TRIPASS XR تري باس

 The proponents of this concept believe in recording the edentulous

ridge in functional form either by using functional impression

technique or by functional reline method.

• During the functional impression, the mucosa covering the

edentulous ridge gets displaced to its functional form.

• Direct retainers and retentive clasp designed with minimum

retention and number of direct retainers is also minimum.

• Denture fabricated with functional impression compresses the soft

tissues even at rest. This can lead to excessive residual ridge

resorption (Fig. 17-5).

• When the partial dentures are at rest, the artificial teeth will be

positioned slightly above the plane of occlusion because of the

rebound of the compressed tissues.

FIGURE 17-5 Dentures made with functional impression

compress the soft tissues even in rest state.

Advantages

• Functional loading has a physiological stimulating effect on the

supporting tissues.

• For proper vertical movement of the partial dentures from rest to

functional position, this direct retainer should be minimum in

number and retention.

• Simplicity of designing and fabrication results in lightweight

prosthesis.

• Less forces are transmitted on the abutment tooth.

Disadvantages

• There are greater chances of premature contacts.

• It is difficult to produce effective indirect retention.

• Greater forces are transmitted to the edentulous ridge.

• There are chances of premature contact, as the teeth will be

positioned slightly above the plane of occlusion.

Broad stress distribution.

This concept advocates wider distribution of stresses by the prosthesis

using additional rests, clasp assembly and broad denture base. The

partial dentures feature maximum coverage of the teeth and the soft

tissues (Fig. 17-6).

FIGURE 17-6 Removable partial denture made with broad

stress distribution concept.

Advantages

• Less concentration of stress

• Greater resistance to lateral stresses

• Less expensive in fabrication

• Increased horizontal stabilization

Disadvantages

• Chances of bulky prosthesis

• Because of wider coverage, difficult to maintain oral hygiene

Factors which influence the amount of stresses on the abutment

tooth.

Factors which influence the amount of stresses on the abutment tooth

are:

Length of edentulous span: Greater the length of the edentulous span,

greater will be the length of the denture base and thus more forces

will be transmitted to the abutment tooth.

Form of residual ridge: Broad, well-formed ridges provide better

support and stability to the prosthesis than thin knife-edged ridges.

Also, firmly bound healthy keratinized mucosa is capable of

resisting the functional stresses better than the loose, atrophic and

flabby tissues.

Clasp quality: More flexible the clasp, greater lateral and vertical

forces will be transmitted to the residual ridge. More flexibility of

the clasp, lesser will be force transmitted to the abutment.

Length of the clasp: Flexibility of the curved clasp is better than the

straight one. Flexibility of the clasp is directly proportional to the

length of the clasp.

Clasp design

• Passively fitting clasp exerts less stress on the abutment tooth than

the active one.

• The framework should be completely seated in order to ensure that

the retentive clasp will be passive.

• Disclosing wax can be used to seat the framework completely.

Clasp material: Greater the rigidity of the clasp material, greater will

be the stress transmitted to the abutment.

• Cast clasp will exert more stress on the abutment

tooth than the gold clasp.

• As the cast clasp has greater rigidity in comparison

to the gold clasp, it should be made of smaller

diameter.

Tooth surface: Intact enamel offers less frictional movement to the

clasp arm than full veneer crown or restoration.

• Greater stress will be created on the tooth restored

with gold or cast metal rather than with tooth

enamel.

Occlusion: Deflective or disharmonious occlusal contact and the type

of opposing occlusion influence the amount of force on the

abutment tooth.

• Type of the occlusion and the area of the denture

base determine the amount of stress transmitted

onto the abutment tooth and the residual ridge.

• Partial denture opposing complete denture will be

subjected to less occlusal stress than if opposed by

natural occlusion.

• The occlusal load should be applied in the centre on

the residual ridge, both anteroposteriorly and

buccolingually. Usually, the second premolar and

the first molar are the best areas to bear the

masticatory load.

Design considerations in controlling stress in an RPD.

The following are the design considerations which are important in

controlling stresses in an RPD:

Direct retention: The retentive clasp arm transmits most of the

leverages forces to the abutment tooth. Clasp retention should be

kept to the minimum but without compromising on the retention of

the prosthesis.

• Retention is enhanced by accurately fitting and

maximal coverage denture base.

• Retention by frictional control is enhanced by

creating guiding planes on as many teeth as

possible.

• Properly extended partial denture can aid in better

neuromuscular control by the patient, therefore,

contributing in retention of the prosthesis.

Clasp position: The position of the clasp in relation to the height of

contour influences the amount of stress on the partial denture. The

number of clasps used is governed by the classification.

Quadrilateral configuration: It is indicated in Kennedy class III cases

with a modification space on the opposite side. A retentive clasp is

placed on all abutment teeth adjacent to the edentulous space (Fig.

17-7).

Tripod configuration: It is indicated in Kennedy class II cases with a

modification space on the opposite side. All the abutment teeth on

both sides are clasped to result in tripod configuration (Fig. 17-8).

Bilateral configuration: It is indicated in Kennedy class I cases

without any modification space. The retentive clasp is located on

abutment on both sides adjacent to the edentulous space (Fig. 17-9).

Clasp design: Circumferential clasp originating from the distal

occlusal rest and engaging into the mesiobuccal undercut should be

avoided in distal extension cases, as it produces harmful leverage

forces on the abutment tooth. Reverse circlet clasp can be used.

• Bar clasp is indicated in distal extension cases when

distal undercut is located. It should never be used

when there is mesiobuccal undercut.

• It is advantageous to place the mesial rest more

anteriorly than the distal rest because of better

mechanical advantage.

• T clasp with disto-occlusal rest and rigid

circumferential reciprocating clasp is thought to

produce least stress on the abutment tooth.

Splinting: Splinting of two or more teeth helps in distributing the

stress over a larger area of support, as it increases the periodontal

ligament attachment area.

• For splint to stabilize teeth in the arch in the

buccolingual direction, it should extend across the

arch curvature.

Indirect retention: It is essential in distal extension cases.

• It resists the rotation of the prosthesis.

• It is usually located anteriorly and perpendicular to

the fulcrum line.

• It should be located as far anteriorly as possible to

provide long lever arm. Also, the indirect retainers

should be located in definite rest seat in order to

transmit the forces along its long axis.

• It also contributes to the stability and support of the

denture.

Occlusion: Harmonious occlusion minimizes the stress on the

abutment teeth and the residual ridges.

• The buccolingual width of the artificial teeth should

be reduced in order to minimize stress on the

abutment and edentulous ridge.

• Steep cuspal inclines should be avoided.

• Posterior teeth should have sharp cutting surfaces

and sluiceways.

Denture Bases: The denture base should be extended over wider ridge

area in order to distribute the stresses.

• Denture base flanges should be made as long as

possible in order to stabilize the denture against

horizontal stresses.

• Denture base should be accurate and closely fitting,

as this will ultimately reduce the stresses

transmitted to the abutment teeth.

• Selective pressure impression technique is useful in

reducing stresses on the ridge and the abutment.

Major connector: It should be rigid.

• In mandibular arch, lingual plate design can

effectively support periodontally compromised

teeth and can distribute stresses to the remaining

teeth, if supported by the rests at the distal

abutments.

• In maxillary arch, the complete palatal design

contributes to stability, support and retention of the

prosthesis. This helps in distributing the functional

forces over wide surface area, thereby reducing the

amount of forces on the abutment and the ridge.

Minor connector: It joins the clasp assembly to the major connector

and the guide planes on the abutment tooth.

• It provides horizontal stability to the partial denture

against the lateral forces.

• Because of its contact with abutment tooth, it

stabilizes the tooth against lateral stresses.

• In order to minimize the stresses on the abutment

teeth, guide planes should be prepared on

additional teeth.

Rests: It helps in directing the stresses along the long axis of the teeth.

• It provides support to the prosthesis.

• It should form an acute angle with the

perpendicular line passing through the long axis of

the tooth.

• In distal extension cases (class I and class II), rest

seat should be saucer-shaped to allow freedom of

movement of the rest within the rest seat. The

action is similar to the ball and socket joint.

• More the number of teeth with rest seat, lesser will

be the stress transmitted to each abutment tooth.

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FIGURE 17-8 Tripodal configuration in Kennedy class II

modification I.

FIGURE 17-9 Bilateral configuration in Kennedy class I

partial dentures.

Design considerations in distal extension partial

denture

Design consideration in a distal extension cases (Kennedy class I and

class II).

Direct retention

• Properly contoured and closely fitting denture base is important to

restore function and appearance.

• Accurate fitting of the framework against the guide planes.

Clasps

• Simplest type of clasp design should be selected.

• Selected clasp should possess a good stabilizing quality and should

be passive in nature.

• These should be strategically positioned so that these can best

control the stress.

In Kennedy class I situation, two retentive clasp arms are required,

one each on the terminal abutment.

• In case retentive undercut is located in the distobuccal region, a bar

type clasp should be used.

• If the retentive undercut is located in the mesiobuccal region,

wrought wire clasp should be used.

• The reciprocal or the bracing arm should always be rigid.

In Kennedy class II situation, the prosthesis should have three

retentive clasps.

• On distal extension side, the terminal abutment has one retentive

clasp.

• On the tooth supported side, one clasp is placed as far anterior and

one clasp is placed as far posterior.

• If modification space is present, retentive clasp is placed on teeth,

both anterior and posterior to the edentulous space.

Rests

• Rests should be placed next to the edentulous space.

• Rest seat should be prepared so that the functional forces are

directed along the long axis of the tooth.

• Teeth which can provide maximum support should be selected.

• Rest seat should be saucer-shaped which should not have any sharp

angles and ledges.

• Rest should freely move in the rest seat in order to release the

stresses which otherwise would have been transferred to the

abutment tooth.

Indirect retention

• It should be located as far anterior to the fulcrum line as possible.

• Two indirect retainers are indicated in class I situation and usually

one is indicated in class II situation.

• It should be prepared with positive rest seats which can direct the

forces along the long axis of the tooth.

• Lingual plate can act as an indirect retainer, if supported at both

ends with rest seats.

Major connectors

• It should always be rigid and should not impinge on the gingival

tissue.

• In maxillary major connector, support should be derived from the

hard palate.

• In mandibular major connector, extensions into the lingual surface

of the teeth should be used in order to increase rigidity and

distribute the lateral stresses.

Minor connectors.

These should be rigid and should be positioned such that they

increase the comfort and cleanliness.

Occlusion

• A harmonious occlusion is desired without any interfering contact.

• Artificial teeth should be arranged such that they minimize the

stresses produced by the prosthesis.

• To minimize the stress, fewer teeth with reduced buccolingual

width are selected:

• Teeth with sharp cutting edges and sluiceways are

selected.

• For better mechanical advantage, the teeth should

be positioned over the ridge.

• The centric relation should be coincided with the

centric occlusion.

Denture base

• Denture base should have broad coverage to distribute the stresses

over wider area.

• The tissues are recorded in functional form using a selective

pressure technique.

• The form and contour of the denture should be highly polished.

Design consideration in tooth-supported partial

denture

Tooth-supported partial denture is included in Kennedy class III

situation. The components of the partial denture should be designed

after surveying the master cast on the surveyor.

Direct retainers

• The location of the retentive undercut is not critical as in distal

extension cases.

• The abutment teeth are not subjected to harmful stresses during

function.

Clasp design

• Quadrilateral configuration of the clasps should be ideal.

• Simplest type of clasp design should be selected.

• The reciprocal arm should always be rigid.

Rests

• Rests are usually placed next to the edentulous ridge.

• The rests provide support to the prosthesis.

Indirect retainers

• These are usually not required.

• If posterior abutment is not clasped, the requirements are similar to

the distal extension cases.

Major, minor connector and occlusion.

These should be rigid and design consideration similar to distal

extension cases.

Denture base

• Functional impression is not needed here.

• Extension of the denture base depends on factors such as comfort

and aesthetics of the patient.

In class IV situation, the aesthetic need may necessitate the

placement of the teeth more anterior to the crest of the ridge. This may

result in transmission of harmful leverage forces on the abutment

teeth. In order to minimize the stresses on the abutment:

• As many teeth as possible should be retained.

• Labial alveolar process should be preserved.

• The edentulous span should be small.

Quadrilateral configuration of the clasp system is desirable. The

broad palatal type of major connector is preferred. In case of extensive

edentulous space, functional impression is indicated.

Biomechanical problems associated with

extension base RPDs and their remedies

Biomechanical problems in distal extension partial dentures

• The distal extension partial denture can be subjected to a number of

movements on functional or parafunctional loading.

• The movements are dependent on the quality of supporting

structures, accuracy and extent of the denture base and the

magnitude, direction, duration and frequency of the functional

force.

• Possible movements of the partial dentures can occur:

• About an axis through the most posterior abutment

teeth.

• Around the longitudinal axis formed by the crest of

the residual ridge.

• Around the vertical axis located near the centre of

the arch.

Rotation about the axis through the most posterior abutments

• On the application of the functional load, the rotation of the distal

extension partial denture occurs around the line joining the occlusal

rests which is called the axis of rotation or the fulcrum line.

• The amount of rotation of the denture base depends on the

resiliency of the mucosa covering the residual alveolar ridge and the

accuracy of the adaptation of the denture base.

• Movement of the denture base in opposite direction is resisted by

the action of the retentive clasp arm of the terminal abutment and

the indirect retainer.

• The indirect retainer should be placed as far as possible from the

distal extension base to afford the best possible mechanical

advantage.

• The denture base should cover as large area as possible in order to

reduce the load per unit area.

• The occlusal rests should be placed mesially on the abutment tooth

in order to move the arc of movement of the saddle more

perpendicular to the mucosa.

• Effect of clasping is that more the rigidity of the clasp, greater the

leverage on the tooth and less the load on the alveolar ridge;

whereas, more the flexibility of the clasp, less leverage on the tooth

and more load on the ridge.

Rotation around the longitudinal axis formed by the crest of the

ridge

• Fulcrum line extends posteriorly distal to the terminal abutment.

• It passes along the crest of the ridge to its posterior extent on the

same side.

• In class I situation, there are two fulcrum lines around which lateral

movement of the partial denture can occur.

• The lateral movement of the extension base can occur due to the

inclined plane of the cusp of the posterior teeth.

• The steeper the cusp, more will be the lateral load.

• The anatomy of the residual ridge will play a significant role in

resisting lateral movement of the denture base.

• Flat ridge with movable submucosa will offer less resistance to

lateral movement as compared to well-formed ridge with firmly

bound mucosa.

• Cast clasp transmits more rotational or lateral force on the abutment

tooth in comparison to the wrought clasp.

• However, the wrought clasp will convey more lateral stress on the

residual ridge in comparison to the cast clasp.

• Lateral loads are also exerted on the denture by the adjacent facial

and lingual musculature during swallowing.

Rotation around the vertical axis located near the centre of the

arch

• The rotation of the prosthesis is along the vertical axis located near

the centre of the arch.

• When vertical forces are acting on the denture base, most of the

periodontal ligament fibres are activated.

• If lateral forces are applied to the denture base, only part of the

periodontal fibres will be activated and this will result in harmful

forces on the abutment.

• This rotation is resisted by the stabilizing component of the partial

denture such as the reciprocal clasp and the minor connectors.

• Stability component on one side of the arch acts to stabilize the

partial denture against the horizontal forces applied on the opposite

side.

• To minimize the movement, the arms of the three-arm clasp should

brace the tooth completely on its buccal and lingual surfaces.

• The minor connector should be made rigid.

• The magnitude of the stress on the abutment will be greater, if the

clasp is made of cast alloy than with wrought alloy.

• The magnitude of stress will be less on the ridge posteriorly, if the

clasp is made of cast alloy than with wrought alloy.

Methods of stress control in RPD

Methods of controlling stress in RPD are:

(i) Reduce the load on the abutment and the ridge

(ii) Distribution of load between the teeth and the residual ridge

• By varying the connector between the clasp and

saddle

• Stress breaking

• Combination of rigid connection and bar clasp

• Combination of rigid connection and Akers’ clasp

• The disjunct denture

• By anterior placement of the occlusal rest

• RPI system

• Balance of the force system

• By mucocompression

(iii) Distributing the load widely

• Over more than one abutment tooth on each side

• Over the maximal area of the edentulous ridge

Reducing load on abutment and the ridge

• By reducing the buccolingual width of the teeth

• By reducing the number of teeth on the denture base particularly the

distal most tooth

• Broad coverage of the denture base

• This ensures decreased load on the ridge and abutment teeth,

thereby increasing the chewing efficiency.

Distribution of load between the teeth and the

ridge

Distribution of load between the teeth and the ridge is done by

varying the connection between the clasp and the denture base.

Stress breaking: It provides a certain degree of movement between the

clasp unit and the denture base.

• Most of the load here is borne by the ridge rather

than the abutment tooth

Combination of rigid connection and the bar clasp: The resiliency of

the portion of the bar clasp which contacts the abutment tooth

depends upon its length, cross-section and the type of alloy used.

• Greater the resiliency of the clasp, lesser will be the

horizontal and lateral stresses borne by the

abutment.

Combination of rigid connection and the Akers’ clasp: Here there is

more load on the abutment tooth and less on the ridge.

• To reduce the stress, saucer-shaped rest seat should

be prepared in distal extension cases.

Disjunct denture: In patient with severe gingival recession and

periodontically weakened teeth, a two-part denture called disjunct

denture is constructed.

• The denture consists separately of tooth-borne and

mucosa-borne segments which act independently

of each other on the supporting tissues.

By anterior placement of the occlusal rest

• By doing so, the stresses on the saddle is changed from class I lever

system to the favourable class II lever system.

• This ensures even distribution of stresses on the ridge and less stress

on the abutment.

• This principle is utilized in RPI system and the balance of force

system.

Distribution of load

• Wider load distribution over the teeth takes place by anterior

placement of the rest on the abutment.

• Broad coverage of the denture base reduces the load over the

edentulous ridge.

Stress breaker

Definition

Stress breaker is defined as ‘a device or system that relieves specific dental

structures of part or all of the occlusal forces and redirects those forces to

other bearing structures or regions’. (GPT 8th Ed)

Stress breaker is a device which allows movement between the

denture base and the clasp assembly.

Role of stress breaker in RPD

• Role of stress breaker is to distribute the load between the ridge and

the teeth.

• Both the vertical and horizontal components of the forces are

favourably distributed.

• The vertical component of the force is to the greater extent

distributed to the edentulous ridge and to lesser extent to the

abutment tooth.

• The horizontal or lateral forces acting on the stress breaker sadly are

greatly distributed to the edentulous ridge and not to the abutment

tooth.

• The magnitude of harmful lateral torquing forces on the abutment

tooth is greatly reduced.

• Since the edentulous ridge bears greater amount of horizontal and

lateral forces, it is more likely to show signs of greater resorption.

• Stress breakers are usually indicated for periodontally compromised

abutment teeth, where it is desirable to distribute the stresses over

the edentulous ridge rather than the teeth.

• Poorer the condition of the teeth, more flexible connection between

the ridge and tooth is desired and vice versa.

Advantages

• Horizontal or lateral forces acting on the abutment teeth are

minimized.

• It is possible to seek a balance of stress between the abutment and

the ridge by proper selection of the flexible connector.

• Intermittent pressure on the denture base massages the mucosa.

• Splinting of the weak teeth by the denture is possible despite the

movement of the distal extension base.

Disadvantages

• Fabrication is complex.

• It is costly.

• Concentration of horizontal and vertical stresses leads to increased

ridge resorption.

• Effectiveness of the indirect retainer is reduced or eliminated.

• Repair and maintenance are difficult.

• There are chances of wear of the attachments.

• The spaces between the components can attract food lodgement.

Precision attachments

Definition

Precision attachments are defined as ‘an interlocking device, one

component of which is fixed to an abutment or abutments, and the other is

integrated into a removable dental prosthesis in order to stabilize and/or

retain it’. (GPT 8th Ed)

Classification of the precision attachments

According to Alan A. Grant and O.A. Wesley:

On the basis of site of attachment to the abutment tooth:

(i) Class 1: Coronal attachments – divided into extracoronal

attachments and intracoronal attachments

(ii) Class 2: Root-face attachments – divided into stud attachments and

bar attachments

According to H.W. Prieskel (1979)

On the basis of shape of attachments:

(i) Intracoronal attachments – frictional type

• Mechanical lock

• Semiprecision attachment (custom-made)

(ii) Extracoronal attachments – projection units

• Connectors

• Combined units

(iii) Stud attachments

(iv) Bar attachments – bar joints

• Bar units

(v) Auxiliary attachments – screw units

• Friction devices

• Bolts, hinged flange

Function of precision attachments

• Labial or buccal clasp arm is eliminated.

• These are aesthetically superior.

• These direct the forces along long axis of the teeth.

• These can provide effective reciprocation.

Indications

• Tooth supported partial dentures

• To break stress in distal extension cases

• To retain hybrid dentures

• To stabilize unilateral saddles

• Use in overdentures

• When few remaining teeth are present

• When splinting of teeth is indicated to aid in their stabilization

• For superior aesthetics, since there is elimination of the clasp.

Contraindications

• In case of teeth which are narrow buccolingually and have short

clinical crown.

• In a patient with poor oral hygiene.

• In a patient with high caries index.

• In case of teeth with large pulp horns.

• In case of decreased patient dexterity.

• In case of inadequate space for the attachment.

• In case of compromised restorative and endodontic treatment.

Advantages

• It improves aesthetics.

• It has better mechanical advantage, as it directs the forces along the

long axis of the tooth.

• Force applications are closer to the fulcrum line.

• In distal extension cases, there is decreased stress to the abutment

tooth.

• In comparison to the clasp, the attachments are less bulky and are

more aesthetic and lead to less food stagnation.

Disadvantages

• It has complexities of design, procedures for fabrication and clinical

treatment.

• Minimum 4–6 mm of occlusogingival abutment height is required to

incorporate attachment without overcontouring.

• Anatomy of the tooth – limited faciolingual tooth width.

• It is expensive.

• Wearing of attachment components is a disadvantage.

Shortened dental arch concept

The primary aim of restorative dentistry is to preserve the complete

dental arch. It may not be possible or affordable for majority of the

elderly patients; therefore, the concept of shortened dental arch (SDA)

can be considered.

Definition: SDA is an arch with reduction of teeth starting posteriorly,

mostly in the permanent molars.

This concept was first developed by A.F. Kayser (1981). The concept

suggests that the minimum number of occluding pairs of teeth

(anterior and the premolars) are required to provide satisfactory oral

functional demands of the patient.

The number of occluding pairs can vary according to age and other

factors illustrated in Table 17-1.

TABLE 17-1

FACTORS ON WHICH OCCLUDING PAIRS VARY

Age Functional Level Occluding Pairs

20–50 I – Optimal 12

40–80 II – Suboptimal 10 (SDA)

70–100 III – Minimal 8 (ESDA) extreme

Note: ESDA, extreme shortened dental arch; SDA, shortened dental arch.

Prognosis of SDA depends on:

• Excellent oral hygiene

• Spatial relationship between the maxillary and mandibular arch

• Age of the patient

• Periodontal condition of the anterior teeth

• Adaptive capacity of the TMJ

• Occlusal load

Indications

• Progressive caries/periodontal disease confined mainly to the

molars

• Periodontal condition of the anterior and premolars favourable

• Financial and other limitation to the restoration of dental arch

Contraindications

• Class III and severe class II skeletal relationship

• Alveolar support of remaining teeth is markedly reduced

• Parafunctional habits

• TMJ disorders

• Excessive or abnormal wear of existing teeth

Advantages

• It results in simplification of oral hygiene maintenance.

• It results in enhanced prognosis of the remaining teeth.

Disadvantages

• Decreased occlusal table.

• No support from the edentulous ridge.

Key Facts

• Gillett Bridge consists of a partial denture which utilizes the Gillett

clasp system. It is composed of an occlusal rest which is notched

deep into the occlusal axial surface with a gingivally placed groove

and a circumferential clasp for retention. The occlusal rest is

custom-made with a cast restoration.

• Angle of gingival convergence is located apical to the height of

contour on the abutment tooth.

• Every denture is an all acrylic type of dentures which restores

multiple edentulous spaces in the maxillary arch. There is minimal

contact between the acrylic teeth and the abutment teeth to reduce

the lateral stresses. The posterior most teeth are bounded by the

wrought clasp which aids in retention and prevents distal tipping of

the posterior teeth.

• Embrasure clasp is best used in Kennedy class II cases.

• Dr A.J. Fortunati was first to use dental surveyor.

• Surveyor is essentially a parallelometer which is used to determine

the relative parallelism of two or more surfaces of the teeth or other

parts of the cast.