Biomechanical principles in Biomechanical principles in Removable Partial DenturesRemovable Partial Dentures

ContentsContents

IntroductionIntroduction

DefinitionsDefinitions

Review of literatureReview of literature

Mechanical principles applicable in removable Mechanical principles applicable in removable Prosthodontics.Prosthodontics.

Stress consideration in a partial dentureStress consideration in a partial denture

Forces acting on partial dentureForces acting on partial denture

Biomechanical consideration of individual Biomechanical consideration of individual componentcomponent

Factors influencing magnitude of stressFactors influencing magnitude of stress

Controlling stress by design Controlling stress by design considerationconsideration

SummarySummary

ConclusionConclusion

ReferencesReferences

Biomechanical consideration Biomechanical consideration of individual componentof individual component

RESTREST

FUNCTIONSFUNCTIONS• Provide vertical support for the partial denture.Provide vertical support for the partial denture.

• Maintains components in planned positions.Maintains components in planned positions.

• Maintains established occlusal relations by Maintains established occlusal relations by preventing settling of the denture base.preventing settling of the denture base.

• Prevents impingement of soft tissue.Prevents impingement of soft tissue.

• Directs and distributes occlusal loads to Directs and distributes occlusal loads to abutment teeth.abutment teeth.

Relation between rest and abutment should be such that forces should be directed apically down along the long axis of the tooth

Stress absorbed by fibers of PDLwithout getting damaged

Rest Rest

Primary restPrimary rest part of direct retainer unit part of direct retainer unit

Secondary / Auxiliary restSecondary / Auxiliary rest

Primary rest:Primary rest:prevent vertical movement of denture base prevent vertical movement of denture base

towards tissuetowards tissueTransmit horizontal forces applied during Transmit horizontal forces applied during

function to abutment teethfunction to abutment teeth

Occlusal RestOcclusal Rest

If angle greater than 90 degrees

Forces not along long axis

Slippage of prosthesis away from the abutment

Orthodontic like forces

Abutment severely tiltedAbutment severely tilted

Extended occlusal rest seatExtended occlusal rest seat

Decrease further tipping Decrease further tipping

To ensure that To ensure that

the forces are the forces are

directed along directed along

the long axis of the tooth.the long axis of the tooth.

Internal occlusal restInternal occlusal rest

Only in tooth supported partial dentureOnly in tooth supported partial denture

Provides both support Provides both support

and horizontal stabilityand horizontal stability

Adv:Adv: Location of rest moreLocation of rest more

favorable in relation tofavorable in relation to

tipping axis of abutmenttipping axis of abutment

No clasp required buccallyNo clasp required buccally

Cingulum rest Vs Incisal restCingulum rest Vs Incisal rest

Cingulum rest nearer to center of rotation

less tipping of tooth

Incisal rest

Longer minor connector required

magnifies the forces being transferred to the abutment tooth

Positioning of occlusal rest in Distal Positioning of occlusal rest in Distal extension base situations extension base situations (JPD 1977: (JPD 1977:

38;261)38;261)

Moves the point of rotation anteriorlyTransmits forces in a more vertical direction

Kratochvil et al (JPD 1963:13;114)Kratochvil et al (JPD 1963:13;114) Suggested using Suggested using mesial occlusal restmesial occlusal rest so that so that

the force delivered to the mesial aspect of the the force delivered to the mesial aspect of the abutment tooth would abutment tooth would tend to tip that tooth tend to tip that tooth forward maintaining a tight contactforward maintaining a tight contact with the with the tooth immediately anteriortooth immediately anterior to it and gaining to it and gaining stabilization and support from the remaining stabilization and support from the remaining anterior teeth.anterior teeth.

They suggested that occlusal rest be moved They suggested that occlusal rest be moved anterior to better use the residual ridge for anterior to better use the residual ridge for support.support.

John W. Mc cartney (JPD 1980;43:15)John W. Mc cartney (JPD 1980;43:15) Did a study on the effect of location of Did a study on the effect of location of

occlusal rest on abutment tooth occlusal rest on abutment tooth movement and concluded that the mesial movement and concluded that the mesial rest placement caused less abutment rest placement caused less abutment movement than distal rest placement.movement than distal rest placement.

Stewart et alStewart et al favored favored distal restdistal rest for distal for distal extension prosthesis primarily to prevent extension prosthesis primarily to prevent food impaction between the tooth and food impaction between the tooth and denture.denture.

ADVANTAGES OF MESIAL REST ADVANTAGES OF MESIAL REST FOR DEB RPDFOR DEB RPD

1.1. Decreases stress on abutment teethDecreases stress on abutment teeth

Distal occlusal restGingival extremity of denture base adjacent to posterior abutment tends to move in an arc towards the tooth

Little or no support from the mucosa near the tooth

Tissue adjacent to tooth may be compressed between denture base and abutment causing tissue ischemia

Mesial restproduces more perpendicular transmission of occlusal forces

Increased dependence of the denture on ridge for support

Force more vertical near abutment tooth

Decreased possibilityof marginal tissue ischemia

2.Mesial rest will apply a mesial force to the abutment tooth, movingIt towards the adjacent tooth.

3.3. Mesially placed rest helps to provide Mesially placed rest helps to provide indirect retention of the denture indirect retention of the denture (Zach et (Zach et al JPD 1975;33;32-35)al JPD 1975;33;32-35)

4.4. A mesial rest provides additional A mesial rest provides additional guiding planes to limit the possible path guiding planes to limit the possible path of withdrawl of withdrawl (Zach et al JPD (Zach et al JPD 1975;33;32-35)1975;33;32-35)

5.5. Mesial rest and its accompanying Mesial rest and its accompanying minor connector can be instrumental in minor connector can be instrumental in preventing distal displacement of RPD. preventing distal displacement of RPD. (Zach et al JPD 1975;33;32-35)(Zach et al JPD 1975;33;32-35)

SITUATIONS WHERE A MESIAL SITUATIONS WHERE A MESIAL REST IS CONTRAINDICATEDREST IS CONTRAINDICATED

1.If the space between the distal abutment and its adjacent tooth does not allow adequate space for the minor connector to rotate occlusal force on the denture base will cause wedging action.

2. If the only occlusal contact is against the rest,

the opposing tooth will move into the space reserved for the rest when partial denture is not in patients mouth.

Returning the prosthesis to place will produce a transient occlusal prematurity.

DIRECT RETAINERDIRECT RETAINER

Any unit of removable dental prosthesis Any unit of removable dental prosthesis that engages an abutment tooth in such a that engages an abutment tooth in such a manner as to resist displacement of the manner as to resist displacement of the prosthesis away from basal seat tissue.prosthesis away from basal seat tissue.

Extra coronal retainers (clasps) operates on Extra coronal retainers (clasps) operates on the principle of the principle of resistance of metal to resistance of metal to

deformdeform

ProtheroProthero Cone theoryCone theory as as basis of clasp retentionbasis of clasp retention

Described shape of crowns of Described shape of crowns of premolar and molar teeth as 2 premolar and molar teeth as 2 cones sharing a common cones sharing a common base . Line where they base . Line where they meetmeet height of contourheight of contour. . (represents the greatest (represents the greatest bulge).bulge).

SuprabulgeSuprabulge InfrabulgeInfrabulge

Height of contour of tooth changes as the Height of contour of tooth changes as the vertical position of tooth changes.vertical position of tooth changes.

Surface is retentive if it is cervical to its height of Surface is retentive if it is cervical to its height of contour.contour.

Clasp tip in infrabulge areaClasp tip in infrabulge area resist the resist the movement in occlusal direction because to movement in occlusal direction because to release from tooth release from tooth it has to undergo it has to undergo deformation.deformation.

Degree of resistance to deformation Degree of resistance to deformation determines the amount of clasp retention.determines the amount of clasp retention.

Retention varied byRetention varied by:: depth of undercutdepth of undercut Flexibility of clasp arm positioned in Flexibility of clasp arm positioned in

undercutundercut

Areas used for retention must be undercut in relation to partial dentures path of insertion and withdrawl

Requirements of clasp design:Requirements of clasp design:RetentionRetentionSupportSupportStabilityStabilityReciprocationReciprocationEncirclementEncirclementpassivitypassivity

1. Retention1. Retention

Retentive arm Retentive arm provides retentionprovides retention

Three parts:Three parts:Terminal thirdTerminal thirdMiddle thirdMiddle thirdProximal thirdProximal third

Retention depends on:Retention depends on:Flexibility of clasp armFlexibility of clasp armDepth that the retentive terminal extends Depth that the retentive terminal extends

into the undercutinto the undercutAmount of clasp arm that extends below Amount of clasp arm that extends below

the height of contourthe height of contour

Retentive undercut has three Retentive undercut has three dimensions:dimensions:

A)A) B-L depth of undercut:B-L depth of undercut:

measured by Undercut gaugemeasured by Undercut gauge (thousands of an inch)(thousands of an inch)

Distance between height of contour Distance between height of contour and a given B-L measurement and a given B-L measurement depends upon the angle formed by depends upon the angle formed by infrabulge and this vertical line.infrabulge and this vertical line.

Angle is called Angle is called angle of cervical angle of cervical convergence.convergence.

Less sharp this angle, greater distance Less sharp this angle, greater distance needed between the height of contour needed between the height of contour and the retentive terminal to achieve and the retentive terminal to achieve the same amount of retention.the same amount of retention.

Clasp flexibility affects its placement

Cast chrome 0.01 inch

Gold 0.015 inch

Cast wrought metal 0.02 inch

B)B) another another dimensiondimension distance distance between survey line between survey line and tip of the and tip of the retentive claspretentive clasp

Effects clasp arm Effects clasp arm lengthlength influences influences flexibility of claspflexibility of clasp

C)C) third dimension third dimension M-D length of clasp M-D length of clasp arm below the height arm below the height of contourof contour

Longer this Longer this measurementmeasurement more more flexible the claspflexible the clasp

Flexibility of clasp depends on:Flexibility of clasp depends on:LengthLengthDiameterDiameterTaperTaperCross sectional formCross sectional formMaterial Material

Length of the claspLength of the clasp

Increase lengthIncrease length

increased flexibilityincreased flexibility

Flexure directly proportional Flexure directly proportional to (length)to (length)

Clasp arm Clasp arm not to cover the not to cover the tooth surface in straight tooth surface in straight

line but be curved with tip line but be curved with tip facing occlusalyfacing occlusaly

3

DiameterDiameter

Flexibility inversely Flexibility inversely proportional to proportional to diameter of clasp diameter of clasp armarm

Uniform taperUniform taper

Essential in both Essential in both width and thickness width and thickness for retentive arm of for retentive arm of clasp clasp

Clasp be half as thick Clasp be half as thick at tip as at origin. at tip as at origin.

Cross sectional form:Cross sectional form:

Round clasp greater flexibility than a half Round clasp greater flexibility than a half round clasp with same diameterround clasp with same diameter

Round clasp flex in all spatial planes, while Round clasp flex in all spatial planes, while half round only in one planehalf round only in one plane

Material:Material: Chrome alloysChrome alloys higher modulus of elasticity than gold higher modulus of elasticity than gold

alloysalloys less flexible. less flexible. Therefore smaller cross sectional form of the clasp and Therefore smaller cross sectional form of the clasp and

less depth of retentive undercut must be used for chrome less depth of retentive undercut must be used for chrome alloyalloy

Wrought wireWrought wire because because

of internal structure i.e.of internal structure i.e.

longitudinal structure as longitudinal structure as

compared to grain compared to grain

structure of cast alloystructure of cast alloy

greater flexibilitygreater flexibility

2. Support2. Support Prime support unit of clasp assembly is the rest.Prime support unit of clasp assembly is the rest.

3. Stability3. Stability All components of clasp except retentive All components of clasp except retentive

terminal contributes to stability.terminal contributes to stability. Cast circumferential claspCast circumferential clasp greatest stability greatest stability

because its shoulder is rigid and aids in because its shoulder is rigid and aids in stabilizationstabilization

Wrought wire claspWrought wire clasp Bar claspBar clasp

Less stability

4. Reciprocation:4. Reciprocation:

Each retentive arm must be opposed by reciprocal arm or other element of partial dentureCapable of resisting these horizontal forces.

It also plays role in stabilizing the denture against horizontal movement.

Reciprocal arm rigid, not tapered

Positioned on tooth that is reasonably parallel to dentures path of insertion and withdrawal

If arm positioned on surface that is tapered occlusally slight movement of denture will cause clasp to loose contact with tooth reciprocal and retention lost

Placed as close to the height of contour as possible, no higher than middle third of the tooth.

Preferably at junction of gingival and middle third

If height of contour high

lowered by recontouring

the enamel surface

5. Encirclement5. Encirclement

Each clasp designed to Each clasp designed to encircle more than 180 encircle more than 180 degrees of abutment degrees of abutment toothtooth

if notif not

clasp act an orthodontic clasp act an orthodontic applianceappliance

forcing abutment tooth to forcing abutment tooth to move out of contact with move out of contact with clasp.clasp.

EncirclementEncirclement In form of In form of continuous contactcontinuous contact

circumferential clasp armcircumferential clasp arm

Broken contactBroken contact Bar clasp.Bar clasp. (assembly must (assembly must contact atleast contact atleast three different tooth three different tooth areasareas {occ rest, retentive terminal, {occ rest, retentive terminal,

reciprocal terminal} that embrace more reciprocal terminal} that embrace more than half the tooth circumference.)than half the tooth circumference.)

6. Passivity:6. Passivity:

Clasp be completely Clasp be completely passivepassive

Retentive function Retentive function activated only when activated only when dislodging forces are dislodging forces are applied to partial applied to partial denture.denture.

Clasp be seated Clasp be seated completelycompletely

Location of retentive terminalsLocation of retentive terminals

Retentive terminal be located at mesial or Retentive terminal be located at mesial or distal line angle of the tooth.distal line angle of the tooth.

If centre of facial or lingualIf centre of facial or lingual surface usedsurface used clasp clasp shortenedshortened decreasing decreasing flexibility and flexibility and compromising encirclement.compromising encirclement.

Retentive terminal generally Retentive terminal generally preferred on facial/ buccalpreferred on facial/ buccal side than lingual side.side than lingual side.

In a Kennedy’s class III situation the In a Kennedy’s class III situation the only function of clasp is to deform only function of clasp is to deform

sufficiently during the insertion and sufficiently during the insertion and removal of theremoval of the restoration to allow restoration to allow them to flex over the them to flex over the height of contour of theheight of contour of the teeth in passing to andteeth in passing to and from undercut area. from undercut area. The retentive clasp is The retentive clasp is not called upon to flex, other than to not called upon to flex, other than to

engage the undercut area of the tooth for engage the undercut area of the tooth for the purpose of resisting vertical the purpose of resisting vertical movement.movement.

The direct retainer for the free end saddle must perform still The direct retainer for the free end saddle must perform still other function in addition to that of resisting vertical other function in addition to that of resisting vertical displacement. displacement.

Because of absence of Because of absence of distal support distal support saddle saddle will move tissueward under will move tissueward under function proportionate to thefunction proportionate to the quality of the supporting quality of the supporting tissues and the accuracy of tissues and the accuracy of the supporting base. the supporting base. Therefore these elements of Therefore these elements of Clasp which lie in an undercut area mustClasp which lie in an undercut area mustflex sufficiently to dissipate theflex sufficiently to dissipate thestresses which would otherwise bestresses which would otherwise betransmitted to the abutment toothtransmitted to the abutment toothas leverage.as leverage.

But – clasp can not effectively dissipate this But – clasp can not effectively dissipate this stress because:stress because:

i)i) Material itself can have only limited Material itself can have only limited flexibility .flexibility .

ii)ii) Clasp are made half round in shape Clasp are made half round in shape it it can flex only in one directioncan flex only in one direction therefore therefore can not effectively dissipate, by flexing can not effectively dissipate, by flexing all stresses placed on it. Therefore most all stresses placed on it. Therefore most of the tilting stress is transmitted to of the tilting stress is transmitted to abutment as leverage.abutment as leverage.

Henderson and Steffel:-Henderson and Steffel:- suggested a suggested a wrought wire retentive arm that would wrought wire retentive arm that would be more flexible and therefore will not be more flexible and therefore will not pull the tooth posteriorly.pull the tooth posteriorly.

Alternate suggestion was to use a bar-Alternate suggestion was to use a bar-type clasp arm in the distal facial type clasp arm in the distal facial undercut to reduce force on the undercut to reduce force on the abutment.abutment.

Kurrasch et alKurrasch et al recommended to recommended to place the retentive place the retentive arm of the clasp at the arm of the clasp at the height of contour height of contour rather than into an rather than into an undercut and rely on undercut and rely on friction for retention, friction for retention, thereby decrease thereby decrease force on the abutment.force on the abutment.

INDIRECT RETAINERINDIRECT RETAINER

Function:Function: 1.1. to prevent the DEB to prevent the DEB from moving away from its seat from moving away from its seat because of cheek and tongue because of cheek and tongue forces, sticky food.forces, sticky food.

It uses mechanical advantage It uses mechanical advantage of leverage by of leverage by moving the moving the fulcrum line farther from the fulcrum line farther from the forceforce

2. Contributes to support and stability of the partial 2. Contributes to support and stability of the partial denturedenture counteracts horizontal forces applied to counteracts horizontal forces applied to the denture.the denture.

Long span mandibular lingual bar major connectorLong span mandibular lingual bar major connector (even if tooth supported), IR (even if tooth supported), IR provides additional provides additional support and rigiditysupport and rigidity for lingual bar. for lingual bar. Prevents impingement of the lingual bar on the Prevents impingement of the lingual bar on the mucosa during function.mucosa during function.

Effectiveness of indirect retainer depends Effectiveness of indirect retainer depends upon:upon:

1)1) Its distance from the fulcrum lineIts distance from the fulcrum line

greater the distance between fulcrum line greater the distance between fulcrum line and IRand IR more effective more effective

(because the (because the

mechanicalmechanical

advantage increases advantage increases

proportionately)proportionately)

2)2) Effectiveness of the direct retainerEffectiveness of the direct retainer

Direct retainer must be effective if the Direct retainer must be effective if the indirect retainer is to function when the indirect retainer is to function when the denture rotatesdenture rotates

otherwise the partialotherwise the partial

denture will be denture will be

dislodged.dislodged.

3)3) The quality of support of the The quality of support of the indirect retainer.indirect retainer.

An indirect retainers effectiveness An indirect retainers effectiveness is reported to decrease when it is is reported to decrease when it is placed on an inclined plane placed on an inclined plane because the tooth moves and the because the tooth moves and the retainer slips. retainer slips. (JPD 1975;34;24-(JPD 1975;34;24-30)30)

Frank and NichollsFrank and Nicholls (JPD 1977:38;494)(JPD 1977:38;494) did a did a study on the effectiveness of indirect retainer study on the effectiveness of indirect retainer

and concluded that use of a mesial restand concluded that use of a mesial rest

instead of a distal rest on the terminalinstead of a distal rest on the terminal

abutment tooth does not decrease indirect abutment tooth does not decrease indirect

retention. Thus the choice of indirect retainer retention. Thus the choice of indirect retainer location should be made mostly on the basis of location should be made mostly on the basis of abutment tooth support, a crown form favoring abutment tooth support, a crown form favoring

adequate rest seat preparation, and the adequate rest seat preparation, and the patients esthetic desires.patients esthetic desires.

Frechette et alFrechette et al (JPD 1956;6:195-212)(JPD 1956;6:195-212) demonstrated that removal of the demonstrated that removal of the

indirect retainers from a RPD results indirect retainers from a RPD results in the application of more pressure to in the application of more pressure to the direct abutment teeth. The indirect the direct abutment teeth. The indirect

retainer probably is effective in retainer probably is effective in distributing forces to teeth other than distributing forces to teeth other than direct abutments, and in preventing direct abutments, and in preventing

denture base lifting.denture base lifting.

Major ConnectorMajor Connector

Connects parts of prosthesis located on Connects parts of prosthesis located on one side of the arch with those on the one side of the arch with those on the

opposite sideopposite side

Should be rigidShould be rigid Distribution of forceDistribution of force Effectiveness of otherEffectiveness of other components components

If flexibleIf flexible forces concentrated on forces concentrated on individual teeth or ridgeindividual teeth or ridge damaging. damaging.

L beam effect L beam effect applicable to the antero-applicable to the antero-posterior palatal bar or strap major connector.posterior palatal bar or strap major connector.

The two bars lying in The two bars lying in two different planestwo different planes produce a structurally strong L beam effect that produce a structurally strong L beam effect that gives excellent rigidity to the prosthesis.gives excellent rigidity to the prosthesis.

Horse shoe shape/ U shape connectorHorse shoe shape/ U shape connector

When vertical forces appliedWhen vertical forces applied tendency to tendency to spread or straighten. (movement can occur at spread or straighten. (movement can occur at open end).open end).

Not a good connector when cross arch Not a good connector when cross arch stabilization is required.stabilization is required.

Green LK et al Green LK et al

((J Prosthet Dent. 2003;89:400-7J Prosthet Dent. 2003;89:400-7 ) )

Did a study to determine the effects of Did a study to determine the effects of changing width, thickness, and shape on changing width, thickness, and shape on the rigidity of U-shaped maxillary major the rigidity of U-shaped maxillary major

connectors. They concluded that Doubling connectors. They concluded that Doubling the thickness of the anterior strap of a U-the thickness of the anterior strap of a U-

shaped maxillary major connector improved shaped maxillary major connector improved the rigidity of the framework to torsional the rigidity of the framework to torsional

loads. loads.

Guiding planesGuiding planes

Definition:Definition: two or more parallel, vertical two or more parallel, vertical surfaces of abutment teeth, so shaped to surfaces of abutment teeth, so shaped to direct a prosthesis during placement and direct a prosthesis during placement and removal.removal.

Functions:Functions: Provides one path Provides one path of placement and of placement and removalremoval Ensure the intended Ensure the intended action of reciprocal, action of reciprocal, and retentive componentsand retentive components Eliminates gross food traps.Eliminates gross food traps.

Guiding planesGuiding planes need to be as need to be as parallel to the long axisparallel to the long axis of of abutment teeth.abutment teeth.

Establishing Establishing guiding planes on several abutment teethguiding planes on several abutment teeth (preferably more than two teeth), located at (preferably more than two teeth), located at widely separatedwidely separated

positions, provides forpositions, provides for a more effective use ofa more effective use of these surfaces.these surfaces.

Effectiveness is increased if Effectiveness is increased if these surfaces are these surfaces are preparedprepared on more than one commonon more than one common axial surfaceaxial surface of the abutment of the abutment teeth.teeth.

Should be located on the abutment Should be located on the abutment surface adjacent to the surface adjacent to the edentulous areaedentulous area..

Guiding planes Guiding planes squarely facingsquarely facing each other each other should should not be preparednot be prepared on lone standing abutment on lone standing abutment place place undue strain on abutment when denture is rotated. undue strain on abutment when denture is rotated.

Unfavorable leverages avoided by Unfavorable leverages avoided by preparing preparing guiding plane surfaces to guiding plane surfaces to slightly diverge in buccal slightly diverge in buccal direction.direction.

STRESS BREAKERS/ STRESS BREAKERS/ STRESS DIRECTORSSTRESS DIRECTORS

GPT 7-GPT 7- A device or system that relieves A device or system that relieves specific dental structures of part or all of specific dental structures of part or all of the occlusal forces and redirects those the occlusal forces and redirects those forces to other bearing structures or forces to other bearing structures or regionsregions

In distal extension situationIn distal extension situation

Rigid connection Rigid connection between denture base between denture base and retainersand retainers

stress on abutment stress on abutment

reduced by using reduced by using functional basingfunctional basing, broad , broad coverage, harmonious coverage, harmonious occlusion and correct occlusion and correct choice of direct retainers choice of direct retainers

stress breakingstress breaking

Allows independent Allows independent movement of the denture movement of the denture base and the direct retainers.base and the direct retainers.

separates the action of the separates the action of the

retaining elements from theretaining elements from the

movement of the denturemovement of the denture

basebase

The need for stress breakers on free end RPDs has The need for stress breakers on free end RPDs has been recognized on the basis that the been recognized on the basis that the resiliency or resiliency or displaceabilitydisplaceability of the of the mucosal tissue ranges between mucosal tissue ranges between 0.4 mm to 2mm0.4 mm to 2mm, while the vertical resiliency of a , while the vertical resiliency of a normal healthy tooth in its socketnormal healthy tooth in its socket

is approx. is approx. 0.1mm.0.1mm.

This tissue resiliency This tissue resiliency

differential of differential of 20 to 4020 to 40

times the axial displaceability times the axial displaceability

of a normal tooth in its socket of a normal tooth in its socket

dictates the dictates the necessity for necessity for

some form of stress directionsome form of stress direction in the partial denture in the partial denture design.design.

2 types of designs:2 types of designs:

1) 1) HINGE DESIGN:HINGE DESIGN:

Base is permitted to move in a vertical plane only. Base is permitted to move in a vertical plane only. The hinge type device spares the tooth virtually The hinge type device spares the tooth virtually all of the stress which results from vertical all of the stress which results from vertical movement of the base, but it is still subjected to movement of the base, but it is still subjected to all the lateral loads and torsional stress. all the lateral loads and torsional stress.

• Eg: Gerber Hinge, DE Hinge type.Eg: Gerber Hinge, DE Hinge type.

2.2. ROTATIONAL TYPE:ROTATIONAL TYPE:

Works on the ball and socket principle, Works on the ball and socket principle, movements of the base is allowed in all planes, movements of the base is allowed in all planes, and the tooth is relieved of virtually all stresses.and the tooth is relieved of virtually all stresses.

• Eg: CRISMANI, DALBO

SPLIT BAR MAJOR CONNECTORSPLIT BAR MAJOR CONNECTORHenderson and SteffelHenderson and Steffel suggested a split lingual bar to suggested a split lingual bar to redirect the force on the abutment tooth.redirect the force on the abutment tooth.

Vertical and diagonal forces applied to tissue-supported Vertical and diagonal forces applied to tissue-supported

base must pass anteriorly along lower bar and then back base must pass anteriorly along lower bar and then back

along more rigid upper baralong more rigid upper bar

to reach abutment.to reach abutment.

Thus tipping forces that wouldThus tipping forces that would otherwise be transmitted otherwise be transmitted directly to abutment tooth aredirectly to abutment tooth are supposedly dissipated bysupposedly dissipated by flexibility of lower bar and distance traveled.flexibility of lower bar and distance traveled.

Reitz PVReitz PV et al did a photoelastic study et al did a photoelastic study and confirmed that split major and confirmed that split major

connectors were effective as stress connectors were effective as stress breakers.breakers.

Philip V. Et al JPD 1991: 66:220) did a Philip V. Et al JPD 1991: 66:220) did a study and study and concluded that use of split concluded that use of split lingual bar decreased the amount of lingual bar decreased the amount of

stresses on abutment teeth and stresses on abutment teeth and increased stresses on residual ridges.increased stresses on residual ridges.

The vertical movement permitted by a hinge in The vertical movement permitted by a hinge in the sagittal plane is such that the distal base the sagittal plane is such that the distal base portion, in effect, becomes a beam that under portion, in effect, becomes a beam that under

load behaves like the moving radius of a load behaves like the moving radius of a circle.circle.

The distal extremity of the distal base will The distal extremity of the distal base will scribe an arc, and because it is furthest scribe an arc, and because it is furthest from its centre of movement at the hinge from its centre of movement at the hinge

will consequently undergo more will consequently undergo more movement than any other point along the movement than any other point along the

length of the denture base.length of the denture base.

Flexible DEB RPD showing a Flexible DEB RPD showing a steep steep arc differentialarc differential when location of when location of stress breaker hinge results in stress breaker hinge results in short-radius beamshort-radius beam..

Long radius designsLong radius designs (like split major (like split major connector) seeks to lengthen the active connector) seeks to lengthen the active

radius of the system by ensuring that the radius of the system by ensuring that the flexible joint is located as far anterior as flexible joint is located as far anterior as possible. Lengthening the beam radius possible. Lengthening the beam radius could could reduce the arcing differentialreduce the arcing differential to a to a

gradient more in keeping with the gradient more in keeping with the inherent displaceability of the ridge inherent displaceability of the ridge

tissues.tissues.

INDICATION FOR THE USE OF INDICATION FOR THE USE OF STRESS BREAKERSTRESS BREAKER

Because the stress breaker does, in far relieveBecause the stress breaker does, in far relieve

the abutment tooth of the forces generated bythe abutment tooth of the forces generated by

the masticatory load, the stress is then borne the masticatory load, the stress is then borne

by the residual ridge. Therefore a by the residual ridge. Therefore a prime indicationprime indication

for the application of this principle would be the for the application of this principle would be the

mouth where in an abutment tooth is inherently mouth where in an abutment tooth is inherently

weakweak..

Factors influencing magnitude of Factors influencing magnitude of stresses transmitted to abutment teethstresses transmitted to abutment teeth

1 quality of support of ridge 1 quality of support of ridge Form of residual ridgeForm of residual ridge type of mucosal coveringtype of mucosal covering

2 Length of span2 Length of span

3 clasp factor3 clasp factor designdesign lengthlength materialmaterial amount of tooth contactamount of tooth contact

4 occlusion 4 occlusion

Type of teethType of teeth

Harmony of occlusionHarmony of occlusion

5 Areas of the base to which load is 5 Areas of the base to which load is appliedapplied

1.1. Quality of support of ridgeQuality of support of ridge

Better support by ridgeBetter support by ridge less stress on less stress on abutment teethabutment teeth

A) Form of residual ridgeA) Form of residual ridge Large well formed ridgesLarge well formed ridges absorb greater absorb greater

stressstress less stress on abutment less stress on abutment Broad ridges with parallelBroad ridges with parallel sidessides longer flangeslonger flanges on on the denture basethe denture base stabilize stabilize the denture against lateral the denture against lateral forces.forces.

B) Type of mucosaB) Type of mucosa

Influences magnitude of stresses transmitted to Influences magnitude of stresses transmitted to abutment teeth.abutment teeth.

Healthy mucosaHealthy mucosa capablecapable of bearing greater functional of bearing greater functional loads than thin atrophic loads than thin atrophic mucosamucosa

Soft, flabby, displaceable mucosaSoft, flabby, displaceable mucosa Contribute little to vertical support of denture allows Contribute little to vertical support of denture allows

excessive movement of dentureexcessive movement of denture stress transmitted stress transmitted to abutment teeth to abutment teeth

2. Length of span2. Length of span

Longer edentulous spanLonger edentulous span

longer denture baselonger denture base

greater force transmitted togreater force transmitted to

abutment teethabutment teeth

Every effort be made to retain a posterior abutment Every effort be made to retain a posterior abutment to avoid class I and class II situation. to avoid class I and class II situation.

3. clasp as a factor in stress3. clasp as a factor in stress

More flexible the retentive arm of More flexible the retentive arm of claspclaspless stress to abutment toothless stress to abutment tooth

But, flexible clasp arm But, flexible clasp arm provides less provides less stability against horizontal forcesstability against horizontal forces increase stress on residual ridge.increase stress on residual ridge.

In examination phase decide whether ridge or In examination phase decide whether ridge or abutment tooth require more protectionabutment tooth require more protection

If periodontal If periodontal support goodsupport good

less flexibleless flexible clasp clasp like vertical like vertical projection clasp projection clasp indicated.indicated.

If periodontal support If periodontal support weakweak

use more use more flexible flexible claspclasp like like combination clasp combination clasp (residual ridge share(residual ridge share more resistance to more resistance to horizontal forces).horizontal forces).

Clasp designClasp design

Clasp be passive Clasp be passive once framework once framework seated completelyseated completely

Length of claspLength of clasp Increase in lengthIncrease in length

increased fexibilityincreased fexibility

Flexure directly Flexure directly proportional to proportional to (length)(length)

3

Material:Material: Chrome alloysChrome alloys higher modulus of elasticity than gold higher modulus of elasticity than gold

alloysalloys less flexible. less flexible. Therefore smaller cross sectional form of the clasp and Therefore smaller cross sectional form of the clasp and

less depth of retentive undercut must be used for chrome less depth of retentive undercut must be used for chrome alloyalloy

Wrought wireWrought wire because because

of internal structure i.e.of internal structure i.e.

longitudinal structure as longitudinal structure as

compared to grain compared to grain

structure of cast alloystructure of cast alloy

greater flexibilitygreater flexibility

Type abutment tooth surfaceType abutment tooth surface

Surface if gold crown offers Surface if gold crown offers more frictional more frictional resistanceresistance to clasp arm movement than to clasp arm movement than does enamel surface of tooth.does enamel surface of tooth.

Greater stress exerted on tooth restored Greater stress exerted on tooth restored with crown than with intact enamel.with crown than with intact enamel.

Amount of clasp surface in contact with Amount of clasp surface in contact with toothtooth

Greater the area of tooth to metal contact Greater the area of tooth to metal contact between clasp and toothbetween clasp and tooth

more will be stress exerted on the tooth.more will be stress exerted on the tooth.

Occlusion as a factorOcclusion as a factor

A) Harmony of occlusion or lack of itA) Harmony of occlusion or lack of it

Disharmonious occlusion Disharmonious occlusion

generate horizontal stressesgenerate horizontal stresses

when magnified by factor of leveragewhen magnified by factor of leverage

can transmit destructive forces to both can transmit destructive forces to both abutment teeth and residual ridge.abutment teeth and residual ridge.

B) type of opposing occlusionB) type of opposing occlusion

Play important role in determining amount of stress Play important role in determining amount of stress generated by occlusiongenerated by occlusion

Natural teethNatural teeth can exert closing force upto 300 can exert closing force upto 300 pounds/inch square, whereas, complete denture pounds/inch square, whereas, complete denture upto 30 pounds/inch square.upto 30 pounds/inch square.

Therefore RPD constructed against removable Therefore RPD constructed against removable prosthesis is subjected to much less occlusal stress prosthesis is subjected to much less occlusal stress than one opposed by natural dentition. than one opposed by natural dentition.

Area of denture base to which load is appliedArea of denture base to which load is applied

Less movement of base if load applied adjacent to Less movement of base if load applied adjacent to the abutment tooth than if it is applied to the distal the abutment tooth than if it is applied to the distal end of the base.end of the base.

movement may be 4 times greater at distal end of movement may be 4 times greater at distal end of base than next to the clasp.base than next to the clasp.

Controlling stress by design Controlling stress by design considerationconsideration

1. Retention as a means of stress control1. Retention as a means of stress control A) forces of adhesion and cohesionA) forces of adhesion and cohesion B) atmospheric pressureB) atmospheric pressure C) Frictional contactC) Frictional contact D) Neuromuscular controlD) Neuromuscular control

2 Strategic clasp positioning2 Strategic clasp positioning a) Quadrilateral configurationa) Quadrilateral configuration b) Tripod configurationb) Tripod configuration c) Bilateral configurationc) Bilateral configuration

3 Clasp design3 Clasp design a) circumferential claspa) circumferential clasp b) bar claspb) bar clasp c) combination claspc) combination clasp

4 Indirect retention4 Indirect retention

5 Functional basing5 Functional basing

6 occlusion6 occlusion a) harmonious occlusiona) harmonious occlusion b) size of food tableb) size of food table c) occlusal pattern of posterior teethc) occlusal pattern of posterior teeth

7 Denture base7 Denture base a) Size & configurationa) Size & configuration b) accuracy of adaptation of base to the tissuesb) accuracy of adaptation of base to the tissues

1.1. Retention as a means of stress controlRetention as a means of stress control

Retention : that quality inherent in the prosthesis acting to resist the forces of dislodgment along the path of placement.

Retentive clasp arm retention transmits destructive forces to abutment teeth.

Any retention by other components of RPD

decrease the amount of retention that clasp need to provide

decrease stress on abutment teeth.

By exploiting retentive potentials in various other areas of mouth, both support and stability may be enhanced at the same time and stress is reduced.

A) Forces of adhesion and cohesionA) Forces of adhesion and cohesion

To secure the maximum possible retention To secure the maximum possible retention through the use of the forces of adhesion and through the use of the forces of adhesion and cohesion, the denture base should:cohesion, the denture base should:

cover the maximum areacover the maximum area

of available support of available support

must be accurately adapted must be accurately adapted

to the underlying mucosa. to the underlying mucosa.

B) Atmospheric pressureB) Atmospheric pressure Although not possible to develop a complete Although not possible to develop a complete

peripheral seal around borders of partial denture peripheral seal around borders of partial denture because of presence of teethbecause of presence of teeth may still may still contribute to a slight amount of retention.contribute to a slight amount of retention.

Noted particularly in maxillary Noted particularly in maxillary complete palate majorcomplete palate major connector when an connector when an accurate metal castingaccurate metal casting is used and margins ofis used and margins of connector are connector are beaded.beaded.

C) Frictional contactC) Frictional contact

Partial denture designed Partial denture designed guide planes created guide planes created on as many teeth as on as many teeth as possiblepossible

Frictional contact of the Frictional contact of the prosthesis against prosthesis against properly planned guide properly planned guide planes can contribute planes can contribute significantly to the significantly to the retention of denture.retention of denture.

D) Neuromuscular controlD) Neuromuscular control

Patients ability to control denture with lips, cheek Patients ability to control denture with lips, cheek and tongueand tongue important factor in retention. important factor in retention.

Design and contour Design and contour

of denture base alsoof denture base also

important factors.important factors.

Any overextension of Any overextension of

denture basedenture base loss loss

of retentionof retention abutment abutment

teeth bearing direct retainers will be stressed more.teeth bearing direct retainers will be stressed more.

2. Strategic clasp positioning as a means 2. Strategic clasp positioning as a means of stress controlof stress control

Leverages can be controlled to a large Leverages can be controlled to a large extent by means of clasps, if there are extent by means of clasps, if there are sufficient abutment teeth and they are sufficient abutment teeth and they are strategically distributed in the dental arch.strategically distributed in the dental arch.

If number and location of potential If number and location of potential abutments is less than idealabutments is less than ideal harmful harmful effects can be decreased by strategic effects can be decreased by strategic placement of clasps.placement of clasps.

a) Quadrilateral configurationa) Quadrilateral configuration

When 4 abutment teeth available for clasping and When 4 abutment teeth available for clasping and partial denture confined within 4 claspspartial denture confined within 4 clasps all all leverages neutralized.leverages neutralized.

Ideal (for support and leverage control)Ideal (for support and leverage control)

Indicated most often in class III arches (with Indicated most often in class III arches (with modification space on opposite side)modification space on opposite side)

Class III with no modification spaceClass III with no modification space

B) Tripod configurationB) Tripod configuration

Class II situationsClass II situations Distal abutment on one Distal abutment on one

side of arch missingside of arch missing leverage controlled to leverage controlled to some extent by creating some extent by creating tripod configuration of tripod configuration of clasp placement.clasp placement.

Class II with no modification Class II with no modification spacespace

Separating two abutments on dentulous side as far as possible largest possible area of denture will be enclosed in a triangle formed by retentive clasps.

Bilateral configurationBilateral configuration

For class I situationsNot considered ideal, but best option availableStress must be controlled by other means.

3. Clasp design as a means 3. Clasp design as a means of stress controlof stress control

a) circumferential cast a) circumferential cast claspclasp

b) Reverse circlet cast b) Reverse circlet cast circumferential claspcircumferential clasp

(torque not transmitted to (torque not transmitted to abutment tooth,abutment tooth,

May produce wedging May produce wedging forces (counteracted by forces (counteracted by rests on approximating rests on approximating surfaces of both teeth)surfaces of both teeth)

b) vertical projection b) vertical projection claspclasp

In DEB RPD when In DEB RPD when undercut on D-B undercut on D-B surfacesurface

C) Combination claspC) Combination clasp In DEB RPD when In DEB RPD when

undercut on M-B undercut on M-B surfacesurface

Wrought wireWrought wire More flexibleMore flexible Flex in all planesFlex in all planes

4 Indirect retention as 4 Indirect retention as a means of stress a means of stress controlcontrol

Essential in class I and Essential in class I and II situationsII situations

5. Functional basing 5. Functional basing as a means of as a means of stress controlstress control

Alveolar ridge can assume two contoursResting or passive contourFunctional form

Functional basing:Functional basing:

This approach includes the impressionThis approach includes the impression

technique that record the tissue surfacetechnique that record the tissue surface

in the shape the residual ridge assumein the shape the residual ridge assume

under functional load.under functional load.

When subjected to occlusal loading When subjected to occlusal loading not displaced to any appreciable extent not displaced to any appreciable extent

Magnitude of stress transmitted to abutment less.Magnitude of stress transmitted to abutment less.

FIG>>>>>>>>>>>>>>>>>>>>>>>FIG>>>>>>>>>>>>>>>>>>>>>>>

6 Occlusion as a means of stress 6 Occlusion as a means of stress controlcontrol

Harmonious occlusionHarmonious occlusion minimum minimum tipping forces on abutment teeth tipping forces on abutment teeth and residual ridge.and residual ridge.

Size of food table:Size of food table: Large food tableLarge food table potential of potential of

transmitting more pressure on transmitting more pressure on both residual ridge and abutment both residual ridge and abutment teeth than a small one.teeth than a small one.

Factor negligible when small Factor negligible when small morsel of food between teeth, but morsel of food between teeth, but would become substantial when would become substantial when the teeth were forced to penetrate the teeth were forced to penetrate a mass of fibrous food resistant to a mass of fibrous food resistant to comminution.comminution.

Occlusal patterns of posterior teethOcclusal patterns of posterior teethCuspless teethCuspless teeth more occlusal load as more occlusal load as

compared to teeth with cusp.compared to teeth with cusp.Steep inclines be avoidedSteep inclines be avoided tend to tend to

introduce horizontal forces .introduce horizontal forces .

7 Denture base as means of stress control7 Denture base as means of stress control

maximum coverage without overextending.maximum coverage without overextending. Accurate adaptation of denture baseAccurate adaptation of denture base

SummarySummary

ConclusionConclusion

In the oral cavity one would find a number In the oral cavity one would find a number of sources of stress generation, the human of sources of stress generation, the human body is built in such a manner that it learns body is built in such a manner that it learns

to adapt to any stressful situation. Great to adapt to any stressful situation. Great caution and reserve are essential caution and reserve are essential

whenever an attempt is made to interpret whenever an attempt is made to interpret biological phenomenon entirely by biological phenomenon entirely by

mathematical computation.mathematical computation.

Removable partial dentures by design are Removable partial dentures by design are intended to be removed from and replaced intended to be removed from and replaced

into the mouth. Because of this, they are not into the mouth. Because of this, they are not rigidly connected to the teeth or tissues, rigidly connected to the teeth or tissues, which means that they are subjected to which means that they are subjected to

movement in response to functional loads. movement in response to functional loads. These movement can cause generation of These movement can cause generation of

stresses which if not controlled , can damage stresses which if not controlled , can damage the remaining teeth and supporting bone.the remaining teeth and supporting bone.

It is very important for us as prosthodontist It is very important for us as prosthodontist to understand these biomechanical to understand these biomechanical principle in order to recognize their principle in order to recognize their application. Efforts must be always application. Efforts must be always

directed towards minimizing the harmful directed towards minimizing the harmful forces on the remaining teeth by proper forces on the remaining teeth by proper

designing of the RPD. designing of the RPD.

ReferencesReferences

• Load distribution in extension saddle partial Load distribution in extension saddle partial dentures:JPD 1952,(2)92-100:George W. dentures:JPD 1952,(2)92-100:George W. Hindels.Hindels.

• A comparison of tooth borne and tooth-tissue A comparison of tooth borne and tooth-tissue borne RPDs. JPD 1953:3;375-381.borne RPDs. JPD 1953:3;375-381.

Case analysis of partial denture problem : Case analysis of partial denture problem : Friedman, JPD 1954; 4;82-86Friedman, JPD 1954; 4;82-86

Abutment sites, spaces in partial denture Abutment sites, spaces in partial denture analysis :Friedman, JPD 1954analysis :Friedman, JPD 1954

Bilateral free end saddle lower denture : Lammie Bilateral free end saddle lower denture : Lammie & Osborne, JPD 1954:4;640-652.& Osborne, JPD 1954:4;640-652.

• A study of partial denture design and masticatory A study of partial denture design and masticatory pressure in a mandibular bilateral distal extension case. pressure in a mandibular bilateral distal extension case. JPD 1958:8;340-350.JPD 1958:8;340-350.

The removable partial denture in the general practice of The removable partial denture in the general practice of tomorrow : Oliver Applegate, JPD 1958tomorrow : Oliver Applegate, JPD 1958

An evaluation of the support for the RPD. JPD An evaluation of the support for the RPD. JPD 1960:10;112-123 1960:10;112-123

Occlusal rest position & clasp design : Kratochvil,JPD Occlusal rest position & clasp design : Kratochvil,JPD 19631963

A comparative study of impression procedures for distal A comparative study of impression procedures for distal extension partial dentures. JPD 1966:16;708-720.extension partial dentures. JPD 1966:16;708-720.

• The rationale of resilient hinge-action stress The rationale of resilient hinge-action stress breaker. JPD 1968:20;204-215.breaker. JPD 1968:20;204-215.

• Clasp designs for RPD. JPD 1973:29;409-415.Clasp designs for RPD. JPD 1973:29;409-415.

• An evaluation of photoelastic stres patterns An evaluation of photoelastic stres patterns produced by various designs of bilatearl DE produced by various designs of bilatearl DE RPDs. JPD 1977:38;261-273.RPDs. JPD 1977:38;261-273.

• Management of loading forces on mandibular Management of loading forces on mandibular distal-extension prosthesis:part I. JPD distal-extension prosthesis:part I. JPD 1984:42;673-6811984:42;673-681

• Motion vector analysis of an abutment for a Motion vector analysis of an abutment for a distal-extension RPD: A pilot study. JPD distal-extension RPD: A pilot study. JPD 1983:43:15-21.1983:43:15-21.

• Direct retainers for distal extension removable partial Direct retainers for distal extension removable partial dentures. JPD 1986:56;562-566.dentures. JPD 1986:56;562-566.

• Factors that augment the role of direct retainers in Factors that augment the role of direct retainers in mandibular DERPDs. JPD 1988:60;696-698.mandibular DERPDs. JPD 1988:60;696-698.

The effect on function of distal extension RPD as The effect on function of distal extension RPD as determines by occlusal rest position : James DeBoer, JPD determines by occlusal rest position : James DeBoer, JPD 19881988

Indirect retention. BDJ 2001:190;128-132Indirect retention. BDJ 2001:190;128-132

The effects of buccolingual width and position of occlusal The effects of buccolingual width and position of occlusal rest seats on load transmission to the abutments for tooth-rest seats on load transmission to the abutments for tooth-supported removable partial dentures. Sato Y et al: Int J supported removable partial dentures. Sato Y et al: Int J Prosthodont. 2001:14:340-3Prosthodont. 2001:14:340-3

McCraken’s Removable partial prosthodontics McCraken’s Removable partial prosthodontics 8th edition8th edition

Clinical Removable partial prosthodontics : Clinical Removable partial prosthodontics : Stewart & Rudd, 2nd editionStewart & Rudd, 2nd edition

Removable partial prosthodontics : Grasso & Removable partial prosthodontics : Grasso & miller, 3rd editionmiller, 3rd edition

Removable partial prosthodontics : BoucherRemovable partial prosthodontics : Boucher Color atlas of Removable partial prosthodontics : Color atlas of Removable partial prosthodontics :

Davenport & HeathDavenport & Heath