The biomechanics of stresses induced by removable partial denture

THE BIOMECHANICS
THE BIOMECHANICS
OF STRESSES INDUCED
OF STRESSES INDUCED
BY REMOVABLE
BY REMOVABLE
PARTIAL DENTURE
PARTIAL DENTURE
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.comwww.indiandentalacademy.com

CONTENTSCONTENTS
IntroductionIntroduction
What Is StressWhat Is Stress
Simple machinesSimple machines
Development of forcesDevelopment of forces
Forces acting on the removable partial dentureForces acting on the removable partial denture
Distribution of stress between the teeth andDistribution of stress between the teeth and
tissuetissue
Induced stresses to be recognizedInduced stresses to be recognized
Consequence of inefficient management of stressesConsequence of inefficient management of stresses
ConclusionConclusion
BibliographyBibliography

INTRODUCTIONINTRODUCTION
 Nature has designed theNature has designed the StomatognathicStomatognathic
system to dissipate the forces of masticationsystem to dissipate the forces of mastication
to the supporting structures.to the supporting structures.
 In a partially edentulous situation theIn a partially edentulous situation the
mechanism of dissipating the force by themechanism of dissipating the force by the
dentition is compromised due to the breach indentition is compromised due to the breach in
integrity of the dental arches.integrity of the dental arches.

 When a removable partial denture is usedWhen a removable partial denture is used
to restore the missing natural teeth and toto restore the missing natural teeth and to
maintain the continuity of the dental archesmaintain the continuity of the dental arches
it is important for the dentist to consider theit is important for the dentist to consider the
direction ,duration and frequency of thedirection ,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 externallyAn 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 stress
Tensile stressTensile stress
Shear stressShear stress

SimpleSimple machinesmachines
With the introduction of the removable partial denturesWith the introduction of the removable partial dentures
into the oral cavity certain simple machines come intointo the oral cavity certain simple machines come into
play. These canplay. These can magnifymagnify the stresses .It is thereforethe stresses .It is therefore
important to know its functioning to maintain the health ofimportant to know its functioning to maintain the health of
the oral structures.the oral structures.
LeverLever
WedgeWedge
ScrewScrew
Wheel and axleWheel and axle
PulleyPulley
Inclined planeInclined plane

Development of forcesDevelopment of forces
The most commonly encountered simpleThe most commonly encountered simple
machines in removable partial denture are:machines in removable partial denture are:
LeverLever
Inclined planeInclined plane
It should be borne in mind that the success of aIt should be borne in mind that the success of a
removable partial denture lies in avoiding theseremovable partial denture lies in avoiding these
to the greatest extent possibleto the greatest extent possible..

LeverLever
AA lever is a rigidlever is a rigid
bar that rests on abar that rests on a
fulcrum (a fixedfulcrum (a fixed
point.)point.)
The farther the forceThe farther the force
is from the fulcrum,is from the fulcrum,
the easier it is tothe easier it is to
work the leverwork the lever
There areThere are threethree
classesclasses of levers.of levers.

Class IClass I
AA class-1class-1 leverlever has itshas its
fulcrum locatedfulcrum located
somewhere betweensomewhere between
the effort and thethe effort and the
resistanceresistance
E.g. Distal extensionE.g. Distal extension
removable partialremovable partial
denturedenture

Class IIClass II
With aWith a class-2 leverclass-2 lever,,
the fulcrum is at onethe fulcrum is at one
end, the effort is at theend, the effort is at the
other end and theother end and the
resistance is in theresistance is in the
middlemiddle
Seen as indirectSeen as indirect
retention in removableretention in removable
partial denturepartial denture

Class IIIClass III
In aIn a class-3 leverclass-3 lever. The. The
fulcrum is at one end andfulcrum is at one end and
the effort is appliedthe effort is applied
between the fulcrum andbetween the fulcrum and
the resistance.the resistance.
Not encountered inNot encountered in
partial denturespartial dentures
TMJ muscles and teethTMJ muscles and teeth
act as a class III leveract as a class III lever

Inclined PlaneInclined Plane
Forces against theForces against the
inclined plane may resultinclined plane may result
in deflection of thatin deflection of that
which is applying thewhich is applying the
force or may result inforce or may result in
the movement of thethe movement of the
inclined plane .inclined plane .
Neither of these resultsNeither of these results
are desirableare desirable
E.g Cingulum rest seatE.g Cingulum rest seat
on cuspidson cuspids

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

Forces acting during placementForces acting during placement
and removal of the partialand removal of the partial
denturedenture
1)1) When a contacting rigid area of a removableWhen a contacting rigid area of a removable
partial denture passes over the surface bulge ofpartial denture passes over the surface bulge of
the abutment teeth.the abutment teeth.
2)2) Movement of the retentive clasp out of theMovement of the retentive clasp out of the
infrabulgeinfrabulge area and over the abutment height ofarea and over the abutment height of
contour.contour.

Forces acting on tooth and tissueForces acting on tooth and tissue
borne removable partial dentureborne removable partial denture
Forces acting on a distal extension partialForces acting on a distal extension partial
denture are a result of a composite of forcesdenture are a result of a composite of forces
arising from the three principal fulcrums.arising from the three principal fulcrums.
1) Fulcrum on the1) Fulcrum on the horizontal plane.horizontal plane.
2) Fulcrum on the2) Fulcrum on the sagittal plane.sagittal plane.
3) Fulcrum on the3) Fulcrum on the vertical plane.vertical plane.

Fulcrum on theFulcrum on the
horizontal planehorizontal plane
Extends through theExtends through the
two principaltwo principal
abutments, one onabutments, one on
each side of the dentaleach side of the dental
arch.arch.
Controls rotationalControls rotational
movement of themovement of the
denture in the sagittaldenture in the sagittal
plane.plane.

Sagittal PlaneSagittal Plane
Extends through theExtends through the
occlusal rest on the terminalocclusal rest on the terminal
abutment and along theabutment and along the
crest of the residual ridgecrest of the residual ridge
on one side of the archon one side of the arch
The fulcrum controls theThe fulcrum controls the
rotational movement of therotational movement of the
denture in the vertical planedenture in the vertical plane

vertical planevertical plane
Located in the vicinity ofLocated in the vicinity of
the midline just lingualthe midline just lingual
to the anterior teeth.to the anterior teeth.
Controls the rotationalControls the rotational
movement of themovement of the
denture in the horizontaldenture in the horizontal
direction.direction.

Forces acting on tooth supportedForces acting on tooth supported
removable partial dentureremovable partial denture
Kennedy’s Class III situationKennedy’s Class III situation
Supported by abutmentsSupported by abutments
Force against it are transmittedForce against it are transmitted
down the long axisdown the long axis
of the abutment teethof the abutment teeth

Distribution of stress betweenDistribution of stress between
teeth and tissue?teeth and tissue?
Two schools of thought exists in the matter ofTwo schools of thought exists in the matter of
allocating the proportionate share of stress thatallocating the proportionate share of stress that
is to be sustained by the tooth and the residualis to be sustained by the tooth and the residual
ridge.ridge.

Stressbreaker SchoolStressbreaker School
Residual ridgeResidual ridge bearbear the stresses and teeth bethe stresses and teeth be
spared.spared.
Use of flexible or movable joint between theUse of flexible or movable joint between the
teeth and the metal framework.teeth and the metal framework.

Wide Distribution of StressWide Distribution of Stress
SchoolSchool
Support provided by aSupport provided by a combinationcombination ofof
abutment tooth and the mucosa.abutment tooth and the mucosa.
Various clinical techniques and designVarious clinical techniques and design
variations have been tried so as to exploit thevariations have been tried so as to exploit the
maximum load bearing capacity of each.maximum load bearing capacity of each.

Induced stresses to beInduced stresses to be
recognizedrecognized

Stress resulting from applianceStress resulting from appliance
inaccuraciesinaccuracies
Orthodontic stressOrthodontic stress
MalocclusionMalocclusion
Impingement of subbasalImpingement of subbasal
structuresstructures

Stress from interference toStress from interference to
appliance insertionappliance insertion
Intermittent disturbance of tooth alignment .Intermittent disturbance of tooth alignment .
Contacting rigid area of a removable prosthesisContacting rigid area of a removable prosthesis
passes over the surface bulge of the abutmentpasses over the surface bulge of the abutment
tooth.tooth.

Gingival impingement by theGingival impingement by the
removable applianceremovable appliance
Gingivae most susceptible to injury by anyGingivae most susceptible to injury by any
pressure induced by a removable prosthesis.pressure induced by a removable prosthesis.

Stress on a sloping tooth surfaceStress on a sloping tooth surface
for the support of an abutmentfor the support of an abutment
occlusal rest.occlusal rest.
Seen while using cuspid tooth for abutmentSeen while using cuspid tooth for abutment
services with steep lingual surface.services with steep lingual surface.
Appliance slips as occlusal load is appliedAppliance slips as occlusal load is applied
which can cause gingival irritation.which can cause gingival irritation.

Stress resulting in the impingementStress 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 ofFactors influencing magnitude of
stress transmitted to thestress transmitted to the
abutment teethabutment teeth

Length Of SpanLength Of Span
Longer Edentulous SpanLonger Edentulous Span==Longer DentureLonger Denture
BaseBase==Greater Force to the Abutment TeethGreater Force to the Abutment Teeth
The fulcrum is located at or near the occlusalThe 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 muchThe 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 areLarge well formed ridges are
capable of absorbing greatercapable of absorbing greater
amounts of stress than are smallamounts of stress than are small
thin or knife edged ridges.thin or knife edged ridges.
Broad ridges with parallel sidesBroad ridges with parallel sides
 longer flanges on denturelonger flanges on denture
basebase stabilizes the denturestabilizes the denture
against the lateral forcesagainst the lateral forces

Approximately 1mm thick mucoperiosteumApproximately 1mm thick mucoperiosteum
can bear greater functional loads thancan bear greater functional loads than
atrophic mucosa.atrophic mucosa.
Soft flabby displaceable tissue allowsSoft 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 abutmentthe less stress is transmitted to the abutment
tooth.tooth.
A passive clasp will exert less force than anA passive clasp will exert less force than an
active clasp.active clasp.

Flexibility can be increased byFlexibility can be increased by lengtheninglengthening
the clasp.the clasp.
Clasp length is increased by using a curvedClasp length is increased by using a curved
path on an abutment tooth.path on an abutment tooth.
Gold clasps will exert less stress on theGold 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 givesThe surface of a gold crown or restoration gives
more frictional resistance to clasp armmore 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 toothHence greater stress is exerted on a tooth
restored with gold than on a tooth with intactrestored with gold than on a tooth with intact
enamel.enamel.

Occlusal harmonyOcclusal harmony
Disharmonious occlusionDisharmonious occlusion
generates horizontalgenerates horizontal
forcesforces
Partial denturePartial denture
constructed to opposeconstructed to oppose
natural dentition will facenatural dentition will face
more stresses.more stresses.

If the occlusalIf the occlusal loadload is applied to the baseis 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 asof the denture base as
compared if the load was applied to the distalcompared if the load was applied to the distal
end.end.
The occlusal load should be applied in theThe occlusal load should be applied in the
center of the denture bearing area.center of the denture bearing area.

Consequence of inefficient management ofConsequence of inefficient management of
stresses induced by partial denturestresses induced by partial denture
GreaterGreater
stressesstresses
Abutment ToothAbutment Tooth
OrthodonticOrthodontic
movement.movement.
Malocclusion.Malocclusion.
Residual RidgeResidual Ridge
Inflammation,Inflammation,
Edema of residualEdema of residual
ridge mucosa.ridge mucosa.
Resorptive loss ofResorptive loss of
adjacent alveolaradjacent alveolar
process .process .

CONCLUSIONCONCLUSION
Removable Partial Dentures can be aRemovable Partial Dentures can be a
boon or a bane based on theboon or a bane based on the
understanding of its biomechanicalunderstanding of its biomechanical
principles.principles.
Blindly done, the removable partialBlindly done, the removable partial
denture can act as a potentiallydenture can act as a potentially
destructive device which could inducedestructive device which could induce
more harm than than the good it wasmore harm than than the good it was
intended for.intended for.

BibliographyBibliography
Removable partial prosthodonticsRemovable partial prosthodontics
- McCRACKEN- McCRACKEN
Clinical Removable Partial ProsthodonticsClinical Removable Partial Prosthodontics
- STEWART- STEWART
Essentials Of Removable Partial Denture ProsthesisEssentials Of Removable Partial Denture Prosthesis
- APPLEGATE- APPLEGATE
Partial denturesPartial dentures
-JOHN-JOHN
OSBORNEOSBORNE
Removable Partial ProsthodonticsRemovable Partial Prosthodontics
-GRASSO-GRASSO
Removable Denture ProsthodonticsRemovable 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.RDr 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 thisWithout whose help and support this
presentation would not have been possiblepresentation would not have been possible

THANK YOUTHANK YOU
Thank you

The biomechanics of stresses induced by removable partial denture

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