0001

ratio

Tendon CSA (mm2) 14.6 ± 0.7 14.2 ± 0.5 15.7 ± 1.4 17.7 ± 1.6 27.4 ± 3.6 p < .01

Tendon stress at P0 (MPa) 4.06 ± 0.29 2.30 ± 0.27 3.36 ± 0.25 3.06 ± 0.32 3.54 ± 0.66 p = .06

Tendon strain at P0 (%) 1.99 ± 0.20 1.78 ± 0.14 2.35 ± 0.30 2.48 ± 0.45 3.68 ± 0.31c p < .005

Modulus at P0 (MPa) 726.1 ± 73.5 438.1 ± 93.7 721.6 ± 167.3 595.4 ± 93.0 448.0 ± 95.7c p > .2

Ultimate stress (MPa) 71.3 ± 6.4 67.9 ± 4.4 70.8 ± 3.4 74.0 ± 13.5 51.6 ± 9.3c p > .4

Tangent modulus (MPa) 904.7 ± 161.2 604.1 ± 113.6 102.1 ± 131.9 857.5 ± 142.1 540.6 ± 152.6c p > .1

Safety factor (× P0) 18.0 ± 1.7 31.8 ± 4.4 21.4 ± 0.6 23.7 ± 2.7 16.8 ± 5.2c p < .05

Biochemical properties

Hydration (% dry mass) 77.0 ± 1.5 74.4 ± 2.9 80.3 ± 2.0 79.3 ± 1.8 83.6 ± 2.0 p = .06

Collagen (% dry mass) 77.0 ± 2.0 78.4 ± 2.1 79.6 ± 1.0 74.0 ± 5.1 69.4 ± 5.4 p > .3

ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris; FCR, flexor carpi radialis; FCU, flexor carpi

ulnaris; CSA, cross-sectional area; P0, muscle maximum tetanic tension. aValues shown are mean ± standard error of n = 5 independent measurements. bSignificance level from one-way analysis of variance (ANOVA).

c

Signifies n = 4.

Reproduced from Loren GJ, Lieber RL. Tendon biomechanical properties enhance human wrist muscle specialization. J Biomech 28:791–799,

1995, with permission.

of the FCR may exist in the proximal forearm, including

slips to or from the biceps tendon, brachialis, bicipital

aponeurosis, coronoid process, or radius. In the distal forearm, the FCR may have slips that attach to the trapezium,

scaphoid, flexor retinaculum, or fourth metacarpal. Partial

or total insertion into the trapezium is the more common

insertional anomaly (11).

An FCR brevis has been described as a small muscle

arising from the radius and usually inserts into the fibrous

sheath of the tendon of the FCR. It was noted in 6 of 70

limbs by Wood, and in 1 of 400 limbs by Gruber (11), as

well as in a more recent case report by Effendy (180). An

additional, different FCR brevis muscle was described as

an anomalous muscle that originates from the anterior surface of the radius and forms a tendon at the radiocarpal

joint. It enters the carpal tunnel and the tendon extends

between the bases of the index and long finger metacarpals

to interconnect with the tendon of the ECRB. The muscle

is innervated by the anterior interosseous nerve (181). In

addition, it was noted that the ECRB had split into two

tendons, one inserted normally into the radial part of the

base of the long finger metacarpal and the other connected

to the anomalous FCR brevis. It was postulated that this

anomaly may cause restricted wrist flexion or extension

(11).

Clinical Correlations: Flexor Carpi

Radialis

The FCR, innervated by the median nerve, is a common

muscle used for transfer to the extensor digitorum communis (EDC) to provide digital extension in patients with

radial nerve palsy (182–185). From an architectural standpoint, its design for greater excursion makes it (architecturally) a better choice than the FCU, which is designed

more for force generation (see earlier, and Fig. 2.4C).

Attritional rupture of the FCR has been noted to occur

in association with scaphotrapezial osteoarthritis (186).

PALMARIS LONGUS

Derivation and Terminology. Palmaris is derived from

the Latin palma, which means “pertaining to the palm.”

Longus is the Latin for “long” (1,2).

Origin. Medical epicondyle through the common flexor

origin.

Insertion. The palmar fascia of the hand.

Innervation. Median nerve (C7, C8).

Vascular Supply. Muscle belly: the ulnar artery, brachial

artery, superior and inferior ulnar collateral arteries, anterior interosseous artery, and variable contributions from the

anterior and posterior ulnar recurrent arteries. Distal tendon: rami from the ends of the superficial arch (3,4,8,11,

187,188).

Principal Action. Flexion of the wrist. It also contributes

to anchoring of the palmar fascia to resist horizontal shearing forces moving distally in the hand. It can assist with

weak pronation of the forearm.

Gross Anatomic Description: Palmaris

Longus

The palmaris longus comprises one of the central muscles

of the superficial flexors of the forearm (along with the

pronator teres, FCR, FDS, and FCU). It lies in the superficial volar muscle compartment of the forearm (Appendix

2.2). The palmaris longus is small but clinically important

(189–194). It also is well documented as one of the most

variable, in terms of presence (or absence) (195–206) as

well as muscle variations and anomalies (207–262). It is

clinically important because of its value as a free tendon

graft. Because absence is relatively common, this variation

is discussed here instead of under Variations and Anomalies. Its absence has been the subject of several anatomic

investigations (197–206). The incidence sometimes is

given in terms of patients (or cadavers), or in terms of

limbs. The frequency of absence in one or both limbs has

been noted from 6% (197) to as high as 31% to 64%

(187,194,198). Most studies indicate an absence in one or

both limbs in approximately 12% to 25% of patients (or

cadavers) (11,196,199), or 5% to 15% of individual limbs

(187,194,231).

In 2001, in a relatively large study, Thompson et al.

examined 300 caucasian subjects (150 male, 150 female)

and found unilateral absence of the palmaris longus in 49

subjects (16%), and bilateral absence in 26 (9%) (199).

The rate of absence of the tendon may be different in

different ethnicities. Reporting in the Indian Journal of

Medical Sciences, Ceyhan and Mavt in 1997 evaluated

7,000 students of the Graduate School at Gaziantep University for absence of the palmaris longus (198). Findings

included, in women, unilateral absence in 23% and bilateral absence in 45.3%. In men, unilateral absence was

found in 19.5% and bilateral absence in 42.1%. The overall percentage of absence was 63.9%. This is among the

highest reported absence rates (198).

One of the lowest rates of absence was reported by

Troha and colleagues in 1990 (197). In 200 caucasian

patients (100 men, 100 women), the tendon was absent in

one extremity in only 3% of patients. Bilateral absence

was seen in 2.5%, for a 5.5% rate of total overall absence

(197). In addition, the frequency of absence has been as

low as 3.5% in the Japanese population and 2% in the

Chinese population (11).

There is disagreement as to the frequency of unilateral

versus bilateral absence. Several studies and authors have

noted a more common occurrence of bilateral absence

(11,198). However, studies do not consistently support this

(197,199). If a patient has a tendon absence on one side, it

2 Muscle Anatomy 115

was shown that there is a 67% chance that the contralateral

tendon also will be absent (196).

Although some suggest that the palmaris longus is absent

more often in women, and more often on the left side (11),

Thompson et al., in a study of 300 caucasian subjects,

showed no statistical differences between the sexes or in

absence in the right versus the left extremity (199).

It has been suggested that there may a higher incidence

of Dupuytren’s disease in patients with a present palmaris

longus tendon (200). Additional investigations with larger

populations are needed to substantiate this association.

From an anatomic standpoint, the tendon arises with the

other superficial flexors (including the pronator, FCR,

FDS, and FCU) from the common flexor origin of the

medial epicondyle of the humerus (see Fig. 2.2A). It is slender, usually fusiform or slightly triangular, and located ulnar

to the FCR and superficial and radial to the FDS. Besides

the medial epicondyle, the palmaris longus has proximal

attachments to the neighboring superficial muscle fascia as

well as from the intermuscular septa and deep antebrachial

fascia. The muscle fibers are aligned in a nearly parallel

course to the tendon. The muscle usually has a fairly abrupt

myotendinous junction located in the mid-portion of the

forearm, giving rise to a long, slender tendon. The tendon

extends distally, superficial to the flexor retinaculum. It

becomes broad and flat to form a sheet that connects or is

continuous with the palmar fascia (palmar aponeurosis) of

the hand. A few connections may interweave with the transverse fibers of the retinaculum, although most of the fibers

are oriented longitudinally in a proximal-to-distal direction.

The radiating fiber bundles on the radial and ulnar aspects

extend distally to attach to the overlying fascia of the thenar

and hypothenar muscles. The more central bundles usually

are more developed and constitute the more substantial

portion of the palmar fascia (3,4,8).

Fahrer has shown that the proximal end of the palmar

fascia receives two important contingents of fibers from the

FCU. A superficial component blends with the fibers of the

palmaris longus; a deep component runs on the surface of

the pisohamate ligament and connects the flexor retinaculum to the palmar fascia (193).

The tendon and palmar fascia continue distally to form

a diverging sheet that splits longitudinally to send thickenings of the fascia to each of the four rays, with variable fiber

bundles extending toward the thumb (3,4). These diverging

fiber bundles form a triangular connective tissue sheet in

the midpalm with the apex proximal. The palmar fascia has

interconnections with the fibroosseous tendon sheaths,

with the skin, and in the fascia of the distal palm and digital webs.

Although the palmaris longus often is absent, absence of

the palmar fascia has not been noted (194). From gross and

microscopic observations, as well as staining properties, the

palmaris longus tendon and palmar fascia appear as tendon

and fascia, respectively. These observations support the idea

that the palmaris longus and palmar fascia are separate

anatomic structures that develop independently and are

associated only by anatomic proximity (194).

The morphology and biomechanical aspects of the palmaris longus tendon have been evaluated in terms of its use

as a tendon graft, and in comparison with other tendons

used as grafts (191). The palmaris longus mean tendon

length is 161 mm, its mean cross-sectional area 3.1 mm2

,

and its mean volume 529 mm3

. The tendon is among the

stiffest at 42.0 N/mm (191). The average width of the palmaris tendon is approximately 3 mm (189,191).

The arterial supply has been studied in detail by Wafae

and associates (187). Most muscles received one or two

arterial branches from the ulnar artery (86%), and less frequently from the brachial artery (23%). The arterial

branches penetrate the muscle through the posterior surface, 63% in the proximal third and 34% in the middle

third of the muscle. The most frequent patterns observed

included one or two branches of the ulnar artery penetrating the proximal third of the muscle (29%), and two

branches of the ulnar artery, one entering the proximal

third and one entering the middle third of the muscle belly

(187).

The architectural properties of the palmaris longus are

listed in Table 2.1 and shown in Fig. 2.4A and B.

The palmaris longus is innervated by the median nerve

(C7 and C8). The nerve branch usually is a common

branch from the median nerve that also supplies the FCR.

It often courses along with the branch supplying the proximal part of the FDS. The nerve to the palmaris longus usually enters in the middle third of the muscle (3,4,8,11).

Actions and Biomechanics: Palmaris

Longus

The palmaris longus is a weak flexor of the wrist. The

muscle also may assist with a relatively weak contribution

to forearm pronation. It may represent an evolutionary

remnant of a flexor of the metacarpophalangeal (MCP)

joints (188) because it appears that the palmar fascia

extends to that level. In addition, the palmaris longus

plays a role in the stabilization of the palmar fascia. A purpose of the palmar fascia is to anchor the skin on the palm

to resist shearing forces (compared with the loose skin on

the dorsum of the hand, the palmar skin is relatively

immobile). This anchoring of the skin assists with grasp

functions, so that objects do not move or shift during

tight grasp. The palmaris longus, which has power to

apply force to the palmar fascia, contributes to this

anchoring of the palmar fascia to resist horizontal shearing

forces moving distally in the hand.

It has been postulated by Fahrer that, in congenital

absence of the palmaris, the FCU takes over as the longitudinal tensor of the palmar fascia through interconnecting

fibers of the tendon and the palmar fascia (192,193).

116 Systems Anatomy

Fahrer and Tubiana suggest that the palmaris longus contribute to opposition and pronation of the thumb under

some circumstances (192). The palmaris longus, however, is

restricted in this motion because it is tethered by its tendon’s medial slip and terminal insertion that attaches to the

palmar fascia (192).

Kaplan and Smith also give credit to the palmaris longus

as a synergist in thumb opposition (195). The tendon

becomes tense when opposition of the thumb is attempted

or maintained. The contraction is thought to produce synergistic tension of the transverse carpal ligament to provide

better fixation at the origins of the thenar muscles (195). In

addition, the palmaris longus tendon often has a slip that

inserts into the abductor pollicis brevis (APB) and can

therefore act directly on the muscle during opposition. It

was concluded by Kaplan and Smith that the palmaris is an

unimportant flexor of the wrist but a strong synergist of

abduction and opposition of the thumb. In paralysis of the

other flexors of the wrist, the palmaris longus may become

a fairly important wrist flexor if it has a firm insertion into

the transverse carpal ligament or the carpal bones (195).

Anomalies and Variations: Palmaris

Longus

The palmaris longus is one of the most variable muscles in

the upper extremity (195). The presence (or absence) of the

palmaris longus is quite variable. In general, there is an

absence in one or both limbs in approximately 12% to 25%

of patients (or cadavers) (196,199), or absence in individual

limbs in 15% to 31% (187,194). Because absence is relatively common, the incidences are discussed in more detail

earlier, under Gross Anatomic Description.

Several variations have been reported (207–262) (Fig.

2.7). These have clinical implications because of the value

of the palmaris as a free graft or transfer. An awareness of

the variability of the palmaris may help avoid difficulty or

confusion in the harvest of the free graft. In addition, many

of the anomalous muscles cause problems with nerve compression, including the median nerve in the forearm

(207–210,224) and the carpal tunnel (229–231), the palmar cutaneous branch of the median nerve (230–232), and

the ulnar nerve (225,233–235). The more common variations and anomalies are as follows:

Distal Belly (Reverse Belly, Palmaris Longus

Inversus)

The palmaris longus can have a distal or reverse muscle

belly (see Fig. 2.7). In the reversed form, the tendon is

proximal and the muscle is distal. Variations of this form

have been referred to as the palmaris longus inversus (11).

The distal muscle can cause median neuropathy in the

forearm (208,209,211). If the muscle reaches or enters the

carpal tunnel, carpal tunnel syndrome can result (212,

219). A reversed muscle also can lead to ulnar nerve compression (234).

Digastric Head

The palmaris longus may have a digastric head (two heads,

one proximal and one distal, separated by an intercalary tendon) (see Fig. 2.7). The distal muscle belly may cause median

neuropathy in the forearm or, if it reaches or enters the carpal

tunnel, can result in carpal tunnel syndrome (227–232).

Split or Double Belly Tendon

The muscle may be split along its course, presenting as two

separate muscle bellies (see Fig. 2.7). When two bellies are

present, there may be several variations of the origin and

insertion attachments (198,237). The tendon itself may be

split or doubled (11,231). Dowdy and colleagues identified

2 specimens of 52 with a split palmaris longus tendon

(231). The palmar cutaneous branch of the median nerve

passed through the split at 1 to 1.5 cm proximal to the

insertion into the palmar fascia. In the presence of this

anomaly, the nerve is at risk for injury in the harvest of the

tendon. The authors recommend transecting the tendon

2 cm proximal to its insertion into the palmar fascia to

avoid possible nerve injury (231). In addition, this split

may place the nerve at risk for compression neuropathy.

Palmaris Longus Profundus

The palmaris profundus is an anomalous palmaris longus

that arises from the lateral edge of the radius, in its middle

third, external to the FDS and deep to the pronator teres.

The tendon passes deep to the flexor retinaculum (to the

radial side of the median nerve) and broadens in the palm

to insert into the deep side of the palmar aponeurosis

(11,215,221,228). It can be noted as an incidental operative finding without any clinical consequences. However, as

it enters the carpal canal, it can result in carpal tunnel syndrome (221). It has been reported to occur bilaterally

(215,221,228). The muscle also can cause ulnar nerve compression at the wrist (261).

Palmaris Bitendinous

The palmaris bitendinous is an anomalous muscle that is

located deep to the palmaris longus and has a distal insertion on the deep surface of the palmar aponeurosis, similar

to the palmaris profundus. It can result in median neuropathy in the forearm and hand (210).

Continuous Muscle

The palmaris longus may have one continuous muscle

from origin to insertion. The distal muscle extension can

2 Muscle Anatomy 117

118

FIGURE 2.7. The normal palmaris longus and some of its clinically relevant variations. The palmaris longus with a distal muscle belly may be responsible for median or ulnar nerve compression. The median nerve can be compressed either in the distal forearm or in the carpal tunnel if

an anomalous portion or slips extend into the canal. The split or duplicated muscle belly of the

palmaris longus and the digastric variation (with a distal belly) may cause difficulty or confusion

during harvest for transfer or free graft if these possible variations are not appreciated or recognized. The digastric form also may contribute to median and ulnar nerve compression in the

forearm (11).

cause median neuropathy in the forearm or carpal tunnel

(195).

Central Belly

The muscle belly may be located centrally between two tendons, so that the origin and insertion are both tendinous

(195,237).

Continuous Tendon

The palmaris longus may exist only as a tendon from origin

to insertion (11,195).

Triple Muscle Bellies

The muscle may exist as three distinct muscle bellies (195).

The tendon also may be split or triplicated (11).

Variable Origin

The site of origin is variable, and has been noted to include

attachments to the fascia of most of the muscles of the ulnar

side of the forearm (including the biceps, brachialis, and

FDS), from the medial intermuscular septum, from the

coronoid process of the ulna, and from the proximal radius

(11,195). With a double muscle belly, one can arise in a

normal fashion from the medial epicondyle, and the other

from the aforementioned muscles, fascia, intermuscular

septum, proximal ulna, or proximal radius (11,195).

Variable Insertion and Accessory Distal Slips

The site of insertion is equally as variable as the site of origin. It may have abnormal extensions, anomalous slips, an

abnormal split, and associated anomalous muscle bellies

(11,195,215,216,222–225,250). The palmaris longus may

insert into the tendon of the FCU, transverse carpal ligament, antebrachial fascia, scaphoid, pisiform, or APB

(195). It commonly has fascial extensions to the fascia of

the base of the thenar and hypothenar muscles (and attachments to these muscles are so common they may be considered part of the normal insertion). The tendon can insert

onto the deep surface of the palmar fascia (260). Several

accessory slips or anomalous muscle heads at the insertional

area have been identified. The accessory slips may attach to

various flexor tendons and extend distally as far as the MCP

joint (11). Median nerve compression in the forearm and

carpal tunnel has been associated with the accessory slips,

especially if the anomalous tendon or muscle enters the

carpal tunnel (215,216,222,224,250). An accessory muscle

inserting into the base of the hypothenar muscles has been

shown to cause carpal tunnel syndrome (213). An ulnarsided palmar accessory muscle was noted to cause ulnar

tunnel syndrome (234,236). An accessory slip has been

noted to split from the palmaris longus tendon and enter

the ulnar tunnel to cause ulnar tunnel syndrome (225,233).

The ulnar artery also may be compressed by an anomalous

palmaris longus slip that enters the ulnar tunnel (11).

Intrapalmar Muscle

An intrapalmar accessory head of the muscle has been identified in the carpal tunnel, causing carpal tunnel syndrome

(223).

Palmaris Longus and the Accessories Ad

Flexoram Digiti Minimi

The tendon of the palmaris longus may give origin to an

additional muscle, the accessories ad flexorum digiti minimi. This muscle usually inserts on the body and head of the

fifth metacarpal between the abductor digiti minimi and

flexor digiti minim brevis (11).

Palmaris Longus Substituting for Digital

Flexors

The palmaris longus can substitute for the ring finger FDS.

In the absence of the FDS, a palmaris longus was found to

extend to the middle phalanx of the ring finger and function as a digital flexor of the proximal interphalangeal joint

(PIP) (259).

Clinical Correlations: Palmaris Longus

The most important anatomic clinical considerations with

the palmaris longus include its variable presence and the

common anomalies. The specific anatomic forms are discussed in detail previously. The possible variations and

anomalies are important both from the standpoint of free

tendon harvest or transfer, as well as with regard to the

many associated nerve compression syndromes caused by

an anomalous palmaris longus tendon. Problems associated with anomalous muscles include median compression

in the forearm (208–210,212,224) and the carpal tunnel

(131–229), compression of the palmar cutaneous branch

of the median nerve (230–232), and compression of the

ulnar nerve in the forearm or ulnar tunnel (233–236,261)

(discussed in detail earlier, under Variations and Anomalies).

The possibility of absence is of clinical significance

because of the common use of the palmaris longus as a free

graft or tendon transfer (263–267). Its presence always

should be tested by having the patient place the pulp of the

thumb in opposition to the pulp of the small finger. When

the wrist is flexed, the tendon of the palmaris becomes

prominent. In general, there is an absence in one or both

limbs in approximately 12% to 25% of patients (or cadavers) (11,196,199) and absence in individual limbs in 15%

2 Muscle Anatomy 119

to 31% (187,199) (discussed in detail earlier, under Gross

Anatomic Description).

Magnetic resonance imaging or ultrasound (UTZ) have

been shown to be capable of detecting the absence of the

palmaris longus or the presence of anomalies (225,250).

Hypertrophy of a normal palmaris longus tendon can

result in median neuropathy simulating carpal tunnel syndrome (11,212,218).

For low median neuropathy, such as with severe, longstanding carpal tunnel syndrome, the Camitz transfer is a

type of opponensplasty used to provide thumb palmar

abduction and opposition. It was popularized by Braun and

uses the palmaris longus, extended by a strip of palmar fascia, to transfer to the thenar muscles (266–272).

FLEXOR DIGITORUM SUPERFICIALIS

(FLEXOR DIGITORUM SUBLIMIS)

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Digitorum is from the

Latin digitus or digitorum, indicating the digits. Superficialis

denotes its superficial location in the forearm. The term

sublimis sometimes is used. This is derived from Latin sublimis, indicating “superficial” (1,2).

Origin. There are two heads with separate origins.

Humeroulnar head: from the medical epicondyle of the

humerus and from the proximal medial ulna. Radial head:

from a long, oblique, linear attachment from the volar

proximal radial shaft, along the proximal third of the diaphysis.

Insertion. To the medial and lateral margins of the volar

shaft of the middle phalanges of the index, long, ring, and

small fingers.

Innervation. Median nerve (C7, C8, T1).

Vascular Supply. The ulnar artery, superior and inferior

ulnar collateral arteries, anterior and posterior ulnar recurrent arteries, superficial palmar arch, common and proper

palmar digital arteries (3,4,8).

Principal Action. Flexion of the PIPs of the index, long,

ring, and small fingers. It also contributes to flexion of the

digital MCP joints, and flexion of the wrist.

Gross Anatomic Description: Flexor

Digitorum Superficialis

The FDS is one of the central muscles of the superficial

flexors of the forearm (along with the pronator teres, FCR,

palmaris longus, and FCU). It lies in the superficial volar

muscle compartment of the forearm (Appendix 2.2). The

FDS is located medial and deep to the palmaris longus and

FCR. The FCU lies ulnar and superficial to the FDS. It is

an important flexor of the digits, and is one of the largest of

the superficial flexor muscles of the forearm (3,4,8,13).

The FDS has two main heads, the humeroulnar head and

the radial head. The humeroulnar head has several origin

sites. It arises, in part, from the medial epicondyle through

the common flexor origin (see Fig. 2.2A). The muscle has

additional origin attachments from the anterior band of the

ulnar collateral ligament, from adjacent intermuscular septa,

and from the medial side of the coronoid process proximal to

the ulnar origin of the pronator teres (Fig. 2.3A). Additional

origin attachments may connect to the fascia of the

brachialis. The radial head is a long, thin, flat muscular sheet.

It arises from the oblique line of the radius, which is a long,

linear, oblique attachment area from the volar radial shaft in

its proximal third (see Fig. 2.3A). The origin extends distally

from the anterior lateral border of the radius, just proximal to

the insertion of the pronator teres. The origin of the FDS

extends along the anterior diaphysis proximally and medially

to reach the medial side of the radial tuberosity. The two

heads form a muscular arch, through which the median nerve

and ulnar artery pass. The muscular arch formed by the FDS

is a well known site for potential median nerve compression,

especially in forearm compartment syndromes or ischemic

contracture (3,4,11,13,18,273–276).

The muscular fibers extend distally, with the fiber bundles of the ulnar head and the upper part of the radial head

converging. The ulnar fiber bundles extend distally in a vertical fashion. The fibers from the radial head extend distally

obliquely to form a common belly. The deep surface of the

FDS on the ulnar side usually is covered by a dense tendinous or fibrous sheet (3,4).

The muscle belly of the FDS forms two separate submuscle bellies (273–284). These resemble planes or sheets of

muscle fibers (4), referred to as strata by Williams (3). There

is a deep and superficial plane of fibers. The superficial plane

of fibers further divides into two parts that end in the tendons for the long and ring fingers. Similarly, the deep plane

of fibers further divides into two parts, which end in the tendons for the index and small fingers (4). Of these muscles,

the FDS belly to the long finger may arise more independently than the others (277). Before dividing, the deep plane

gives off a muscular slip to join the portion of the superficial

plane associated with the tendon of the ring finger. The

arrangement of deep and superficial muscle planes is retained

at the wrist level. As the four tendons continue distally in the

forearm and pass deep to the flexor retinaculum, they still are

arranged in pairs, the superficial and deep. The superficial

pair, located superficial and the central of the four tendons,

continues to the long and ring fingers. The deep pair, located

deep and at the radial and ulnar margins of the four tendons,

continues to the index and small fingers, respectively. (Note:

This arrangement of the tendons at the distal forearm and in

the carpal tunnel can be simulated on one’s own hand. If one

touches the index and small fingers behind the ring and long

fingers, the pattern of tendons is roughly simulated, with the

ring and long tendons located superficial and central, and the

index and small finger tendons located deep and to the radial

120 Systems Anatomy

and ulnar margins, respectively.) The tendons diverge from

one another in the palm and extend distally deep to the

superficial palmar arterial arch and the digital branches of the

median and ulnar nerves. At the level of the base of the proximal phalanges, each tendon divides into two slips. The divergence of the two slips forms an interval through which the

associated tendon of the FDP passes. The two slips of the

FDS then rotate 90 to 180 degrees, flattened against the profundus tendon. The slips thus encircle the profundus tendon.

At the side of the profundus tendon, the spiraling, flat bands

of the FDS tendon have rotated such that the fibers that were

nearest to the midline in the undivided tendon become the

most volar at the sides of the middle phalanx. These anterior

fibers continue on the same side of the profundus tendon

attached to the proximal part of the ridge on the margin of

the middle phalanx. The posterior fibers sweep around the

profundus tendon to reunite dorsal to the profundus. The

two portions of the FDS reunite at Camper’s chiasma. In this

area, the FDS slips form a grooved channel for passage of the

profundus. At the level of Camper’s chiasma, the FDS slips

decussate in an “X” pattern (behind the profundus, on the

volar surface of the middle phalanx) and pass distally to

attach to the distal part of the ridge on the opposite margins

of the middle phalanx (282). Each slip of the tendon of the

FDS inserts into the medial and lateral aspects of the volar

shaft of the associated digit (see Fig. 2.6A). The chiasma can

be variable in terms of anatomy and morphology (3,4,284).

The vascular supply to the tendons comes from several

sources. These include the longitudinal vessels (some of

which may originate in the muscle belly) that enter in the

palm and extend down intratendinous channels; vessels that

enter at the level of the proximal synovial fold in the palm;

segmental branches from the paired digital arteries that

enter in the tendon sheaths by means of the long and short

vincula; and the vessels that enter the FDS and FDP tendons at their osseous insertions (285–299). In the digital

sheath, the segmental vascular supply to the flexor tendons

is through long and short vincular connections. These

include the vinculum brevis superficialis, the vinculum brevis profundus, the vinculum longum superficialis, and the

vinculum longum profundus. The vincula often are variable

in presence and configuration (299). In addition to the vascular supply, the tendons in the synovial sheath receive

nutrition through synovial fluid diffusion.

The vinculum longum superficialis arises at the level of

the base of the proximal phalanx. Here, the digital arteries

give rise to branches on either side of the tendons that interconnect anterior to the phalanx, but deep (dorsal) to the

tendons. These branches form the vinculum longum superficialis that connects to the FDS at the floor of the digital

sheath. The vinculum longum superficialis supplies the

FDS at the level of the proximal phalanx (285–299).

The vinculum brevis superficialis and the vinculum brevis profundus consist of small triangular mesenteries near

the insertion of the FDS and FDP tendons, respectively.

The vinculum brevis superficialis arises from the digital

artery, at the level of the distal part of the proximal phalanx. It supplies the FDS tendon near its insertion into the

middle phalanx. A portion of the vinculum brevis superficialis continues anteriorly, at the level of the PIP joint

toward the FDP to form the vinculum longum profundus

(285–299).

Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the

digits is richer than the palmar half.

Architectural features of the FDS include the physiologic

cross-sectional area of the muscle, the fiber bundle length,

muscle length, muscle mass, and pennation angle. Skeletal

muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FDS to each digit

(135–139,174) (see Table 2.1 and Fig. 2.4). As can be seen

in the figures, the digital extrinsic flexor and extensor muscles have similar architectural features (see Fig. 2.4A and B).

The relative difference index values compare the FDS with

other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3.

The FDS is innervated by a branch from the median

nerve (C7, C8, T1). The nerve branch usually exits the

median nerve trunk proximal to the pronator teres and

accompanies the median nerve trunk through the two

heads of the pronator teres. The branch then divides into

multiple smaller motor branches that supply the radial head

of the muscle. The muscle portions that ultimately form the

tendons to the index and small fingers each may receive a

separate motor branch. On occasion, the motor branches

may exit the median nerve more distally, in the distal third

of the forearm, to supply the FDS (3,4,11,18).

Actions and Biomechanics: Flexor

Digitorum Superficialis

The FDS functions to flex the PIP joints of the index, long,

ring, and small fingers. It also contributes to flexion of the

digital MCP joints and flexion of the wrist. During flexion,

there is a slight adduction component as the FDS draws the

digits together, as in making a fist. The tendon of the small

finger has a minor rotatory (opposing) action at the carpometacarpal joint (11).

The FDS has independent muscle components to each

of the four digits. It therefore can flex each PIP independently (unlike the FDP, which has a common muscle group

to the middle, ring, and small fingers). The ability of the

FDS to flex one PIP at a time is useful in assessing tendon

lacerations.

Anomalies and Variations: Flexor

Digitorum Superficialis

Among the many described muscle variations and anomalies of the FDS (300–333), the more common involve

2 Muscle Anatomy 121

muscle slips that interconnect the FDS with the other forearm flexors. These include slips to the flexor pollicis longus

(FPL), the palmaris longus, or the brachioradialis

(324,325). The variations seem to be more common in the

index and small fingers (11,311,313,319,326,332). As

much as 10% of 70 cadaver hands showed an anatomic

variation of the small finger that would preclude its independent function (284). Some anomalies have been noted

to occur repeatedly in families or in different generations

(326), or to occur bilaterally (312,321,333).

A muscle slip, the radiopalmaris, may arise directly from

the radius deep to the FDS and attach to the palmar

aponeurosis or to the common sheath of the flexor tendons

(11,319).

Several variations of the radial head of the muscle have

been noted. These include complete absence of the radial

head (3,4), or absence of one or more of the distal divisions

(to form specific tendons) (300). The entire muscle may

originate from the radius (11). The muscle belly and tendon

to the small finger may be absent (301,326).

A rare anomaly is a digastric FDS, consisting of an additional distal muscle belly separated from the main muscle

belly by an intercalary tendon (11,302–304,329). The muscle may occur as an accessory FDS, in the presence of a normal FDS (305,306).

An anomalous muscle, the palmar FDS accessories, may

arise from the palmar fascia and distal border of the transverse carpal ligament and end in a tendon that joins the

flexor tendon of the index finger at the level of the MCP

(11,309,319). In a literature review by Elias and SchulterEllis, the muscle was shown to be more common in women

than men in a 13:2 ratio, and involved the right hand in 12

of 13 cases (309). It was seen bilaterally in 4 of 13 cases.

The muscle involved the index finger in all cases; however,

a somewhat similar anomalous muscle involving the small

finger has been reported (312). This anomaly may present

as a painful palmar wrist mass (310,311,313). The muscle

usually can be identified with magnetic resonance imaging

(311,313). An additional variation of this anomaly includes

a palmar muscle belly that originates from the FDS to the

index finger by way of an accessory tendon (314). An

“accessory” FDS also has been noted, causing a volar soft

tissue wrist mass and ulnar neuropathy (307). The FDS

may be associated with Gantzer’s muscle (330).

Clinical Implications: Flexor Digitorum

Superficialis

The median nerve passes deep to the arch formed by the

heads of the FDS. This is a potential site of nerve compression, and should be considered in compartment syndrome

decompression or nerve exploration in ischemic contracture

(273,274).

Because the muscle bellies for each FDS tendon usually

are separate, it is possible independently to flex each of the

PIP joints. By holding the other three digits in extension,

the function of the remaining FDS can independently be

tested by having the patient attempt to flex the digit at the

PIP joint. Note that because the FDP muscle belly usually

consists of one belly supplying the four tendons (instead of

the four separate muscle bellies supplying the four tendons

of the FDS), holding the digits in extension helps to eliminate function of the FDP. Thus, any flexion of the digit at

the PIP joint is performed by the FDS, and each digit can

be evaluated independently (3).

Carpal tunnel syndrome can be precipitated by anomalies and variations of the FDS. The anomalous muscles

bellies in the forearm can cause direct encroachment of the

median nerve. In addition, an anomalous muscle belly or a

belly from a normal muscle that extends abnormally distally

into the carpal tunnel can contribute to carpal tunnel syndrome (303,316–320,322,327,331). Holtzhausen and colleagues have shown the prevalence of the FDS and FDP

muscle bellies that extend into the carpal tunnel to be as

high as 46% in women and 7.8% in men (323). Intermuscular slips that pass between the FDS and the palmaris

longus can cause carpal tunnel symptoms (324). Bilateral

occurrence of carpal tunnel syndrome due to an anomalous

FDS has been reported (331).

Ulnar neuropathy has been reported by Robinson, due

to an accessory FDS that produced a palpable mass in the

volar forearm as well as ulnar nerve encroachment (307).

A painful mass in the palm along the tendon course to

the index finger may represent an anomalous muscle, the

palmar FDS accessories. This muscle may arise from the

palmar fascia and distal border of the transverse carpal ligament and end in a tendon that joins the flexor tendon of

the index finger at the level of the MCP (309–313,319).

The mass usually can be identified as muscle by magnetic

resonance imaging (311,313). A fibroma in association

with an anomalous FDS tendon also has been the cause of

a painful palmar mass (315).

Agee and colleagues have studied the FDS, and note that

the muscle to the long finger may be anatomically the most

independent, arising separately. Therefore, this tendon may

be the most suitable for nonsynergistic tendon transfers

(277).

Progressive flexion contracture of the PIP (resembling

camptodactyly) of the right ring finger has been noted to

occur from an anomalous origin of the FDS. Operative

excision of the aberrant tendon restored normal range of

motion at the PIP joint (333).

In the absence of the FDS, a palmaris longus has been

found to extend to the middle phalanx of the ring finger

and function as a digital flexor of the PIP joint (259).

Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the

digits is richer than the palmar half. This has implications

for placement of sutures in the repair of lacerated tendons.

Sutures placed in the palmar half of the tendon should dis122 Systems Anatomy

rupt the intratendinous vascularity to a lesser degree than

those in the dorsal half. The vincular system should be

appreciated and protected as much as possible in the exploration or repair of the flexor tendons.

FLEXOR CARPI ULNARIS

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Carpi is from the Latin

carpalis and the Greek karpos, both of which indicate

“wrist” (the carpus). Ulnaris is derived from Latin ulna,

indicating “arm” (1,2).

Origin. From two heads. Humeral head: from the medical epicondyle through the common flexor origin. Ulnar

head: extensive origin from the medial margin of the olecranon and proximal two-thirds of the posterior border of

the ulna by an aponeurosis shared with the ECU and FDP,

and from the adjacent intermuscular septum.

Insertion. To the pisiform; a few fibers may attach to the

flexor retinaculum.

Innervation. Ulnar nerve (C7, C8, T1).

Vascular Supply. The ulnar artery, superior and inferior

ulnar collateral arteries, anterior and posterior ulnar recurrent arteries, ulnar end of the superficial palmar arch

(3,4,334,335).

Principal Action. Flexion and ulnar deviation of the

wrist.

Gross Anatomic Description: Flexor Carpi

Ulnaris

The FCU is the most medial muscle of the superficial flexors of the forearm (along with, from radial to ulnar, the

pronator teres, FCR, palmaris longus, and FDS)

(3,4,8,11,13). It lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The FCU is

located medial and superficial to the FDS. It has two heads

of origin (336). A smaller humeral head originates from the

distal part of the medial epicondyle through the common

flexor origin (see Fig. 2.2A). There also are fascial attachments from the humeral head to the adjacent intermuscular septum and deep fascia of the forearm. The larger ulnar

head has a more extensive origin, arising from the medial

margin of the olecranon and proximal two-thirds of the

posterior border of the ulna by a fascial sheet or aponeurosis (see Fig. 2.3B). It shares this aponeurotic origin with the

ECU and FDP. The FCU also has attachments to the

neighboring intermuscular septum between it and the FDS.

The two heads of the FCU create a muscular arch extending from the olecranon to the medial epicondyle. The ulnar

nerve and posterior ulnar recurrent artery pass through this

fibromuscular arch. The muscle belly from the humeral

head extends distally in a nearly longitudinal fashion. The

muscle fibers from the larger ulnar head, however, extend

distally obliquely and anteriorly. This muscle belly, which is

highly pennated, may continue nearly the entire length of

the muscle–tendon unit, almost to the insertion site. (This

is very different from the FCR, which has a fairly abrupt

myotendinous junction in the central portion of the forearm, and a long solitary tendon that extends distally without attaching muscle fibers.) The FCU has a long, thick

tendon that forms along the anterolateral border of the

muscle in its distal half. The tendon usually is more than 10

mm long (337). As the tendon extends distally, it usually

retains muscle fibers to the distal portion of the forearm

almost to the level of its insertion onto the pisiform (337).

Rarely, there is a discrete tendon without accompanying

muscle fibers (337). At the level of insertion, all the muscle

fibers insert in a penniform manner. The pisiform is a

sesamoid bone, and therefore is within a tendon (or ligament). The FCU thus inserts primarily into the pisiform

(see Fig. 2.6A), but is also, to an extent, extended distally

through the pisiform to the hamate through the pisohamate

and pisometacarpal ligaments. In addition, a few fibers

attach to the flexor retinaculum and to the palmar aponeurosis, and, possibly, to the base of the third, fourth, and fifth

metacarpals (338). As the muscle inserts into the pisiform,

the ulnar nerve and artery are located deep and radial to the

tendon.

Architectural features of the FCU include the physiologic cross-sectional area of the muscle, the fiber bundle

length, muscle length, muscle mass, and pennation angle

(angle of the muscle fibers from the line representing the

longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the

data for the FCU (135–139,174) (see Table 2.2 and Fig.

2.4). The FCU has a relatively small fiber length and relatively large physiologic cross-sectional area. This indicates

that its design is more optimal for force generation (proportional to cross-sectional area) than for excursion or

velocity (proportional to fiber length). Its relative difference

index values compare it with other upper extremity muscles, based on architectural features. These values are listed

in Appendix 2.3. In comparing the architectural features of

the FCU with the FCR, the FCR muscle length is shorter

than the FCU, but the muscle fibers of the FCR are longer

(136,174). The relatively longer fiber length indicates that

the FCR is designed more for excursion and velocity of contraction (because excursion and velocity are proportional to

fiber length) compared with the FCU. The FCU, in contrast, has a higher pennation angle, with a larger physiologic

cross-sectional area. This indicates the FCU is designed

more for force production and less for excursion and velocity, compared with the FCR (because cross-sectional area is

proportional to force production) (174,175) (see Table 2.2

and Fig. 2.4).

The FCU is innervated by the ulnar nerve (C7, C8, T1).

The muscle usually receives two to three muscular branches

2 Muscle Anatomy 123

in its proximal portion, although there may be up to six

separate branches (11,339). These branches usually leave

the ulnar nerve near the level of the elbow joint or in the

distal portion of the cubital tunnel. Rarely, a branch can

exit the ulnar nerve proximal to the elbow (339). The

motor branches often are visualized during cubital tunnel

decompression or ulnar nerve transposition. Each head of

the FCU receives a separate motor branch (336). There

occasionally is a single branch that leaves the ulnar nerve

trunk, enters the proximal FCU on the deep surface, and

then branches in the muscle to send long, slender motor

branches through the muscle to reach the middle third

(3,4,11,13,340).

Actions and Biomechanics: Flexor Carpi

Ulnaris

The FCU functions primarily to flex the wrist, and usually

works with the FCR. It also ulnarly deviates the wrist, especially working with the ECU. The FCU takes an important

role in stabilizing the wrist during strong power grip, as in

the tight grasp of a hammer. The wrist usually is held in

slight ulnar deviation during these functions because the

wrist is stabilized, in large part, by the FCU. The FCU also

helps stabilize the pisiform, and thus can assist the abductor

digiti minimi, which has its origin on the pisiform. The

FCU therefore can assist indirectly with abduction of the

small digit.

From its insertion on the medial epicondyle, and from

its course that positions the muscle directly over the medial

collateral ligament, it has been postulated that the FCU

(along with the FDS) functions to support or stabilize the

medial elbow joint (170).

As stated earlier, the FCU is architecturally designed

more for force generation than for excursion or velocity

compared with its radial counter part, the FCR. This is due

to the FCU having a larger physiologic cross-sectional area

(proportional to force generation), being highly pennated

(which helps increase the physiologic cross-sectional area),

and having a shorter fiber length (which is proportional to

excursion or velocity) (135–138,174,341,342).

Anomalies and Variations: Flexor Carpi

Ulnaris

The muscle and tendon arrangement of the FCU occurs in

three general types. The most common is a large muscle

belly that runs distally almost to the insertion on the pisiform. The next most common is a muscle belly that ends

more proximally, with some large muscle fibers that run

parallel to the tendon and almost reach the pisiform. Rarely,

the musculotendinous junction ends more proximally, with

only single muscle fibers that continue distally. These different muscle–tendon patterns should be kept in mind

when interpreting magnetic resonance images, during general operative exploration for penetrating trauma, or when

performing muscle–tendon lengthening procedures

(135–138,174,341–344).

Among the most common variations of the FCU is an

accessory tendon or muscle slip that extends from the coronoid process and joins the muscle belly in the proximal

third of the muscle (3,11). An accessory muscle may extend

the entire length of the FCU and resemble a duplicated

muscle (345).

Distally, there are several possible variations of the insertion of the tendon. It may send tendinous slips to the flexor

retinaculum. It may have extensions to the metacarpals of

the small, ring, or long fingers, or to the capsules of the carpometacarpal joints (3,4,11). A distal slip inserting into the

proximal phalanx of the ring finger has been described

(346). A distal anomalous muscle belly and a “reversed”

muscle belly located predominantly distally have been

noted and associated with ulnar nerve compression, either

in the forearm or in the ulnar tunnel (346–350). These

anomalous muscles entering the ulnar tunnel also have been

associated with ulnar artery thrombosis (351).

The insertional tendon may extend to the proximal portion of the abductor digiti minimi.

The epitrochleoanconeus [epitrochleoolecranonis or

anconeus sextus of Gruber (11)] is a small anomalous muscle closely associated with the FCU. It originates from the

posterior surface of the medial epicondyle of the humerus

and inserts into the olecranon process. It is superficial to the

ulnar nerve (from which it is innervated), and takes the

place of the fibrous arch of the deep fascia usually found in

the same location. The muscle has a frequency of approximately 25% in cadaver dissections (11). The muscle

restricts mobility of the ulnar nerve in the forearm, thereby

contributing to the development of neuropathy (especially

with trauma as a precipitating factor) (347).

A split FCU tendon has been noted, with the ulnar nerve

passing between the split. Ulnar nerve compression symptoms were produced with wrist hyperextension (352–354).

Clinical Implications: Flexor Carpi Ulnaris

Because the FCU is designed most optimally for force generation and less for excursion or velocity, it may be a less

optimal tendon transfer for use in radial nerve palsy. The

FCR, which is designed more for excursion, may be a more

appropriate transfer to achieve digital extension. (In radial

nerve transfers, great tendon motor power strength usually

is not as important as excursion because the antigravity

function of the transfer usually is sufficient to achieve good

functional results) (135–138,174,341–343).

The ulnar nerve and artery lie deep and radial to the

FCU tendon in the distal forearm (the artery is radial to the

nerve). This is a reasonable site for ulnar nerve local anesthetic block, by infiltration of the nerve deep to the palpable FCU tendon. For complete block of the ulnar portion

124 Systems Anatomy

of the hand, the dorsal branch of the ulnar nerve, which

leaves the ulnar nerve trunk proximal to the wrist, should

be blocked as well. The dorsal branch of the ulnar nerve can

be blocked by a wheal of subcutaneous local anesthetic

injected circumferentially along the ulnar and dorsal borders of the wrist in the area just distal to the ulnar head.

As noted previously, variations and anomalies of the

FCU, either in the forearm with accessory slips, fibrous

bands, or muscles, or distally with extended muscle bellies

or anomalous bellies extending into the ulnar tunnel, can

result in ulnar neuropathy (346–350,355,356). In addition,

a split FCU tendon pierced by the ulnar nerve or one of its

branches can lead to neuropathy (352–354). An anomalous

muscle extending into the ulnar tunnel also has been associated with ulnar artery thrombosis (351).

Sarcomere length of a muscle can be measured using

intraoperative laser diffraction techniques. With these techniques, it is possible to show and measure the change of sarcomere length after muscle transfer. When the FCU was

transferred to the EDC (to restore digital extension), the

absolute sarcomere length and sarcomere length operating

range of the FCU increased. It also was shown that despite

good clinical results, a more desirable result could be

obtained if the FCU sarcomere length was increased (by

approximately 5 µm) by further stretching of the muscle

during the transfer. The authors were able to quantify the

relationship between the passive tension chosen for transfer,

sarcomere length, and the estimated active tension that

could be generated by the muscle. These findings demonstrate the feasibility of using intraoperative laser diffraction

techniques during tendon transfer as a guide for setting tension and the optimal placement and sarcomere length of the

transferred muscle (341–343).

FLEXOR DIGITORUM PROFUNDUS

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Digitorum is from the

Latin digitus or digitorum, indicating the digits. Profundus is

from the Latin profundus, indicating “deep,” and refers to

the muscle’s location deep in the forearm (1,2).

Origin. From the median and anterior surface of the

ulna, interosseous membrane, and deep fascia of the forearm.

Insertion. To the base of the distal phalanges.

Innervation. Anterior interosseous nerve (from the

median nerve) to the index and long finger; ulnar nerve to

the ring and small fingers.

Vascular Supply. Posterior ulnar recurrent artery, posterior and anterior interosseous arteries, palmar carpal

arch, palmar metacarpal arteries, common and proper digital palmar arteries. In addition, the lateral portion is supplied by the ulnar collaterals and the deep palmar arch,

and the medial part is supplied by the ulnar artery

(3,4,11,13,68).

Principal Action. Flexion of the distal and interphalangeal joints and flexion of the MCP joints. The FDP also

contributes to wrist flexion and functions as the origin for

the lumbrical muscles (3,4,11).

Gross Anatomic Description: Flexor

Digitorum Profundus

The FDP, with the FPL, is one of the deep flexors of the

forearm and lies in the deep volar muscle compartment of

the forearm (Appendix 2.2). The muscle is situated deep in

the forearm, lying against the ulnar portion of the

interosseous membrane. The FDP is covered anteriorly by

the FCU and the FDS. The median nerve courses between

the deep flexor muscle group and the superficial flexor

muscle group of the forearm. It is a strong, broad, somewhat flat muscle. The FDP arises deep to the superficial

flexors from an extensive origin (see Fig. 2.3). The origin

includes attachments to the proximal two-thirds of the

anterior and medial surface of the ulna. There also are

attachments of origin to a depression on the medial side of

the coronoid process. Some of its origin extends medially

and posteriorly around the ulna to reach the posterior surface of the ulna, and there are connections through an

aponeurosis shared with the flexor carpi ulnaris and ECU.

In addition, the FDP has attachments from the ulnar half

of the anterior surface of the interosseous membrane.

There also may be an inconsistent origin from a small area

on the radius distal to the bicipital tuberosity. The extensive origin then forms what resembles a single large muscle

belly, although the belly to the index finger usually is separate and may be discernible. The muscle then divides into

four parts that are more distinct. The myotendinous junction usually is in the central third of the forearm. At the

junction, the muscle attaches to the dorsal surface of the

tendon, so that more of the tendon is visible on the volar

aspect. The myotendinous junction gives rise to four separate tendons usually aligned parallel to each other, from

radial to ulnar, to extend distally to the index, long, ring,

and small fingers, respectively. This is in contrast to the

FDS tendons, which, at the level of the wrist, have a

“stacked” pattern, with the FDS tendons to the long and

ring fingers located palmar and central to the FDS tendons

of the index and small fingers, which are located dorsal and

radial (for the index) or dorsal and ulnar (for the small finger) (3,4,11) (see earlier, under Gross Anatomic Description: Flexor Digitorum Superficialis). The muscle belly to

the long, ring, and small fingers remain interconnected to

some extent from the forearm to the palm through areolar

tissue and tendinous slips. The muscle and tendon to the

index finger usually remain separate and distinct throughout their course from the muscle belly to the palm. In

some, the FDP tendon to the small finger may be more

2 Muscle Anatomy 125

independent, and resemble that of the index finger. The

tendons to the long and ring finger are the least independent and more often are connected by areolar tissue. The

tendons then extend distally, deep to the tendons of the

FDS, to cross through the carpal tunnel. At the distal

extent of the carpal tunnel, the tendons diverge to cross the

palm in the direction of each digit. Just proximal to the

MCP joints, the FDP tendons enter the A1 pulley of the

fibroosseous tunnel. In the digits, at the level of the proximal phalanx, the FDS tendons split and the associated

FDP tendon passes through the split. Each tendon continues distally to insert on the base of each of the distal phalanges (3,4,11,357) (see Fig. 2.6A).

At the level of the distal margin of the carpal tunnel, the

lumbricals arise from the radial aspect of each FDP tendon.

As discussed earlier (see under Gross Anatomic Description: Flexor Digitorum Superficialis), the vascular supply to

the FDP and FDS tendons comes from several sources.

These include the longitudinal vessels (some of which may

originate in the muscle belly) that enter in the palm and

extend down intratendinous channels. There also are vessels

that enter at the level of the proximal synovial fold in the

palm to supply the tendons. In addition, there is the vincular supply, supplied by segmental branches from the 

paired

digital arteries that enter into the tendon sheaths. The most

distal vascular supply to the flexor tendons includes vessels

that enter the FDS and FDP tendons at their osseous insertions (285–299).

The vinculum longum superficialis arises at the level of

the base of the proximal phalanx. Here, the digital arteries

give rise to branches on either side of the tendons that interconnect anterior to the phalanx, but deep (dorsal) to the

tendons. These branches form the vinculum longum superficialis that connects to the superficialis at the floor of the

digital sheath. The vinculum longum superficialis supplies

the FDS, at the level of the proximal phalanx.

In the digital sheath, the segmental vascular supply to

the flexor tendons is by means of long and short vincular

connections. These include the vinculum brevis superficialis, the vinculum brevis profundus, the vinculum

longum superficialis, and the vinculum longum profundus.

The vincula often are variable in presence and configuration

(285–299). In addition to vascular sources, the tendons in

the synovial sheath receive nutrition through synovial fluid

diffusion.

The vinculum brevis superficialis and the vinculum

brevis profundus consist of small, triangular mesenteries

near the insertion of the FDS and FDP tendons, respectively. The vinculum brevis superficialis arises from the

digital artery, at the level of the distal part of the proximal

phalanx. It supplies the FDS tendon near its insertion into

the middle phalanx. A portion of the vinculum brevis

superficialis continues anteriorly, at the level of the PIP

joint, toward the FDP to form the vinculum longum profundus (299).

Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the

digits is richer than that of the palmar half.

Architectural features of the FDP include the physiologic cross-sectional area of the muscle, the fiber bundle

length, muscle length, muscle mass, and pennation angle.

Skeletal muscle architectural studies by Lieber, Friden, and

colleagues provide the data for the FDP to each digit

(135–139,174) (see Table 2.1 and Fig. 2.4). The digital

extrinsic flexor and extensor muscles have similar architectural features. The relative difference index values compare

the FDP with other upper extremity muscles, based on

architectural features. These values are listed in Appendix

2.3.

The FDP is innervated by both the median nerve

(through the anterior interosseous nerve to supply the belly

of the index and long fingers) and by the ulnar nerve (to

supply the bellies of the ring and small fingers). The anterior

interosseous nerve usually exits the median nerve trunk

proximal to the nerve trunk entering the interval between

the heads of the pronator teres. The anterior interosseous

nerve branch usually accompanies the main median nerve

trunk through the interval between the humeral and ulnar

heads of the pronator teres, then through the interval created

by the fibromuscular arch of the origins of the FDS. The

anterior interosseous nerve then divides into several motor

branches to supply the muscle portions of the FDP to the

index and long fingers. The nerve branches enter the muscle

bellies on the radial border in the middle third of the muscle. A branch of the anterior interosseous nerve continues

distally along the anterior surface of the interosseous ligament to reach and enter the proximal border of the pronator quadratus. The ulnar nerve innervation of the FDP is

from a motor branch that arises approximately the level of

the elbow joint. The nerve branch enters the anterior surface

of the muscle in the region of the junction of the proximal

and middle thirds. This branch supplies the part of the muscle that provides tendons to the ring and small fingers. Considerable variation exists as to the innervation of the muscle

bellies of the FDP. In only approximately 50% of extremities

do the median nerve and ulnar nerve specifically innervate

the index and long, and the ring and small finger muscle bellies, respectively (3,4,11,358,359).

Actions and Biomechanics: Flexor

Digitorum Profundus

The FDP functions mainly to flex the digits. Through its

insertion onto the distal phalanx, it exerts powerful flexion

on the distal phalanx at the distal interphalangeal (DIP)

joint. However, by passing across the PIP and MCP joints,

the FDP tendons assist the FDS to flex the PIP joints, and

the FDP assists both the FDS and the interossei and lumbricals to flex the MCP joints. The FDP also assists with

flexion of the wrist. The FDP provides the origins for the

126 Systems Anatomy

lumbricals muscles. When the FDP contracts and moves

proximally, there is a dynamic action on the lumbricals.

Anomalies and Variations: Flexor

Digitorum Profundus

There commonly are accessory muscles or tendinous slips

from the FDP to the radius, to the FDS, FPL, the medial

epicondyle, or to the coronoid process (3,4,11,360–365).

Flexor indicis profundus or flexor digitorum profundus

indicis. There may be more than four muscle bellies of the

FDP, and the separation between the tendons can occur to

varying degrees. The separation to the index finger usually

is the greatest, but also is variable. If the FDP to the index

exists as a separate muscle and tendon, it has been referred

to as the flexor indicis profundus or flexor digitorum profundus indicis (11).

An anomalous accessory FDP tendon may exist as a separate muscle–tendon unit lying ulnar to the main flexor

digitorum profundus indicis. It has been noted then to join

the main tendon at the level of the distal palmar crease

(363).

Other rare described anomalies of the FDP include an

anomalous muscle in association with a fibroma of a tendon

sheath causing triggering of the wrist (364), and a rare congenital abnormality of the FDP causing a flexion deformity

of the long and ring fingers (365).

Clinical Implications: Flexor Digitorum

Profundus

Flexor tendon rupture can occur in the carpal tunnel from

several causes, including chronic abrasion against a hook of

the hamate fracture or nonunion, attrition against the radial

side of the pisiform affected by osteoarthritis of the pisotriquetral joint (366), and in the patient with rheumatoid

arthritis.

Avulsion of the FDP most commonly involves the ring

finger. This is due to its relatively greater length during

grasp. During grip, the ring fingertip becomes 5 mm more

prominent than any other digit in 90% of subjects, and it

absorbs more force than any other finger during pull-away

testing (367).

The anterior interosseous nerve syndrome involves paresis or palsy of the FDP to the index and long (and occasionally the ring) fingers, as well as paresis of the FPL and

pronator quadratus. The syndrome often is associated with

trauma, tight-fitting casts, neuritis, or anatomic structures

that impinge on the anterior interosseous nerve, including

fascial bands, adhesions, and muscle impingement (i.e.,

fibrous bands of the pronator teres) (368–376).

In 1979, Linburg and Comstock described an anomalous tendon slip from the FPL to the FDP to the index finger (360). It appears that the anomaly is present in at least

one extremity of 25% to 31% of individuals, and in both

extremities in 6% to 14%. If present it can be demonstrated

when a patient attempts to independently flex the interphalangeal joint of the thumb, and there is coexisting flexion at the DIP joint of the index finger (360,361), called

Linburg’s sign. This anomaly may be associated with chronic

tenosynovitis or carpal tunnel symptoms.

Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the

digits is richer than in the palmar half. This has implications for placement of sutures in the repair of lacerated tendons. Sutures placed in the palmar half of the tendon

should disrupt the intratendinous vascularity to a lesser

degree than those in the dorsal half. The vincular system

should be appreciated and protected as much as possible in

the exploration or repair of the flexor tendons.

Tendon excursion of the FDP relative to the tendon

sheath has been shown to be greatest in zone II during PIP

joint rotation. This suggests that PIP joint motion may be

most effective in reducing adhesions after tendon repair in

zone II (377).

After laceration of the FDP distal to the superficialis

insertion, tendon advancement of the proximal cut end of

the tendon to the insertion has been used as a means of

repair. Anatomic studies suggest that 1 cm is approximately

the maximum amount that the tendon can be safely

advanced, without causing problematic shortening (378).

FLEXOR POLLICIS LONGUS

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Pollicis is from the Latin

pollex, indicating “thumb.” Longus is the Latin for “long.” It

is the longest flexor of the thumb (1,2).

Origin. From the anterior surface of the middle third of

the radius, the anterior interosseous ligament.

Insertion. To the base of the distal phalanx of the thumb.

Innervation. Median nerve through anterior interosseous

branch (C6, C7, C8) (3,4).

Vascular Supply. From the radial artery through direct

muscular branches, anterior interosseous artery, princeps

pollicis artery, and palmar carpal arch. The tendon receives

vascularity, in part, through a vincular system, originating

from the digital arteries (3,4,11,379–384).

Principal Action. Flexion of the thumb interphalangeal

joint and MCP.

Gross Anatomic Description: Flexor

Pollicis Longus

The FPL, with the FDP, is one of the deep flexors of the

forearm and lies in the deep volar muscle compartment of

the forearm (Appendix 2.2). The FPL is located radial to

the FDP, roughly in the same deep plane. Like the FDP, it

2 Muscle Anatomy 127

is a relatively large and flat muscle. It has a large area of

origin, arising from an obliquely oriented groove on the

anterior surface of the radius that extends from just below

the tuberosity to the proximal attachment of the pronator

quadratus (see Fig. 2.3A). The origin often extends as far

proximal as to within approximately 5 cm of the wrist

joint. The muscle belly thus attaches and covers the middle third of the anterior surface of the radial diaphysis. It

also has attachments from the adjacent interosseous ligament, and there often is an attachment by a variable slip

from either the lateral or medial border of the coronoid

process. There also can be attachments from the medial

epicondyle of the humerus (385). The muscle fibers

extend distally and obliquely to attach in a penniform

manner on the tendon at the myotendinous junction. The

muscle has a relatively long and variable myotendinous

junction. At this junction, there usually is more tendon

that extends along the ulnar border of the muscle, on its

anterior surface. The muscle blends with its broad, flat

tendon, usually in the distal third of the forearm. The tendon extends distally, usually in the plane of the tendons of

the FDP. The adjacent anterior interosseous nerve also

continues distally, between the FPL and the FDP. The

FPL then enters the carpal tunnel. Some muscle fibers

may accompany the tendon to the level of the proximal

edge of the flexor retinaculum. As the tendon passes

through the carpal tunnel, it is located radial to the tendons of the FDP and median nerve. It passes deep to the

superficial head of the flexor pollicis brevis (FPB). After

passing through the carpal canal, the tendon emerges deep

to the superficial palmar arch, between the opponens pollicis and the oblique head of the adductor pollicis. It continues between the thumb sesamoid bones, entering its

own synovial sheath. The tendon enters the fibroosseous

tunnel of the thumb through the A1 pulley at the level of

the MCP joint (386). The tendon continues distally to

insert onto the palmar surface of the base of the distal phalanx of the thumb (see Fig. 2.6A).

Architectural features of the FPL include the physiologic

cross-sectional area of the muscle, the fiber bundle length,

muscle length, muscle mass, and pennation angle (angle of

the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the

FPL (135–139,174) (see Table 2.1 and Fig. 2.4). The digital extrinsic flexor and extensor muscles have similar architectural features. The relative difference index values compare the FPL with other upper extremity muscles, based on

architectural features. These values are listed in Appendix

2.3 (15).

The FPL is innervated by the anterior interosseous nerve

from the median nerve (C6, C7, C8) (387–395). There

usually are at least two motor branches that enter the proximal half of the muscle at its ulnar aspect.

Actions and Biomechanics: Flexor Pollicis

Longus

The FPL is the only muscle that flexes the thumb interphalangeal joint (396,397). It assists the thenar muscles with

flexion of the thumb at the MCP joint. In addition, the

FPL assists with flexion and adduction at the carpometacarpal joint.

If a load is applied to the FPL, the moment arm of the

tendon in the carpal tunnel can change as the tendon shifts

its position in the carpal tunnel (398).

Anomalies and Variations: Flexor Pollicis

Longus

Several anomalies of the FPL have been described

(399–427). The FPL can have interconnections of tendon

slips or muscle extensions with the FDS, the FDP, or the

pronator teres (360,361,399–401). The FPL actually may

coalesce and blend with the muscle belly of the FDP, FDS,

or pronator teres (402). The origin may extend proximally

to the medial epicondyle of the humerus. This anomalous

belly is the epitrochlear bundle of the FPL (11).

The best documented accessory head of the FPL is

Gantzer’s muscle. It has been noted in up to 52% to 66%

of limbs and is supplied by the anterior interosseous nerve

(403,404). It usually arises from either the medial humeral

epicondyle (in 85%) or from a dual origin from the epicondyle and coronoid process (15%). The muscle usually

inserts into the ulnar aspect of the FPL and its tendon.

Gantzer’s muscle usually is posterior to the median nerve

and either anterior or posterior to the anterior interosseous

nerves. Anatomic variations of Gantzer’s muscle have contributed to median nerve compression in the forearm

(403–407).

Most commonly, there can be a tendon slip that connects the tendons of the FPL to the FDP. Attempts at independent flexion of the thumb interphalangeal produce concurrent flexion of the distal phalanx of the index finger.

This is referred to as Linburg’s sign, or the Linburg syndrome

(360,361) and may be associated with tendonitis or carpal

tunnel syndrome.

The original portion of the FPL that arises from the

interosseous ligament may be absent. The entire FPL may be

absent (11,408–417). Congenital absence of the FPL often

is associated with a hypoplastic thumb (408,416), and has

been noted bilaterally (413). The FPL may exist as a double

tendon or malpositioned tendon, or may have an accessory

tendon accompanying the normal tendon (420,421). This

has been associated with triggering of the thumb (422).

Various anomalous insertions of the FPL have been

noted and usually result in poor flexor power of the distal

phalanx (423–425). Of clinical significance, the FPL may

insert onto the proximal as well as the distal phalanx of the

128 Systems Anatomy

thumb. This may appear to be congenital absence of the

FPL because of the lack of flexion on the distal phalanx

(423). This insertion can be bilateral. The FPL also may

insert into the soft tissue of the carpal tunnel, with the

muscle power diverted to flex the wrist. Inadequate flexion power of the thumb will then be present (424). The

FPL may be conjoined to the extensor pollicis longus

(EPL) (425–427).

Clinical Implications: Flexor Pollicis

Longus

Neuropathy of the anterior interosseous nerve (anterior

interosseous nerve syndrome) results in paresis or palsy of

the FPL and the FDP to the index and long (and occasionally the ring) fingers, as well as paresis of the pronator quadratus. The syndrome may be caused by trauma, tight-fitting

casts, neuritis, or anatomic structures that impinge on the

anterior interosseous nerve, including fascial bands, adhesions, or normal or anomalous muscle impingement (i.e.,

fibrous bands of the pronator teres, Gantzer’s muscle)

(368–377).

The anomalous tendon slip from the FPL to the FDP to

the index finger appears to be present in at least one extremity of 25% to 31% of individuals, and in both extremities

in 6% to 14% (360). It can be demonstrated when a patient

attempts independently to flex the interphalangeal joint of

the thumb, and there is coexisting flexion at the DIP joint

of the index finger (360,361), (Linburg’s sign). This anomaly may be associated with chronic tenosynovitis or carpal

tunnel symptoms.

PRONATOR QUADRATUS

Derivation and Terminology. Pronator is derived from

the Latin pronus, meaning “inclined forward” (the Latin

pronatio refers to the act of assuming the prone position or

the state of being prone). Quadratus is a Latin term indicating “squared” or “four sided” (based on the muscle’s shape)

(1,2).

Origin. There are two heads. The superficial head and

deep head originate from the anterior distal ulnar diaphysis.

Insertion. The superficial head inserts onto the anterior

distal radial diaphysis and anterior metaphysis. The deep

head inserts proximal to the ulnar notch of the distal radius.

Innervation. Anterior interosseous nerve of the median

nerve.

Vascular Supply. The radial artery, anterior interosseous

artery, anterior descending branch, recurrent branches of

the palmar carpal arch (3,4,11,13).

Principal Action. Pronation of the forearm. It usually

works with the pronator teres. The pronator quadratus may

be the principal pronator of the forearm; the pronator teres

appears to function more during rapid or forceful pronation.

Gross Anatomic Description: Pronator

Quadratus

The pronator quadratus is a flat, quadrangular muscle

that covers the distal 25% of the palmar surface of the

radius and ulna. In textbooks, it usually is grouped with

or discussed under the section on deep flexors of the forearm. The muscle more accurately belongs in its own section. It is now considered to occupy a separate compartment of the forearms, and should be addressed as such

with compartment syndromes (273,274,428–432)

(Appendix 2.2).

The origin of the pronator quadratus is along a relatively

narrow, oblique ridge on the anterior surface of the distal

ulnar diaphysis (see Fig. 2.3A). Some muscle fibers also

originate from the medial surface of the distal ulna and

from a thick aponeurosis that attaches to the medial third

of the muscle. The muscle fibers pass from medial to lateral,

and slightly distally, to reach the radius. The muscle fibers

are roughly transverse to the axis of the forearm. The muscle inserts onto the palmar 20% of the distal radius, covering a portion of the distal diaphysis and a portion of the

metaphysis (see Fig. 2.3A). The deep (dorsal) fibers insert

into a triangular area proximal to the ulnar notch of the

radius. Both heads also have fibers that insert into the capsule of the distal radioulnar joint (433).

The pronator teres appears to have two distinct heads: a

superficial oblique head and a deep head. The superficial

head originates from the ulna and passes transversely to an

insertion into the radius. It averages 5.1 cm in length, 4.5

cm in width, and 0.2 cm in thickness, and has a mean crosssectional area of 0.95 cm2

. The superficial head has a contractile volume of 2.6 cm3

. The superficial head entirely

covers the deep head, whose muscle fibers are oblique from

their ulnar origin to the distal volar surface of the radius.

The deep head runs obliquely from a more proximal origin

on the ulna to a distal insertion on the radius. It has an average length of 4.0 cm, average width of 3.2 cm, and a thickness of 0.4 cm. Its mean cross-sectional area is 1.64 cm2 and

its contractile volume is 2.5 cm3 (434). A group of fibers

occasionally has been noted deep to both heads, running at

right angles to them and paralleling the direction of the

fibers of the interosseous membrane (434).

The fibers of both heads are somewhat oblique to the

axis of rotation. From this orientation, both heads, by contracting, develop a rotatory and a stabilizing force. The

superficial head is thought to provide the major force for

rotation in supination and pronation. The deep head functions more to provide maintenance of transverse forces at

the distal radioulnar joint. The deep head coapts the joint

surfaces and stabilizes the joint (431,433,434).

2 Muscle Anatomy 129

The pronator quadratus, located in the distal palmar

forearm, has been shown to occupy a functionally separate

fascial compartment (428–430,432). The muscle is

enclosed anteriorly by a well defined fascial sheath that

measures 0.4 to 0.5 mm in thickness. This sheath, along

with the relatively rigid posterior boundaries of the

interosseous ligament and distal radius and ulna, forms a

distinct fascial space. Experimentally injected dye into this

compartment does not communicate with the other forearm compartments (430,432). Clinical correlations of

compartment syndrome involving the pronator quadratus

support the concept of the muscle occupying its own compartment (428–430,434).

The architectural features of the pronator quadratus,

including the fiber length and physiologic cross-sectional

area, are listed in Table 2.1 and depicted in Fig. 2.4.

The pronator quadratus is innervated by the anterior

interosseous nerve and receives its blood supply from the

anterior interosseous artery. The anterior interosseous nerve

extends distally along the anterior surface of the

interosseous ligament, passes dorsal (deep) to the middle of

the proximal margin of the muscle, and gives off several

branches to the muscle in its substance. The nerve fibers are

derived from C8 (mostly) and C7 (3,4,11,13).

Actions and Biomechanics: Pronator

Quadratus

The pronator quadratus appears to be the principal pronator of the forearm. It usually works with the pronator teres.

The pronator teres appears to function more during rapid

or forceful pronation. The deeper fibers of the pronator stabilize the distal ulna and radius by preventing or opposing

separation of their distal ends, especially during loading of

the carpus (3,4,434).

Anomalies and Variations: Pronator

Quadratus

The deep and superficial heads may exist as separate muscle

bellies (completely separated) (11). The pronator quadratus

may be absent (11). An anomalous head may extend proximally, either to the radial shaft, pronator quadratus, or to

the FCR brevis (11). An anomalous head may extend distal

to the carpus, either to the radiocarpal or ulnocarpal capsule, to the base of the thenar muscles, or to the adductor

pollicis (11).

Clinical Implications: Pronator Quadratus

The pronator quadratus, although situated in the volar forearm, is considered to occupy a separate compartment.

Anatomic dye injection studies by Sotereanos and colleagues have demonstrated a distinct fascial space without

communications to the deep or superficial volar compartment of the forearm (273,274,430–433). Decompression

of the volar compartment of the forearm without specifically addressing the pronator quadratus may not consistently decompress the muscle (429,430).

The pronator quadratus is a potential pedicle flap, either

with or without a portion of attached, vascularized bone; it

also can serve as a free muscle flap (435–442).

From the standpoint of the use of the pronator as a muscle–bone flap, the vascular anatomy has been studied in

detail (442). The anterior interosseous artery divides into a

muscular branch and a dorsal branch 1 to 3.5 cm from the

proximal margin of the pronator quadratus. There is a rich

periosteal plexus to which the anterior interosseous artery

also contributes. Both the anterior interosseous artery and

the dorsal branch can perfuse the muscle and the portion of

radial cortex used for the transfer. The dorsal branch, which

provides good perfusion of the distal radius, allows the

pedicle muscle flap to be mobilized a farther distance if the

dorsal branch is left intact (432). A muscle–bone pedicle

graft with a portion of the anteromedial cortex of the distal

radius that is mobilized with an intact anterior interosseous

artery can be mobilized less than 2 cm. After ligating and

dividing the anterior interosseous artery, blood supply to

the distal radius bone flap relies on flow through the dorsal

branch, and a bone flap can then be mobilized distally up

to 4 to 6 cm (442).

The pronator quadratus has been used successfully to

receive a relocated sensory nerve of the palm after resection

of a painful end-neuroma (443).

To test pronation strength of the pronator quadratus, the

elbow can be flexed past 90 degrees. Pronation strength is

then tested. This flexed elbow position helps isolate the

pronator strength of the pronator quadratus by eliminating

the contribution of the pronator teres (which is lax when

the elbow is passively flexed).

After stroke or brain injury, the forearm often is held in

spastic pronation by both the pronator teres and the pronator quadratus. For correction, operative recession of the

pronator quadratus (along with the pronator teres) can be

performed by releasing the muscle off the insertion on the

distal anterior radius. This usually is performed in combination with digital and wrist flexor lengthening.

EXTENSOR CARPI RADIALIS LONGUS

Derivation and Terminology. The ECRL derives its

name from several sources. Extensor is from the Greek and

Latin ex, which indicates out of, and the Latin tendere, “to

stretch,” thus extension indicates a motion to stretch out,

and extensor usually is applied to a force or muscle that is

involved in the “stretching out or straightening out” of a

joint. Carpi is derived from the Latin carpalis or the Greek

130 Systems Anatomy

karpos, both of which indicate “wrist” (the carpus). Radialis

is from the Latin radii, which means “spoke” (used to

describe the radius of the forearm). Longus is the Latin for

“long.” Therefore, extensor carpi radialis longus indicates a

long radial wrist extensor (1,2).

Origin. From the lateral epicondylar ridge, just proximal

to the lateral epicondyle. Additional areas of origin include

the lateral intermuscular septum, and the anterior fascia of

the muscles that arise from the common extensor origin at

the lateral epicondyle.

Insertion. To the dorsal base of the index metacarpal.

Innervation. Radial nerve (C6, C7).

Vascular Supply. The radial recurrent artery, interosseous

recurrent artery, posterior interosseous artery, and radial

collateral continuation of the profunda brachii artery

(3,4,11,13).

Principal Action. Extension and radial deviation of the

wrist. Assistance with weak flexion of the elbow. The ECRL

also helps stabilize the wrist (with cocontractions of the

wrist flexors) during powerful grasp functions.

Gross Anatomic Description: Extensor

Carpi Radialis Longus

The ECRL arises from the lateral epicondylar ridge, just

proximal to the lateral epicondyle (see Fig. 2.2A). It comprises part of the mobile wad muscle compartment, along

with the ECRB and the brachioradialis (Appendix 2.2)

(12). Its origin includes the distal third of the lateral

supracondylar ridge of the humerus, and the muscle is

partly overlapped by the brachioradialis. The ECRL also

has attachments of origin that include the common extensor origin of the lateral epicondyle, the lateral intermuscular septum, and the anterior fascia of the ECRB and

EDC (both of which arise from the common extensor origin at the lateral epicondyle). The superficial surface of

the muscle at first faces radially in the proximal portion

near its origin. The muscle then twists slightly so that the

superficial surface faces dorsally. The muscle belly extends

approximately one-third to one-half the way down the

forearm to reach the myotendinous junction, usually

noted at the junction of the proximal third and distal twothirds. In this area, the tendinous portion first appears on

the lateral and deep surface of the muscle. It then forms a

stout, flat, thick tendon that usually is devoid of muscle

tissue the entire length. The tendon of the ECRL travels

along the lateral surface of the radius, located radial and

adjacent to the ECRB. The ECRL and ECRB pass deep to

the APL and EPB in the distal third of the forearm to

reach its own tunnel as a part of the extensor retinaculum.

The tendon lies in a groove on the dorsal surface of the

radius just proximal to the styloid process. The ECRL,

along with the ECRB, forms the second dorsal compartment. [Editor’s note: The dorsal compartments of the wrist

are as follows: the APL and EPB comprise the first dorsal

compartment; the ECRL and ECRB form the second; the

EPL forms the third; the EDC and extensor indicis proprius (EIP) form the fourth; the extensor digiti minimi

(EDM, also called extensor digiti quinti [EDQ]) forms

the fifth; and the ECU forms the sixth (6).] The tendon

of the ECRL continues distally deep to the tendon of the

EPL as the tendons exit the extensor retinaculum. The

tendon of the ECRL then inserts onto the base of the dorsal surface of the index metacarpal (see Fig. 2.6B). The

tendon is not centralized on the metacarpal, but rather

attaches off center on the radial aspect of the dorsal surface of the metacarpal base. The insertion may have slips

that extend to the metacarpals of the thumb, index, or

long fingers, as well as possible slips to the intermetacarpal

ligaments (3,4,11,13).

Architectural features of the ECRL include the physiologic cross-sectional area of the muscle and the fiber bundle

length. Skeletal muscle architectural studies by Lieber and

colleagues provide the data for the ECRL (135–139,174)

(see Table 2.2 and Fig. 2.4). The relative difference index

values compare the ECRL with other upper extremity muscles, based on architectural features. These values are listed

in Appendix 2.3 (15).

The ECRL is innervated by the radial nerve. The branch

leaves the radial nerve trunk proximal to the elbow joint.

There may be two nerve branches to the muscle. The motor

branches enter the muscle on the deep surface of the proximal third of the muscle belly. The nerve fibers are derived

from C6 (mostly) and C7.

Actions and Biomechanics: Extensor Carpi

Radialis Longus

The ECRL functions mainly to provide extension of the

wrist. It works in conjunction with the ECRB and ECU.

The ECRL, by its insertion onto the radial aspect of the

hand, also provides radial deviation of the wrist. In addition, the ECRL gives assistance with weak flexion of the

elbow because the muscle’s origin is proximal to the elbow.

The ECRL (along with the ECRB and ECU) also helps stabilize the wrist (with cocontractions of the wrist flexors)

during powerful grasp functions or heavy lifting (3,4,11,

13).

Anomalies and Variations: Extensor Carpi

Radialis Longus

The ECRL may coalesce with the ECRB, or have several

variations where muscle fibers are interconnected between

the two muscles. Muscle interconnections also may exist

between the APL or to the interosseous muscles (11,444).

The ECRL may have a split tendon or multiple tendons

that insert into the index metacarpal. There may be an anom2 Muscle Anatomy 131

alous insertion into the long finger metacarpal, or even to the

ring finger metacarpal or to the adjacent carpal bones.

The extensor carpi radialis intermedius is an anomalous

muscle situated between the ECRL and ECRB (Fig. 2.8). It

is a rare muscle that may arise independently from either

the lateral epicondyle of the humerus or more proximally

on the distal humeral diaphysis. It inserts into the index or

long finger metacarpal. The muscle also may present as a

muscle slip of variable size that arises from either the ECRL

or ECRB and inserts into the index or long finger

metacarpal, or both (445–447).

The extensor carpi radialis accessorius is an anomalous

muscle that arises from the humerus adjacent to the origin

of the ECRL. The muscle lies deep to the ECRL and

extends the length of the forearm. It usually inserts onto

either the base of the thumb metacarpal, the proximal phalanx of the thumb, or into the tendon of the APB. It also

may originate as a muscle slip from the tendon of the ECRL

to insert as noted previously (11,446).

Clinical Implications: Extensor Carpi

Radialis Longus

Injury to the posterior interosseous nerve, including compression at the arcade of Frohse (at the proximal edge of the

supinator muscle) does not effect the ECRL because the

motor nerve of the ECRL leaves the radial nerve trunk

proper, usually proximal to the elbow (and therefore proxi132 Systems Anatomy

FIGURE 2.8. The anomalous muscle, the extensor carpi radialis intermedius. It is situated

between the extensor carpi radialis longus and extensor carpi radialis brevis. It originates from

the lateral epicondylar region (A), or more proximally, on the lateral aspect of the distal humeral

diaphysis (B). The muscle inserts into the base of either the index or long finger metacarpal, or

both.

A B

mal to the branching of the posterior interosseous nerve).

Complete laceration or dense neuropathy of the posterior

interosseous nerve usually presents clinically with loss of

digital and thumb extension, and weak wrist extension.

Residual wrist extension, produced by the intact ECRL, is

possible, but the wrist also deviates radially during extension because of the ECRL insertion into the index

metacarpal on the radial side of the hand. ECRB function

may be preserved because its motor branch usually exits the

radial nerve trunk or off of the posterior interosseous nerve

proximal to the arcade of Frohse (448–450).

Intersection syndrome is a condition of pain and

swelling in the region of the muscle bellies of the APL and

EPB. As noted by Wolfe, this area lies approximately 4 cm

proximal to the wrist joint, and may show increased

swelling of a normally prominent area (451). In severe

cases, redness and crepitus have been noted. The syndrome

originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More

recently, Grundberg and Reagan have demonstrated that

the basic pathologic process appears to be tenosynovitis of

the ECRL and ECRB (455).

EXTENSOR CARPI RADIALIS BREVIS

Derivation and Terminology. The ECRB derives its

name from several sources. Extensor is from the Greek and

Latin ex, which indicates “out of,” and from the Latin tendere, “to stretch”; thus, extension indicates a motion to

stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening

out” of a joint. Carpi is derived from the Latin carpalis and

the Greek karpos, both of which indicate “wrist” (the carpus). Radialis is from the Latin radii, which means “spoke”

(used to describe the radius of the forearm). Brevis is the

Latin for “short.” Therefore, extensor carpi radialis brevis

indicates a short radial wrist extensor (1,2).

Origin. From the lateral epicondyle of the humerus

through the common extensor origin (additional attachments to the radial collateral ligament of the elbow, surrounding intermuscular septum; see later).

Insertion. To the dorsal base of the long finger

metacarpal.

Innervation. Posterior interosseous nerve or directly

from the radial nerve (C7, C8).

Vascular Supply. The radial recurrent artery, interosseous

recurrent artery, posterior interosseous artery, radial collateral continuation of the profunda brachii artery (3,4,11,

13).

Principal Action. Extension of the wrist. Assistance with

weak flexion of the elbow. The ECRB, along with the

ECRL and ECU, also helps stabilize the wrist (with cocontractions of the wrist flexors) during powerful grasp functions (3,4,11).

Gross Anatomic Description: Extensor

Carpi Radialis Brevis

The ECRB originates from the lateral epicondyle of the

humerus, as part of the common extensor origin (see Fig.

2.2A). It comprises part of the mobile wad muscle compartment of the forearm (12) (Appendix 2.2). The muscle

origin also includes attachments to the intermuscular septum, to the radial collateral ligament of the elbow joint,

and to a strong aponeurosis that covers the surface of the

muscle. The muscle is shorter than the ECRL and is in

part covered by it. The muscle belly, lying adjacent to that

of the ECRL, extends to the mid-portion of the forearm.

At the myotendinous junction, the tendinous portion is

seen first at the dorsolateral surface of the muscle. The

myotendinous junction also is in close proximity to that

of the ECRL. The tendon of the ECRL is a strong, flat

tendon, similar in size to that of the ECRL, and travels

with it to the wrist. The ECRB, along with the ECRL,

passes deep to the APL and EPB, and then enters the second dorsal extensor compartment of the extensor retinaculum. [Editor’s note: The dorsal compartments of the wrist

are as follows: the APL and EPB comprise the first dorsal

compartment; the ECRL and ECRB form the second; the

EPL forms the third; the EDC and EIP form the fourth;

the EDM forms the fifth; and the ECU forms the sixth

(6).] As the tendon extends through the second compartment, it lies in a shallow groove on the dorsal surface of

the radius, medial to the tendon of the ECRL, and separated from it by a low ridge. The tendon of the ECRB

continues distally to reach the base of the long finger

metacarpal (see Fig. 2.6B). Similar to the ECRL, the tendon does not insert centrally on the metacarpal, but rather

attaches off center on the radial aspect of the dorsal surface of the metacarpal base. The insertion may have slips

that extend to the base of the adjacent index metacarpal

(3,4,11,13).

Architectural features of the ECRB include the physiologic cross-sectional area of the muscle and the fiber

bundle length. Skeletal muscle architectural studies by

Lieber, Friden, and colleagues provide the data for the

ECRB (135–139,174) (see Table 2.2 and Fig. 2.4). The

relative difference index values compare the ECRB with

other upper extremity muscles, based on architectural

features. These values are listed in Appendix 2.3 (15,

456–458).

The ECRB is innervated by either the posterior

interosseous nerve or by branches directly from the radial

nerve. The muscle may receive several motor branches, several of which enter the muscle at the medial margin of the

central third. The nerve fibers usually are derived from C6

(mostly), C7, and occasionally C5 (3,4,11,13).

2 Muscle Anatomy 133

Actions and Biomechanics: Extensor Carpi

Radialis Brevis

The ECRB functions mainly to provide extension of the

wrist. It works in conjunction with the ECRL and ECU. It

may provide some radial deviation of the wrist, working

with the ECRL. In addition, the ECRB gives assistance

with weak flexion of the elbow because the muscle’s origin

is proximal to the elbow. The ECRB (along with the ECRL

and ECU) also helps stabilize the wrist (with cocontractions

of the wrist flexors) during powerful grasp functions or

heavy lifting (3,4,11,13,68).

Anomalies and Variations: Extensor Carpi

Radialis Brevis

The ECRB may coalesce with the ECRL, or have several

variations where muscle fibers are interconnected between

the two muscles (11).

The ECRB may have a split tendon or multiple tendons

that insert into the long finger metacarpal. There may be an

anomalous insertion into the adjacent metacarpal bases, or

to the adjacent carpal bones (11).

The extensor carpi radialis intermedius is an anomalous

muscle situated between the ECRL and ECRB (see Fig.

2.8). It is a rare muscle that may arise independently from

the lateral epicondyle of the humerus, and inserts into the

index or long finger metacarpal. The muscle also may present as a muscle slip of variable size that arises from either

the ECRL or ECRB and inserts into the index or long finger metacarpal, or both (445).

The FCR brevis muscle is a rare anomalous muscle associated with the ECRB. The FCR brevis originates from the

anterior surface of the radius and forms a tendon at the

radiocarpal joint. The muscle is innervated by the anterior

interosseous nerve (181). It enters the carpal tunnel and the

tendon extends between the bases of the index and long finger metacarpals to interconnect with the tendon of the

ECRB. The ECRB, in addition, splits into two tendons,

one that inserts normally into the radial part of the base of

the long finger metacarpal, and the other connected to the

anomalous FCR brevis. It has been postulated that this

anomaly causes restricted wrist flexion or extension (11).

Clinical Implications: Extensor Carpi

Radialis Brevis

Because of the central location of its insertion on the wrist

(between the ECRL and ECU), the ECRB often is used as

a recipient muscle for transfers to restore wrist extension

after nerve or spinal injury.

In lateral epicondylitis (tennis elbow), the ECRB is usually implicated as the principal muscle affected. Several

methods for operative management have been described,

including muscle release, lengthening or debridement of its

tendinous origin (459–463), or lengthening of the muscle

at the musculotendinous junction (462,463). Friden and

Lieber have studied the physiologic consequences of surgical lengthening of the ECRB at the tendon junction. The

authors found that the ECRB develops near-maximal isometric force at full wrist extension. This decreases to 20%

maximum at full wrist flexion. Operative lengthening of the

tendon by 9.1 mm results in a mean 10% passive shortening of the fibers, and ECRB sarcomere shortening of 0.3

µm. This 0.3-µm sarcomere shortening, in turn, was predicted to have two primary biomechanical effects: (a) a

25% decrease in muscle passive tension that could lead to

reduced insertional tension and decrease pain; and (b) a

25% increase in active muscle force, which is in opposition

to the notion that tendon lengthening necessarily results in

muscle weakness (457,458).

Intersection syndrome is a condition of pain and

swelling in the region of the muscle bellies of the APL and

EPB. As noted by Wolfe, this area lies approximately 4 cm

proximal to the wrist joint, and may show increased

swelling of a normally prominent area (451). In severe

cases, redness and crepitus have been noted. The syndrome

originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–454). More

recently, Grundberg and Reagan have demonstrated that

the basic pathologic process appears to be tenosynovitis of

the ECRL and ECRB (455).

EXTENSOR DIGITORUM COMMUNIS

Derivation and Terminology. Extensor is from the Greek

and Latin ex, which indicates “out of,” and from the Latin

tendere, “to stretch”; thus, extension indicates a motion to

stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening

out” of a joint. Digitorum is from the Latin digitus or digitorum, indicating the digits. Communis is derived from the

Latin communis, meaning “common,” and is used to indicate a structure serving or involving several branches or sections (1,2).

Origin. From the lateral epicondyle as part of the common extensor origin.

Insertion. To the base of the phalanges of the index,

long, ring, and small fingers.

Innervation. The posterior interosseous nerve, from the

radial nerve (C7, C8).

Vascular Supply. Posterior interosseous artery (which is a

branch of the common interosseous artery); interosseous

recurrent artery and the surrounding anastomoses; the distal continuation of the anterior interosseous artery after it

passes through the interosseous ligament to reach the dorsal

aspect of the forearm; the dorsal carpal arch; dorsal

metacarpal, digital, and perforating arteries (3,4,11,13).

134 Systems Anatomy

Principal Action. Extension of the digits, primarily at the

MCP joints. The EDC also assists with extension of the PIP

and DIP joints, working with the interossei and lumbricals.

The tendons can assist with wrist extension.

Gross Anatomic Description: Extensor

Digitorum Communis

The EDC, with its associated extensor mechanism, juncturae, and anatomic variability, is a complex structure and

the subject of many investigations (464–498). It lies in the

dorsal muscle compartment of the forearm (Appendix 2.2).

It arises from the common extensor origin at the lateral epicondyle of the humerus (see Fig. 2.2A). The muscle also has

attachments that arise from the adjacent intermuscular

septa and from the fascia of the neighboring forearm muscles (3,4,11,13). It is a relatively large muscle, and its muscle belly is close to the muscle of the EDM. At the junction

of the proximal two-thirds and the distal one-third of the

forearm, the myotendinous junction arises and four separate tendons are formed. The tendons may be partially

attached in the forearm, but more distally, at the level of the

extensor retinaculum, four discrete tendons are present.

The tendons pass deep to the extensor retinaculum in a tunnel with the EIP. The tendons of the EDC and EIP form

the fourth dorsal compartment. [Editor’s note: The dorsal

compartments of the wrist are as follows: the APL and EPB

comprise the first dorsal compartment; the ECRL and

ECRB form the second; the EPL forms the third; the EDC

and EIP form the fourth; the EDM forms the fifth, and the

ECU forms the sixth (6).] The tunnel also provides a synovial sheath. The tendons exit the retinaculum and diverge

on the dorsum of the hand, one or more tendon of the

EDC to each digit. The tendon of the EIP extends to the

index finger, along the ulnar margin of the EDC to the

index. Juncturae tendinum interconnect the tendons, with

fewer and thinner juncturae located on the radial aspect of

the hand. The ulnar tendons tend to have more, and thicker

juncturae (discussed later) (492–498). The tendons then

continue into the digits to form the extensor mechanism of

each digit. The EDC tendon, through the extensor mechanism, inserts into the base of each distal phalanx (see Fig.

2.6B), the base of each middle phalanx (through the central

slip), and, to varying degrees, into the bases of the proximal

phalanges. Substantial tendon variability and multiplicity

exits with the extensor tendons (Table 2.3).

The extensor mechanism is complex, and is referred to as

the extensor aponeurosis, dorsal aponeurosis, or extensor expansion (Fig. 2.9). Each of the four digits has a similar extensor

mechanism, and it intimately involves the intrinsic muscles

of the hand as well. Smith and von Schroeder and Botte

have described the mechanism in detail (484,494). Each

extrinsic extensor tendon enters the dorsal aponeurosis at

the level of the MCP joint. The tendon is joined by the

sagittal bands from the medial and lateral aspects. The

transverse lamina of the sagittal bands arise from the palmar

aspect of the MCP joint, attaching to the volar plate, to

intermetacarpal ligaments at the neck of the metacarpals,

and to a portion of the fibroosseous tunnel. The sagittal

bands extend over the medial and lateral aspects of the

MCP joint to envelop the EDC (and EIP) tendons. The

sagittal bands help stabilize and centralize the extrinsic

extensor tendons. (Injury to the sagittal bands may result in

extensor tendon subluxation.) The tendon continues distally, and in the fibrous expansion, the tendon divides into

a central slip and two lateral slips. The central slip inserts

into the base of the middle phalanx and provides extension

of the middle phalanx partially through the central slip. The

intrinsic tendons from the lumbricals and interosseous

muscles join the extensor mechanism at the level of the

proximal and mid-portion of the proximal phalanx. A portion of the lateral band extends dorsally to join the central

slip. It is through this portion of the extensor tendon that

the intrinsic muscles contribute to PIP joint extension. A

portion of the lateral bands also continues distally to join

the terminal tendon, to insert onto the base of the distal

phalanx. The lateral slips join the tendons of the intrinsic

muscles to form the conjoined lateral bands, which continue distally to form the terminal tendon. The lumbricals

to the index and long fingers arise from the radial sides of

the associated profundus tendons (467,468). The lumbricals to the ring and small finger arise from the adjacent sides

of the profundus tendons to the long, ring, and small fingers. Variation of the lumbricals is common, and similar to

the extrinsic extensor tendons there is more variability on

the ulnar side of the hand. The lumbricals and interosseous

muscles are discussed in greater detail under their respective

muscle sections (464,465).

As mentioned previously, the extensor tendons are interconnected on the dorsum of the hand by the juncturae

tendinum and intertendinous fascia. These structures have

been studied in detail and classified by Wehbe and von

Schroeder et al. (489,493). The juncturae tendinum consist

of narrow connective tissue bands or slips that extend

between the EDC tendons as well as to the EDM. Very

rarely does the EIP have a connecting junctura (493). The

function of the junctura remains not entirely understood.

The juncturae may assist with spacing of the EDC tendons

or with force redistribution (486,487), or may help with

coordination of extension or stabilization of the MCP joints

(488). The junctura prevent independent extension of the

digits and are clinically important because they may bridge

and therefore mask tendon lacerations. Juncturae also may

cause snapping by subluxating across the metacarpal head.

The juncturae also may aid in the surgical identification of

the tendons of the hand and has been used in repair of the

dorsal aponeurosis. Complete transection of a juncturae

and the intertendinous fascia may lead to subluxation of the

EDC tendon over a flexed MCP joint (494). The juncturae

tendinum are variable, and become progressively thicker

2 Muscle Anatomy 135

from the radial to the ulnar side of the hand. Three distinct

type of juncturae tendinum have been identified (493) (Fig.

2.10). A thin filamentous junctura is defined as type 1, and

is found primarily between the EDC tendons to the index

and middle fingers and between the tendons to the middle

and ring fingers. Type 2 juncturae are thicker and well

defined and are present between extensor tendons to the

long and ring fingers and between the tendons to the ring

and small fingers. Type 3 juncturae consist of a thick, tendon-like slip between the extensor tendons to the middle

and ring fingers and between the tendons to the ring and

small fingers. Two subtypes of type 3 juncturae have been

identified, a “y” and an “r” type, based on the interconnections. The presence of certain juncturae appears to be associated with the presence or absence of tendons. For

instance, the type of juncturae in the fourth intermetacarpal

space depends on the presence of an EDC tendon to the

small finger. Absence of the EDC small finger tendon has

been found to be associated with a double EDC ring finger

tendon and a thick type 3 junctura that substitutes for the

absent EDC small finger tendon (492,493). Although multiple EDC ring finger tendons usually are present, the ulnar

portion of the double EDC ring finger tendon and, as mentioned, the type 3 junctura may represent a developmental

remnant of the EDC small finger tendon. The presence of

juncturae between the extension tendons and adjacent ten136 Systems Anatomy

FIGURE 2.9. The extensor aponeurosis (see text).

2 Muscle Anatomy 137

FIGURE 2.10. Juncturae tendinum of the extensor tendons. A: Type 1. The type 1 junctura is a

thin, filamentous connection between the extensor digitorum communis (EDC) of the long and

index fingers. It sometimes is present between the EDC of the ring and long fingers. B: Type 2.

The type 2 junctura has morphologic features between types 1 and 3. It typically is present

between the EDC tendons of the long and ring fingers and sometimes between the tendons of

the ring and small fingers. C: Type 3y. The type 3y junctura is a tendon slip most commonly present between the EDC tendons of the small and ring fingers. D: Type 3r. The type 3r junctura is a

tendon slip most commonly present between the EDC of the ring finger and the extensor digiti

quinti. Its presence is associated with an absent EDC to the small finger. (Adapted from von

Schroeder HP, Botte MJ, Gellman H. Anatomy of the juncturae tendinum of the hand. J Hand Surg

[Am] 15:595–602, 1990, with permission.)

A B

C D

dons should be appreciated when tendon transfer or harvesting is used (492,493).

Architectural features of the EDC include the physiologic cross-sectional area of the muscle, the fiber bundle

length, muscle length, muscle mass, and pennation angle

(angle of the muscle fibers from the line representing the

longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the

data for the EDC (135–139,174) (see Table 2.1 and Fig.

2.4). The digital extrinsic extensor and flexor muscles have

similar architectural features. In general, the EDC muscles

do have smaller physiologic cross-sectional areas compared

with the extrinsic flexors, indicating that the EDC is not

optimally designed for force generation. The relative difference index values compare the EDC with other upper

extremity muscles, based on architectural features. These

values are listed in Appendix 2.3 (15).

The EDC is innervated by the posterior interosseous

nerve, derived mostly from C7 as well as from C6 and C8.

The posterior interosseous nerve passes through the supinator muscle, and branches into several motor branches that

enter the deep surface of the middle third of the muscle.

There is variation among the motor branches, and there

may be a common branch or branches that also innervate

the EDM or ECU. The EDC muscle may receive a variable

number of branches.

Actions and Biomechanics: Extensor

Digitorum Communis and the Associated

Extensor Mechanism

The EDC functions mainly to extend the digits, primarily

at the MCP joints (494). The EDC also assists with extension of the PIP and DIP joints, working with the interossei

and lumbricals. The tendons also can assist with wrist

extension.

Extension of the digits is a complex function, involving simultaneous actions of the intrinsic and extrinsic

extensor muscles (464,465,467,468,474,494). The

interossei and lumbricals extend the PIP and DIP joints

and flex the MCP joints. The extrinsic digital extensor

138 Systems Anatomy

TABLE 2.3. EXTENSOR TENDON VARIATIONS AND MULTIPLICITY

Incidence as %

Tendons von Schroeder/ Mestdagh Leslie Ogura

Tendon or Slips Botte (492) Schenk et al. (479) (478) et al. (500)

EIP Absent 0 1 0

1 77 93 96

2 16 6 4

3 7

EIP to middle Present 5 3 2

EMP Present 12 5

EDC—index 1 98 95

2 2 5

EDC—long 1 51 61

2 28 39

3 16

4 5

EDC—ring 1 12 63

2 63 31

3 16 5

4 9

5

EDC—small Absent 54 56 91

1 19 44 9 97

2 26 3

3 2

EDQ 1 2 7 16 4

2 84 84 84 94

3 77 2

4 7 2

EDQ to ring Present 2 2

EDBM Present 0 0 3

EIP, extensor indicis proprius; EMP, extensor medii proprius; EDC, extensor digitorum communis; EDQ, extensor digiti quinti; EDBM, extensor

digitorum brevis manus.

Reprinted from von Schroeder HP, Botte MJ. Functional anatomy of the extensor tendons of the digits. Hand Clin 13:51–62, 1997, with

permission.

muscles, including the EDC, EIP, and EDM, function

primarily to extend the MCP joints, but do have extensor

function at the PIP and DIP joints. The flexor muscles

and respective tendons on the palmar aspect of the hand

are important in stabilizing and balancing the phalangeal

joints during extension. Despite all the separate tendons

involved in finger extension, complete independent

extension of each finger is not always possible. This is due

in part to the juncturae tendinum and intertendinous fascia between the extrinsic tendons on the dorsum of the

hand (491–496).

As noted previously, the EDC muscles have smaller

physiologic cross-sectional areas than the extrinsic flexors,

indicating that the EDC is not optimally designed for force

generation (466) (see Table 2.1 and Fig. 2.4). Although the

EDC appears as a single muscle belly that forms four tendons, each tendon usually can be traced back to a muscle

belly that can be separated from the remaining EDC muscle. Each of these four muscle bellies are similar, however.

The EDCs to the long and ring fingers have a relatively

larger cross-sectional area than the EDCs to the index and

small fingers. The cross-sectional area of the EDC muscles

to the long and ring fingers also are larger that those of the

EIP or EDM.

At the level of the extensor retinaculum, the EDC usually exits as four tendons. Distal to the wrist, many of the

tendons divide into double or triple tendons. These

anatomic variations as well as their arrangement and incidences have been recognized in clinical and anatomic studies (462,475,478,479,481) (Table 2.3). Because these variations are so common, it is difficult to label these as

anomalies; they perhaps are best considered as normal variations. In a study of 43 hands, the most common pattern

on the dorsum of the hand was a single EIP tendon (77%)

that inserted ulnar to the index finger EDC on the dorsal

aponeurosis of the index finger; a single index finger EDC

(98%); a single long finger EDC (51%); a double ring finger EDC tendon (63%) with a single insertion; an absent

small finger EDC (54%); and a double EDQ tendon (84%)

with a double insertion into the dorsal aponeurosis of the

small finger (492). The extensor tendons typically have longitudinal fissures or striae, but tendons that can be readily

divisible along fissures without sharp dissection are defined

as tendon slips (492).

Anomalies and Variations: Extensor

Digitorum Communis

The EDC tendons are extremely variable as to number and

presence (see Table 2.3). Double and triple tendons exist.

An EDC tendon may be absent (in 54% to 56% of

hands) (481,492). Absence of the EDC to the small finger

often is associated with a double EDM to the small finger

(492). There commonly are thick juncturae from the ring

finger EDC to the small finger (493).

The most common extensor tendon pattern is as follows:

a single EIP tendon (77%), a single index finger EDC

(98%), a single long finger EDC (51%), a double ring finger EDC (63%), an absent small finger EDC (54%), and a

double EDM (84%) (492).

Additional frequent variations include a double EIP

(16%), a double (28%) or triple (16%) long finger EDC, a

single (12%) or triple (16%) ring finger EDC, and a single

(19%) or double (26%) small finger EDC (492).

The juncturae tendinum of the EDC are variable, with

fewer and thinner juncturae on the radial side of the hand

compared with the ulnar (493). There is more tendon variability and multiplicity (along with more juncturae) toward

the ulnar side of the hand (492).

The muscle belly of the EDC may exist as a single or

double muscle, or as four separate bellies (11). The EDC

may have a tendon slip or a junctura that extends to the

extensor tendon of the thumb (11).

The extensor medii proprius (EMP), also known as the

extensor medii digiti or extensor medii communis, is a deeply

situated anomalous muscle that is analogous to the EIP but

inserts into the ulnar aspect of the dorsal aponeurosis of the

long finger (Fig. 2.11A). The EMP and EIP muscles usually

have a common origin on the distal ulna and adjacent

interosseous ligament. The EMP is encountered in 0.8% to

10.3% of hands (11,479,485,490), but is rarely described

or noted (478). The EMP is commonly found in Old

World monkeys, whereas the EMP is variably present in the

chimpanzee and gorilla, as it is in humans. Because of these

findings, von Schroeder and Botte speculate that the EMP

is an evolutionary remnant and not a variation of a normal

arrangement (494).

The extensor indicis et medii communis (EIMC) muscle is

an anomalous muscle similar to the EIP muscle, except that

it splits to insert into both the index and long fingers (see

Fig. 2.11B). It has been studied in detail by von Schroeder

and Botte, who observed an incidence of 3.4% (490). Similar to the EMP, the EIMC commonly is found in Old

World monkeys, whereas the EIMC is variably present in

the chimpanzee and gorilla, as it is in humans. Because of

these findings, the EIMC (like the EMP) may be an evolutionary remnant and not a variation of a normal arrangement (494).

The extensor medii et annularis communis is an anomalous EIP muscle that splits to insert into both the long and

ring fingers (490).

The extensor digitorum brevis manus is an anomalous

muscle that originates from the distal radius, radiocarpal

ligament, or the distal ulna (Fig. 2.12). The tendon

inserts into the index finger or, less commonly, into the

long finger. It is innervated by a branch of the posterior

interosseous nerve. Most of the muscle belly is located on

the dorsum of the hand, and can cause local discomfort

or tendon dysfunction. It can be mistaken for a ganglion

or other tumor. The muscle may become symptomatic

2 Muscle Anatomy 139

140 Systems Anatomy

FIGURE 2.11. Anomalous extensor tendons of the hand and forearm. A: Schematic illustration

of the extensor medii proprius (EMP). The EMP originates in the forearm and inserts into the dorsal aponeurosis of the long finger. The EMP is similar to the extensor indicis proprius (EIP); however, the EMP inserts into the aponeurosis of the long finger, not the index finger. The insertions

(cut) of the extensor digitorum communis (arrows) to the index and long fingers also are shown.

B: Schematic illustration of the extensor indicis et medii communis (EIMC). The EIMC consists of

one muscle belly and two tendons that insert into the index and long fingers. The EIP is absent.

The insertions (cut) of the extensor digitorum communis (arrows) to the index and long fingers

also are shown. (From von Schroeder HP, Botte MJ. The extensor medii proprius and anomalous

extensor tendons to the long finger. J Hand Surg [Am] 16:1141–1145, 1991, with permission.)

A B

FIGURE 2.12. The extensor digitorum brevis

manus is an anomalous muscle that originates

from the distal radius, radiocarpal ligament, or

the distal ulna. It can resemble a dorsal wrist

ganglion.

deep to the extensor retinaculum. Excision of this anomalous muscle or decompression under the extensor retinaculum may be performed if symptoms warrant (494,

499–504).

Clinical Implications: Extensor Digitorum

Communis

Their frequent multiplicity and variability and the possible

presence of many anomalous extensor tendons should be

appreciated during extensor tendon exploration for trauma

repair or tendon transfer.

The index finger has the greatest independent motion

in extension. It has two independent tendons (index finger EDC and EIP) that are the least variable of the extensor tendons. It also has the lowest frequency of interconnecting juncturae tendinum. These anatomic findings

help explain its relatively independent extension capabilities compared with the more ulnarly located digits (e.g.,

the ring finger).

Occasionally, an anomalous junctura tendinum may

cross between the EDC and the EPL tendons. This junctura

restricts digital motion, making it impossible actively to

extend the digits fully while maintaining the interphalangeal joint of the thumb in flexion (483).

The extensor digitorum brevis manus (see earlier) can be

mistaken for a ganglion or other tumor. It may become

symptomatic deep to the extensor retinaculum. Excision of

this anomalous muscle or decompression under the extensor retinaculum may be performed if symptoms warrant

(494,499–504) (see Fig. 2.12).

EXTENSOR INDICIS PROPRIUS

Derivation and Terminology. Extensor is derived from

the Greek and Latin ex, which indicates “out of,” and from

the Latin tendere, “to stretch”; thus, extension indicates a

motion to stretch out, and extensor usually is applied to a

force or muscle that is involved in the “stretching out or

straightening out” of a joint. Indicis is from the Latin to

indicate the index finger (1,2).

Origin. The dorsal surface of the distal ulna and adjacent

interosseous ligament.

Insertion. The extensor hood of the index finger.

Innervation. Posterior interosseous nerve (C7, C8).

Vascular Supply. The posterior interosseous artery,

interosseous recurrent artery and its communicating vessels,

continuation of the anterior interosseous artery after it

passes through the interosseous ligament; the dorsal carpal

arch; dorsal metacarpal, digital and perforating arteries

(3,4,11,13).

Principal Action. Extension of the index finger. As with

the index finger EDC, the principal action is on the MCP

joint.

Gross Anatomic Description: Extensor

Indicis Proprius

The EIP is a relatively small and short extensor located deep

to the EDC, EDM, and ECU. It lies in the dorsal muscle

compartment of the forearm (Appendix 2.2). The EIP originates from a diagonally oriented origin on the ulnar aspect

of the distal forearm (see Fig. 2.3B). The muscle arises from

the dorsal surface of the distal ulna and from a portion of the

adjacent interosseous ligament. Additional attachments

include the fascia or septum between the EIP and EPL. The

muscle belly of the EIP lies next to and ulnar to the muscle

belly of the EPL. The tendon of the EIP passes deep to the

EDM and EDC tendons in an oblique fashion as it extends

distally toward the index finger. It joins the tendons of the

EDC in the fourth dorsal compartment as it passes deep to

the extensor retinaculum. As the EIP enters the extensor

retinaculum, it is located on the ulnar margin of the retinaculum, and positioned ulnar and deep to the EDC. In the

extensor retinaculum, the EIP tendon continues in a diagonal course to cross deep to the EDC tendons, so that when

the EIP emerges from the extensor retinaculum, it is on the

lateral aspect of the retinaculum. The tendon continues distally and laterally toward the dorsal aspect of the index finger, and remains in close proximity and ulnar to the EDC to

the index finger. (This ulnar position of the tendon is important in identification of the tendon for harvest for tendon

transfer during such procedures as opponensplasty.) At the

level of the index metacarpal head and neck, the tendon of

the EIP joins the tendon of the index EDC to form a continuous extensor hood (3,4,11,13,68) (Fig. 2.6B).

Architectural features, including the physiologic crosssectional area and the muscle fiber length of the EIP, are

listed in Table 2.1 and depicted in Fig. 2.4.

The EIP is innervated by the posterior interosseous nerve,

predominantly C7, as well as C8. Anatomic studies have

shown that the branch to the EIP usually is the last or terminal motor branch of the posterior interosseous nerve (505).

Actions and Biomechanics: Extensor

Indicis Proprius

The EIP assists with extension of the index finger. It also

assists with wrist extension. The separate muscle of the EIP

provided to the index finger assists with the strong independent motion of the index finger. Principal action is on

the MCP joint.

Anomalies and Variations: Extensor

Indicis Proprius

See also Anomalies and Variations: Extensor Digitorum

Communis.

Despite the variability and multiplicity of the extensor

tendons (see Table 2.3), the EIP usually exists as a single

2 Muscle Anatomy 141

tendon. In a study of 43 hands, the most common pattern

on the dorsum of the hand was a single EIP tendon (77%)

that inserted ulnar to the index finger EDC on the dorsal

aponeurosis of the index finger, and a single index finger

EDC (98%) (492).

The EIP may be absent (11). The muscle or tendon of

the EIP may be doubled (11).

Muscle or tendon slips can pass to the thumb or adjacent

digits, including additional anomalous insertions into the

base of the long finger metacarpal or base of the long finger

proximal phalanx.

The extensor medii proprius (EMP) and the extensor

indicis et medii communis (EIMC) are anomalous muscles

similar to the EIP that attach to the index or long fingers,

and are seen in 2% to 6.5% of hands (478,490,506–508)

(see also under Anomalies and Variations: Extensor Digitorum Communis, and Fig. 2.10).

The EIP tendons usually insert ulnar to the index finger

EDC tendon (81% to 87% of specimens). However, they

may be located or insert directly palmar to the index finger

EDC in 10% to 11%, and radial to the index finger EDC

in 3% to 8% (11,479,490).

Clinical Implications: Extensor Indicis

Proprius

Their frequent multiplicity and variability, and the possible

presence of many anomalous extensor tendons should be

appreciated during extensor tendon exploration for trauma

or for tendon transfer (492,493,509).

The EIP usually is located along the ulnar aspect of the

index finger EDC tendon (3,4,11,13,492). This positioning helps identify the tendon for repair or for harvest for

transfer (i.e., for opponensplasty).

EXTENSOR DIGITI MINIMI (EXTENSOR

DIGITI QUINTI)

Derivation and Terminology. Extensor is from the Greek

and Latin ex, which indicates “out of,” and from Latin tendere, “to stretch”; thus, extension indicates a motion to

stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening

out” of a joint. Digiti is the plural of the Latin digitus,

“digit.” Minimi is from the Latin minima, “the minimum,”

referring to the small finger (1,2).

Origin. From the lateral epicondyle through the common extensor origin, as well as from the adjacent intermuscular septum (between it and the ECU), and from the overlying fascia.

Insertion. To the extensor mechanism of the small finger.

Innervation. Posterior interosseous nerve (C7, C8).

Vascular Supply. The posterior interosseous artery;

interosseous recurrent artery and its communicating

branches; from the continuation of the anterior

interosseous artery after it passes through the interosseous

ligament; the dorsal carpal arch; dorsal metacarpal, digital,

and perforating arteries (3,4,11,13,68).

Principal Action. Extension of the MCP joint of the

small finger, extension of the PIP and DIP joints. The

EDM also assists with wrist extension.

Gross Anatomic Description: Extensor

Digiti Minimi

The EDM is a relatively small, slender muscle. It lies in the

dorsal muscle compartment of the forearm (Appendix 2.2).

It originates from the lateral epicondyle of the humerus as

part of the common extensor origin tendon (3,4,11,13)

(Fig. 2.2A). In addition, fibers arise from the adjacent intermuscular septum (between the EDM and the EDC) as well

as from the overlying deep antebrachial fascia. The narrow

muscle is formed and blends to some extent to that of the

EDC. The tendon forms in a manner similar to those of the

EDC in the distal third of the forearm. The tendon passes

deep to the extensor retinaculum, comprising the fifth dorsal compartment. [Editor’s note: The dorsal compartments

of the wrist are as follows: the APL and EPB comprise the

first dorsal compartment; the ECRL and ECRB form the

second; the EPL forms the third; the EDC and EIP form

the fourth; the EDM forms the fifth; and the ECU forms

the sixth (6).] The fifth dorsal compartment is located dorsal to the distal radioulnar joint. The tendon continues distally to reach the dorsal surface of the small finger

metacarpal. It remains on the ulnar side of the EDC tendon

to the small finger. The EDM inserts, in part, into the base

of the proximal phalanx of the small finger (Fig. 2.6B). The

tendon also is joined by the slip from the EDC to the small

finger. The tendon often is split or doubled, and exhibits

variability, as do the EDC tendons (492) (see Table 2.3).

The architectural features of the EDM are listed in Table

2.1 and depicted in Fig. 2.4.

The EDM is innervated by the posterior interosseous

nerve, mostly from C7 and C8. The nerve branch or

branches enter the muscle belly of the EDM in the middle

third of the muscle on the deep surface.

Actions and Biomechanics: Extensor

Digiti Minimi

The EDM provides extension of the MCP joint of the small

finger, as well as extension of the PIP and DIP joints. The

EDM also assists with wrist extension. It works with the

small finger EDC, and may be the only digital extensor of

the small finger if the small finger EDC tendon is absent

(3,4,11,13).

Anomalies and Variations: Extensor Digiti

Minimi

See also Anomalies and Variations: Extensor Digitorum

Communis.

142 Systems Anatomy

The EDM exhibits variability similar to that of the EDC

tendons (see Table 2.3). The tendon may be absent, or exist

as a double or triple tendon. Its most common pattern is

that of a double tendon, seen in 84% (492,495,497,498).

The muscle belly may be doubled, or have an accessory

head. An accessory head may originate from the ulna (11).

The muscle belly may blend or coalesce with the EDC muscle belly (11).

Several variations in the insertion can exist. A tendon

slip to the base of the ring finger proximal phalanx has been

noted in 6% to 10% (11,446). An ulnar slip has been noted

to insert onto the base of the small finger metacarpal (11).

Clinical Implications: Extensor Digiti

Minimi

The EDM may provide the principal digital extension for

the small finger in the absence of the EDC to the small finger. The most common pattern of the extensor tendons

actually is an absent small finger EDC, and a double tendon of the EDM (492,497,498) (Table 2.3).

EXTENSOR CARPI ULNARIS

Derivation and Terminology. Extensor is derived from

the Greek and Latin ex, which indicates “out of,” and from

the Latin tendere, “to stretch”; thus, extension indicates a

motion to stretch out, and extensor usually is applied to a

force or muscle that is involved in the “stretching out or

straightening out” of a joint. Carpi is from the Latin carpalis

or the Greek karpos, both of which indicate “wrist” (the carpus). Ulnaris is derived from the Latin ulna, “arm,” and

ulnaris, “pertaining to the arm” (1,2).

Origin. The lateral epicondyle of the humerus through

the common extensor origin. Additional attachments

include the posterior border of the ulna by an aponeurosis

that wraps around the ulna and is shared with the FCU and

FDP. The ECU also has attachments of origin from the

overlying fascia.

Insertion. Base of the small finger metacarpal, dorsal

aspect.

Innervation. Posterior interosseous nerve (C6, C7, C8).

Vascular Supply. The posterior interosseous artery;

interosseous recurrent artery (3,4,11,13).

Principal Action. Extension of the wrist. It contributes to

ulnar deviation of the wrist. The ECU also helps stabilize

the wrist during forceful grip or lifting, or production of a

clenched fist.

Gross Anatomic Description: Extensor

Carpi Ulnaris

The ECU originates mainly from the lateral epicondyle of the

humerus through the common extensor origin (see Fig. 2.2A).

It lies in the dorsal muscle compartment of the forearm

(Appendix 2.2). In addition, there may be several other sites

of origin (3,4,11,13). The ECU usually also has attachments

to the posterior border of the ulna that connect to an aponeurosis that wraps around the ulna and is shared with both the

FCU and FDP (see Fig. 2.3B). The ECU also has attachments

of origin from the overlying fascia of the forearm muscles.

Two heads may be present. One head originates from the distal dorsal portion of the lateral epicondyle of the humerus and

from the investing fascia and septa between the ECU and

EDM, anconeus, and supinator. The other head originates

from the proximal dorsal border of the ulna. The muscle fibers

extend distally along the dorsal ulnar portion of the forearm

in an osteofascial compartment consisting of the dorsal surface

of the ulna, the fascia of the forearm, dense fascia lying on the

ulnar origin of the muscles of the thumb, and the origin of the

extensor indicis. The muscle usually extends the distal threefourths of the forearm to end in a thick tendon. The tendon

first appears on the dorsal surface of the muscle or deep in the

muscle on the radial border of the middle third of the posterior surface of its belly (3,4,11). The tendon reaches the extensor retinaculum to form the sixth dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows:

the APL and EPB comprise the first dorsal compartment; the

ECRL and ECRB form the second; the EPL forms the third;

the EDC and EIP form the fourth; the EDM forms the fifth;

and the ECU forms the sixth (6).] In the sixth compartment,

the tendon is stabilized by traversing a groove in the distal

ulna. The groove is located lateral to the styloid process of the

ulna, but medial to the head of the ulna. The dorsal retinaculum holds the tendon in place. The tendon extends distally in

close proximity to the dorsomedial portion of the triangular

fibrocartilage (510,511). The tendon continues across the

ulnar carpus to reach the base of the fifth metacarpal (see Fig.

2.6B). It inserts onto a tubercle located on the medial aspect

of the dorsal base of the metacarpal (3,4,11,13) (Fig. 2.6B).

Architectural features of the ECU include the physiologic cross-sectional area of the muscle and the fiber length.

Skeletal muscle architectural studies by Lieber and colleagues provide the data for the ECU (135–139,174) (see

Table 2.2 and Fig. 2.4). The relative difference index values

compare the ECU with other upper extremity muscles,

based on architectural features. These values are listed in

Appendix 2.3 (15).

The ECU is innervated by the posterior interosseous

nerve, comprising contributions from the C6, C7, and C8

nerve roots. The branch to the ECU usually leaves the posterior interosseous nerve just distal to the distal edge of the

supinator muscle. The nerve may branch into several

smaller branches that enter the middle third of the muscle

belly on its deep surface.

Actions and Biomechanics: Extensor Carpi

Ulnaris

The main function of the ECU is extension of the wrist. It

also contributes to ulnar deviation of the wrist. The ECU

2 Muscle Anatomy 143

helps stabilize the wrist during forceful gripping or lifting,

or producing a clenched fist. It is a dynamic stabilizer of the

distal radioulnar joint and distal ulna. In stabilizing the distal radioulnar joint complex, the ECU works with the

interosseous ligament, the extensor retinaculum and competence of the sigmoid notch of the distal radius, and the

dynamic forces of the pronator quadratus (510–515).

Anomalies and Variations: Extensor Carpi

Ulnaris

The ECU may consists of a double muscle belly, or terminate in a double tendon (516,517). With a double tendon,

one slip may insert onto the base of the fourth metacarpal

(517).

The ulnaris digiti minimi (or ulnaris digiti quinti) is an

anomalous muscle closely associated with the ECU. It arises

distally in the forearm from the dorsal surface of the distal

ulna. This small muscle extends distally along the ulnar

wrist and hand to insert into the base of the distal phalanx

of the small finger. The ulnaris digiti minimi may represent

an extension or accessory belly of the ECU. It may be a separate tendon slip arising from the tendon of the ECU. The

ulnaris digiti minimi also may have insertions into the dorsal fascia of the fifth metacarpal, capsule of the MCP joint,

or proximal phalanx of the small finger (11).

The ECU may be absent (518). This is rare, occurring in

0.55% (11). Absence has been noted to be bilateral (518).

Clinical Implications: Extensor Carpi

Ulnaris

Duplication of the ECU tendon, or a double tendon that

extends to the base of the small finger distal phalanx, may

impair simultaneous extension of the wrist and the small

finger. Synovitis has been associated with this anomaly

(517).

Dislocation, subluxation, and stenosing tenosynovitis

are potential problems of the ECU tendon as it passes to

and through the dorsal retinaculum (519–522).

SUPINATOR

Derivation and Terminology. Supinator is derived from

the Latin supinatio, which denotes the act of assuming the

supine position, or the state of being supine. Applied to the

hand, it is the act of turning the palm forward (anteriorly)

or upward, performed by lateral rotation of the forearm

(1,2).

Origin. From the lateral epicondyle and the lateroposterior ulna.

Insertion. To the proximal radius, along the lateral, posterior, and anterior surface.

Innervation. Posterior interosseous nerve (C6, C7).

Vascular Supply. The radial artery; posterior interosseous

artery; radial recurrent artery; interosseous recurrent artery;

middle collateral artery (3,4,11,13).

Principal Action. Supination of the forearm (lateral rotation of the forearm so that the palm faces anteriorly, or

superiorly if the elbow is flexed).

Gross Anatomic Description: Supinator

The supinator is a relatively broad and flat muscle of the

proximal deep forearm. It comprises one of the many muscles of the dorsal muscle compartment of the forearm

(Appendix 2.2). It arises from two main areas: the lateral

epicondyle and the proximal lateral ulna (3,4,11,13) (see

Fig. 2.3). From the lateral epicondyle, it arises from the dorsal aspect from a tendinous band that joins the deep surface

of the tendons of origin of the ECRL, ECRB, and EDC. It

also has attachments to the radial collateral ligament of the

elbow joint. The other main area of origin is from the proximal ulna, on its lateral aspect. Some fibers arise from a

depression distal to the radial notch and others from a crest

on the proximal ulna known as the supinator crest. The

fibers extend radially and slightly distally to the radius, to

insert onto the proximal radius (see Fig. 2.3). The insertion

area surrounds the proximal third of the radius, from the

radial tuberosity to the attachment of the pronator teres, or

to the upper part of the radius between the anterior and

posterior oblique lines. The muscle has two layers, a superficial and a deep layer. These layers are separated by a connective tissue septum through which the posterior

interosseous nerve courses. The two layers arise together,

the superficial by tendinous origin and the deep by muscular fibers from the lateral epicondyle of the humerus, from

the radial collateral ligament of the elbow joint and the

annular ligament of the superior radioulnar joint, from the

supinator crest of the ulna, and from the posterior aponeurosis covering the muscle (523). The proximal portion of

the muscle contains an opening in the superficial layer, the

arcade of Frohse. The arcade of Frohse allows the passage of

the posterior interosseous nerve as it enters between the two

heads. There is variability of the anatomy pertaining to the

tendinous or membranous nature of the rim of the arcade

of Frohse (524–529). Thomas and colleagues noted that the

arcade of Frohse was lined by a tendinous rim in 32% and

a membranous rim in 68% (523). Conversely, Ozkan and

colleagues reported that the rim of the arcade was fibrous in

80% and membranous in 20% of specimens (524). In addition, Debouck and Rooze noted that the arcade was tendinous in 64% (525) and Papadopoulos et al. noted a tendinous arcade in 90% (528). The arcade of Frohse is a well

known area of possible nerve impingement resulting in posterior interosseous neuropathy (526). It remains unclear if a

fibrous rim of the arcade predisposes the posterior

interosseous nerve to impingement (529). After the nerve

enters the supinator, it continues obliquely through the

144 Systems Anatomy

muscle, with the direction of the nerve roughly perpendicular to the fibers of the muscle. The nerve often branches

within the muscle, and several branches often are seen exiting the distal edge of the muscle. The nerve also may be

compressed at the distal edge of the supinator (530).

The supinator is innervated by the branches of the posterior interosseous nerve before the nerve passes through

the arcade of Frohse. Theses branches usually carry contributions from C5, C6, and C7 (3,4,11,13,505,531).

Actions and Biomechanics: Supinator

The supinator functions mainly for supination of the forearm (lateral rotation of the forearm so that the palm faces

anteriorly, or superiorly if the elbow is flexed). It works in

conjunction with the biceps for forearm supination, and is

thought to provide approximately half the power of the

biceps muscle for supination (11). It may act alone in slow,

unopposed supination and together with the biceps in fast

or forceful supination (3,4,11,13).

Anomalies and Variations: Supinator

The supinator may exist as only one muscle head, without

a superficial and deep layer (11).

Accessory slips of muscle or tendon may interconnect

the supinator with the biceps tendon, annular ligament of

the elbow, tuberosity of the radius, and neighboring areas

(11,532).

The tensor ligamenti anularis anterior muscle is an

anomalous muscle that connects the supinator to the annular ligament in 5% of individuals (11).

Clinical Implications: Supinator

The arcade of Frohse is the opening of the superficial layer

of the supinator. It often is lined by a fibrous rim, and provides the opening of the muscle through which the posterior interosseous nerve passes. The arcade of Frohse is a well

known area of possible nerve impingement resulting in posterior interosseous neuropathy (526).

ABDUCTOR POLLICIS LONGUS

Derivation and Terminology. Abductor is derived form

the Latin ab, meaning “away from,” and from ducere, which

means “to draw”; therefore, abductor is “that which draws

away from.” Pollicis is from the Latin pollex, indicating

“thumb.” Longus is derived from the Latin longus, indicating “long.” The APL is the longest abductor of the thumb

(1,2).

Origin. The mid-dorsal radial diaphysis and adjacent

portion of the interosseous ligament, and from the lateral

edge of the middle third of the ulnar diaphysis.

Insertion. The base of the thumb metacarpal, dorsal

aspect.

Innervation. Posterior interosseous nerve (C7, C8).

Vascular Supply. The posterior interosseous artery; perforating arteries and continuation of the anterior

interosseous artery; radial artery in the anatomic snuff-box;

first dorsal metacarpal artery; dorsal carpal arch (3,4,11,13).

Principal Action. Abduction of the thumb metacarpal

(abduction of the thumb in the radial direction in the plane

of the palm).

Gross Anatomic Description: Abductor

Pollicis Longus

The APL is located in the deep layer of the posterior forearm. The muscle comprises one of the many muscles of the

dorsal muscle compartment of the forearm (Appendix 2.2).

It arises from the lateral edge of the dorsal radial diaphysis,

from a portion of the interosseous ligament, and from the

proximal part of the middle third of the ulna

(3,4,11,13,68). Its origin from the radius is distal and central to the supinator, but proximal to the origins of the EPL

and EPB (see Fig. 2.3B). Additional areas of origin include

the septa between the APL and the supinator, the ECU, and

the EPL. The muscle fibers converge in a penniform manner to join in a muscle belly that extends distally in an

oblique fashion, coursing radially in the direction of the

thumb. The muscle then forms the myotendinous junction

in the distal third of the forearm, joined by the tendon of

the EPB, which lies immediately ulnar to the APL. The tendon becomes more superficial in the distal third of the forearm. The tendon of the APL is round and thick. At the level

of the extensor retinaculum, the APL and EPB enter their

own fibroosseous tunnel to comprise the first dorsal compartment (3,4,533). [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB

form the second; the EPL forms the third; the EDC and

EIP form the fourth; the EDM forms the fifth; and the

ECU forms the sixth (6).] The first dorsal compartment is

located on the dorsolateral surface of the distal radius, just

lateral to the tendons of the ECRL and ECRB of the second dorsal compartment. The APL exits the first dorsal

compartment, remaining on the lateral side of the EPB, and

continues toward the base of the thumb to insert onto the

base of the thumb metacarpal on its radial surface (see Fig.

2.6A). The tendon often splits into two slips, one attaching

to the radial side of the thumb metacarpal base and the

other to the trapezium. Variations in the number and

course of the tendon are so numerous that the normal pattern of a single APL and EPB occurs less than 20% of the

time (451,534). This variability has implications for the etiology and treatment of de Quervain’s tenosynovitis

(534–553). The first dorsal compartment may have more

variations in tendon structure and organization than most

2 Muscle Anatomy 145

other muscles in the upper extremity (451). This is discussed in detail later, under Anomalies and Variations:

Abductor Pollicis Longus.

The APL is innervated by the posterior interosseous

nerve, usually by one or more branches. The branches enter

the muscle just after the nerve exits the supinator muscle.

The branches then enter the proximal third of the muscle

belly, usually on the superficial surface. The motor branches

usually have contributions mainly from C7, but also from

C6 and C8 (3,4,11,13,68).

Actions and Biomechanics: Abductor

Pollicis Longus

The APL functions mainly to abduct the thumb metacarpal

from the hand in the radial direction and in the plane of the

palm. During maximal contraction, it also may contribute

to flexion of the wrist or radial deviation of the wrist. It is

considered an antagonist to the opponens pollicis (11). The

APL works in conjunction with the APB to abduct the

thumb; it works in conjunction with the EPL and EPB to

assist with extension at the thumb carpometacarpal joint.

Anomalies and Variations: Abductor

Pollicis Longus

The tendon of the APL often is doubled. It may have multiple tendons. With double tendons, both often still insert

to the base of the thumb metacarpal. In several studies, a

double tendon was more common that a single tendon,

with the single APL and EPL pattern occurring less than

20% of the time (451,534,537,538,540,542,544,546,549,

550,554,555). Failure to recognize these variations potentially leads to persistence or recurrence of pain after operative decompression because of incomplete surgical release of

the tendon sheath (535,536,545).

The muscle belly may be split or doubled, or there may

be multiple bellies or slips (11).

Multiple accessory muscles or tendon slips have been

noted, including those that extend to the trapezium,

scaphoid, opponens pollicis, proximal phalanx of the

thumb, flexor retinaculum (volar carpal ligament), superficial muscles on the thenar eminence, other areas of the

thumb metacarpal, APB, or FPB (451,544,549,550).

The septum in the first dorsal compartment may have several variations as well. In 24% to 34% of specimens in

anatomic studies, the first compartment was found to be subdivided by a longitudinal ridge and septum into two distinct

osteofibrous tunnels, an ulnar one for the EPB and a radial

one containing one or more slips of the APL (451,541–543).

The reported incidence of separate compartments at surgery

is higher than that seen in anatomic specimens in several

series (539,548,550–552,554), which, as noted by Wolfe,

raises the possibility that septationw increases the probability

that nonsurgical treatment will fail (451).

The abductor pollicis tertius (extensor atque abductor

pollicis accessorius) is a rare anomalous muscle that arises

from the dorsal aspect of the radius with the APL and

inserts, after coalescing with the APB, onto the thumb

metacarpal (11).

Clinical Implications: Abductor Pollicis

Longus

The APL and EPB often are afflicted with tendonitis,

resulting in the well known de Quervain’s tenosynovitis

(534–553). The disease often is referred to as stenosing tenovaginitis of the first dorsal compartment. As noted earlier

under Anomalies and Variations, several studies have shown

a double tendon was more common that a single tendon,

with the single APL and EPL pattern occurring less than

20% of the time (451,534,537,538,540,542,544,546,549,

550,554,555). The number of variations in tendon structure and organization in the first dorsal compartment are

among the greatest of the upper extremity muscles. Failure

to recognize these variations potentially leads to persistence

or recurrence of pain after operative procedures because of

incomplete surgical release of the tendon sheath (535,536,

545).

The variability of the septa in the first dorsal compartment may also be related to the incidence of stenosing

tenosynovitis. The reported incidence of separate compartments at surgery is higher than that seen in anatomic specimens in several series (539,548–552,554). Wolfe has noted

that this raises the possibility that septation of the EPB

increases the probability that nonsurgical treatment will

fail. Harvey et al. reported success with one or two steroid

injections in 80% of patients and found separate compartments for the APL and EPB in 10 of 11 wrists that failed

injection and required surgical release (539). It also has

been noted that observations at surgical release suggest that

either one of both subdivisions of the first dorsal compartment may be stenotic (544).

The radial nerve and, to a lesser extent, the radial artery

are at risk for injury during surgical release of the first dorsal compartment (451). The radial artery passes diagonally

across the anatomic snuffbox from the volar aspect of the

wrist to the dorsum of the web space deep to the APL,

EPB, and EPL. It is separated from the first dorsal compartment by areolar tissue, and usually is not at risk if the

floor of the compartment sheath is not perforated distal to

the radial styloid. The radial nerve, however, has two or

three terminal divisions that lie superficial to the first dorsal compartment and must be identified and protected

during the surgical procedure (451,534,547). Radial neuroma is a not uncommon complication, and can result in

failure of treatment.

Intersection syndrome is a condition of pain and

swelling in the region of the muscle bellies of the APL and

EPB. As noted by Wolfe, this area lies approximately 4 cm

146 Systems Anatomy

proximal to the wrist joint, and may show increased

swelling of a normally prominent area (451). In severe

cases, redness and crepitus have been noted. The syndrome

originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More

recently, Grundberg and Reagan have demonstrated that

the basic pathologic process appears to be tenosynovitis of

the ECRL and ECRB (455).

EXTENSOR POLLICIS BREVIS

Derivation and Terminology. Extensor is derived from

the Greek and Latin ex, which indicates “out of,” and the

Latin tendere, “to stretch”; thus, extension indicates a motion

to stretch out, and extensor is usually applied to a force or

muscle that is involved in the “stretching out or straightening out” of a joint. Pollicis is derived from the Latin pollex,

“thumb.” Brevis is the Latin for “short.” Therefore, extensor

pollicis brevis indicates a short thumb extensor (1,2).

Origin. The distal end of the middle third of the radius,

on the medial portion of the posterior surface of the radius

and adjacent interosseous ligament. This origin is distal to

the origins of the APL and EPL. The muscle also may have

origin attachments to the ulna.

Insertion. The base of the proximal phalanx of the

thumb.

Innervation. Posterior interosseous nerve (C7, C8).

Vascular Supply. The posterior interosseous artery, continuation and the perforating branches of the anterior

interosseous artery. The tendon receives vascularity from

the radial artery in the anatomic snuffbox from branches to

the radial side of the thumb, and from the first dorsal

metacarpal artery and dorsal carpal arch (3,4,11,13).

Principal Action. Extension of the proximal phalanx of

the thumb. It also assists with extension of the thumb

metacarpal.

Gross Anatomic Description: Extensor

Pollicis Brevis

The EPB lies close to the APL, and takes origin from the

radial diaphysis and adjacent interosseous ligament just distal to that of the APL (see Fig. 2.3B) (3,4,11,13,68). It

comprises one of the many muscles of the dorsal muscle

compartment of the forearm (Appendix 2.2). The area on

the radius includes a portion of the distal part of the middle third, along the medial border of the dorsal surface.

Approximately half of the origin is also from the adjacent

interosseous ligament. There may be rare attachments to

the adjacent ulna. The muscle fibers converge in a radial

direction toward the thumb, just distal to and adjacent to

the path of the APL. The EPB usually is thinner than the

APL. The EPB along with the APL crosses obliquely and

superficially to the ECRB and ECRL. In the distal forearm,

the EPB and APL are superficial to the most distal portion

of the brachioradialis. The myotendinous junction of the

EPB forms just proximal to the extensor retinaculum. The

EPL enters the extensor retinaculum with the APL to comprise the first dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and

EPB comprise the first dorsal compartment; the ECRL and

ECRB form the second; the EPL forms the third; the EDC

and EIP form the fourth; the EDM forms the fifth; and the

ECU forms the sixth (6).] The tendon anatomy and presence of septa in the first dorsal compartment commonly

show anatomic variations and anomalies (see earlier, under

Abductor Pollicis Longus, Gross Anatomic Description and

Anomalies and Variations). In general, in approximately

24% to 34% of specimens in anatomic studies, the first

compartment has been found to be subdivided by a longitudinal ridge and septum into two distinct osteofibrous

tunnels, the ulnar one for the EPB and the radial one containing one or more slips of the APL (451,541–543). Muscle fibers often extend to the proximal edge of the extensor

retinaculum. In the first dorsal compartment, the tendon is

located on the radial side of the radial metaphysis. The tendon is parallel with the ulnar border of the APL tendon,

and together the tendons pass through the fibroosseous

compartment. The EPL then crosses the dorsoradial carpus

to extend distally on the dorsal aspect of the thumb

metacarpal (see Fig. 2.6B). It remains radial to the EPL tendon. The EPB then inserts into the base of the proximal

phalanx of the thumb. It also may send slips to the capsule

of the MCP joint (3,4,11,13).

The EPB is innervated by the posterior interosseous

nerve, mostly from C7 with additional contributions from

C8. There usually is a single motor branch that supplies

the EPB. The nerve branch usually arises in common with

or near the nerve to the APL. The nerve may cross the APL

to reach the EPB. The motor nerve to the EPB enters the

muscle in the proximal third, usually along the radial border (11).

Actions and Biomechanics: Extensor

Pollicis Brevis

The EPB functions mainly to extend the proximal phalanx

of the thumb. Because it crosses the thumb carpometacarpal joint, the tendon also assists with extension of

the thumb metacarpal. In addition, at extremes of contraction, it assists with radial deviation of the wrist (3,4).

Anomalies and Variations: Extensor

Pollicis Brevis

Several variations of the septa and tendon slips in the first

dorsal compartment exist (see earlier, under Anomalies and

Variations: Abductor Pollicis Longus).

2 Muscle Anatomy 147

The EPB is absent in 5% to 7% of individuals

(544,549,550). The EPB may have an anomalous tendon

slip that extends to the base of the thumb distal phalanx as

well the normal insertion into the base of the proximal phalanx. Rarely, it inserts only onto the distal phalanx. The

muscle also may have a tendon slip to the thumb

metacarpal (11,556,557). The EPB may coalesce with the

APL, forming one muscle, and inserts into the thumb

metacarpal (11). The EPB may exist as a double tendon

(11). Rarely, the tendon coalesces with the EPL (3).

Clinical Implications: Extensor Pollicis

Brevis

See also Clinical Implications: Abductor Pollicis Longus.

The EPB and APL are the tendons involved with de

Quervain’s tenosynovitis (see earlier, under Clinical Implications: Abductor Pollicis Longus). Yuasa and Kiyoshige

have suggested that the EPB is the main tendon involved,

and have demonstrated successful resolution of symptoms

after decompression of the EPB alone (558).

Intersection syndrome is a condition of pain and

swelling in the region of the muscle bellies of the APL and

EPB. As noted by Wolfe, this area lies approximately 4 cm

proximal to the wrist joint, and may show increased

swelling of a normally prominent area (451). In severe

cases, redness and crepitus have been noted. The syndrome

originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More

recently, Grundberg and Reagan have demonstrated that

the basic pathologic process appears to be tenosynovitis of

the ECRL and ECRB (455).

EXTENSOR POLLICIS LONGUS

Derivation and Terminology. Extensor derived is from

the Greek and Latin ex, which indicates “out of,” and the

Latin tendere, “to stretch”; thus, extension indicates a motion

to stretch out, and extensor usually is applied to a force or

muscle that is involved in the “stretching out or straightening out” of a joint. Pollicis is derived from the Latin pollex,

“thumb.” Longus is derived from the Latin longus, indicating “long.” Therefore, extensor pollicis longus indicates the

long extensor of the thumb (1,2).

Origin. The dorsal middle third of the ulna and adjacent

interosseous ligament.

Insertion. The base of the distal phalanx of the thumb.

Innervation. Posterior interosseous nerve (C7, C8).

Vascular Supply. The posterior interosseous artery,

continuation and the perforating branches of the anterior interosseous artery. The tendon receives vascularity

from the radial artery in the anatomic snuffbox from

branches to the radial side of the thumb, and from the

first dorsal metacarpal artery and dorsal carpal arch (3,4,

11,13,559).

Principal Action. Extension of the distal phalanx of the

thumb. Also contributes to extension of the proximal phalanx and the thumb metacarpal through the MCP and carpometacarpal joints, respectively.

Gross Anatomic Description: Extensor

Pollicis Longus

The EPL is a deep extensor of the dorsal forearm situated

between the EIP (ulnarly) and the EPB (radially) (3,4,11,14).

It is one of the many muscles that comprise the dorsal muscle compartment of the forearm (Appendix 2.2). It is much

larger than the EPB. The EPL arises from the dorsal middle

third of the ulna, chiefly on its radial border (see Fig. 2.3B).

In addition, at least half of the muscle takes origin from the

adjacent interosseous ligament. Portions of the muscle also

arise from the septa between the EPL and the EIP and ECU.

The muscle courses obliquely in a radial direction as it

extends distally, in the direction of the thumb. The muscle

fibers converge in a bipenniform manner on the two sides of

a flattened tendon that first appears proximally on the dorsal

surface of the muscle. The EPL is initially deep to the EDC,

crossing obliquely toward the thumb to emerge from the

EDC and enter the extensor retinaculum just radial to the

EDC. The muscle belly usually is fusiform (7,8). The

myotendinous junction is located deep to the EDC, proximal

to the extensor retinaculum, and muscle fibers may continue

with the tendon as far distally as the extensor retinaculum.

The EPL then enters its own fibroosseous tunnel at the extensor retinaculum to form the third dorsal compartment. [Editor’s note:The dorsal compartments of the wrist are as follows:

the APL and EPB comprise the first dorsal compartment; the

ECRL and ECRB form the second; the EPL forms the third;

the EDC and EIP form the fourth; the EDM forms the fifth;

and the ECU forms the sixth (6).] The path through the third

compartment continues in an oblique direction toward the

thumb. It is stabilized in part by a narrow groove in the distal

radius, and passes ulnar to Lister’s tubercle before taking a

more oblique direction. The tendon in this region appears to

have a slightly smaller cross-sectional area (560), and is relatively poorly vascularized (561). This area also coincides with

an area commonly affected by closed rupture (see later, under

Clinical Implications: Extensor Pollicis Longus). The tendon

exits the third compartment over the distal radius or radiocarpal joint. It passes across the dorsal surface of the carpus,

superficial to the tendon of the ECRL and ECRB, to the dorsum of the thumb metacarpal. It is located ulnar to the EPB,

and, in the region of the radial styloid and scaphoid, the EPL

and EPB form a triangular depression (when the thumb is in

full extension). This depression, referred to as the anatomic

snuffbox, lies over scaphoid, and point tenderness in this area

usually indicates injury to the scaphoid (or possibly the radial

styloid). The EPL remains ulnar to the EPB but becomes

148 Systems Anatomy

adjacent to the EPB just proximal to the MCP joint. The EPL

tendon continues distally on the dorsal surface of the proximal phalanx and expands to insert onto the base of the distal

phalanx (see Fig. 2.6B). The tendon becomes an aponeurosis

as it is joined by the tendon of the APB laterally and the first

palmar interosseous and adductor pollicis medially. Together,

the EPL with the thumb intrinsic muscles form the aponeurosis that comprises the extensor mechanism of the thumb

(3,4,11,562).

The EPL is innervated by the posterior interosseous

nerve, chiefly from C7, but also from C8 and C6. There

usually initially is one branch to the EPL that may divide

before entering the muscle belly. The motor branches usually enter the muscle in the proximal third, usually into the

radial border.

Actions and Biomechanics: Extensor

Pollicis Longus

The EPL functions mainly for extension of the distal phalanx of the thumb. It also contributes to extension of the

proximal phalanx (working with the EPB) and to extension

of the thumb metacarpal (working with the APL) (563). In

extremes of contraction, it can contribute to radial deviation of the wrist. When the thumb is in full extension, the

EPL also can contribute to adducting the thumb toward the

index metacarpal.

Anomalies and Variations: Extensor

Pollicis Longus

Most of the variations of the EPL involve variations in the

distal tendon. There may be an accessory slip to the base of

the carpal bones (especially the capitate), to the index finger (distal phalanx), to the EPB, or to the extensor retinaculum (11,564–569). A double tendon or double muscle

belly may exist. An accessory EPL in the third dorsal compartment has caused dorsal wrist pain that resolved after

excision of the accessory EPL (570).

Extensor communis pollicis et indicis is an anomalous

muscle found in approximately 6% of dissected specimens.

It crosses between the EIP and the EPL. The muscle may

have two tendons that insert into the distal phalanges of the

thumb and the index finger. The muscle may replace the

EPL or EIP (11,564).

Clinical Implications: Extensor Pollicis

Longus

Closed rupture of the EPL is well documented (571–588),

and has been associated with tendon injury after fractures of

the distal radius (589–605), or inflammatory conditions

such as rheumatoid arthritis (590,606,607). Nonunion also

has been associated with EPL ruptures. Ruptures often

occur even after nondisplaced fractures, usually in the

region of the distal radius. Engkvist and Lundborg have

shown that in the common area of the rupture, there is a

relatively poorly vascularized portion of tendon (561). In

addition, Wilhelm and Qvick have shown that the crosssectional area of the tendon in this area is slightly smaller

(560). These factors may play a role in the closed or delayed

ruptures of the EPL (especially those associated with

nondisplaced fractures, where tendon injury or attrition

from uneven bone edges is unlikely).

In patients with rheumatoid arthritis, the EPL is at risk for

rupture at the level of Lister’s tubercle, due to either chronic

tenosynovitis (590) at the dorsal wrist or local attrition

against the friction point at the tubercle (especially if there is

bony irregularity from chronic arthritis). Closed rupture also

has occurred after use of anabolic steroids (608).

The EPL also is subject to subluxation or dislocation,

usually associated with rupture or damage to the radial side

of the extensor hood on the dorsum of the MCP joint of

the thumb. The EPL subluxates to the ulnar side (609,610).

Dislocation also can occur after fracture of the distal radius

(611,612).

The EPL can be affected by tenosynovitis, or triggering,

as it courses through the third dorsal compartment

(613–616).

ABDUCTOR POLLICIS BREVIS

Derivation and Terminology. Abductor is derived from

the Latin ab, meaning “away from,” and ducere, which

means “to draw”; therefore, abductor is “that which draws

away from.” Pollicis is derived from the Latin pollex,

“thumb.” Brevis is Latin for “short” (1,2). Therefore, abductor pollicis brevis indicates a short thumb abductor.

Origin. From the flexor retinaculum, scaphoid tubercle,

trapezial ridge or tubercle.

Insertion. To the base of the thumb proximal phalanx,

palmar surface.

Innervation. Recurrent branch of the median nerve (C8,

T1).

Vascular Supply. The radial artery and superficial palmar

arch.

Principal Action. Palmar abduction of the thumb (pulling

the thumb away from the palm) at right angles to the palm.

In addition, the APB contributes to flexion of the proximal

phalanx of the thumb. Through the superficial layer of the

APB that continues distally and dorsally to reach the EPL,

the APB contributes to extension of the thumb distal phalanx

as part of the extensor mechanism (3,4,68).

Gross Anatomic Description: Abductor

Pollicis Brevis

The APB, along with the opponens pollicis, FPB, and

adductor pollicis, comprises one of the thenar muscles (3,4,

2 Muscle Anatomy 149

11,13,68). In terms of muscle compartments, it is one of

the three muscles that comprise the thenar muscle compartment of the hand. (The adductor pollicis has a separate

compartment, Appendix 2.2). The APB is located subcutaneously on the radial aspect of the thenar eminence, and

constitutes the shape and contour of the radial border of the

thenar eminence. The muscle is flat and broad, and covers

the opponens pollicis and approximately 30% of the FPB.

The ABP arises mostly from the flexor retinaculum (see Fig.

2.6A). Fibers also arise from the scaphoid tubercle, the

trapezial tubercle, and possibly from the terminal tendon or

tendon sheath of the APL, as the APL inserts onto the base

of the thumb metacarpal (496,617). The muscle courses

distally and radially toward the thumb, located as a superficial thenar muscle, in line with the thumb metacarpal. The

muscle fibers converge into a flat tendon. It joins the fibers

of the FPB. The muscle of the APB often consists of two

layers or bellies, a deep (or medial layer) and a superficial

(or lateral) layer. The deep layer inserts onto the radial

sesamoid and radial side of the base of the proximal phalanx

of the thumb (see Fig. 2.6A). The superficial layer continues radially and dorsally to join the aponeurosis of the EPL

as part of the extensor mechanism of the thumb.

The APB is innervated by the recurrent branch of the

median nerve. This usually is the first branch from the lateral side of the median nerve in the hand. The nerve

receives contributions mostly from T1 and C8. The nerve

takes a recurrent course proximally and laterally superficial

to or through the superficial division of the FPB and enters

the deep surface of the APB in the middle third near its

ulnar border (11).

Actions and Biomechanics: Abductor

Pollicis Brevis

The APB functions mainly to provide palmar abduction of

the thumb (pulling the thumb away from the palm, at right

angles to the palm. The APB also contributes to flexion of

the proximal phalanx of the thumb. A superficial layer of

the distal tendon of the APB continues radially and dorsally

past the MCP joint of the thumb to reach and attach to the

tendon of the EPL. Through this aponeurosis, the APB

becomes part of the extensor mechanism of the thumb and

contributes to extension of the distal phalanx of the thumb.

Its extensor function of the distal phalanx is relatively weak

(496).

From architectural studies on the muscle’s physiologic

cross-sectional area, muscle length, muscle fiber length, and

muscle mass, it can be seen that the muscle architecture is

fairly close to that of the other thenar muscles (466). It therefore would have similar relative abilities for force generation,

velocity, and excursion (466) (Table 2.4 and Fig. 2.13).

150 Systems Anatomy

TABLE 2.4. ARCHITECTURAL FEATURES OF INTRINSIC MUSCLES OF THE HAND

Muscle Muscle Mass Muscle Length Fiber Length Pennation Angle Cross-Sectional Area Fiber Length/

(n = 9) (g) (mm) (mm) (Degrees) (cm2) Muscle Length Ratio

ADM 3.32 ± 1.67 68.4 ± 6.5 46.2 ± 7.2 3.9 ± 1.3 0.89 ± 0.49 0.68 ± 0.10

APB 2.61 ± 1.19 60.4 ± 6.6 41.6 ± 5.6 4.6 ± 1.9 0.68 ± 0.28 0.69 ± 0.09

APL 9.96 ± 2.01 160.4 ± 15.0 58.1 ± 7.4 7.5 ± 2.0 1.93 ± 0.59 0.36 ± 0.05

AP 6.78 ± 1.84 54.6 ± 8.9 34.0 ± 7.5 17.3 ± 3.4 1.94 ± 0.39 0.63 ± 0.15

DI 1 4.67 ± 1.17 61.9 ± 2.5 31.7 ± 2.8 9.2 ± 2.6 1.50 ± 0.40 0.51 ± 0.05

DI 2 2.65 ± 1.01 62.8 ± 8.1 25.1 ± 6.3 8.2 ± 3.1 1.34 ± 0.77 0.41 ± 0.13

DI 3 2.01 ± 0.60 54.9 ± 4.6 25.8 ± 3.4 9.8 ± 2.8 0.95 ± 0.45 0.47 ± 0.07

DI 4 1.90 ± 0.62 50.1 ± 5.3 25.8 ± 3.4 9.4 ± 4.2 0.91 ± 0.38 0.52 ± 0.11

EPB 2.25 ± 1.36 105.6 ± 22.5 55.0 ± 7.5 7.2 ± 4.4 0.47 ± 0.32 0.54 ± 0.13

FDM 1.54 ± 0.44 59.2 ± 10.4 40.6 ± 13.7 3.6 ± 1.0 0.54 ± 0.36 0.67 ± 0.17

FPB 2.58 ± 0.56 57.2 ± 3.7 41.5 ± 5.2 6.2 ± 4.5 0.66 ± 0.20 0.73 ± 0.08

Lum 1 0.57 ± 0.19 64.9 ± 10.0 55.4 ± 10.2 1.2 ± 0.9 0.11 ± 0.03 0.85 ± 0.03

Lum 2 0.39 ± 0.22 61.2 ± 17.8 55.5 ± 17.7 1.6 ± 1.3 0.08 ± 0.04 0.90 ± 0.05

Lum 3 0.37 ± 0.16 64.3 ± 8.9 56.2 ± 10.7 1.1 ± 0.8 0.08 ± 0.04 0.87 ± 0.07

Lum 4 0.23 ± 0.11 53.8 ± 11.5 50.1 ± 8.4 0.7 ± 1.0 0.06 ± 0.03 0.90 ± 0.05

ODM 1.94 ± 0.98 47.2 ± 3.6 19.5 ± 4.1 7.7 ± 2.9 1.10 ± 0.43 0.41 ± 0.09

OP 3.51 ± 0.89 55.5 ± 5.0 35.5 ± 5.1 4.9 ± 2.5 1.02 ± 0.35 0.64 ± 0.07

PI 2 1.56 ± 0.22 55.1 ± 5.0 25.0 ± 5.0 6.3 ± 2.2 0.75 ± 0.25 0.45 ± 0.08

PI 3 1.28 ± 0.28 48.2 ± 2.9 26.0 ± 4.3 7.7 ± 3.9 0.65 ± 0.26 0.54 ± 0.08

PI 4 1.19 ± 0.33 45.3 ± 5.8 23.6 ± 2.6 8.2 ± 3.5 0.61 ± 0.23 0.52 ± 0.10

ADM, abductor digiti minimi; APB, abductor pollicis brevis; APL, abductor pollicis longus; AP, adductor pollicis; DI 1–4, dorsal interosseous

muscles; EPB, extensor pollicis brevis; FDM, flexor digiti minimi; FPB, flexor pollicis brevis; Lum 1–4, lumbrical muscles; ODM, opponens digiti

minimi; OP, opponens pollicis; PI 2–4, palmar interosseous muscles.

Values represent mean ± standard deviation.

Reproduced from Jacobson MD, Raab R, Fazeli BM, et al. Architectural design of the human intrinsic hand muscles. J Hand Surg [Am]

17:804–809, 1992, with permission.

2 Muscle Anatomy 151

FIGURE 2.13. Architectural features of the intrinsic muscles of the hand. A: Intrinsic muscle

lengths. Note the short, uniform lengths. B: Intrinsic muscle fiber lengths. There is more disparity in fiber length than in muscle length. This illustrates the relatively large excursions of the relatively short intrinsic muscles. C: Intrinsic muscle masses. The intrinsic muscles have low masses,

with the exception of the first dorsal interosseous (DI1) and the AddP. D: Intrinsic muscle crosssectional areas. The interossei have greater cross-sectional areas than the smaller lumbrical muscles, and in general the lumbrical fibers are longer. This would indicate that the lumbricals are

designed more for excursion or velocity and less for force generation. E: Intrinsic muscle fiber

length/muscle length (FL/ML) ratios. Note the high FL/ML ratio of the intrinsic muscles, especially

the lumbricals, demonstrating their relative design for excursion and velocity. The lumbricals

have among the highest FL/ML ratios of all muscles studied (both extrinsic and intrinsic), and this

indicates their specialization for excursion (and velocity) and their relatively poor design for force

production. Bars represent mean ± standard deviation (SEM). AbDM, abductor digiti minimi;

AbPB, abductor pollicis brevis; AbPL, abductor pollicis longus; AddP, adductor pollicis; DI1–DI4,

dorsal interosseous muscles 1–4; EPB, extensor pollicis brevis; FDM, flexor digiti minimi; FPB,

flexor pollicis brevis; L1–L4, lumbrical muscles 1–4; ODM, opponens digiti minimi; OpP, opponens

pollicis; PI2–PI4, palmar interosseous muscles 2–4. (From Jacobson MD, Raab R, Fazeli BM, et al.

Architectural design of the human intrinsic hand muscles. J Hand Surg [Am] 17:804–809, 1992,

with permission.)

A B

C D

E

Anomalies and Variations: Abductor

Pollicis Brevis

The APB may have two separate heads (besides the two distal layers, as discussed previously) (11). The APB may be

absent (11,618).

The muscle may have attachments to several other

neighboring structures. These include the scaphoid, the

radial styloid, the adductor pollicis, the EPL or EPB, opponens pollicis, palmaris longus, ECRL (accessory ECR), or

FPL (11,619,620). An entire third head may arise from the

opponens pollicis (11).

Clinical Implications: Abductor Pollicis

Brevis

Paralysis or laceration of the distal median nerve usually

results in thenar paralysis (as well as loss of sensibility on

the radiopalmar hand). Loss of thenar function results in

difficulty with attempted palmar abduction of the thumb

(bringing the thumb out of the palm). Therefore, despite

functioning of the adductor pollicis and FPL, thumb

opposition with the digits remains difficult. To restore

thumb opposition, several opponensplasty procedures

have been described. Muscles used for transfer for opponensplasty include the EIP, the FDS to the ring finger, the

abductor digiti minimi (Huber transfer), or the palmaris

longus elongated by a strip of the palmar fascia (Camitz or

Braun transfer, more commonly used with severe carpal

tunnel syndrome and thenar dysfunction) (266–272,

621–624).

FLEXOR POLLICIS BREVIS

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Pollicis is derived from

the Latin pollex, “thumb.” Brevis is the Latin for “short.”

Therefore, flexor pollicis brevis indicates a short thumb

flexor (1,2).

Origin. From two heads, superficial and deep. Superficial head: from the trapezium, adjacent flexor retinaculum,

and the tendon sheath of the FCR. Deep head: from the

trapezoid and capitate, and from the palmar ligament from

the distal carpal row.

Insertion. Superficial head: to the radial side of the anterior aspect of the proximal thumb phalanx. Deep head:

inserts into a tendon that connects with the superficial

head.

Innervation. Variable; classically, the recurrent branch of

the median nerve supplies the superficial head; the terminal

branch of the ulnar nerve supplies the deep head. Either

head may be supplied by either the recurrent branch of the

median nerve or by the ulnar nerve (see later).

Vascular Supply. The radial artery, superficial palmar

branch, branches from the opponens pollicis, and the radialis indicis (3,4,11).

Principal Action. Flexion of the MCP joint of the

thumb.

Gross Anatomic Description: Flexor

Pollicis Brevis

The FPB lies medial and slightly deep to the APB (3,4,7,8).

It helps comprise the thenar muscle compartment of the

hand (Appendix 2.2). It has two heads, a superficial and a

deep (625). The superficial head arises from the distal border of the flexor retinaculum and the distal part of the

tubercle of the trapezium (see Fig. 2.6A). The superficial

head also may have origin attachments to the tendon sheath

of the FCR. The superficial head courses obliquely toward

the base of the thumb to reach the radial side of the base of

the proximal phalanx (Fig. 2.6A).

The deep head arises from the trapezoid and capitate and

from the palmar ligaments of the distal row of the carpus

(see Fig. 2.6A). The deep head passes deep to the tendon of

the FPL and joins the superficial head on the sesamoid bone

and base of the first phalanx.

An additional muscle head or fascicle has been described

by Tountas and Bergman (11). It arises from the ulnar side

of the base of the thumb metacarpal and the adjacent carpal

ligaments. It inserts onto the ulnar side of the base of the

proximal phalanx (see Fig. 2.6A). This fascicle sometimes is

considered to be the deep head of the FPB. It is closely

joined to the carpal head of the adductor pollicis, and the

two muscles share a common tendon. Some fibers of the

medial division of the tendon may be traced into the

aponeurosis of the extensor tendon. It has been suggested

that this portion of the muscle represents a first palmar

interosseous. This component of the FPB remains controversial (11).

The architectural features of the muscle are listed in

Table 2.4.

The innervation of the FPB appears to be quite variable

(625). Classic descriptions suggest that the superficial head

usually is supplied by the lateral terminal branch of the

median nerve, and the deep head by the deep branch of the

ulnar nerve (3,4,68). More recently, the variable innervation has been described, and various combinations exist.

The muscle usually is supplied chiefly by branches that

originate from the recurrent branch of the median nerve.

The branch penetrates the muscle in the region of the

carpal tunnel. Additional branches derived from the ulnar

nerve also often are found, and usually supply the deep

portion. Contributions from both the median and ulnar

nerve were found in 19 of 29 cases. In 5 cases, the median

nerve alone supplied FPB, and in 5 the ulnar nerve alone

supplied the FPB muscles. In addition, when evaluating

152 Systems Anatomy

innervation specifically of the deep head, the deep head

was supplied by the ulnar nerve in 16 of 24 cases, by the

median nerve in 3 of 24 cases, and by both nerves in 5 of

24 cases (11,625).

Actions and Biomechanics: Flexor Pollicis

Brevis

The FPB functions primarily to provide flexion of the MCP

joint of the thumb, as well as flexion of the carpometacarpal

joint of the thumb. It also contributes to rotation of the

thumb in the medial direction (in preparation for opposition). From its contributions into the extensor mechanism

of the thumb, the FPB contributes to extension of the distal phalanx of the thumb (3,4,68).

Anomalies and Variations: Flexor Pollicis

Brevis

A relatively common observation is the coalescing of the

superficial head with the opponens pollicis. The deep head

is variable in size and may be absent. The entire FPB may

be absent (11).

Clinical Implications: Flexor Pollicis

Brevis

Paralysis or laceration of the distal median nerve usually

results in thenar paralysis (as well as loss of sensibility on

the radiopalmar hand). Loss of thenar function results in

difficulty with attempted palmar abduction of the thumb

(bringing the thumb out of the palm). Therefore, despite

functioning of the adductor pollicis and FPL, thumb

opposition with the digits remains difficult. To restore

thumb opposition, several opponensplasty procedures have

been described. Muscles used for transfer for opponensplasty include the EIP, the FDS to the ring finger, the

abductor digiti minimi (Huber transfer), or the palmaris

longus elongated by a strip of the palmar fascia (Camitz or

Braun transfer, more commonly used with severe carpal

tunnel syndrome and thenar dysfunction) (266–272,

621–624).

OPPONENS POLLICIS

Derivation and Terminology. Opponens is the Latin indicating the movement against or toward an opposing structure. Pollicis is derived from the Latin pollex, “thumb” (1,2).

Origin. From the tubercle of the trapezium and from the

flexor retinaculum.

Insertion. To the radial and palmar aspect of the thumb

metacarpal.

Innervation. Recurrent branch of the median nerve (T1

and C8). A branch from the deep branch of the ulnar nerve

also may contribute.

Vascular Supply. The radial artery, superficial palmar

branch, first palmar metacarpal artery, arteria princeps pollicis, arteria radialis indicis, deep palmar arch (3,4,11,13,14).

Principal Action. Flexion, adduction, and median rotation of the thumb metacarpal (contributing to the motion

of opposition).

Gross Anatomic Description: Opponens

Pollicis

The opponens pollicis is a deep thenar muscle covered anteriorly by the APB (Appendix 2.2). It originates from the

tubercle of the trapezium and from the flexor retinaculum

(see Fig. 2.6A). It courses obliquely toward the thumb

metacarpal to insert onto the lateral and anterior aspects of

the diaphysis of the thumb metacarpal (see Fig. 2.6A). The

muscle usually covers the entire lateral part of the palmar

surface of the shaft (3,4).

The architectural features of the muscle are listed in

Table 2.4.

The opponens pollicis is innervated by the recurrent

branch of the median nerve. The branch takes a recurrent

course proximally and laterally, superficial to or through the

superficial divisions of the FPB near its origin. The nerve

provides one or two branches that enter the palmar surface

of the proximal third of the opponens pollicis near its ulnar

border (11). The nerve arises from C6, C7, and C8. As with

the FPB, the deep branch of the ulnar nerve can provide

various contributions. A double innervation of both the

recurrent branch of the median nerve and the deep branch

of the ulnar nerve was noted in 92 of 120 hands (625–627).

Because of the frequent duel innervation, it has been suggested that double innervation with the median and ulnar

nerves be considered the normal (3).

Actions and Biomechanics: Opponens

Pollicis

The opponens pollicis functions mainly to provide flexion,

adduction, and medical rotation of the thumb metacarpal

(contributing to the motion of opposition) (3,4). Opposition occurs when the thumb is flexed, palmarly abducted,

and rotated medially so that the palmar surface of the

thumb opposes the palmar surface of the digits.

The opponens pollicis does not cross the MCP joint (as

does the APB and FPB), and therefore does not contribute

to flexion of the proximal phalanx of the thumb.

Anomalies and Variations: Opponens

Pollicis

The opponens pollicis may coalesce with the FPB (11). Two

heads of the opponens pollicis may be present (11). Complete absence has been reported, but is rare (11).

2 Muscle Anatomy 153

Clinical Implications: Opponens Pollicis

Paralysis or laceration of the distal median nerve usually

results in thenar paralysis (as well as loss of sensibility on the

radiopalmar hand). Loss of thenar function results in difficulty with attempted palmar abduction of the thumb

(bringing the thumb out of the palm). Therefore, despite

functioning of the adductor pollicis and FPL, thumb opposition with the digits remains difficult. To restore thumb

opposition, several opponensplasty procedures have been

described. Muscles used for transfer for opponensplasty

include the EIP, the FDS to the ring finger, the abductor

digiti minimi (Huber transfer), or the palmaris longus elongated by a strip of the palmar fascia (Camitz or Braun transfer, more commonly used with severe carpal tunnel syndrome and thenar dysfunction) (266–272,621–624).

ADDUCTOR POLLICIS

Derivation and Terminology. Adductor is derived from

the Latin adducere, which means “to draw toward.” Pollicis

is derived from the Latin pollex, “thumb” (1,2).

Origin. Two heads. Oblique head: arises from the capitate, bases of the second and third metacarpals, intercarpal

ligaments, and sheath of the FCR. Transverse head: arises

from the distal two-thirds of the palmar surface of the third

metacarpal.

Insertion. Oblique and transverse heads unite to insert

into ulnar side of the base of the proximal phalanx of the

thumb.

Innervation. Deep branch of the ulnar nerve (C8, T1).

Vascular Supply. Arteria princeps pollicis, arteria radialis

indicis, or combined artery as the first palmar metacarpal

artery, deep palmar arch (3,4,11).

Principal Action. Moves the thumb proximal phalanx

from an abducted position toward the palm of the hand. It

therefore adducts the thumb proximal phalanx. It also

assists with adduction of the thumb metacarpal.

Gross Anatomic Description: Adductor

Pollicis

The adductor pollicis lies deep to the extrinsic flexor tendons and radial lumbricals. It occupies its own muscle compartment (Appendix 2.2). The muscle consists of two

heads, an oblique and a transverse. The oblique head (carpal

head) takes origin from several slips, including the palmar

capitate, the base of the second and third metacarpals, the

intercarpal ligaments, the sheath of the FCR, and possibly

from a slip from the flexor retinaculum (3,4,7,8,11,13,14)

(see Fig. 2.6A). From this origin, the muscle fibers converge

and pass distally and radially toward the base of the proximal phalanx of the thumb. The fibers converge into a common tendon (joined by the transverse head). The tendon

usually contains a sesamoid bone. The tendon inserts into

the ulnar side of the base of the proximal phalanx of the

thumb (see Fig. 2.6A). Additional fibers may pass more

obliquely deep to the tendon of the FPL to attach to the lateral portion of the FPB and the APB (3,4).

The transverse head (deep head, metacarpal head) arises

from the long finger metacarpal. Its origin is a broad attachment that includes the distal two-thirds of the palmar surface of the long metacarpal along the palmar ridge. It also

may arise from the deep palmar fascia of the third interspace and, occasionally, from the deep fascia of the fourth

interspace and from the capsules of the second, third, and

fourth MCP joints. It is more deeply situated than the

thenar muscles. The transverse head is triangular and converges in a radial direction toward the base of the proximal

phalanx of the thumb. Its distal border usually lies transverse to the axis of the upper limb. The tendon continues

toward the proximal thumb phalanx to join the tendon of

the oblique head. The common tendon inserts onto the

ulnar side of the base of the proximal phalanx of the thumb

(3,4,7,8,11,13,14) (Fig. 2.6A). A sesamoid bone usually is

found in the tendon, just proximal to the MCP joint.

The architectural features of the muscle are listed in

Table 2.4.

The adductor pollicis is innervated by the deep branch

of the ulnar nerve, from T1 and C8. The deep branch of the

ulnar nerve, along with the deep palmar arterial arch, passes

through the interval created between the oblique and transverse heads of the muscle (3,4).

Actions and Biomechanics: Adductor

Pollicis

The two heads usually work together. The muscle moves

the thumb proximal phalanx from an abducted position

toward the palm of the hand. It therefore adducts the

thumb proximal phalanx. It also assists with adduction of

the thumb metacarpal. The adductor pollicis works with

greatest advantage when the thumb is abducted (3,4,11).

Anomalies and Variations: Adductor

Pollicis

The two heads of the adductor pollicis vary in size. The two

heads can be coalesced to various degrees. The muscle also

can be split into additional bellies (11).

The transversus manum muscle is an anomalous muscle

closely related to the adductor pollicis. It arises from the

palmar MCP ligaments and connects to the base of the

thumb proximal phalanx, or in its vicinity (11).

Clinical Implications: Adductor Pollicis

The adductor pollicis may contribute to thumb-in-palm

deformity in patients with muscle spasticity (cerebral palsy,

154 Systems Anatomy

traumatic brain injury, stroke). Release of the origin of the

adductor pollicis (muscle recession) often is incorporated in

muscles lengthened or released to help correct the deformity. Care must be taken to protect the deep palmar arterial arch and the deep branch of the ulnar nerve, both of

which pass through the interval created by the two heads of

the muscle.

PALMARIS BREVIS

Derivation and Terminology. Palmaris is derived from

the Latin palma, which means “pertaining to the palm.”

Brevis is the Latin for “short” (1,2).

Origin. From the flexor retinaculum and medial border

of the central part of the palmar fascia.

Insertion. Inserts into dermis on the ulnar border of the

hand.

Vascular Supply. The superficial palmar arch.

Principal Action. The palmaris brevis wrinkles the skin

on the ulnar side of the palm of the hand. It deepens the

hollow of the palm by accentuating the hypothenar eminence.

Gross Anatomic Description: Palmaris

Brevis

The palmaris brevis is a small, thin muscle located in the

skin and subcutaneous tissue of the ulnar palm. It is quadrangular and arises from the flexor retinaculum and medial

border of the central part of the palmar aponeurosis. The

fibers are perpendicular to the axis of the upper extremity,

and insert into the dermis on the ulnar border of the hand.

This muscle is superficial to the ulnar artery and terminal

branches of the ulnar nerve (3,4,11).

The palmaris brevis is innervated by the superficial

branch of the ulnar nerve, from C8 and T1.

Actions and Biomechanics: Palmaris

Brevis

In wrinkling the skin on the ulnar side of the palm of the

hand and deepening the hollow of the palm, the palmaris

brevis may assist with cupping the hands for holding

water and may contribute to the security of the palmar

grip (3).

ABDUCTOR DIGITI MINIMI (ABDUCTOR

DIGITI QUINTI)

Derivation and Terminology. Abductor is derived form

the Latin ab, meaning “away from,” and ducere, which

means “to draw”; therefore, abductor is “that which draws

away from.” Digiti is the plural of the Latin digitus, “digit.”

Minimi is from the Latin minima or minimum, indicating

the smallest. Abductor digiti minimi therefore indicates the

abductor of the smallest digit(s). Quinti is from the Latin

quintus, indicating “fifth.” Therefore, the abductor digiti

quinti is the abductor of the fifth digit (1,2).

Origin. From the pisiform, terminal tendon of the FCU,

and the pisohamate ligament.

Insertion. Two slips: one slip to the ulnar side of the base

of the proximal phalanx of the small finger. The other slip

continues dorsally to the ulnar border of the dorsal digital

aponeurosis of the EDM.

Innervation. Deep branch of the ulnar nerve (C8, T1).

Vascular Supply. The ulnar artery, deep palmar branch,

ulnar end of the superficial palmar arch, palmar digital

artery (3,4,7,8,11,13,14).

Principal Action. Abduction of the small finger (proximal phalanx) from the ring finger (thus spreading the

fourth web space when the digits are extended). Through

its contribution to the extensor mechanism, the abductor

digiti minimi may contribute to extension of the middle

phalanx (and possibly of the distal phalanx) of the small

finger.

Gross Anatomic Description: Abductor

Digiti Minimi

The abductor digiti minimi is the most medial of the three

hypothenar muscles (which also include the flexor digiti

minimi and opponens digiti minimi; Appendix 2.2). The

abductor digiti minimi lies on the ulnar border of the

palm. The muscle arises from the pisiform, from the FCU

(at the FCU insertion), and from the pisohamate ligament

(496) (see Fig. 2.6A). The muscle extends distally along

the ulnar palm and splits into two slips. One slip inserts

into the ulnar side of the base of the proximal phalanx of

the small finger (see Fig. 2.6A). The other slip continues

distally and dorsally to join the ulnar border of the EDM

(in the dorsal digital aponeurosis) so that it contributes to

the extensor mechanism of the digits (3,4,7,8,11,13,14).

The architectural features of the muscle are listed in Table

2.4.

Actions and Biomechanics: Abductor

Digiti Minimi

The abductor digiti minimi functions mainly to provide

abduction of the small finger (proximal phalanx) from the

ring finger (thus spreading the fourth web space when the

digits are extended). It also provides some abduction when

the digits are tightly adducted in flexion and extension.

Through its connection to the extensor mechanism

(through the ulnar dorsal slip), the abductor digiti minimi

may contribute to extension of the middle phalanx (and

possibly of the distal phalanx) of the small finger (3,4,7,8,

11,13,14).

2 Muscle Anatomy 155

Anomalies and Variations: Abductor

Digiti Minimi

Accessory slips may join the muscle from the tendon of the

FCU, the flexor retinaculum, the fascia of the distal forearm, or the tendon of the palmaris longus (11). A part of

the muscle may insert onto the metacarpal of the small finger (11).

Clinical Implications: Abductor Digiti

Minimi

The abductor digiti minimi can be used to help restore

thumb opposition as a donor muscle for opponensplasty.

This transfer often is referred to as the Huber transfer,

described in 1921 (621–624).

FLEXOR DIGITI MINIMI (FLEXOR DIGITI

MINIMI BREVIS)

Derivation and Terminology. Flexor is derived from the

Latin flexus, indicating “bent” (and flexor, which indicates

“that which bends,” or “bending”). Digiti is the plural of

the Latin digitus, “digit.” Minimi is from the Latin minima

or minimum, indicating “the smallest.” Brevis is the Latin

for “short.” Flexor digiti minimi therefore indicates the short

flexor of the smallest digit(s) (1,2).

Origin. From the hook of the hamate and flexor retinaculum.

Insertion. To the ulnar aspect of the base of the proximal

phalanx of the small finger.

Innervation. Deep branch of the ulnar nerve (T1, C8).

Vascular Supply. The ulnar artery, deep palmar branch,

ulnar end of the superficial palmar arch, palmar digital

artery (3,4,11).

Principal Action. Flexion of the proximal phalanx of the

small finger.

Gross Anatomic Description: Flexor Digiti

Minimi

The flexor digiti minimi, along with the abductor digiti

minimi and opponens digiti minimi, helps form the

hypothenar muscles (Appendix 2.2). The muscle lies deep

and adjacent to the abductor digiti minimi, along the radial

border of the abductor and coursing in the same direction.

The muscle takes origin from the convex surface of the

hook of the hamate and the palmar surface of the flexor

retinaculum (see Fig. 2.6A). The point of origin is slightly

more distal than that of the abductor digiti minimi. The

muscle extends distally in the same direction and plane as

the abductor digiti minimi to reach the insertion at the

ulnar side of the base of the proximal phalanx of the small

finger. The muscle inserts onto the lateral tubercle of the

proximal phalanx (see Fig. 2.6A). The insertion also is adjacent to that of the abductor digiti minimi, but located

slightly palmar. By this more palmar insertion point, the

muscle exerts a flexor force on the proximal phalanx. The

flexor digiti minimi is separated from the abductor digiti

minimi at its origin by the deep branches of the ulnar nerve

and ulnar artery (3,4,7,8,11,13,14). The architectural features of the muscle are listed in Table 2.4.

Actions and Biomechanics: Flexor Digiti

Minimi

The flexor digiti minimi functions mainly to provide flexion of the proximal phalanx at the MCP joint. It may assist

with lateral rotation of the proximal phalanx (3,4,11,13,

14). As noted earlier, because the flexor digit minimi inserts

onto the proximal phalanx at a point adjacent to but more

palmar than that of the abductor digiti minimi, the flexor

digiti minimi is able to exert a flexor force on the proximal

phalanx.

Anomalies and Variations: Flexor Digiti

Minimi

The flexor digiti minimi may be very small. If so, the

abductor digiti minimi usually is larger than normal (11).

The flexor digiti minimi may be absent (11). The flexor

digiti minimi may coalesce with the abductor digiti minimi

(11). The flexor digiti minimi may have a tendinous slip

that attaches to the metacarpal of the small finger (11).

OPPONENS DIGITI MINIMI

Derivation and Terminology. Opponens is the Latin term

indicating movement against or toward an opposing structure. Digiti is the plural of the Latin digitus, “digit.” Minimi

is from the Latin minima or minimum, indicating “the

smallest” (1,2).

Origin. The hook of the hamate and adjacent flexor retinaculum.

Insertion. The ulnar and anterior margin of the

metacarpal of the small finger.

Innervation. Deep branch of the ulnar nerve.

Vascular Supply. Ulnar artery, deep palmar branch,

medial end of the deep palmar arch (3,4).

Principal Action. Opposition of the small finger to the

thumb. This is a combination movement of abduction,

flexion, and lateral rotation of the metacarpal of the small

finger. It thereby brings the small finger in opposition to the

thumb.

Gross Anatomic Description: Opponens

Digiti Minimi

The opponens digiti minimi, along with the abductor digiti minimi, and flexor digiti minimi, form the hypothenar

156 Systems Anatomy

muscles (Appendix 2.2). The opponens digiti minimi lies

deep to the flexor digiti minimi and abductor digiti minimi

(3,4,7,8,11,13,14). It is triangular, broad at its base and

tapering to an apex distally. The muscle arises from the convex surface of the hook of the hamate, the adjacent pisohamate ligament, and the adjacent part of the palmar surface

of the flexor retinaculum (496) (see Fig. 2.6A). The muscle

becomes wider distally, to form a wide expansion for its

insertion. The muscle inserts along most of the ulnopalmar

surface of the diaphysis of the small finger metacarpal (see

Fig. 2.6A).

The architectural features of the muscle are listed in

Table 2.4.

The opponens digiti minimi is innervated by the deep

branch of the ulnar nerve, containing fibers from T1 and

from C8.

Actions and Biomechanics: Opponens

Digiti Minimi

The opponens digiti minimi permits opposition of the

small finger to the thumb. This is a combination movement

of abduction, flexion, and lateral rotation of the metacarpal

of the small finger. It thereby brings the small finger in

opposition to the thumb. This motion also is referred to as

supination of the small finger (496). Unlike the flexor digiti

minimi and abductor digiti minimi, the opponens digiti

minimi does not normally cross the MCP joint, and therefore does not act on the proximal phalanx of the small finger (3,4,7,8,11,13,14).

Anomalies and Variations: Opponens

Digiti Minimi

The opponens digiti minimi may be divided into two layers by the deep branches of the ulnar artery and ulnar nerve

(11). The opponens digiti minimi may coalesce with the

abductor digiti minimi or the flexor digiti minimi (11).

LUMBRICALS

Derivation and Terminology. Lumbrical is derived from

the Greek lumbricus, which means “earthworm.” The lumbrical muscles resemble the earthworm in shape, size, and

color (1,2).

Origin. From the FDP tendon.

Insertion. To the tendinous expansion of the EDC (into

the extensor hood).

Innervation. The first and second lumbricals are innervated by the median nerve (C8, T1). The third and fourth

are innervated by the deep branch of the ulnar nerve (C8,

T1). The third may receive variable innervation from the

median or ulnar nerve (3,4,628).

Vascular Supply. First and second lumbricals: first and

second dorsal metacarpal and dorsal digital arteries; arteria

radialis indicis, first common palmar digital artery. Third

and fourth lumbricals: second and third common palmar

digital arteries, third and fourth dorsal digital arteries and

their anastomoses with the palmar digital arteries (3,4).

Principal Action. Through the extensor mechanism, the

lumbricals function to provide extension at the PIP and

DIP joints. In addition, they provide assistance with flexion

of the MCP joint (629–634).

Gross Anatomic Description: Lumbricals

The lumbricals consist of four small, somewhat cylindrical

muscle bellies. They arise from the FDP tendons and insert

into the extensor hood. The muscles lie in the central palmar compartment of the hand (Appendix 2.2; see Table

2.4). The first and second lumbricals take origin from the

radial sides and palmar surfaces of the FDP tendons of the

index and long finger, respectively (3,4,7,8,11,13,14). The

third lumbrical arises from the adjacent sides of the FDP

tendons of the long and ring fingers. The fourth lumbrical

arises from the adjacent sides of the FDP tendons of the

ring and small fingers. The muscles pass volar to the deep

transverse metacarpal ligament. Each lumbrical passes to

the radial side of the corresponding digit. At the level of the

MCP joint, the tendon of each lumbrical passes in a dorsal

direction to reach the radial lateral bands of the extensor

mechanism. The tendon of each muscle approaches the

digit at approximately a 40-degree angle before insertion

into the radial lateral band (484) (see Fig. 2.9).

The lumbricals are unique in that they originate from a

flexor tendon in the palm and insert into the dorsal aponeurosis on the radial side of the four digits. These functions

have been studied and discussed in detail by von Schroeder

and Botte and Lieber and colleagues (466,496). Because the

lumbricals originate on the flexor side and insert into the

extensor side of the fingers, they provide unique proprioceptive sensory information.

Each lumbrical muscle also is unique in that, by originating from the FDP tendon, it is the only muscle that is

able to relax the tendon of its own antagonist (484). Smith

has recommended that when considering lumbrical action,

it is best not to focus on its origin and insertion, but rather

on its two attachments—to the profundus tendon and to

the lateral band. Thus, if the profundus contracts and the

lumbrical relaxes, the interphalangeal joints of the fingers

flex. If the profundus is relaxed, contraction of the lumbrical pulls the lateral band proximally and the profundus tendon distally. Thus, the flexion or tension of the profundus

is lessened, and the lumbrical is able to extend the proximal

and interphalangeal joints (484). Hence, the lumbrical has

relaxed its own antagonist. When both the profundus and

the lumbrical contract, the interphalangeal joints and MCP

flex simultaneously (484,617,631,632,635–637).

In addition, the lumbricals have a unique architectural

design. Their muscle fibers extend 85% to 90% of the

length of the muscle (466) and are designed for excursion

2 Muscle Anatomy 157

(Table 2.4, Fig. 2.13). The actual length of the muscle fibers

is similar to that of the extrinsic extensors on the dorsum of

the forearm, but the lumbricals have a very small pennation

angle and cross-sectional area and are ideally suited for creating an even contractile force (466,496). The lumbricals of

the index and long fingers arise from their respective FDP

tendons, which allows a greater independent motion compared with the lumbrical of the ring finger, which originates

from the adjacent sides of the two FDP tendons (long and

ring), or the lumbrical to the small finger, which originates

from the adjacent sides of the FDP tendons to the ring and

small fingers. Variation of the lumbricals is common (638)

and, as with the extensor tendons; more variability is

observed on the ulnar side of the hand (492,497,498). All

lumbricals insert into the lateral band on the radial side of

their respective fingers (Table 2.5). The architectural features of the lumbricals are listed in Table 2.4 (466).

The innervation of the lumbricals is split. The median

nerve innervates the index and long finger lumbricals,

which corresponds to the innervation of the FDP to these

two fingers (496). The ring and small finger lumbricals are

innervated by the ulnar nerve, which also innervates the

FDP to the same fingers (496).

Actions and Biomechanics: Lumbricals

The function of the lumbricals is complex and has been discussed in detail by Smith and von Schroeder and Botte

(484,496). Roughly stated, the lumbricals provide extension of the proximal and interphalangeal joints and flexion

of the MCP joint. From origin to insertion, the lumbricals

pass volar to the deep transverse metacarpal ligaments. As

such, they are volar to the axis of rotation of the MCP joint

and therefore can act as MCP flexors (3,4,13,14,617).

158 Systems Anatomy

TABLE 2.5. INTRINSIC MUSCLES OF THE HAND: ORIGIN, INSERTION, AND FUNCTION OF THE DEEP AND

SUPERFICIAL BELLIES OF THE DORSAL INTEROSSEI, THE VOLAR INTEROSSEI, AND THE LUMBRICALS

Muscle Group Origin Insertion Function

Interossei (7)a

Dorsal (4)

Deep belly (3) Index and long MC Lat tendon to lat band of DA, Abduct and flex MCP joint, extend IP

radial side of long finger joints long finger

Long and ring MC Lat tendon to lat band of DA, Abduct and flex MCP joint, extend IP

ulnar side of long finger joints long finger

Ring and small MC Lat tendon to lat band of DA, Abduct and flex MCP joint, extend IP

ulnar side of ring finger joints ring finger (abduction of small

finger by ADQ)

Superficial belly (3) Index MC Med tendon to lat tubercle of Abduct and weak flexion MCP joint,

prox phalanx, radial side of index index finger

finger

Index and long MC Med tendon to lat tubercle of Abduct and weak flexion MCP joint,

prox phalanx, radial side of long long finger

finger

Ring and small MC Med tendon to lat tubercle of Abduct and weak flexion MCP joint,

prox phalanx, ulnar side of ring ring finger

finger

Volar (3) Index MC Lat band of DA, ulnar side of Adduct and flex MCP joint, extend IP

index finger joints index finger

Ring MC Lat band of DA, radial side of Adduct and flex MCP joint, extend IP

ring finger joints ring finger

Small MC Lat band of DA, radial side of Adduct and flex MCP joint, extend IP

small finger joints small finger

Lumbricals (4) FDP index Lat band of DA, radial side of Extension IP joints, weak flexion MCP

index finger joint index finger

FDP long Lat band of DA, radial side of Extension IP joints, weak flexion MCP

long finger joint long finger

FDP long and ring Lat band of DA, radial side of Extension IP joints, weak flexion MCP

ring finger joint ring finger

FDP ring and small Lat band of DA, radial side of Extension IP joints, weak flexion MCP

small finger joint small finger

aNumbers in parentheses denote number of muscles.

ADQ, abductor digitorum quiti; DA, dorsal aponeurosis; FDP, flexor digitorum profundus tendon; IP, interphalangeal; lat, lateral; MC,

metacarpal bone; MCP, metacarpophalangeal; med, medial; prox, proximal.

Reprinted from von Schroeder HP, Botte MJ. The dorsal aponeurosis, intrinsic, hypothenar and thenar musculature of the hand. Clin Orthop

383:97–107, 2001, with permission.

However, as noted by several authors, the interossei and the

FDP and FDS tendons are primary flexors of the MCP

joints, whereas the lumbricals function primarily to extend

the interphalangeal joints through the dorsal aponeurosis

(496,629,630,633,638–642). The origins, insertions, and

functions of the lumbricals are summarized in Table 2.5

(496).

The role of the lumbricals in interphalangeal joint extension has been emphasized by Smith and others, who have

credited the lumbricals as the “workhorse of the extensor

apparatus” (484,633,634,640). Electromyography of the

lumbricals reveals high levels of activity whenever there is

active extension of the interphalangeal joints. In addition,

strong electrical stimulation of the lumbrical produces

interphalangeal joint extension followed by MCP joint flexion. Low levels of electrical stimulation produce only interphalangeal joint extension (629,630). Although the lumbricals are located on the radial side of the fingers, they

apparently do not function as abductors or adductors of the

MCP joints because of their relatively parallel paths along

the axis of the fingers (496). There is no radial deviation of

the digits when the lumbricals contract (484,631).

Although interphalangeal joint extension is an important part of lumbrical function, the lumbrical contributes

relatively less or little to flexion of the proximal phalanx

(484). This may seem at first inherently somewhat odd

because the lumbrical tendon passes volar to the axis of the

MCP joint (and volar to the interossei). However, electromyographic studies performed by Long and Brown indicated that under normal circumstances, the lumbrical contributes little to MCP joint flexion (633). When the

interossei are paralyzed, however, the lumbrical can initiate

flexion at this joint. Flexion of the proximal phalanx also

may be achieved through contraction of the FDS and FDP.

When these muscle contract, they first flex the interphalangeal joints. After full interphalangeal joint flexion is

achieved, the long flexors flex the MCP joint until the digit

is completely flexed (484,642). If finger flexion were performed solely by the FDP and FDS, MCP joint flexion

would occur only after interphalangeal joint flexion was

complete (643–654).

The fact that the lumbricals originate from the FDP tendons but antagonize FDP flexion at the interphalangeal

joint is an interesting phenomenon. Although it seems to

contradict the respective functions of the muscle units, the

lumbricals can relax the FDP tendons and thereby enhance

their own function toward interphalangeal extension (496).

When the FDP and lumbricals contract simultaneously,

flexion of the interphalangeal and MCP joints occurs. This

cocontraction enhances stability and occurs in power grip.

The end result is simultaneous MCP and interphalangeal

joint flexion (496,633,635), compared with a sequential

contraction (DIP to PIP, then MCP contraction) that

occurs with FDP and FDS contraction (636). The interossei also contribute to flexion of the MCP joints.

Anomalies and Variations: Lumbricals

Variations in sites of attachments of the lumbricals are relatively common. Each muscle may originate by varying

amounts from the adjacent FDP tendons. The first lumbrical may have attachments that extend to the FPL tendon.

Accessory tendon slips that attach to the adjacent FDS tendon may be present (11).

Clinical Correlations: Lumbricals

The lumbricals and interossei work together to provide flexion of the MCP joints and simultaneous extension of the

PIP and DIP joints (see earlier, under Actions and Biomechanics: Lumbricals, and later, under Actions and Biomechanics: Dorsal Interossei, for specific differences and

nuances of function of these muscles). Both muscles often

are grouped together and referred to as the intrinsics or

intrinsic muscles of the hand. In a spastic deformity or

inflammatory condition with chronic spasm, with relative

overactivity of the intrinsic muscles, the hand assumes a

position dictated by these muscles—that is, flexion of the

MCP joints and extension of the PIP and DIP joints. This

position often is referred to as the intrinsic plus position,

indicating overactivity of these intrinsic muscles. In contrast, with paralysis of the intrinsics (due to ulnar nerve laceration or neuropathy), the hand assumes a position opposite to what the muscles would provide (secondary to

muscle imbalance of the functioning muscles). This results

in a position of extension of the MCP joints and flexion of

the PIP and DIP joints. This often is referred to as the

intrinsic minus position, indicating lack of intrinsic function. Intrinsic minus also can occur with relative overpull of

the extrinsic flexors and extensors, in conditions such as

ischemic contractures after severe compartment syndrome

(643– 654).

Although the thenar and hypothenar muscle are true

intrinsic muscles of the hand, the terms intrinsic plus and

intrinsic minus do not pertain to these muscles. Dysfunction of the thenar muscles is referred to simply as thenar

paralysis or (if present) thenar atrophy.

After amputation of the distal phalanx (or untreated distal FDP tendon laceration or rupture), the detached FDP

tendon may migrate proximally along with its lumbrical.

This initially may increase tension of the lumbrical on the

intrinsic extensor mechanism. If active flexion of the digit is

attempted, the detached FDP tendon migrates proximally

and pulls the lumbrical with it. Instead of digital flexion,

the tension of the lumbrical on the extensor apparatus

results in PIP joint extension. The hand is considered to

have a lumbrical plus digit. The undesired PIP extension

often is referred to as a paradoxical extension (because the

person actually is attempting to flex the digit). The lumbrical plus digit does not occur consistently. If it does develop,

elective operative resection of the lumbrical eliminates the

2 Muscle Anatomy 159

paradoxical extension and allows the FDS to assume flexion

control of the PIP joint (641).

DORSAL INTEROSSEI

Derivation and Terminology. Dorsal is derived from the

Latin dorsalis or dorsum, which indicates “the back.” Dorsal

usually is used to indicate the same side as the back, or the

“back side.” Interossei is derived from the Latin inter, which

indicates “between” or “among”; ossei is derived from ossis,

which means “bone.” The dorsal interossei are the muscles

between the bones, on the back side of the hand (1,2).

Origin. There are four dorsal interossei. The first arises

from adjacent sides of the thumb and index metacarpal, the

second from the adjacent sides of the index and long

metacarpal; the third from the adjacent sides of the long

and ring metacarpals, and the fourth from the adjacent

sides of the ring and small metacarpals (3,4,6,7,11,13).

Insertion. The first dorsal interosseous inserts into the

radial side of the base of the index proximal phalanx and

into the dorsal aponeurosis of the extensor hood of the

index finger. The second inserts into the radial side of the

base of the long finger proximal phalanx and into the dorsal aponeurosis of the extensor hood of the long finger. The

third inserts into the ulnar side of the base of the proximal

phalanx of the long finger and into the dorsal aponeurosis

of the extensor hood of the long finger. The fourth inserts

into the ulnar side of the base of the proximal phalanx of

the ring finger and into the dorsal aponeurosis of the extensor hood of the ring finger. The relative amounts of insertion into the associated proximal phalanx versus the

amount reaching the extensor are not the same for each

digit. The first dorsal interosseous inserts mainly into the

proximal phalanx, with a lesser component inserting into

the extensor hood. The second, third, and fourth have variable insertions, but, in general, the second and fourth have

substantial contributions to both the associated proximal

phalanx and to the dorsal aponeurosis. The third dorsal

interosseous inserts mainly into the dorsal aponeurosis of

the long finger, with a minimal component inserting into

the base of the proximal phalanx (484,631,635) (for additional details, see later, under Gross Anatomic Description).

Innervation. Deep branch of the ulnar nerve (C8, T1).

Vascular Supply. Dorsal metacarpal arteries, second to

fourth palmar metacarpal arteries; small branches of the

radial artery; arteria princeps pollicis; arteria radialis indicis;

perforating branches from the deep palmar arch (proximal

perforating arteries); three distal perforating arteries; dorsal

digital arteries (3,4,6,7,11,13).

Principal Action. The dorsal interossei draw the index,

long, and ring finger proximal phalanges away from the

mid-axis of the long finger. The muscles also flex the MCP

joints. Through the extensor hood, the dorsal interossei

help to extend the PIP and DIP joints (475,484,496).

Because each dorsal interosseous muscle varies in the relative amounts of insertion into the proximal phalanx or into

the dorsal aponeurosis, the functions of the interossei vary

among the digits. The first dorsal interosseous inserts

mainly into the proximal phalanx of the index finger (usually nearly 100%); it tends to function more for abduction

of the proximal phalanx than it does for extension of the

PIP or DIP joints. Conversely, the third interosseous usually inserts more into the extensor hood (approximately

94%), and therefore functions more for interphalangeal

joint extension of the long finger. The second and forth

dorsal interosseous have variable but substantial insertions

into both the associated proximal phalanx and the dorsal

aponeurosis, and therefore the second and fourth dorsal

interossei contribute both to abduction of the associated

proximal phalanx and extension of the proximal and interphalangeal joints. There also is a component of flexion of

the MCP joint provided by the dorsal interossei (see later,

under Actions and Biomechanics). The first dorsal

interosseous also adducts the thumb metacarpal toward the

index metacarpal during key pinch functions. This is combined with simultaneous abduction of the index proximal

phalanx, which helps stabilize the MCP joint during forceful pinch. This provides simultaneous adduction of the

thumb (metacarpal) toward the index finger, and allows the

index finger (proximal phalanx) to oppose the force of the

thumb. Thus, a strong key pinch can be generated

(3,4,6,7,11,13,475,484,496,655).

Gross Anatomic Description: Dorsal

Interossei

There are four dorsal interossei and three palmar interossei.

The palmar interossei are described later in a separate section. In general, the dorsal interossei are larger and have a

more complex anatomic arrangement than the palmar

interossei (484,496). The four dorsal interossei also comprise four separate dorsal interosseous muscle compartments of the hand (Appendix 2.2). The dorsal interossei

originate from and lie between the metacarpals (see Fig.

2.6B). Cross-sections of the hand in this area show the muscles occupying the space from the dorsal to palmar extent of

the metacarpal, although the space is shared by the palmar

interossei, which take origin more from the palmar portion

of the metacarpal shaft (656). Each dorsal interosseous

muscle is bipennate, with two muscle heads, each of which

arises from the adjacent metacarpal. The two bellies join in

a central longitudinal septum and the fibers course distally

toward the associated digit. Three of the four dorsal interossei then form a deep muscle belly and three have a superficial muscle belly (484,496,655). The first and second dorsal interossei pass the radial side of the associated MCP

joints to reach their respective digits; the third and forth

dorsal interossei pass the ulnar side of the associated MCP

joints to reach their respective digits (496) (see Table 2.5).

160 Systems Anatomy

The muscle bellies of the dorsal interosseous should not

be confused with the two heads of each muscle. Each muscle head arises from the adjacent metacarpal and joins to its

associated partner head at the septum to form a bipennate

muscle. In contrast, the deep and superficial bellies are

more distally located divisions of the muscle. The superficial and deep head of each muscle usually form just proximal to the MCP joint and are the terminal divisions of each

muscle. The deep and superficial muscle bellies have different final destinations for insertion, either into the associated

proximal phalanx (superficial belly) or into the associated

extensor hood (deep belly; see Fig. 2.9). The size, insertions,

and amount of muscle fibers of the deep and superficial bellies ultimately determine the function of the specific dorsal

interosseous. The superficial and deep muscle bellies have

been studied and discussed in detail by Smith, Kaplan, von

Schroeder and Botte, Landsmeer, and others (475,484,496,

642,655–657). The origin, insertion, and function of the

deep and superficial bellies of the interossei and lumbricals

are summarized in Table 2.5 (496).

The deep belly of each dorsal interosseous muscle is the

portion of the muscle that continues to join the lateral

bands to reach dorsal aponeurosis and become part of the

extensor mechanism of the associated index, long, and ring

finger. Like the superficial belly, the deep belly arises as part

of the main dorsal interosseous muscle from the adjacent

surfaces of the midshafts of the adjacent metacarpals. Just

proximal to the MCP joint, the dorsal interosseous splits

into a deep and superficial belly. The deep belly continues

distally to form or terminate into the lateral tendon. This

lateral tendon of the deep belly is potentially larger than the

medial tendon (which is derived from the superficial belly).

The lateral tendon continues distally to pass superficial to

the sagittal bands. The lateral tendon passes the MCP joint

and continues distally and dorsally to become part of the

extensor aponeurosis. The lateral tendon of the deep belly

forms part of the transverse fibers of the dorsal aponeurosis

(of the intrinsic muscle apparatus; see Fig. 2.9). The lumbrical tendon joins the extensor aponeurosis just distal to

the joining point of the lateral tendon of the dorsal

interosseous. The lumbricals help form the oblique fibers of

the extensor aponeurosis. Through the deep belly and its

insertion into the extensor apparatus, the dorsal

interosseous assists interphalangeal joint extension. This

muscle also provides flexion and assists with abduction of

the proximal phalanges. When the MCP joint is flexed to

approximately 90%, no significant abduction can be performed by the deep belly (496). The deep and superficial

bellies of each dorsal interosseous muscle are of different

sizes; and, the relative insertion into the extensor mechanism versus insertion into the proximal phalanx differs

among the interossei. These differences, in turn, influence

their respective functions of interphalangeal joint extension

versus proximal phalanx abduction. These issues are discussed later.

The superficial belly of each dorsal interosseous muscle

is the portion that inserts into the base of the associated

proximal phalanx and functions mostly for digital abduction. Although the superficial belly is a terminal division of

the dorsal interossei, the muscle belly arises from the adjacent surfaces of the midshafts of the contiguous metacarpals

as part of the main dorsal interosseous muscle. The fibers

form a bipennate muscle that continues distally to converge

into either a deep belly (described previously) or a superficial belly. The superficial belly splits from the main dorsal

interosseous muscle just proximal to the MCP joint. It then

forms or terminates into the medial tendon. The medial

tendon is a small tendon that continues distally and passes

deep to the sagittal bands of the MCP joint. The medial

tendon continues past the MCP joint to insert onto the lateral tubercle at the base of the proximal phalanx. Through

this osseous insertion, the muscle belly functions primarily

as an abductor of the proximal phalanx. It also is a weak

flexor of the proximal phalanx (484). This weak flexion

component increases in power as the MCP joint is increasingly flexed because the tendon passes volar to the axis of

rotation of the joint, and increasing flexion increases its

flexion moment arm. The superficial belly has no direct

effect on interphalangeal joint extension (655).

The first dorsal interosseous also is known as the abductor indicis, and is the largest of the dorsal interossei (3,4).

The first dorsal interosseous is triangular, thick, and flat. As

described earlier, there are two heads, each arising from the

adjacent metacarpal. The radial (lateral) head of the first

dorsal interosseous arises from the proximal half or threefourths of the ulnar border of the thumb metacarpal. The

ulnar (medial) head arises from the major portion of the

radial border of the second metacarpal. The origin from the

index metacarpal usually is slightly larger that that from the

thumb, but each covers approximately two-thirds to threefourths of the associated sides of the metacarpals (11). As a

bipennate muscle, there is a septum that separates the two

heads, in which the muscle fibers converge in an oblique

and distal direction. There also is a fibrous arch in the proximal aspect of the first dorsal interosseous that forms an

interval through which the radial artery passes from the

dorsal aspect of the hand to form the deep palmar arterial

arch. The muscle fibers converge toward the septum, running centrally and longitudinally through the muscle. Just

proximal to the MCP joint, on the radial side of the joint,

the first dorsal interosseous muscle divides into the superficial and deep bellies, which in turn give rise to the medial

and lateral tendons, respectively (484,496). The first dorsal

interosseous is unique in that most of the muscle consists of

the superficial belly, which gives rise to a median tendon

that inserts into the base of the proximal phalanx. The deep

belly is small or inconsistent, and few, if any, fibers form

this deep belly to give rise to a lateral tendon to insert into

the dorsal aponeurosis (635). Therefore, the first dorsal

interosseous inserts almost entirely into the proximal pha2 Muscle Anatomy 161

lanx of the index finger. The first dorsal interosseous thus

functions largely in abduction of the index finger proximal

phalanx. Through the proximal phalanx insertion, the first

dorsal interosseous also contributes to flexion of the MCP

joint. The first dorsal interosseous provides little, if any,

contribution toward PIP or DIP joint extension. The

abduction of the index proximal phalanx helps stabilize the

MCP joint, especially during key pinch function, where

index finger abduction action helps oppose the force of the

thumb.

The first dorsal interosseous also provides an important

function for the thumb metacarpal. The muscle adducts the

thumb metacarpal toward the index metacarpal. This function is used constantly during the pinch function, especially

in key pinch, where the thumb metacarpal is pulled toward

the index metacarpal in the plane of the palm. The simultaneous abduction of the index proximal phalanx helps stabilize the index finger during the key pinch maneuver.

The second dorsal interosseous, like the other dorsal

interossei, has two heads. The radial (lateral) head arises

from the ulnar side of the index metacarpal. The ulnar

(medial) head arises from the radial side of the long

metacarpal. Each of these muscle origins covers approximately the proximal two-thirds to three-fourths of the sides

of the shafts of each associated metacarpal. The origin from

the long finger usually is slightly larger than that from the

index metacarpal (11). The fibers converge into a central

septum, with the fibers oriented obliquely distally and

toward the central septum, forming the bipennate muscle.

Proximal to the MCP joint, on the radial aspect of the

joint, the fibers of the second dorsal interosseous divide

into superficial and deep bellies (described previously).

Approximately 60% of the fibers insert into the proximal

phalanx of the radial aspect of the base of the long finger.

The remaining 40% of the fibers reach the extensor hood

(634). (Thus, functionally, the muscle’s contribution to

abduction of the long finger is approximately equal or

slightly greater compared with its function in extension of

the PIP and DIP joints.) Through the dorsal hood, the second dorsal interosseous also contributes to flexion of the

MCP joint.

The third dorsal interosseous also has two heads. The

radial (lateral) head arises from the ulnar side of the long

metacarpal. The ulnar (medial) head arises from the radial

side of the ring metacarpal. As with the second dorsal

interosseous, the muscle origins of the third dorsal

interosseous attach to the proximal two-thirds to threefourths of the sides of the shafts of each associated

metacarpal. The origin from the long metacarpal usually is

slightly larger than that from the ring metacarpal (11). The

fibers converge into a central septum, with the fibers oriented obliquely distally and toward the central septum,

forming the bipennate muscle. Proximal to the MCP joint,

on the ulnar aspect of the joint, the fibers of the third dorsal interosseous divide into superficial and deep bellies

(described previously). Approximately 6% of the fibers

insert into the proximal phalanx of the ulnar aspect of the

base of the long finger. The remaining 94% of the fibers

reach the extensor hood (635). (Thus, functionally, the

muscle’s contribution to abduction of the long finger is

minimal compared with its major function of extension of

the PIP and DIP joints.) Through the dorsal hood, the

third dorsal interosseous also contributes to flexion of the

MCP joint.

The fourth dorsal interosseous also has two heads. The

radial (lateral) head arises from the ulnar side of the ring

metacarpal. The ulnar (medial) head arises from the radial

side of the small finger metacarpal. As with the other dorsal interossei, the muscle origin covers the proximal twothirds to three-fourths of the sides of the shafts of each

associated metacarpal. The origin from the ring metacarpal

usually is slightly larger than that from the small finger

metacarpal (11). Similar to the other dorsal interossei, the

fibers of the fourth dorsal interosseous converge into a central septum, with the fibers oriented obliquely distally and

toward the central septum, forming the bipennate muscle.

Proximal to the MCP joint, on the ulnar aspect of joint,

the fibers of the fourth dorsal interosseous divide into

superficial and deep bellies (described previously). Approximately 40% of the fibers insert into the proximal phalanx

of the ulnar aspect of the base of the long finger. The

remaining 60% of the fibers reach the extensor hood

(635). (Thus, functionally, the muscle’s contribution to

abduction of the long finger is slightly less than its function in extension of the PIP and DIP joints.) Through the

dorsal hood, the fourth dorsal interosseous also contributes

to flexion of the MCP joint. It also may contribute to

adduction of the small finger metacarpal if the ring finger

metacarpal is fixed.

The dorsal interossei usually all are innervated by the

deep branch of the ulnar nerve. For each of the muscles, the

deep and superficial bellies are separately innervated by distinct small nerve branches (484). It therefore is possible to

contract the deep belly of a dorsal interosseous without contracting the superficial belly, or vice versa (484).

Several variations in innervation are possible. The first

dorsal interosseous may be innervated by either the

median nerve, radial nerve, or musculocutaneous nerve.

Median nerve innervation is through the Martin-Gruber

or Riche-Cannieu anastomosis (see later, under Anomalies

and Variations).

Actions and Biomechanics: Dorsal

Interossei

In general, the dorsal interossei usually are credited with the

function of abduction of the associated digit (as well as flexion of the MCP joint), along with and extension of the PIP

and DIP joints. The function of each dorsal interosseous is

different and depends on the relative amounts of insertion

162 Systems Anatomy

into bone (the associated proximal phalanx), which provide

digital abduction, compared with the relative amounts of

insertion into the dorsal aponeurosis of the extensor hood,

which provide flexion of the MCP joint and extension of

the PIP and DIP joints (475,484). Studies have investigated

the relative insertions of each dorsal interosseous into the

proximal phalanx versus the extensor aponeurosis. Eyler

and Markee noted the following insertion ratios: first dorsal interosseous, 100% proximal phalanx, 0% extensor

aponeurosis; second dorsal interosseous, 60% proximal

phalanx, 40% extensor aponeurosis; third dorsal

interosseous, 6% proximal phalanx, 94% extensor aponeurosis; forth dorsal interosseous, 40% proximal phalanx,

60% extensor aponeurosis (635). Given these relative

amounts of insertion into the proximal phalanx versus the

dorsal aponeurosis, the relative amounts of digital abduction versus interphalangeal joint extension provided by the

muscle can be extrapolated (475,484,496,655)

When the function of abduction of the digits is examined, it is understood that abduction refers to “a drawing

away from the midline.” In the digits, this refers to the midline of the hand, and the mid-axis of the long finger usually

is used as the reference line. The first dorsal interosseous

functions to abduct the index finger, or draw it away from

the mid-axis of the long finger in the radial direction. The

second dorsal interosseous abducts the long finger, drawing

it away from the midline in a radial direction. The third

dorsal interosseous abduction component (although relatively weak) abducts the long finger, drawing it away from

the midline in the ulnar direction. The fourth dorsal

interosseous abducts the ring finger, drawing it away from

the mid-axis of the long finger in the ulnar direction (496).

The long finger only abducts from the mid-axis, and therefore there are two abductors present on either side. There is

no such movement of adduction of the long finger when it

is in a normal resting position. It is, however, possible for

the long finger to adduct back to a normal position from a

position of abduction (radial or ulnar deviation). Returning

back to the normal position can, in a sense, be considered

as adduction of the long finger. Abduction of the small finger is performed by the abductor digiti minimi (quinti).

Abduction of the thumb is performed primarily by the APL

and APB (496). The first dorsal interosseous also functions

to adduct the thumb metacarpal toward the index

metacarpal in the plane of the palm.

Based on muscle architecture, the dorsal and palmar

interossei (and lumbricals as well) are all highly specialized

muscles with similar architectural features (see Table 2.4

and Fig. 2.13). These muscles, with their relatively long

fiber length and relatively small physiologic cross-sectional

areas, are designed more optimally for excursion (and velocity) than force generation.

The small finger has no dorsal interosseous muscle

inserting into it. Abduction of the small finger is performed

by the abductor digiti minimi.

Anomalies and Variations: Dorsal

Interossei

The deep branch of the ulnar nerve normally innervates all

of the dorsal interosseous muscles. Infrequently, the median

nerve may innervate the first dorsal interosseous (in 3% of

limbs) (11,484,628,639). This variation may be associated

with the Martin-Gruber anastomosis, which is the medianto-ulnar nerve crossover in the forearm (658,659), or may

be associated with the Riche-Cannieu anastomosis, which is

the median-to-ulnar nerve crossover in the palm (475,660).

These anomalies are not uncommon, and their presence

explains continued function of the interosseous muscle(s) in

the presence of ulnar nerve laceration or severe neuropathy.

Rarely, the dorsal interosseous may be innervated by the

radial nerve or, more infrequently, there may be intercommunication between the musculocutaneous and median

nerves (628). The presence of these anomalies also explains

continued function of the interosseous muscle in the presence of ulnar nerve laceration or severe neuropathy.

The interossei may have additional muscle bellies or may

be completely absent in one or two of the interspaces (11).

Clinical Correlations: Dorsal Interossei

Because the first dorsal interosseous inserts mainly into the

proximal phalanx of the index finger, its principal function

is to abduct the proximal phalanx of the index finger (compared with its contribution to extension of the PIP or DIP

joints). By abducting the index proximal phalanx away

from the long finger, the first dorsal interosseous is able to

help stabilize the index MCP joint by opposing the thumb

during key pinch. During key pinch, the first dorsal

interosseous can visibly be seen and felt contracting. The

second dorsal interosseous also has a substantial insertion

into the proximal phalanx (60%), and therefore this muscle

probably also contributes to opposing the force of the

thumb or stabilizing the long finger MCP joint. This is

functionally advantageous when the long finger participates

in pinch, such as in three-jaw chuck-type pinch (484,635).

As opposed to the first dorsal interosseous, most of the

fibers of the third dorsal interosseous continue to the dorsal aponeurosis to reach the extensor hood. Thus, functionally, the third dorsal interosseous contributes much

more to extension of the PIP and DIP joint, compared

with its minimal contribution toward abduction of the

long finger. From a functional standpoint, this is advantageous because abduction of the long finger (in the ulnar

direction) is relatively unimportant. However, extension of

the long finger PIP and DIP joints and flexion at the MCP

joint are useful and important movements provided by the

dorsal aponeurosis.

The interossei and lumbricals work together to provide

simultaneous extension of the PIP and DIP joints and flexion of the MCP joints (475,484,496) (see earlier, under

2 Muscle Anatomy 163

Actions and Biomechanics, for specific differences and

nuances of function of these muscles). This is known as

intrinsic function, and is a complex and important component of hand movement required for everyday tasks. At the

initiation of a grasping maneuver, simultaneous extension

of the interphalangeal joints and flexion of the MCP joint

allows the digits to “wrap around” a relatively large object

such as a milk carton, doorknob, or orange-sized object.

Without the intrinsics providing the initial extension of the

interphalangeal joints, extrinsic tendon flexion function of

the digits results in flexion at the MCP, PIP, and DIP joints.

The flexion of the digits often starts at the DIP joint, followed by the PIP and MCP. The digits flex and tend to “roll

up” onto themselves and into the palm, similar to the way

a party blower toy roles up on itself after it is blown out and

inflated into a straight position and allowed passively to roll

back up. When the fingers flex or “role up” into the palm,

grasping of large objects is impossible. The digits are unable

to wrap around the object (which requires interphalangeal

joint extension at the initiation of the maneuver). This is

demonstrated when the intrinsic minus hand (or claw

hand) attempts to grasp a large object, and is a major functional problem of the intrinsic minus hand.

Function of an intrinsic minus hand can be roughly simulated in a cadaver. Flexion of the digits by the extrinsic

muscle in the absence of intrinsic muscle can be created in

a cadaver by grasping an extrinsic FDP tendon in the forearm and pulling proximally. This produces extrinsic flexion

without intrinsic function. The digit flexes at the DIP, PIP,

and MCP joints, but tends to roll up onto itself, as

described previously. The difficulties of the intrinsic minus

hand in grasp can thus be demonstrated.

Both the interossei and lumbrical muscles often are

grouped together and referred to as the intrinsics or intrinsic

muscles of the hand (484,496,661). In a spastic deformity, or

an inflammatory condition with chronic spasm, with relative overactivity of the intrinsic muscles, the hand assumes a

position that the muscles normally produce or provide, that

is, flexion of the MCP joints and extension of the PIP and

DIP joints. This position often is referred to as the intrinsic

plus position, indicating overactivity of these intrinsic muscles. In contrast, with paralysis of the intrinsics (due to ulnar

nerve laceration or neuropathy), the hand assumes a position

opposite to that which the muscles would provide (secondary to imbalance of the functioning muscles). This

results in a position of extension of the MCP joint and flexion of the PIP and DIP joints. This often is referred to as the

intrinsic minus position, indicating lack of intrinsic function.

The intrinsic minus position also can be produced by relative overactivity or contracture of the extrinsic muscles. This

can be seen with ischemic contracture after compartment

syndrome of the forearm (662–664).

Although the thenar and hypothenar muscles are true

muscles intrinsic to the hand, the terms intrinsic plus and

intrinsic minus do not pertain to these muscles. Dysfunction of the thenar muscles is referred to simply as thenar

paralysis or (if present) thenar atrophy.

PALMAR INTEROSSEI

Derivation and Terminology. Palmar is derived from the

Latin palma, which means “palm,” or palmaris, which

means “pertaining to the palm.” Interossei is derived from

the Latin inter, which indicates “between” or “among”; ossei

is derived from ossis, which means “bone.” The palmar

interossei are the muscles between the bones, on the palm side

of the hand (1,2).

Origin. There are usually three palmar interossei

attached to the index, ring, and small fingers. Four palmar

interossei are sometimes described (see Anomalies and Variations, Palmar Interossei). The first arises from the ulnar

side of the index metacarpal. The second arises from the

radial side of the ring metacarpal. The third arises from the

radial side of the small finger metacarpal. The origins are

located palmar to the dorsal interossei, and both sets of

muscles share the metacarpals for their origins.

Insertion. The palmar interossei insert into the dorsal

aponeurosis of the associated digit. The first inserts into the

dorsal aponeurosis on the ulnar side of the index finger. The

second inserts into the dorsal aponeurosis on the radial side

of the ring finger. The third inserts into the dorsal aponeurosis on the radial side of the small finger (3,4,484,496).

Innervation. From the deep branch of the ulnar nerve

(C8, T1).

Vascular Supply. Deep palmar arch, arteria princeps pollicis, arteria radialis indicis, palmar metacarpal arteries,

proximal and distal perforating arteries, common and

proper digital (palmar) arteries, common and proper palmar digital arteries, dorsal digital arteries (3,4,6,7,11,13).

Principal Action. The first, second, and third palmar

interossei adduct the proximal phalanx of the index, ring,

and small finger, respectively.

Gross Anatomic Description: Palmar

Interossei

The three palmar interossei are smaller and more uniform

than the dorsal interossei, and occupy the palmar portion of

the intermetacarpal spaces, also shared with the dorsal

interossei.The three palmar interossei comprise three separate palmar interosseous compartments of the hand

(Appendix 2.2). Each palmar interosseous arises form the

associated side of its metacarpal, covering the base to the

head and neck region of the bone (see Fig. 2.6A). The second and third palmar interossei tend to arise from the entire

surface, whereas the first originates from and covers a

slightly smaller area (3,4,11,13). Each belly converges to a

tendon at the level of the MCP joint and passes the joint on

the adductor side (which corresponds to the ulnar side of

164 Systems Anatomy

the joint for the first, and the radial side for the second and

third palmar interossei). In classic anatomy textbooks and

descriptions of the insertions of the palmar interossei, the

muscles have been described as inserting into both the lateral bands of the extensor aponeurosis as well as into the

base of the proximal phalanx (3,4,13,14,662). From the

studies of Eyler and Markee, and as emphasized by Smith

and von Schroeder and Botte, it appears that the palmar

interossei have few, if any, significant insertions into the

proximal phalanx (484,496,635). Eyler and Markee studied

the relative insertions of each palmar interosseous into the

proximal phalanx versus into the extensor aponeurosis. The

relative ratios of muscle insertion for each palmar

interosseous were as follows: first palmar dorsal

interosseous, 0% proximal phalanx (of index finger), 100%

dorsal aponeurosis; second palmar interosseous, 0% proximal phalanx (of ring finger), 100% dorsal aponeurosis;

third palmar interosseous, 10% proximal phalanx (of small

finger), 90% dorsal aponeurosis (634). Smith has emphasized that the palmar interossei have no distinct deep and

superficial bellies (as do the dorsal interossei), and thus

none is inserted onto the proximal phalanx. Each of the palmar interossei can still adduct and flex the proximal phalanx and can extend the distal two phalanges of the finger.

But these functions are performed through insertions into

the lateral bands of the dorsal aponeurosis (and not through

bone insertions into the proximal phalanges) (484,496).

The origin, insertion, and function of the interossei and

lumbricals are summarized in Table 2.5 (496). Architectural

features are shown in Table 2.4 and Figure 2.13.

The first palmar interosseous arises from the ulnar side

of the second metacarpal diaphysis. The fibers converge

into its tendon at the level of the MCP joint, on its ulnar

aspect. The tendon then inserts into the lateral band of the

dorsal aponeurosis on the ulnar side of the proximal phalanx of the index finger (3,4,484,496).

The second palmar interosseous arises from the radial

side of the ring metacarpal diaphysis. The fibers converge

into its tendon at the level of the MCP joint, on its radial

aspect. The tendon then inserts into the lateral band of the

dorsal aponeurosis on the radial side of the proximal phalanx of the ring finger (3,4,484,496).

The third palmar interosseous arises from the radial side

of the small finger metacarpal diaphysis. The fibers converge into its tendon at the level of the MCP joint, on its

radial aspect. The tendon then inserts into the lateral band

of the dorsal aponeurosis on the radial side of the proximal

phalanx of the small finger. According to Eyler and Markee,

a small amount, approximately 10% of the muscle, of the

third palmar interosseous also may insert into the base of

the proximal phalanx of the small finger (635).

Some authors describe four palmar interossei (3,13). In

usual descriptions, however, this muscle is considered as

part of the adductor pollicis (see later, under Anomalies and

Variations).

Actions and Biomechanics: Palmar

Interossei

Each palmar interosseous adducts and flexes the proximal

phalanx of the associated digit, and extends the middle and

distal phalanges (484). The first, second, and third palmar

interossei act on the proximal phalanx of the index, ring,

and small finger, respectively. Adduction of the digits refers

to drawing the digit toward the midline of the hand (toward

the mid-axis of the long finger). This movement is performed by the muscles’ insertion into the dorsal aponeurosis (3,4).

Anomalies and Variations: Palmar

Interossei

Variations of the palmar interosseous muscles are rare. A

muscle can be duplicated. Most of the variations are related

to innervation, such as with the median nerve (see earlier,

under Anomalies and Variations: Dorsal Interossei).

Although three palmar interossei usually are present,

occasionally a fourth palmar interosseous is present or

described (13). This may represent an alternative description of basically normal anatomy, or may be a variant of the

adductor pollicis. The authors who describe a fourth palmar

interosseous usually attach the term first palmar interosseous

to a muscle or fibers that passes from the base of the thumb

metacarpal to the base of the thumb proximal phalanx. This

muscle usually inserts with the adductor pollicis. In their

description, the remaining palmar interossei (as described

previously) become the second, third, and fourth palmar

interossei, respectively. Because the thumb has a large

adductor muscle of its own, these fibers have been considered as part of that muscle in most descriptions (3,13,14).

Clinical Correlations: Palmar Interossei

The thumb and the long finger do not have or need a palmar interosseous muscle. The long finger lies in the midline

of the hand, and therefore does not need to be “adducted.”

If it is in a position of abduction in the ulnar or radial direction, it can be brought back to the midline (adducted, in a

sense) by the second or third dorsal interossei, respectively.

The thumb does not require a palmar interosseous because

it has the adductor pollicis (3,13,14).

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2 Muscle Anatomy 179

180 Systems Anatomy

APPENDIX 2.1. MUSCLES OF THE HAND AND FOREARM AND ARM: ORIGIN, INSERTION, ACTION,

INNERVATION

Muscle Origin Insertion Action Innervation (Nerve Roots)

Deltoid Lateral one-third Deltoid tuberosity of Abduction of humerus, Axillary n. (C5, C6)

clavicle, acromion, humerus forward flexion or

spine of scapula extension of humerus

Coracobrachialis Coracoid process of Medial humeral Forward flexion, adduction Musculocutaneous n. (C5, C6)

scapula diaphysis of humerus

Biceps brachii Short head from Radial tuberosity, Flexion, supination of Musculocutaneous n. (C5, C6)

coracoid process, lacertus fibrosis forearm

long head from

supraglenoid

tuberosity

Brachialis Distal two-thirds of Coronoid process of Flexion of forearm Musculocutaneous, (and

anterior humerus ulna occasionally radial) n. (C5,

C6, C7)

Triceps brachii Long head from Olecranon, deep fascia Extension of forearm, Radial n. (C6, C7)

infraglenoid of forearm adduction of arm

tuberosity of scapula, (long head)

lateral head from

posterolateral

humerus, medial

head from distal

posterior humerus

Anconeus Lateral epicondyle of Lateral side of Extension of forearm Radial n. (C7, C8)

humerus, posterior olecranon and

capsule of elbow posterior surface of

ulna

Brachioradialis Lateral supracondylar Lateral, distal radius, Flexion or forearm, Radial n. (C5, C6)

ridge of humerus, styloid process assistance of pronation

lateral intermuscular of forearm (when

septum forearm is supinated),

assistance of forearm

supination (when

forearm is pronated)

Pronator teres Humeral head from Central lateral radial Pronation of forearm, Median n. (C6, C7)

medial epicondylar diaphysis assistance of flexion

ridge of humerus, of forearm

ulnar head from

medial side of

coronoid process

of ulna

Flexor carpi Medial epicondyle of Base of metacarpals of Flexion, radial deviation Median n. (C6, C7)

radialis humerus (common index and long of wrist, assistance

flexor origin) fingers with flexion and

pronation of forearm

Palmaris longus Medial epicondyle of Palmar fascia Flexion of wrist, assists Median n. (C6, C7)

humerus (common (aponeurosis) flexion, pronation of

flexor origin) forearm

Flexor carpi Humeral head from Pisiform (possible Flexion, ulnar deviation Ulnar n. (C8, T1)

ulnaris medial epicondyle extensions to of wrist, assistance with

of humerus hamate and base flexion of forearm

(common flexor metacarpal of little

origin), ulnar head finger)

from proximal

dorsal ulna

Flexor digitorum Humeral head from Palmar middle Flexion of middle and Median n. (C7, C8)

superficialis medial epicondyle phalanges of digits proximal phalanges,

of humerus assistance with forearm

(common flexor and wrist flexion

origin), ulnar head

from coronoid

process of ulna,

radial head from

oblique line of

radial diaphysis

2 Muscle Anatomy 181

APPENDIX 2.1. (continued)

Muscle Origin Insertion Action Innervation (Nerve Roots)

Flexor digitorum Medial anterior Palmar distal phalanges Flexion of distal (and Median n. to radial 2 digits,

profundus surface of ulna, middle and proximal) ulnar n. to ulnar 2 digits

interosseous phalanges, assistance (C7, C8)

membrane, deep with wrist flexion

fascia of forearm

Flexor pollicis Palmar surface of Base, palmar distal Flexion of distal (and Median n. (C8, T1)

longus radius, interosseous phalanx of thumb proximal) phalanx of

membrane, medial thumb

border of coronoid

process

Pronator Distal palmar ulna Distal palmar radius Pronation of forearm Median n. (C8, T1)

quadratus

Extensor carpi Lateral supracondylar Dorsal base of index Extension, radial Radial n. (C6, C7)

radialis longus ridge of humerus, metacarpal deviation of wrist

lateral intermuscular

septum

Extension carpi Common extensor Dorsal base of long Extension, radial Posterior interosseous

radialis brevis origin from lateral finger metacarpal deviation of wrist or radial n. (C6, C7)

epicondyle of

humerus, radial

collateral ligament

of elbow joint,

intermuscular

septum

Extensor Common extensor Dorsal bases of middle Extension of digits, Posterior interosseous of

digitorum origin from lateral and distal phalanges assistance with radial n. (C6, C7)

communis epicondyle of wrist extension

humerus,

intermuscular

septum

Extensor digiti Common extensor Dorsal base of distal Extension of little Posterior interosseous of

minimi origin from lateral phalanx of little finger radial n. (C7, C8)

epicondyle of finger

humerus,

intermuscular

septum

Extensor carpi Common extensor Dorsomedial base of Extension, ulnar Posterior interosseous of

ulnaris origin from lateral little finger deviation of wrist radial n. (C6, C7)

epicondyle of metacarpal

humerus, posterior

border of ulna

Supinator Lateral epicondyle of Radiopalmar surface Supination of forearm Radial n. (deep branch)

humerus, lateral of proximal radius (C6, C7)

capsule of elbow,

supinator crest and

fossa of ulna

Abductor Dorsal surface of Radial base of thumb Abduction of thumb, Posterior interosseous or

pollicis longus mid-diaphysis of metacarpal assistance of wrist radial n. (C6, C7)

radius and ulna, abduction

interosseous

membrane

Extensor pollicis Dorsal surface of Base, proximal Extension of proximal Posterior interosseous or

brevis radial diaphysis, phalanx of thumb phalanx (and radial n. (C6, C7)

interosseous metacarpal) of thumb

membrane

Extensor pollicis Dorsal surface of Dorsal base, distal Extension of distal Posterior interosseous or

longus ulnar diaphysis, phalanx of thumb phalanx of thumb, radial n. (C6, C7)

interosseous assists extension of

membrane proximal phalanx and

metacarpal of thumb

Extensor indicis Dorsal distal ulnar Dorsal proximal Extension of proximal Posterior interosseous or

proprius diaphysis, phalanx of index phalanx of index radial n. (C6, C7)

interosseous finger finger

membrane

182 Systems Anatomy

APPENDIX 2.1. (continued)

Muscle Origin Insertion Action Innervation (Nerve Roots)

Abductor Transverse carpal Radial side, base of Palmar abduction of Recurrent branch of median

pollicis brevis ligament, scaphoid proximal phalanx proximal phalanx of n. (C8, T1)

tubercle, palmar of thumb thumb

trapezium

Opponens Transverse carpal Radiopalmar surface Opposition of thumb to Recurrent branch of median

pollicis ligament, palmar of thumb digits (palmar n. (C8, T1)

trapezium abduction, pronation

metacarpal of thumb)

Flexor pollicis Transverse carpal Base proximal phalanx Flexion of proximal Recurrent branch median n.

brevis ligament, palmar of thumb phalanx of thumb (C8, T1)

trapezium

Adductor Oblique head from Ulnar side, base of Adduction of thumb, Deep branch of ulnar n.

pollicis palmar trapezium, proximal phalanx assistance with (C8, T1)

trapezoid, and of thumb opposition

capitate

Transverse head

from palmar

surface of long

finger metacarpal

Palmaris brevis Ulnar side of Skin on ulnar border Corrugation of skin on Superficial branch of ulnar n.

transverse carpal of palm ulnar palm (deepening (C8, T1)

ligament, palmar of palm)

aponeurosis

Adductor digiti Pisiform, tendon of Ulnar side, base of Abduction of little finger Deep branch of ulnar n.

minimi flexor carpi ulnaris proximal phalanx from palm (C8, T1)

of little finger,

aponeurosis of

extensor digiti

minimi

Flexor digiti Transverse carpal Ulnar side, base of Flexion of proximal Deep branch of ulnar n.

minimi ligament, hook of proximal phalanx of phalanx of little (C8, T1)

hamate little finger finger

Opponens Transverse carpal Ulnar side of metacarpal Opposition of little Deep branch of ulnar n.

digiti minimi ligament, hook of of little finger finger to thumb, (C8, T1)

hamate flexion of metacarpal

of little finger

anteriorly out of palm

Lumbricals Four lumbricals Join with interossei to Extension of the middle Median n. to radial two

arise from tendons form lateral bands phalanges, flexion of lumbricals, ulnar n. to

of flexor digitorum that become dorsal the proximal phalanges ulnar two lumbricals

profundus hood with the extensor (C8, T1)

digitorum communis

tendons; ultimate

insertions include base

of the middle phalanx

(central slip) and base

of distal phalanx

Dorsal interossei Four dorsal interossei First into radial side of Abduction of index, Deep branch ulnar

each from sides of proximal phalanx of long, ring fingers n. (C8, T1)

adjacent two index finger; second from midline of hand,

metacarpals into radial side of flexion of proximal

proximal phalanx of phalanges, extension

long finger; third into of middle phalanges

ulnar side of proximal

phalanx of long finger;

fourth into ulnar side

of proximal phalanx

of ring finger

All interossei also with

variable contributions

to lateral bands to

form part of the dorsal

hood

APPENDIX 2.1. (continued)

Muscle Origin Insertion Action Innervation (Nerve Roots)

Palmar interossei Three palmar First into ulnar side of Adduction of digits Deep branch ulnar n. (C8, T1)

interossei: First proximal phalanx of

from ulnar side of index; second into

index metacarpal, radial side of proximal

second from radial phalanx of ring finger;

side of ring third into radial side

metacarpal, third of proximal phalanx

from radial side of of little finger

little finger

metacarpal

2 Muscle Anatomy 183

APPENDIX 2.2. MUSCLE COMPARTMENTS AND FASCIAL SPACES OF THE

UPPER EXTREMITY

Compartment Principal Muscles

Deltoid compartment Deltoids

Anterior compartment of the arm Coracobrachialis

Biceps brachii

Brachialis

Posterior compartment of the arm Triceps muscle (three heads)

Mobile wad compartment of the forearm Brachioradialis

Extensor carpi radialis longus

Extensor carpi radialis brevis

Superficial volar compartment of the forearm Pronator teres

Flexor carpi radialis

Palmaris longus

Flexor digitorum superficialis

Flexor carpi ulnaris

Deep volar compartment of the forearm Flexor digitorum profundus

Flexor pollicis longus

Pronator quadratus compartment Pronator quadratus

Dorsal compartment of the forearm Extensor digitorum communis

Extensor indicis proprius

Extensor carpi ulnaris

Extensor digiti quinti

Extensor pollicis longus

Supinator

Abductor pollicis longus

Extensor pollicis brevis

Carpal tunnela Extrinsic digital flexor tendons

Central palmar compartment of the hand Extrinsic flexor tendons

Lumbricals

Thenar compartment Abductor pollicis brevis

Flexor pollicis brevis

Opponens pollicis

Hypothenar compartment Abductor digiti minimi

Flexor digiti minimi

Opponens digiti minimi

Adductor compartment of the hand Adductor pollicis

Interosseous compartments of hand Dorsal interossei (four)

Palmar interossei (three)

aAlthough not a true muscle compartment, the carpal tunnel is listed here because it can have the

physiologic properties of a closed compartment in the presence of compartment syndrome.

APPENDIX 2.3. HUMAN FOREARM MUSCLE DIFFERENCE INDEX VALUES: A COMPARISON OF ARCHITECTURAL FEATURES OF SELECTED

SKELETAL MUSCLES OF THE UPPER EXTREMITY

A

FCR FCU PL ECRB ECRL ECU FDSI FDSM FDSR FDSS FDPI FDPM FDPR FDPS FPL EDCI EDCM EDCR EDCS EDQ

FCR 0.00

FCU 0.63 0.00

PL 0.63 1.23 0.00

ECRB 0.36 0.65 0.87 0.00

ECRL 0.94 1.40 0.94 0.86 0.00

ECU 0.27 0.39 0.90 0.33 1.06 0.00

FDSI 0.31 0.62 0.78 0.56 0.99 0.34 0.00

FDSM 0.42 0.46 1.02 0.38 1.00 0.23 0.37 0.00

FDSR 0.20 0.80 0.52 0.43 0.78 0.43 0.34 0.51 0.00

FDSS 0.84 1.44 0.25 1.03 1.03 1.10 1.01 1.23 0.73 0.00

FDPI 0.22 0.77 0.62 0.35 0.73 0.39 0.34 0.43 0.12 0.82 0.00

FDPM 0.46 0.51 1.02 0.49 1.02 0.31 0.30 0.14 0.52 1.25 0.45 0.00

FDPR 0.24 0.63 0.71 0.50 0.95 0.32 0.08 0.38 0.26 0.95 0.27 0.33 0.00

FDPS 0.27 0.66 0.79 0.23 0.78 0.29 0.37 0.28 0.29 0.99 0.18 0.34 0.32 0.00

FPL 0.15 0.61 0.65 0.44 1.07 0.30 0.39 0.50 0.32 0.84 0.36 0.54 0.32 0.40 0.00

EDCI 0.77 1.38 0.20 0.96 0.86 1.03 0.91 1.13 0.63 0.21 0.71 1.14 0.84 0.88 0.81 0.00

EDCM 0.59 1.21 0.28 0.73 0.68 0.84 0.74 0.92 0.43 0.40 0.49 0.93 0.67 0.65 0.65 0.25 0.00

EDCR 0.58 1.20 0.10 0.80 0.87 0.85 0.75 0.97 0.46 0.27 0.56 0.98 0.68 0.73 0.61 0.20 0.20 0.00

EDCS 0.78 1.39 0.16 1.01 0.98 1.05 0.92 1.17 0.66 0.15 0.76 1.17 0.86 0.93 0.80 0.13 0.34 0.21 0.00

EDQ 0.61 1.19 0.12 0.89 1.01 0.87 0.72 1.00 0.51 0.34 0.62 0.99 0.67 0.79 0.62 0.30 0.36 0.20 0.24 0.00

EIP 0.77 1.39 0.20 0.96 0.91 1.04 0.94 1.15 0.65 0.12 0.74 1.17 0.87 0.90 0.80 0.11 0.28 0.20 0.12 0.31

EPL 0.53 1.11 0.19 0.79 1.04 0.79 0.72 0.95 0.48 0.35 0.58 0.96 0.65 0.74 0.51 0.38 0.38 0.21 0.32 0.19

PT 0.71 0.57 1.26 0.45 1.24 0.54 0.87 0.58 0.84 1.41 0.77 0.72 0.83 0.63 0.72 1.36 1.15 1.19 1.39 1.27

PQ 0.92 1.42 0.75 0.87 0.86 1.10 1.18 1.20 0.86 0.69 0.87 1.28 1.11 0.95 0.95 0.70 0.62 0.68 0.76 0.86

BR 1.03 1.30 1.24 1.02 0.74 1.05 0.86 0.89 0.91 1.43 0.84 0.82 0.88 0.84 1.16 1.23 1.06 1.19 1.32 1.23

FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; PL, palmaris longus; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris; FDSI, FDSM,

FDSR, FDSS, flexor digitorum superficialis to the index, middle, ring, and small fingers; FDPI, FDPM, FDPR, FDPS, flexor digitorum profundus to the index, middle, ring, and small fingers;

FPL, flexor pollicis longus; EDCI, EDCM, EDCR, EDCS, extensor digitorum communis to the index, middle, ring, and small fingers; EDQ, extensor digiti quinti; EIP, extensor indicis proprius;

EPL, extensor pollicis longus; PT, pronator teres; PQ, pronator quadratus; BR, brachioradialis.

aEditors Note: Architectural features of a muscle include the physiologic cross-sectional area of the muscle, fiber bundle length, muscle length, muscle mass, and pennation angle (angle

of the muscle fibers from the line representing the longitudinal vector of its tendon.) This table lists each of the difference index values, which is a number that compares a pair of

muscles. The difference index is the amount that the two muscles differ from each other, and has been determined based on the architectural features (15). A lower number (index

value) indicates a lesser difference, a larger index value indicates a greater difference. The mean architectural difference index among the upper extremity muscles is 0.74.

Reproduced from Lieber RL, Brown CG. Quantitative method for comparison of skeletal muscle

architectural properties. J Biomech 25:557–560, 1992, with permission.

EIP. EPL PT PQ BR

0.00

0.34 0.00

1.34 1.15 0.00

0.64 0.77 1.14 0.00

1.31 1.30 1.34 1.48 0.00

184

3

NERVE ANATOMY

MICHAEL J. BOTTE

The gross anatomy of the upper extremity peripheral nerves

is described in the following sections. The physical course of

each nerve and its associated branches is outlined, followed

by descriptions of nerve anomalies or variations, and clinical correlations. For descriptive purposes, each nerve discussion is divided into the regions of the arm, forearm, and

wrist and hand, if applicable. Sensory nerve organelles are

discussed at the end of the chapter. The dermatomes of the

upper extremity are depicted for reference in Appendix 3.1.

MEDIAN NERVE

Origin of the Median Nerve

The median nerve arises from the lateral and medial cords

of the brachial plexus, and comprises fibers from the anterior rami of C5, C6, C7, C8, and T1 (Fig. 3.1). The median

nerve originates from two branches, one each from the lateral and medial cords of the brachial plexus. The two

branches, referred to as the lateral and medial roots, unite

adjacent to and anterior or anterolateral to the third portion

of the axillary artery, in the vicinity of the medial border of

the coracobrachialis. This occurs approximately at the longitudinal level of the surgical neck of the humerus with the

shoulder abducted 90 degrees (1–5).

Median Nerve in the Axilla and Arm

The median nerve continues distally in the arm, posterior to

the pectoralis major, anterior to the coracobrachialis, lateral to

the brachial artery, and medial to the biceps brachii. In the

arm, and along most of its course, it lies anteromedial to the

brachialis muscle and posteromedial to the biceps brachii muscle. The median nerve does not normally supply motor

branches to any muscle in the arm. In the mid-portion of the

arm, in the vicinity of the insertion of the coracobrachialis

muscle, the median nerve crosses anterior to the brachial artery

to lie on the medial side of the artery. The nerve continues to

the cubital fossa, remaining medial to the brachial artery. Both

the nerve and artery remain close to the biceps tendon just

proximal to the lacertus fibrosus. The mnemonic, MAT, helps

in remembering the relationship (from medial to lateral) of the

median nerve, brachial artery, and the biceps tendon in this

area (6). The nerve usually gives off several small vascular

branches, but does not provide innervation to muscles in the

arm (7,8) (Fig. 3.2). In the distal third of the arm, the brachial

artery gives off several muscular arteries, including the supratrochlear artery (inferior ulnar collateral arteries). These

branches cross anteriorly or posteriorly to the nerve, often in

close proximity. Adjacent to the brachial artery are venae comitantes, two to three of which lie between the artery and the

median nerve (6). Throughout the course of the median nerve

in the arm, the ulnar nerve remains posterior and somewhat

parallel to the median nerve, diverging slightly from the

median nerve as the two nerves descend along the arm. The

ulnar nerve continues distally to reach the cubital fossa.

Anomalies and Variations: Median Nerve

in the Axilla and Arm

Although the median nerve usually is formed by the union

of the lateral and medial cords anterior or lateral to the axillary artery, the nerve also has been noted rarely to be formed

by the branches of these cords uniting posterior to the axillary artery (7).

The median nerve usually is formed at the level of the

third portion of the axillary artery. The nerve also can originate from the union of the lateral and medial cords more

distally, in the proximal third of the arm (7,9,10).

Fibers from C7 may leave the lateral root in the distal

part of the axilla and pass distomedially posterior to the

medial branch from the medial cord. The nerve usually

passes anterior to the axillary artery, to join the ulnar nerve.

These fibers are believed to be mainly motor fibers to the

flexor carpi ulnaris (3,11).

If the lateral cord is small, the musculocutaneous nerve

(C5, C6, and C7), which usually arises from the lateral

cord, can arise directly from the median nerve (1,3,11).

A branch from the musculocutaneous nerve occasionally

joins the median nerve after the musculocutaneous nerve

pierces the coracobrachialis muscle. This variation has been

reported in 8% to 36% of dissected specimens (12). The

fibers enter the musculocutaneous nerve from the lateral

cord rather than passing into the lateral root of the median

nerve. The communicating branch leaves the musculocutaneous nerve, descends from lateral to medial between the

brachialis and biceps muscles, and joins the median nerve

in the mid-portion of the arm. When this anomaly occurs,

the branch (or branches) of the lateral cord that joins the

medial cord is smaller than normal.

Fibers may cross from the median to musculocutaneous

nerve. This anomaly is rare.

A nerve to the pronator teres muscle may leave the main

median nerve trunk in the arm as high as 7 cm proximal to

the epicondyles (7,13).

Clinical Correlations: Median Nerve in the

Axilla and Arm

The median nerve may be compressed at several points in

the upper extremity. These are well described by Siegal and

Gelberman (7), and include the following areas:

At the level of the coracoid process, the nerve (or lateral

cord) may be compressed by the pectoralis minor muscle.

The muscle lies on the anterior surface of the nerve, and can

cause nerve compression, especially when the arm is hyperabducted (7,14).

An anomalous muscle known as Langer’s muscle can

cause median nerve compression. This muscle arises from

the latissimus dorsi tendon, crosses the axillary neurovascular bundle, and inserts on the pectoralis major (7,15).

Median nerve compression can occur in the axilla and

arm from anomalous vascular arches, or perforations of the

nerve by anomalous vessels. The vascular anomalies may be

arterial or venous in origin. An 8% incidence of abnormal

relationships between the vascular and neural elements in

the axilla has been reported (7,16).

186 Systems Anatomy

FIGURE 3.1. Schematic illustration

of the brachial plexus and associated major branches. A, nerve to

subclavius; B, lateral pectoral nerve;

C, subscapular nerves; D, thoracodorsal nerve; E, medial antebrachial cutaneous nerve; F, medial

brachial cutaneous nerve; G, medial

pectoral nerve.

3 Nerve Anatomy 187

FIGURE 3.2. Schematic illustration of the median nerve and the musculocutaneous nerve, with

associated branches and innervated muscles.

The deltopectoral fascia, when thickened and fibrotic,

may occasionally compress the median nerve at its distal

edge. This has been noted after blunt trauma to the shoulder (7,17).

The supracondylar process and associated ligament of

Struthers may compress the median nerve in the distal arm

(7,18–23). The supracondylar process, a hook-shaped projection from the medial aspect of the distal humerus, usually is located 3 to 5 cm proximal to the medial epicondyle.

This anomalous protrusion provides attachment for an

anomalous ligament, the ligament of Struthers. The ligament spans between the supracondylar process and medial

epicondyle, forming a fibroosseous tunnel, which is present

in 1% of limbs. It may represent an accessory origin of the

pronator teres muscle. The nerve passes through the tunnel

with either the ulnar or brachial artery and veins, medially

to the vessels. Nerve compression may be caused by either

the supracondylar process itself or by the ligament (7,24).

Just proximal to the elbow, in the area of the medial epicondyle, there is a constant relationship of the median nerve,

brachial artery, and the biceps tendon. The mnemonic, MAT,

describes this relationship (from medial to lateral) of the

median nerve, brachial artery, and biceps tendon (6).

Median Nerve in the Forearm

In the cubital fossa, the nerve dives deep to the lacertus

fibrosus, lying anterior to the brachialis muscle and medial

to the brachial artery. As the nerve crosses the level of the

elbow joint, one to two articular branches are given off to

supply the proximal radioulnar joint (25) (see Fig. 3.2).

The median nerve in the proximal third of the forearm

supplies the flexor pronator group of muscles that arise

from the medial epicondyle. These include the pronator

teres, the flexor carpi radialis, and the palmaris longus. The

proximal portion of the flexor superficialis, which arises

from the medial epicondyle and the thickened fascia

(raphe) in the proximal third of the forearm, obtains its

motor supply from the motor branches supplying the flexor

carpi radialis and the palmaris longus. The motor branches

supplying the medial portion of the flexor pronator mass

usually enter the muscles on their deep (posterior) surface

(6). When the anterior surface of the antecubital region is

exposed, these branches usually are not readily visible

because of their deep course. On deeper exposure and

inspection, three to four motor branches can be found traversing deep to the muscles to innervate the pronator teres,

flexor carpi radialis, palmaris longus, and the humeral portion of the flexor digitorum superficialis (see Fig. 3.2).

The nerve enters the forearm between the superficial

(humeral) and deep (ulnar) heads of the pronator teres muscle. The nerve passes deep to the humeral head even when

there is a congenital absence of the ulnar head, as noted in

6% of cases (7,26). As the nerve passes through the muscle

bellies, it crosses the ulnar artery anteriorly, from medial to

lateral, separated from the artery by the deep head of the

pronator teres. Most commonly, the pronator teres motor

nerve has a common branch with nerve branches to the

superficial and deep heads (60% of specimens). Alternatively, two separate nerve branches may be found, one going

to the superficial head and one going to the deep head of

the pronator teres (7,26,27).

After emerging from the pronator teres, the median

nerve passes deep to an arch created by the two heads of the

flexor digitorum superficialis. In the region of the superficialis arch, the median nerve usually provides three motor

branches to the flexor digitorum superficialis. These

branches are located on the deep surface of the muscle (6).

The nerve continues distally in the forearm between the

flexor digitorum superficialis and flexor digitorum profundus (28). The nerve usually is in the fascia of the flexor digitorum superficialis, or may occasionally lie in the substance

of the muscle (7,29). The nerve usually becomes superficial

approximately 5 cm proximal to the wrist, emerging

between the flexor digitorum superficialis and flexor carpi

radialis, dorsal and slightly radial to the tendon of the palmaris longus (7,30). In the proximal forearm, the median

nerve innervates the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis (see Fig.

3.2). The branch to the pronator teres arises from 7 cm

above the medial epicondyle to 2.3 cm distal to the medial

epicondyle (31). In 45% of studied specimens, Sunderland

and Ran noted two branches to the pronator teres, in 30%

one branch, and in 25%, three or four branches (32). The

anterior interosseous nerve usually branches from the dorsoradial surface of the median nerve trunk, usually arising

immediately distal to the flexor digitorum superficialis arch,

5 cm distal to the medial epicondyle (see later). Proximal to

the anterior interosseous nerve branch, the median nerve

supplies the flexor carpi radialis, palmaris longus, and flexor

digitorum superficialis. There usually is only a single nerve

to the flexor carpi radialis and only one to the palmaris

longus, but often from two to seven branches to the flexor

digitorum superficialis. The branch to the index finger portion of the flexor digitorum superficialis arises in the midportion of the forearm, up to 20 cm distal to the medial epicondyle. (7). The muscular branches of the median nerve

arise primarily from its medial surface (7,33).

The median nerve and its branches supply the sympathetic fibers to the portions of the vascular structures of the

forearm and hand in a segmental fashion. At the elbow, the

median nerve provides a branch to the region of the bifurcation of the brachial artery. The nerve arborizes in the

proximal few centimeters of the radial and ulnar arteries.

The anterior interosseous nerve provides fibers to the anterior interosseous artery (see later). The sympathetic

branches from the median nerve continue distally to provide sympathetic fibers into the palm to supply the superficial palmar arch, and, with the ulnar nerve, partially supply

the deep palmar arch of the hand (see later) (6).

188 Systems Anatomy

Anterior Interosseous Nerve

The anterior interosseous nerve is the largest muscular branch

that originates from the median nerve. The anterior

interosseous nerve provides innervation to the flexor digitorum profundus to the index and long fingers (i.e., the radial

half of the muscle), the flexor pollicis longus, and the pronator quadratus (34) (see Fig. 3.2). The terminal portion of the

nerve also provides sensory fibers to the carpal joints.

The nerve typically arises from the trunk of the

median nerve on the dorsoradial surface at a level of

approximately 5 to 8 cm distal to the medial epicondyle.

Sunderland has demonstrated that the interosseous nerve

actually becomes a separate group of fascicles at a point

approximately 2.5 cm proximal to its branching from the

median nerve trunk and at approximately 22 to 23 cm

proximal to the radial styloid process (35). After leaving

the median nerve, the anterior interosseous nerve initially

lies between the flexor digitorum superficialis and flexor

digitorum profundus. The nerve passes dorsally, in the

interval between the flexor pollicis longus and the flexor

digitorum profundus, providing two to six branches to

each of these muscles. The nerve reaches the anterior surface of the interosseous ligament (interosseous membrane) and continues distally, usually close to the anterior

interosseous artery. The nerve eventually reaches the

pronator quadratus, where it penetrates the muscle proximally and passes deep to the belly to innervate the muscle. The nerve continues distally to the wrist, containing

sensory afferent fibers for the intercarpal, radiocarpal,

and distal radioulnar joints (6).

The anterior interosseous nerve also supplies sympathetic

nerve fibers to the proximal forearm. The sympathetic nerve

fibers exit the anterior interosseous nerve and join with the

anterior interosseous artery to continue distally (6).

Palmar Cutaneous Branch of the Median

Nerve

The palmar cutaneous branch of the median nerve is the

last major branch of the median nerve in the forearm (see

Fig. 3.2). This nerve provides sensory fibers to the base of

the thenar eminence. It contains no motor fibers. The nerve

usually arises from the anteroradial aspect of the median

nerve trunk, 5 to 7 cm proximal to the wrist (6,36). This is

in the vicinity of the radial margin of the flexor digitorum

superficialis (37). The palmar cutaneous nerve usually consists initially of only one nerve branch as it exits the main

median nerve trunk, and usually can be identified approximately 5.5 cm proximal to the radial styloid. Before branching, the nerve usually continues in or adjacent to the

epineurium of the median nerve trunk for 16 to 25 mm

before separating from the median nerve. The nerve courses

distally in the very distal forearm along the ulnar side of the

flexor carpi radialis tendon, adherent to the undersurface of

the antebrachial fascia. At the proximal edge of the transverse carpal ligament, the nerve deviates ulnarly and enters

its own short fibrous tunnel in the ligament. The tunnel

through the transverse carpal ligament is usually 9 to 16

mm long (6,37). The nerve pierces the transverse carpal ligament in line with the ring finger and enters the ligament,

dividing into ulnar and medial branches. These branches

supply the skin of the proximal two-fifths of the palm on

the radial side and the thenar eminence (7,24,38).

Anomalies and Variations: Median Nerve

in the Forearm

The most common nerve anomalies in the forearm are connections between the median and ulnar nerves. A connection

often exists between the anterior interosseous and ulnar nerves

in the substance of the flexor digitorum profundus. This intramuscular communication leads to multiple variations in patterns of innervation of the muscle. Dual innervation is most

common in the long finger flexor, but may occur in all the

digits. The median nerve, or rarely the ulnar nerve, may innervate the entire flexor digitorum profundus (7,39). When the

median nerve supplies the entire flexor digitorum profundus,

it usually is through fibers from the anterior interosseous

nerve. (The anterior interosseous nerve normally supplies the

flexor digitorum profundus to the index and long finger, but

in the “all median nerve hand,” the anterior interosseous nerve

also supplies the flexor digitorum profundus to the ring and

small fingers.)

The complete median- and complete ulnar-innervated

hand:There are several described clinical situations where the

hand appears to be completely innervated by the median or

ulnar nerve. Within these described conditions, there are several variations of reported findings. These variations probably

are due to gradations between median and ulnar innervations, representing individual differences in anatomic

arrangements. Fibers may pass between the ulnar and median

nerves in the forearm or hand. Their terminal branches may

send communicating fibers within the hand. The median

nerve sometimes innervates the interosseous muscles, particularly the first dorsal interosseous, either alone or jointly with

the ulnar nerve (40,41). In the extreme “all-median hand,”

the anterior interosseous nerve (from the median nerve) supplies the flexor digitorum profundus to the ring and small

fingers (which normally are supplied by the ulnar nerve) (6).

The ulnar nerve more often provides dual or replacement

innervation to muscles usually innervated by the median

nerve (36,41–44). Less often, the median nerve innervates

muscles that usually are innervated by the ulnar nerve (44).

Each of the lumbrical muscles can have dual innervation

from both the median and ulnar nerves (45,46). Double

innervation of the flexor pollicis brevis is relatively common.

Several patterns with ulnar innervation of the thenar muscles

have been noted (43,45,47).

The Martin-Gruber anastomosis: The Martin-Gruber

anastomosis is an anomalous or variant communication that

3 Nerve Anatomy 189

contains motor, sensory, or mixed fibers from the median or

anterior interosseous nerve to the ulnar nerve in the proximal

forearm (6,48,49). This anastomosis has been found in

approximately 15% (range, 10% to 44%) of dissected forearms (39,50–52). Several variations of this anastomosis are

recognized, although most of the communications consist of

a communication branch that originates from either the

trunk of the median nerve or from the anterior interosseous

nerve and crosses ulnarly to reach the ulnar nerve in the proximal, middle, or distal forearm. Approximately half of the

communications are recognized to arise from the anterior

interosseous nerve (6). Mannerfelt cites the earliest known

description of the anomaly by Martin in 1763 (46,53). Gruber made similar findings in 1870 (51). The connections

usually pass distally and ulnarly, dorsal and adjacent to the

ulnar artery, in the plane between the flexor digitorum superficialis and flexor digitorum profundus muscle bellies. In

addition, a variant of the Martin-Gruber anastomosis consists of motor fibers from the motor branches of the flexor

digitorum profundus crossing over to the ulnar nerve in the

muscle of the flexor digitorum profundus. The Martin-Gruber communication occasionally sends branches to the flexor

digitorum profundus or the flexor digitorum superficialis

(54). There may be a loop-shaped connection, with convexity distally, that contains motor fibers. Straight connections

usually are sensory (7). Electrophysiologic and electrodiagnostic studies have supported these anatomic findings, where

investigators have identified Martin-Gruber communications

carrying median nerve fibers to the hand through the ulnar

nerve (55–59).

There is an increased incidence of the Martin-Gruber

communication in some families, and an autosomal dominant inheritance pattern of median–ulnar connections has

been observed (60).

Comparative anatomy studies have shown that a communication between the median and ulnar nerves exists in

the proximal forearm in all baboons, rhesus monkeys, and

certain (cynomolgus) monkeys (27,61).

The Martin-Gruber communication presents several distinctly different types of anomalous motor innervation of

the hand muscles. These have been studied and outlined by

Meals, Spinner, and others (6,34,36,41,62).

Of 226 ulnar or median nerve–injured patients, Rowntree found evidence of anomalous innervation of hands in

20% (41). These included cases where the median nerve

innervated the first dorsal interosseous muscle, and where

the ulnar nerve innervated the abductor pollicis brevis. He

also noted cases of the “complete median” or “complete

ulnar” innervation of the hand.

The so-called all-ulnar or all-median hand probably is

represented in situations where one or the other nerve is cut

without evident functional impairment of the hand

(36,41).

There is a pattern of variation that consists of motor

fibers that pass from the median to ulnar nerve, proceeding

to innervate muscles of the hand usually innervated directly

by median nerve branches (44,46,47). In this case, an additional crossover occurs in the palm for these fibers to reach

the thenar muscles.

There is a pattern of variation where fibers pass from the

median to ulnar nerve, eventually terminating in muscles

that usually are ulnar nerve innervated (6,34,46,47,58).

Here, the Martin-Gruber communication provides a pathway for redirecting nerve fibers that were not completely

sorted in the brachial plexus.

There is a pattern of variation where ulnar nerve–derived

fibers targeted for muscles normally innervated by the ulnar

nerve sometimes cross over into the median nerve (ulnar to

median). This is a variation of the Martin-Gruber communication, and the fibers therefore must cross over again in

the palm to reach their targets (6,41).

Nerve anastomoses from the ulnar nerve to the median

nerve also are observed, but are much more infrequent than

from the median nerve to ulnar nerve. When present, the

connections usually are located in the distal forearm, palmar to the flexor digitorum profundus (12).

Overlapping of territory in the innervation of the flexor

digitorum profundus by the median and ulnar nerves has

been noted in up to 50% of specimens. It is twice as common for the median nerve to encroach on the ulnar nerve

compared with ulnar encroachment on median-innervated

muscles (63,64). The portion of the flexor digitorum profundus to the index finger is the only part of that muscle

constantly supplied by one nerve, the median nerve

(63,64). In most specimens, the flexor digitorum profundus

and the lumbrical of a particular digit are innervated by the

same nerve. Encroachment of the median on the ulnar

nerve is less common for the lumbricals than for the flexor

digitorum profundus (63,64).

In 16% of specimens studied, the relation of the median

nerve to the two heads of the pronator teres varies from that

traditionally described (65,66). Some of these variations have

been found to be associated with congenital absence of the

ulnar head of the pronator teres. When the ulnar head is

absent, the nerve (which usually passes between the ulnar and

humeral heads) has been found to pass either deep to the

humeral head in 6% or through the humeral head in 2% (26).

Variations of the Anterior Interosseous Nerve

Several variations of the anterior interosseous nerve have

been described.

Anterior Interosseous Nerve Innervation to the Flexor

Digitorum Superficialis

Sunderland has noted that in 30% of 20 specimens studied,

the anterior interosseous nervesupplied a branch to the flexor

digitorum superficialis (35,63). The specimens also had separate nerve innervation from the main trunk of the median

nerve supplying the flexor digitorum superficialis. Thus, in a

190 Systems Anatomy

dense anterior interosseous syndrome, there may be some

variable weakness of the flexor digitorum superficialis (6).

Anterior Interosseous Nerve Innervation to Gantzer’s

Muscle

Gantzer’s muscle is an accessory head to the flexor pollicis

longus (67–69). Its presence is variable, but it has been

noted in up to two-thirds of limbs. It is innervated by the

anterior interosseous nerve in most specimens (69).

Gantzer’s muscle is of clinical significance because it may be

a causative factor in anterior interosseous nerve syndrome

by muscle/fibrous entrapment; in addition, fibrosis of the

muscle with secondary contraction can produce a flexion

contracture of the thumb distal phalanx (69).

High Division of the Median Nerve and Bifid

Median Nerve in the Forearm

The median nerve may aberrantly divide into two components at the level of the wrist or forearm. Subsequently, two

separate nerve “branches,” a medial and a lateral component, extend down the forearm and enter the carpal tunnel.

The two branches can be of equal or unequal size. Early

descriptions of this anomaly, as noted by Sunderland, were

by Gruber, who described four cases in which the median

nerve branch to the third web space originated in the proximal forearm (6,44). In addition, Amadio found high

branching of the median nerve in 3% of cases (70). Hartmann and Winkelman and Spinner also have reported a

similar high branching of the median nerve in the forearm

(71,72). In most of the cases studied by Amadio, the bifid

median nerve had two branches that remained independent

of one another. However, two of nine cases had a loop communication in which one or the other median nerve branch

received a communicating branch from the other in or just

distal to the carpal canal (70). This communicating loop

was also noted in 3 of 29 cases reported in the literature at

the time of Amadio’s study (70). The variant branch of the

nerve may pass through the muscle mass or anterior to the

flexor digitorum superficialis (instead of its usual course

deep to the muscle) (6). At the level of the division, a small

or large ellipse or opening can occur, in which a tendon,

muscle, or vascular structure can pass (6,71). The high division of the median nerve can be accompanied with multiple other variants, including the Martin-Gruber anastomosis, a communication between the ulnar and median nerves

distal to the flexor retinaculum, and two components to the

median nerve crossing the distal half of the forearm and

carpal canal (6). The high division of the median nerve is a

true division of the nerve into two separate components. It

therefore probably is incorrect to describe this variant as a

“duplication” of the median nerve, as it is sometime referred

to in the literature (see also later discussion of bifid median

nerve, under Anomalies and Variations: Median Nerve in

the Wrist and Hand).

Accessory Motor Supply to the Flexor

Digitorum Superficialis

Spinner has noted several variations to the flexor digitorum

superficialis (6). An accessory nerve to the flexor superficialis

can arise from the motor branches to the flexor carpi radialis

or palmaris longus. The accessory branch usually crosses

between the superficial and deep head of the pronator teres.

This branch then crosses deep to the flexor digitorum superficialis arch to innervate a portion of the flexor digitorum

superficialis muscle. Similarly, the anterior interosseous

nerve, which supplies the flexor pollicis longus and flexor digitorum profundus to the index and long fingers, also may at

times supply a portion of the flexor digitorum superficialis.

Variations of the Palmar Cutaneous Branch of

the Median Nerve

The palmar cutaneous branch of the median nerve usually

divides from the median nerve trunk approximately 5 to 7

cm proximal to the wrist (approximately 5.5 cm proximal

to the styloid) and traverses the transverse carpal ligament

through its own fibrous tunnel (6,17). Several variations of

the palmar cutaneous branch have been noted.

Two Separate Branches of the Palmar Cutaneous

Branch

Two separate nerves of the palmar cutaneous branch may

exist. One can arise at the usual location. The other can

arise more proximally, from 9 cm or more proximal from

the median nerve (44). In addition, two palmar cutaneous

nerves may exit the median nerve trunk at the normal location, approximately 5.5 cm proximal to the styloid (73,74).

Distal Exit of the Palmar Cutaneous Branch of the

Median Nerve

The palmar cutaneous branch of the median nerve may exit

the median nerve trunk more distally than usual. It may continue with the median nerve trunk to the very distal forearm

flexor compartment before crossing the transverse carpal ligament (70). It also has been observed to arise from the median

nerve at the radial styloid or in the proximal end of the carpal

tunnel. It penetrates the transverse flexor retinaculum and palmar fascia to reach the skin at the base of the thenar muscles.

Absence of the Palmar Cutaneous Branch of the

Median Nerve

Complete absence of the palmar cutaneous branch of the

median nerve has been noted (6,44). In its absence, it has

been replaced with either an anterior division of the musculocutaneous nerve, a branch of the superficial radial

nerve, a branch of the palmar cutaneous nerve from the

ulnar nerve, or a combination of these branches (75).

Palmar Cutaneous Nerve Deep to the Palmaris Longus

The palmar cutaneous branch of the median nerve may lie

deep to the tendon of the palmaris longus, especially if the

3 Nerve Anatomy 191

palmaris longus is abnormal. At the level of the wrist crease,

the palmaris longus tendon may have a broad insertion into

the palmar fascia or a variant muscular attachment. In these

cases, the palmar cutaneous nerve has been noted to be deep

to or adjacent to the palmaris longus tendon (6).

Clinical Correlations: Median Nerve in the

Forearm

Martin-Gruber Anastomosis

The Martin-Gruber anastomosis consists of an anomalous

communication that carries motor fibers from the median

nerve to the ulnar nerve in the forearm (6,49) (see earlier,

under Anomalies and Variations: Median Nerve in the

Forearm). The motor fibers from the median nerve cross

from either the median nerve trunk or from the anterior

interosseous nerve, and travel to reach the ulnar nerve in the

proximal, middle, or distal forearm. The Martin-Gruber

communicating fibers from the median nerve often carry

the motor innervation of several of the intrinsic muscles of

the hand. These muscles include the first dorsal

interosseous, the adductor pollicis, the abductor digiti

quinti, and, less commonly, the second and third dorsal

interosseous muscles (46). Both anatomic and electrical

studies have noted these findings (6,46).

If a high ulnar nerve laceration (at or proximal to the

proximal forearm) is accompanied with preservation of

intrinsic muscle function, along with loss of function of the

flexor carpi ulnaris and flexor digitorum profundus to the

little finger, a Martin-Gruber communication should be

suspected distal to the area of nerve injury.

If a high median nerve laceration (at or proximal to the

proximal forearm) is accompanied with loss of some of the

intrinsic muscles (usually innervated by the ulnar nerve), a

Martin-Gruber communication should be suspected distal

to the area of nerve injury. Additional support for this

occurrence is provided if normal sensibility to the ring and

little fingers remains (innervated by the ulnar nerve).

Spinner has reported a patient with a complete ulnar

nerve laceration at the wrist that did not develop clawing.

That same patient did develop transient clawing only after

blocking the ulnar nerve at the elbow with local anesthetic

(6). A Martin-Gruber communication distally may have

been the pathway through which ulnar nerve–derived fibers

reached the intrinsic muscles (36).

Electrophysiologic Studies and the MartinGruber Anastomosis

Electrophysiologic studies have been used to evaluate and

confirm the presence of Martin-Gruber connections

(55–59). When the Martin-Gruber connection carries

median nerve fibers to the hand through the ulnar nerve,

this can result in varying degrees of anomalous innervation

of the intrinsic muscles. This also effects or confuses the

findings from electrodiagnostic studies, when evaluation for

nerve compression is sought at specific sites. A patient with

carpal tunnel syndrome with median-to-ulnar nerve communication may have normal latency from the elbow to the

thenar muscles, but prolonged latency across the wrist (36).

Because the incidence of the Martin-Gruber connection is

high (10% to 44%), it is not surprising that inconsistencies

occur between the clinical examination and electrodiagnostic studies (39,50–52).

Compression of the Median Nerve in the

Forearm

The median nerve is at risk for compression at several sites

in the forearm. These include the lacertus fibrosus, the two

heads (humeral and ulnar heads) of the pronator teres muscle, and the proximal origin or deep fascia of the flexor digitorum superficialis (17,76–80).

Pronator Syndrome

The pronator syndrome is a result of median nerve compression in the proximal forearm, most often caused by or related

to the pronator teres muscle (6,77,81–84). The clinical syndrome includes several findings: pain in the proximal volar

forearm that is increased with pronation against resistance;

paresthesias or numbness in the palmar thumb, index, long,

and radial ring finger; negative Phalen’s test (wrist flexion

does not produce median nerve paresthesias); variable weakness of the median-innervated intrinsic muscles (thenar muscles and radial lumbricals); normal extrinsic function of muscles innervated by the anterior interosseous nerve (flexor

pollicis longus, flexor digitorum profundus to the index and

long, and pronator quadratus); and electrodiagnostic studies

suggestive of localized sensory and motor conduction delay

in the proximal forearm (and absence of generalized polyneuropathy). (Electrodiagnostic studies may be variable and

unreliable.) Although the pronator teres muscle most often is

the site of compression of the median nerve, compression at

two other adjacent sites also has been included in the pronator syndrome (6). These include compression by the lacertus

fibrosus and by the fibrous arch of the flexor digitorum

superficialis. Reproduction of forearm pain with elbow flexion and forearm supination against resistance suggests

involvement of the lacertus fibrosus. Forearm pain reproduced by flexion of the long finger proximal interphalangeal

joint (flexor digitorum superficialis) suggests a site of compression at the arch of the flexor digitorum superficialis.

Causes of Pronator Syndrome

Anatomic abnormalities and related problems that have

been observed with the pronator syndrome include (6,66,

81,85–92):

n Hypertrophied pronator teres

n Fibrous bands in the pronator teres or associated tendons

(93)

192 Systems Anatomy

n Median nerve passing posterior to both heads of the

pronator teres

n Thickened lacertus fibrosus (94)

n Hematoma deep to the lacertus fibrosus, resulting from

blood sample drawn from antecubital fossa with difficulty in patient on renal dialysis or anticoagulant therapy

n Thickened flexor digitorum superficialis arch

n An accessory tendinous origin of the flexor carpi radialis

from the ulna

n Tightness of the lacertus fibrosus from serial casting to

correct elbow flexion contractures

Anterior Interosseous Nerve Syndrome

Compression or injury causing neuropathy of the anterior

interosseous nerve usually is associated with a classic clinical

presentation referred to as the anterior interosseous syndrome

(6,95–113). Because of Kiloh and Nevin’s early description

of neuritis of the anterior interosseous nerve (114), the syndrome also has been referred to as the Kiloh-Nevin syndrome,

especially in the international literature (115–119). The

clinical findings consist of paralysis or weakness of the flexor

pollicis longus, flexor digitorum profundus to the index and

long fingers, and pronator quadratus. An episode of pain in

the proximal forearm may precede the clinical paresis. When

the patient attempts to perform a thumb-to-index pulp

pinch or a three-jaw chuck pinch, the interphalangeal joint

of the thumb and the distal interphalangeal joints of the

index and long collapse into extension (owing to weakness

of the associated flexor muscles to the distal joints). Forearm

pronation may be weak because of involvement of the

pronator quadratus, although the pronator teres is intact and

still provides some pronation. There is no detectable sensibility abnormality or involvement of other muscles supplied

by the median nerve. Variations in clinical presentation can

exist depending on the extent of the nerve lesion, whether

partial or complete, and the specific site of involvement

along the course of the nerve. In addition, specific anatomic

variations in a particular limb may contribute to variations

in clinical presentation. Spinner has noted that in the

extreme all-median hand, the anterior interosseous nerve

supplies all of the flexor profundus muscles. Thus, in this

variant, there would be weakness of flexion of the distal phalanx of the ring and small fingers as well (6). Conversely, in

variations where the ulnar nerve innervates more of the profundi, the flexor digitorum profundus of the long finger may

be unaffected or only partially weakened by loss of function

of the anterior interosseous nerve (6). To test for insolated

function of the pronator quadratus in the presence of anterior interosseous nerve syndrome, the pronation power of

the pronator teres must be eliminated. This can be accomplished by testing for forearm pronation strength with the

elbow fully flexed. In this position, most of the pronation

strength of the pronator teres is eliminated as the muscle is

shortened and slack. This can by corroborated by direct electrodiagnostic studies.

Causes and Sites of Anterior Interosseous Nerve

Compression or Injury

Several causes of anterior interosseous nerve compression or

injury have been recognized, including injury by penetrating trauma, external compression, intrinsic compression by

either muscle/tendon structures or vascular structures, and

iatrogenic causes (6,120,121). Penetrating injuries of the

proximal forearm have included glass and metal lacerations,

stab wounds, injections by drug abusers, and gunshot

injuries. Fractures also have been known to result in anterior interosseous syndrome (122), and usually consist of

either supracondylar fractures in children or forearm fractures treated in either an open or closed fashion (6,123,

124). Iatrogenic injury also has been reported after cutdown catheterization in the forearm (125) and from the

flexor pronator slide procedure (126). Causes of external

compression include tight-fitting casts, especially the proximal rim of the short arm cast. Several causes of intrinsic

compression have been noted. Those involving compression by muscle or tendon structures include (6):

n A tendinous origin of the deep head of the pronator teres

(a tendinous loop encircling the median nerve at the level

of the origin of the anterior interosseous nerve) (6)

n A tendinous origin of the flexor superficialis to the long

finger

n An accessory head of the flexor pollicis longus (Gantzer’s

muscle)

n An accessory muscle and tendon from the flexor superficialis to the flexor pollicis longus

n A tendinous origin of anomalous muscles such as the palmaris profundus or the flexor carpi radialis brevis (127)

n An enlarged bicipital bursa encroaching on the median

nerve near the origin of the anterior interosseous nerve

n Vascular structures such as thrombosis or dilation of crossing ulnar collateral vessels, and an aberrant radial artery

Anterior Interosseous Nerve and the Martin-Gruber

Anastomosis

The Martin-Gruber anastomosis (between the median and

ulnar nerves) occurs in 15% of limbs (54). In approximately

half of these anastomoses, the communication branch arises

from the anterior interosseous nerve. The communicating

branch from either the median nerve or anterior interosseous

nerve often carries fibers to various intrinsic muscles, including the first dorsal interosseous, adductor pollicis, abductor

digiti minimi, and, less commonly, the second and third dorsal interosseous. Therefore, as noted by Spinner, in the presence of a Martin-Gruber communication, a patient with

dense anterior interosseous nerve syndrome also may show

some dysfunction of the intrinsic muscles of the hand (6).

Anterior Interosseous Nerve and the Flexor Digitorum

Superficialis

Sunderland has noted that in 30% of 20 specimens studied,

the anterior interosseous nerve supplied a branch to the

3 Nerve Anatomy 193

flexor digitorum superficialis (35,63). The specimens also

had separate innervation from the main trunk of the median

nerve supplying the flexor digitorum superficialis. Spinner

thus has pointed out that in a complete anterior interosseous

nerve syndrome, there also may be some variable weakness

of the flexor digitorum superficialis muscles (6).

Differential Diagnosis in Anterior Interosseous Nerve

Syndrome

Several clinical conditions can produce loss of flexion of the

distal joints of the thumb, index, and long finger. These

include brachial plexus compression, traumatic lesions, or

neuritis (Parsonage-Turner syndrome; see later), compartment

syndrome or Volkmann’s contracture, attritional rupture of

the radial flexor tendons, and congenital absence of the flexor

tendons (128,129). Chronic inflammatory conditions such as

rheumatoid arthritis can produce carpal subluxation or tendon-damaging irregularities involving the scaphoid or lunate.

These can produce attritional ruptures of the radial digital

flexors of the hand. Congenital absence of the deep flexors of

the hand can involve the flexor pollicis longus and the flexor

digitorum profundus, thus resulting in a pinch similar to that

seen in anterior interosseous syndrome. A history of weakness

since birth, along with electrodiagnostic studies, helps confirm the diagnosis of the congenital condition (6).

Anterior Interosseous Nerve Palsy and the Neuritis of

Parsonage and Turner

In the patient presenting with weakness of flexion of the

interphalangeal joint of the thumb and the distal interphalangeal joints of the index and long fingers, the differential

diagnosis includes, besides the anterior interosseous syndrome, the neuritis described by Parsonage and Turner (129).

In the Parsonage-Turner syndrome, there often is weakness of

the distal phalanges of the thumb and index fingers. However, there usually is an associated variable weakness of the

scapular muscles, which distinguishes this form of brachial

plexopathy from anterior interosseous nerve palsy.

High Division of the Median Nerve (Bifid

Median Nerve)

High division of the median nerve can subject the nerve to

potential injury during forearm dissection, especially if one

of the two branches is not recognized. The variant branch

of the nerve may pass through the muscle mass or anterior

to the flexor digitorum superficialis (instead of its usual

course deep to the muscle) (6). If unrecognized, the anomalous nerve branch is at additional risk for injury during

operative procedures in the region.

High Division of the Median Nerve (Bifid

Median Nerve) and Forearm Lacerations

Laceration of the forearm associated with numbness of the

third web space and accompanying loss of sensibility in the

ulnar half of the long finger and radial half of the ring finger suggests the occurrence of a bifid median nerve with laceration to the ulnar component (or perhaps a partial laceration of a normal median nerve) (6). Conversely, forearm

laceration with sparing of sensibility to the third web space

suggests either incomplete median nerve laceration (in a

normal nerve) or laceration to the radial component of a

bifid median nerve.

Injury to the Palmar Cutaneous Branch of the

Median Nerve

Surgery adjacent to or along the ulnar border of the flexor

carpi radialis must be performed with caution to avoid

injury to the palmar cutaneous branch of the median nerve.

The flexor carpi radialis and the radial styloid can be used

to help identify the palmar cutaneous branch of the median

nerve. The nerve usually exits the median nerve trunk as

one branch, approximately 5.5 cm proximal to the radial

styloid. The exit point is along the radial margin of the

flexor digitorum superficialis and continues along the ulnar

margin of the flexor carpi radialis longus tendon. If the

nerve is injured, the resulting loss of sensibility may be of

secondary concern compared with problems associated with

a painful neuroma (37,130). A painful neuroma can be disabling. For this reason, if the palmar cutaneous branch of

the median nerve is inadvertently injured, or if the nerve is

found injured from penetrating trauma, nerve repair, if possible, usually is warranted (more from the standpoint of

neuroma prevention than from that of sensibility restoration). If the nerve is not reparable, it can be transected

cleanly at its point of exit from the nerve trunk, or can be

placed in an area of protection deep to or inside a muscle

belly (37,130).

Isolated Compression of the Palmar

Cutaneous Branch of the Median Nerve

Entrapment of the palmar cutaneous nerve has been

reported, caused by or associated with an abnormal palmaris longus tendon. Associated symptoms included localized pain, and numbness at the base of the thenar muscles.

Nerve decompression may be indicated (6,131).

Absence of the Palmar Cutaneous Branch of

the Median Nerve

With absence of the palmar cutaneous branch of the

median nerve, sensibility at the base of the thenar muscles

usually is provided by the anterior division of the musculocutaneous nerve, a branch of the superficial branch of radial

nerve, a branch of the palmar cutaneous nerve from the

ulnar nerve, or a combination of these branches (75). In

these situations, lacerations of any of these nerves results in

anesthesia at the base of the thenar muscles.

194 Systems Anatomy

Peripheral Block of the Palmar Cutaneous

Branch of the Median Nerve

To provide adequate local anesthesia for procedures in the

region of the palmar thenar muscles, block of the palmar

cutaneous branch must be included along with block of the

median nerve (unless the median nerve is blocked proximal

to the origin point of the palmar cutaneous nerve). Usually,

infiltration of anesthetic solution along the ulnar border of

the flexor carpi radialis anesthetizes the palmar cutaneous

branch of the median nerve.

Median Nerve in the Wrist and Hand

The median nerve becomes superficial in the distal forearm

approximately 5 cm proximal to the wrist, surfacing from the

radial border of the flexor digitorum superficialis. The nerve

continues distally, deep and slightly radial to the palmaris

longus (if present). The nerve is ulnar to the flexor carpi radialis and anterior and ulnar to the flexor pollicis longus. In the

very distal forearm or at the level of the wrist, the median

nerve comes to lie palmar to the flexor digitorum superficialis, and continues into the carpal region by entering deep

to the transverse carpal ligament (flexor retinaculum). The

median nerve enters the carpal tunnel at a level that corresponds to the volar flexion crease of the wrist. The carpal tunnel boundaries comprise the transverse carpal ligament on

the palmar aspect, the scaphoid and trapezium on the radial

aspect, the hook of the hamate and pisiform on the ulnar

aspect, and the palmar radiocarpal ligaments on the dorsal

aspect. The median nerve usually enters the carpal tunnel as

one nerve trunk. At this level, the internal topography of the

nerve is well organized and consistent. Within the

epineurium, the groups of fascicles are arranged linearly

according to their destination. The motor fibers are anterior.

The sensory fascicles for each of the web spaces and the radial

three and one-half digits are located from lateral to medial in

progressive sequence in the nerve (6,35,44).

Recurrent Motor Branch

After passing through the carpal tunnel, the recurrent

motor branch to the thenar muscle arises from the radial

surface of the median nerve (132–134). Variations of the

point of branching are well appreciated (see later, under

Anomalies and Variations: Median Nerve in the Wrist and

Hand). Most commonly, an extraligamentous recurrent

branch leaves the main nerve trunk at the distal margin of

the transverse carpal ligament. The nerve branch curves

proximally and radially to enter the thenar muscles. This

pattern has been noted in 46% of studied specimens. The

first muscle branch usually is to the flexor pollicis brevis,

followed by a branch to the abductor pollicis brevis. The

nerve then passes deeply to innervate the opponens pollicis

from the ulnar border of the muscle. The motor branch of

the median nerve rarely may supply innervation to the first

dorsal interosseous muscle (7).

The median nerve usually passes through the carpal tunnel as the most palmar structure (volar to the flexor tendons), with the transverse carpal ligament lying immediately against the palmar surface of the nerve. The median

nerve then divides into three common palmar digital nerves

(discussed later). In general, the common digital nerves

divide at the junction of the middle and distal third of the

metacarpal shafts to form the proper digital nerves. This

branch point usually is approximately 1 cm distal to the

superficial palmar arch.

Common Palmar Digital Nerves

The first common palmar digital nerve divides into three

proper palmar digital nerves, two of which supply sensibility

to the palmar aspects of the thumb and one that continues

as the proper palmar digital nerve for the radial aspect of the

index finger (after supplying a small nerve branch to the first

lumbrical) (1,2,4,11). This branch to the first lumbrical

branches off just distal to the edge of the transverse carpal

ligament, in the proximal or middle palm (Fig. 3.2).

The second common palmar digital nerve supplies a small

nerve branch to the second lumbrical, and continues to the

web between the index and long fingers. The nerve splits into

proper digital nerves for the ulnar aspect of the index finger

and the radial aspect of the long finger (1–4,11) (Fig. 3.2).

The third common palmar digital nerve occasionally

gives a small branch to the third lumbrical (in which the

muscle receives double innervation from both the ulnar and

median nerves). The third common palmar digital nerve

also often communicates with a branch of the ulnar nerve,

and continues to the web space between the long and ring

fingers. The nerve then splits into proper digital nerves to

supply the ulnar aspect of the long finger and radial aspect

of the ring finger (Fig. 3.2).

Proper Digital Nerves

The proper digital nerves of the median nerve supply the

skin of the palmar surface and the dorsal surface of the distal phalanx of the respective digits. At the end of each digit,

the nerve terminates in two or three branches. One branch

usually innervates the pulp of the digit, another usually supplies the tissue deep to the nail. These nerves often communicate with the dorsal digital branches of the superficial

radial nerve.

In the palm, the median nerve branches usually are

located deep to the associated arterial structures, but superficial (palmar) to the flexor tendons. These branches pass

deep to the superficial palmar arch and usually cross deep to

the common digital arteries as the nerves and arteries course

distally. The division of the common digital nerves into

proper digital nerves usually occurs at the level of the

metacarpal necks. At this level, the proper digital nerves

course more palmarly, to come to lie palmar (superficial) to

the digital arteries. The nerves enter the digits between the

3 Nerve Anatomy 195

deep and superficial transverse metacarpal ligaments,

maintaining their palmar relationship to the digital arteries (7).

Anomalies and Variations: Median Nerve

in the Wrist and Hand

Because of its clinical relevance, the anatomy of the median

nerve has received substantial attention in anatomic studies.

As a result, several variations and anomalies have been

noted (36,44,134–139). In general, the anomalies usually

are one of the various patterns of the median nerve in the

carpal tunnel, or involve the median-to-ulnar or ulnar-tomedian nerve anastomosis in the palm.

Median Nerve Variations in the Carpal Tunnel

In the carpal tunnel, several variations of median nerve

anatomy have been described (6,41,71,72,137,138,

140–149) (Table 3.1). Lanz has described eight patterns

(138). These variations also have been classified by Spinner

(6) and by Amadio, based on evaluation of 275 carpal tunnel releases (70) (see Table 3.1). The variations described by

Lanz (138) include the following:

Among the most common patterns is the usual form and

course where the recurrent motor branch exits from the radial

aspect of the median nerve trunk just distal to the transverse

carpal ligament. This is a relatively safe pattern when performing carpal tunnel release because the recurrent motor

branch courses distal to the area of ligament transection.

Although this is the most common pattern of the recurrent

branch of the median nerve, Amadio found an overall 19%

incidence of variations in a study of 275 patients undergoing

carpal tunnel release (70) (see Table 3.1).

A Transligamentous Passage of the Recurrent Motor

Branch

A transligamentous (transretinacular) passage of the recurrent motor branch is a pattern where the recurrent branch

penetrates the transverse carpal ligament, usually in the distal half. This pattern is the second most common type, and

is potentially problematic because the motor branch may

travel in the ligament and is at risk for injury when the ligament is transected during carpal tunnel release (70,134,

135–139). The relatively high frequency of this transligamentous course of the recurrent branch has been well documented by several authors (134,135–139). Spinner

describes a separate tunnel for the nerve in its transligamentous course, where the nerve passes through the transverse retinaculum 2 to 6 mm from the distal margin of the

ligament (6,137,149). The length of the transligamentous

tunnel is 15 to 30 mm (134,135–139). When the transligamentous pattern is encountered during carpal tunnel

release, the nerve branch should be decompressed throughout its tunnel through the ligament.

Subligamentous Origin of the Recurrent Motor Branch

Subligamentous origin of the recurrent motor branch is a

pattern where the recurrent motor branch leaves the median

nerve trunk more proximally, within the carpal tunnel, but

continues in a distal direction to the distal edge of the transverse carpal ligament and curves back to the thenar muscles

in a retrograde fashion. The nerve branch does not penetrate the transverse carpal ligament.

Multiple Recurrent Motor Branches

Multiple recurrent motor branches is a pattern where the

nerves originate from the median nerve trunk in the more

common site just distal to the transverse carpal ligament,

but more than one branch is present (70,138,139,143,

146,147). This anomaly was found in 4% of patients

undergoing carpal tunnel release (70). When there are

multiple branches present, it is not uncommon for some

branches to pass through the ligament (70,143,147). The

nerve branches also may course either in their usual recurrent course or through different aberrant paths. On occasion, an accessory motor branch can arise in the distal

forearm or proximal wrist. It can pass through the carpal

tunnel or through the flexor retinaculum (138,146–148).

In Amadio’s study, when multiple recurrent branches were

present, approximately half of the branches were found to

pass through the retinaculum (70). Mumford et al. found

2 branches in 1 of 10 dissections; one of the branches

passed through the retinaculum (134). An accessory

thenar nerve arising from the first common digital nerve

or the radial proper digital nerve was noted and reported

by Mumford et al. These findings were seen in 15 of 20

hands dissected (134). The accessory thenar nerve was the

only median nerve supply to the flexor pollicis brevis in

eight specimens.

196 Systems Anatomy

TABLE 3.1. CLASSIFICATION OF MEDIAN NERVE

ANOMALIES IN THE CARPAL TUNNEL

High division

Open branching

Closed loop

Motor branch

Transretinacular

Multiple

Multiple and transretinacular

Palmar cutaneous branch

Transretinacular

Multiple

Multiple and transretinacular

Median–ulnar sensory ramus

(Arising on median nerve proximal to superficial arch)

Unclassified

From Amadio PC. Anatomic variation of the median nerve w

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