the biomechanics of stresses induced by removable partial denture/ fixed orthodontic training

11

THE BIOMECHANICS THE BIOMECHANICS OF STRESSES OF STRESSES INDUCED BY INDUCED BY REMOVABLE PARTIAL REMOVABLE PARTIAL DENTUREDENTURE

INDIAN DENTAL ACADEMY

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CONTENTSCONTENTS

IntroductionIntroductionWhat Is StressWhat Is StressSimple machinesSimple machinesDevelopment of forces Development of forces Forces acting on the removable partial dentureForces acting on the removable partial dentureDistribution of stress between the teeth and Distribution of stress between the teeth and tissuetissueInduced stresses to be recognizedInduced stresses to be recognizedConsequence of inefficient management of Consequence of inefficient management of stressesstressesConclusionConclusionBibliographyBibliography


INTRODUCTIONINTRODUCTION

Nature has designed the Nature has designed the StomatognathicStomatognathic system to dissipate the forces of mastication system to dissipate the forces of mastication to the supporting structures.to the supporting structures.

In a partially edentulous situation the In a partially edentulous situation the mechanism of dissipating the force by the mechanism of dissipating the force by the dentition is compromised due to the breach in dentition is compromised due to the breach in integrity of the dental arches.integrity of the dental arches.


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When a removable partial denture is used When a removable partial denture is used to restore the missing natural teeth and to to restore the missing natural teeth and to maintain the continuity of the dental arches maintain the continuity of the dental arches it is important for the dentist to consider the it is important for the dentist to consider the direction ,duration and frequency of the direction ,duration and frequency of the force application as well as the magnitude.force application as well as the magnitude.


What is stress?What is stress?

An internal force that resists an externally An internal force that resists an externally applied load or force. GPTapplied load or force. GPT

Different types of stresses areDifferent types of stresses are

Compressive stressCompressive stressTensile stressTensile stressShear stressShear stress


Simple Simple machinesmachines

With the introduction of the removable partial dentures With the introduction of the removable partial dentures into the oral cavity certain simple machines come into into the oral cavity certain simple machines come into play. These canplay. These can magnify magnify the stresses .It is therefore the stresses .It is therefore important to know its functioning to maintain the health of important to know its functioning to maintain the health of the oral structures.the oral structures.

LeverLeverWedgeWedgeScrew Screw Wheel and axleWheel and axlePulleyPulleyInclined planeInclined plane


Development of forcesDevelopment of forces

The most commonly encountered simple machines The most commonly encountered simple machines in removable partial denture are: in removable partial denture are:

LeverLeverInclined planeInclined plane

It should be borne in mind that the success of a It should be borne in mind that the success of a removable partial denture lies in avoiding these removable partial denture lies in avoiding these to the greatest extent possibleto the greatest extent possible..


LeverLeverA A lever is a rigid lever is a rigid bar that rests on a bar that rests on a fulcrum (a fixed fulcrum (a fixed point.) point.) The farther the force The farther the force is from the fulcrum, is from the fulcrum, the easier it is to the easier it is to work the leverwork the lever There are There are three three classesclasses of levers. of levers.


Class IClass I

A A class-1class-1 leverlever has its has its fulcrum located fulcrum located somewhere between somewhere between the effort and the the effort and the resistanceresistanceE.g. Distal extension E.g. Distal extension removable partial removable partial denturedenture


Class IIClass II

With a With a class-2 leverclass-2 lever, , the fulcrum is at one the fulcrum is at one end, the effort is at the end, the effort is at the other end and the other end and the resistance is in the resistance is in the middlemiddle

Seen as indirect Seen as indirect retention in removable retention in removable partial denturepartial denture


Class IIIClass III

In a In a class-3 leverclass-3 lever. The . The fulcrum is at one end and fulcrum is at one end and the effort is applied the effort is applied between the fulcrum and between the fulcrum and the resistance.the resistance.Not encountered in Not encountered in partial denturespartial dentures

TMJ muscles and teeth TMJ muscles and teeth act as a class III lever act as a class III lever


Inclined PlaneInclined Plane

Forces against the Forces against the inclined plane may result inclined plane may result in deflection of that in deflection of that which is applying the which is applying the force or may result in force or may result in the movement of the the movement of the inclined plane .inclined plane .

Neither of these results Neither of these results are desirableare desirableE.g Cingulum rest seat E.g Cingulum rest seat on cuspidson cuspids


Forces acting on the Forces acting on the partial denture.partial denture.


Forces acting during Forces acting during placement and removal of the placement and removal of the partial denturepartial denture

1)1) When a contacting rigid area of a removable When a contacting rigid area of a removable partial denture passes over the surface bulge of partial denture passes over the surface bulge of the abutment teeth.the abutment teeth.

2)2) Movement of the retentive clasp out of the Movement of the retentive clasp out of the infrabulgeinfrabulge area and over the abutment height of area and over the abutment height of contour.contour.


Forces acting on tooth and Forces acting on tooth and tissue borne removable partial tissue borne removable partial

denturedentureForces acting on a distal extension partial Forces acting on a distal extension partial denture are a result of a composite of forces denture are a result of a composite of forces arising from the three principal fulcrums. arising from the three principal fulcrums.

1) Fulcrum on the 1) Fulcrum on the horizontal plane.horizontal plane.2) Fulcrum on the 2) Fulcrum on the sagittal plane.sagittal plane. 3) Fulcrum on the 3) Fulcrum on the vertical plane.vertical plane.


Fulcrum on Fulcrum on the horizontal the horizontal planeplaneExtends through the Extends through the two principal two principal abutments, one on abutments, one on each side of the dental each side of the dental arch.arch.

Controls rotational Controls rotational movement of the movement of the denture in the sagittal denture in the sagittal plane.plane.


Fulcrum on the Fulcrum on the Sagittal PlaneSagittal Plane

Extends through the Extends through the occlusal rest on the terminal occlusal rest on the terminal abutment and along the abutment and along the crest of the residual ridge crest of the residual ridge on one side of the archon one side of the arch

The fulcrum controls the The fulcrum controls the rotational movement of the rotational movement of the denture in the vertical planedenture in the vertical plane


Fulcrum on Fulcrum on the vertical the vertical planeplane

Located in the vicinity of Located in the vicinity of the midline just lingual the midline just lingual to the anterior teeth. to the anterior teeth.

Controls the rotational Controls the rotational movement of the movement of the denture in the horizontal denture in the horizontal direction.direction.


Forces acting on tooth Forces acting on tooth supported removable partial supported removable partial denturedenture

Kennedy’s Class III situation Kennedy’s Class III situation Supported by abutmentsSupported by abutmentsForce against it are transmittedForce against it are transmitted down the long axis down the long axis of the abutment teeth of the abutment teeth


Distribution of stress between Distribution of stress between teeth and tissue?teeth and tissue?

Two schools of thought exists in the matter of Two schools of thought exists in the matter of allocating the proportionate share of stress that allocating the proportionate share of stress that is to be sustained by the tooth and the residual is to be sustained by the tooth and the residual ridge.ridge.


Stressbreaker SchoolStressbreaker School

Residual ridge Residual ridge bearbear the stresses and teeth be the stresses and teeth be spared.spared.

Use of flexible or movable joint between the Use of flexible or movable joint between the teeth and the metal framework.teeth and the metal framework.


Wide Distribution of Wide Distribution of Stress SchoolStress School

Support provided by a Support provided by a combinationcombination of of abutment tooth and the mucosa. abutment tooth and the mucosa.

Various clinical techniques and design Various clinical techniques and design variations have been tried so as to exploit the variations have been tried so as to exploit the maximum load bearing capacity of each.maximum load bearing capacity of each.


Induced stresses to be Induced stresses to be recognizedrecognized


Stress resulting from appliance Stress resulting from appliance inaccuraciesinaccuracies

Orthodontic stressOrthodontic stress

MalocclusionMalocclusion

Impingement of subbasal Impingement of subbasal structuresstructures


Stress from interference to Stress from interference to appliance insertionappliance insertion

Intermittent disturbance of tooth alignment .Intermittent disturbance of tooth alignment .

Contacting rigid area of a removable prosthesis Contacting rigid area of a removable prosthesis passes over the surface bulge of the abutment passes over the surface bulge of the abutment tooth.tooth.


Gingival impingement by the Gingival impingement by the removable applianceremovable appliance

Gingivae most susceptible to injury by any Gingivae most susceptible to injury by any pressure induced by a removable prosthesis.pressure induced by a removable prosthesis.


Stress on a sloping tooth surface Stress on a sloping tooth surface for the support of an abutment for the support of an abutment occlusal rest.occlusal rest.

Seen while using cuspid tooth for abutment Seen while using cuspid tooth for abutment services with steep lingual surface.services with steep lingual surface.

Appliance slips as occlusal load is applied Appliance slips as occlusal load is applied which can cause gingival irritation.which can cause gingival irritation.


Stress resulting in the impingement Stress resulting in the impingement by a major connectorby a major connector

Trauma from flexingTrauma from flexing

Trauma from lateral appliance movementTrauma from lateral appliance movement

Trauma from connector settlement.Trauma from connector settlement.


Factors influencing magnitude Factors influencing magnitude of stress transmitted to the of stress transmitted to the

abutment teethabutment teeth


Length Of SpanLength Of SpanLonger Edentulous SpanLonger Edentulous Span==Longer Denture Longer Denture BaseBase==Greater Force to the Abutment TeethGreater Force to the Abutment Teeth

The fulcrum is located at or near the occlusal The fulcrum is located at or near the occlusal rest of the terminal abutment tooth. rest of the terminal abutment tooth.

The denture base determines how much The denture base determines how much force the abutment tooth must withstand.force the abutment tooth must withstand.


Quality of support of ridgeQuality of support of ridge

Large well formed ridges are Large well formed ridges are capable of absorbing greater capable of absorbing greater amounts of stress than are small amounts of stress than are small thin or knife edged ridges.thin or knife edged ridges.

Broad ridges with parallel sides Broad ridges with parallel sides longer flanges on denture longer flanges on denture basebase stabilizes the denture stabilizes the denture against the lateral forcesagainst the lateral forces


Approximately 1mm thick mucoperiosteum Approximately 1mm thick mucoperiosteum can bear greater functional loads than can bear greater functional loads than atrophic mucosa.atrophic mucosa.

Soft flabby displaceable tissue allows Soft flabby displaceable tissue allows excessive movement.excessive movement.


Qualities of claspQualities of clasp

The more flexible the retentive arm of the clasp , The more flexible the retentive arm of the clasp , the less stress is transmitted to the abutment the less stress is transmitted to the abutment tooth.tooth.

A passive clasp will exert less force than an A passive clasp will exert less force than an active clasp.active clasp.


Flexibility can be increased by Flexibility can be increased by lengtheninglengthening the clasp. the clasp.

Clasp length is increased by using a curved Clasp length is increased by using a curved path on an abutment tooth.path on an abutment tooth.

Gold clasps will exert less stress on the Gold clasps will exert less stress on the abutment than chrome alloy.abutment than chrome alloy.


Abutment tooth surfaceAbutment tooth surface

The surface of a gold crown or restoration gives The surface of a gold crown or restoration gives more frictional resistance to clasp arm more frictional resistance to clasp arm movement than the enamel surface of the tooth.movement than the enamel surface of the tooth.

Hence greater stress is exerted on a tooth Hence greater stress is exerted on a tooth restored with gold than on a tooth with intact restored with gold than on a tooth with intact enamel.enamel.


Occlusal harmonyOcclusal harmony

Disharmonious occlusion Disharmonious occlusion generates horizontal generates horizontal forcesforces

Partial denture Partial denture constructed to oppose constructed to oppose natural dentition will face natural dentition will face more stresses.more stresses.


If the occlusal If the occlusal load load is applied to the base is applied to the base adjacent to the abutment toothadjacent to the abutment tooth , there will be , there will be less movementless movement of the denture base as of the denture base as compared if the load was applied to the distal compared if the load was applied to the distal end.end.

The occlusal load should be applied in the The occlusal load should be applied in the center of the denture bearing area.center of the denture bearing area.


Consequence of inefficient management of Consequence of inefficient management of stresses induced by partial denturestresses induced by partial denture

Greater Greater stressesstresses

Abutment ToothAbutment Tooth

Orthodontic Orthodontic movement.movement.

Malocclusion.Malocclusion.

Residual RidgeResidual Ridge

Inflammation, Inflammation, Edema of residual Edema of residual ridge mucosa.ridge mucosa.Resorptive loss of Resorptive loss of adjacent alveolar adjacent alveolar process .process .


CONCLUSIONCONCLUSIONRemovable Partial Dentures can be a boon Removable Partial Dentures can be a boon or a bane based on the understanding of its or a bane based on the understanding of its biomechanical principles.biomechanical principles.

Blindly done, the removable partial denture Blindly done, the removable partial denture can act as a potentially destructive device can act as a potentially destructive device which could induce more harm than than which could induce more harm than than the good it was intended for. the good it was intended for.


BibliographyBibliographyRemovable partial prosthodontics Removable partial prosthodontics

- McCRACKEN- McCRACKENClinical Removable Partial ProsthodonticsClinical Removable Partial Prosthodontics

- STEWART- STEWARTEssentials Of Removable Partial Denture Prosthesis Essentials Of Removable Partial Denture Prosthesis

- APPLEGATE- APPLEGATEPartial dentures Partial dentures

-JOHN OSBORNE-JOHN OSBORNERemovable Partial ProsthodonticsRemovable Partial Prosthodontics

-GRASSO-GRASSORemovable Denture Prosthodontics Removable Denture Prosthodontics

- GRANT- GRANT


AcknowledgementsAcknowledgements

I would like to thankI would like to thank Dr Geetha K PatilDr Geetha K Patil Dr Malathi DayalanDr Malathi Dayalan Dr Nagaraj K.R Dr Nagaraj K.R Dr Shilpa ShettyDr Shilpa Shetty Dr Manjunath .S.BDr Manjunath .S.B Dr Bharath ShettyDr Bharath Shetty

Without whose help and support this Without whose help and support this presentation would not have been possiblepresentation would not have been possible


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