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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178 Systems Anatomy
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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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