FPD

1. Biomechanical principles of tooth preparations
PRESENTED BY:
SAPAMSONIA
BDS FINALYEAR
ROLL NO.15
CONTENTSOF THE SEMINAR
 INTRODUCTION
 PRINCIPLESOFTOOTH PREPARATION
 REQUIREMENTS OFPREPARATION
 RETENTION FORM
GEOMETRY OF THE TOOTH PREPARATION
 RESISTANCEFORM
 STRUCTURAL DURABILITY
 MARGINALINTEGRITY
FINISHLINES
 CONCLUSION
 REFERENCES
INTRODUCTION
 Teethdonot possessthe regenerative abilityfoundinmostothertissues.Therefore,once
enamel ordentinislostas a resultof caries, trauma,or wear,restorative materialsmustbe
usedto reestablishformandfunction.
 Teethrequire preparation toreceiverestorations,andthese preparationsmustbe basedon
fundamental principles.
The designandpreparationof a tooth are governedbyfive principles:
1)Preservationof toothstructure
2)RetentionandResistanceform
3)Structural durabilityof the restoration
4)Marginal integrity

2. 5)Preservationof the periodontium
REQUIREMENTS OFTOOOTH PREPARATION
Preservationof tooth structure
 Excessremoval of toothstructure can have many ill effects.If atooth isovertaperedor
shortened toomuch,there will be unnecessarysacrifice of retentionandresistance.
 Thermal hypersensitivity,pulpal inflammationandnecrosiscanresultfromapproachingthe
pulptoo closely.
1.USE OF PARTIALCOVERAGERATHER THAN FULL COVERAGERESTORATIONS
2. Preparationof toothwithminimal practical convergenceangle (taper) betweenaxial walls
3. Preparationof occlusal surface followingthe anatomical contourtogive uniformreduction
4. Reductionof axial wallswithmaximumthicknessof remainingdentin surroundedthe pulp
5. Selectionof marginal geometrywhichisconservative andcompatible withotherprinciples
6. Avoidance of unnecessaryapical extensionof the preparation
Alsoyoushouldbe careful about;
1. Adjacenttooth-
 By usingmetal matrix band
 Use of proximal enamel of the tooth
2. Soft tisues
 By careful retractionwithanaspiratortipmouth mirroror flangedsalivaejector.
Pulp:
Tooth preparationmustnotinvolve orirritate the pulpchamber.
Pulpsize ismore inyoungand adolescentindivisualsanddecrease withage.
Causesof pulpinjury:
 Increaseddepthof preparation
 Thermal
 Chemicals
 Bacterial
 Preservationof toothstructure requiresdesigningthe restorationsothatitwill reinforce
and protectthe remainingenamel anddentin.

3. Retentionform
 The feature of a toothpreparationthatresistsdislodgementof acrownin a vertical
directionoralongthe path of placement(GPT9).
 Retentionpreventsremoval of the restorationalongthe pathof insertionorlongaxisof the
toothpreparation.
 The essential elementof retentionistwoopposingvertical surfacesinthe same preparation.
 These maybe external surfaces,suchasthe buccal and lingual wallsof afull veneercrown.
 It can alsobe internal,suchasbuccal and lingual wallsof the proximal box of aproximo-
occlusal inlay.
GEOMETRY OF THE TOOTH PREPARATION
 Degree of taper
 Total surface areaof the cementfilm
 Areaof cementundershear,and
 Roughnessof the toothsurface.
1)Taper and Retention
 Taper isthe convergence of two opposingexternal wallsof atooth preparationasviewedin
a givenplane.
 Taper isthe angle,measuredindegreesasviewedinagivenplane,formedbetweenan
external wall andthe pathof placementof a toothpreparationormachinedsurfacesona
metal or ceramicmaterial whenpreparedforfixeddental prosthesis.(GPT9)
 More nearlyparallel the opposingwallsof apreparation,the greaterwillbe the retention.
 Thishas beenverifiedexperimentallybyJorgensen,whofoundthatretentiondecreasesas
taperincreases.
 The most retentive preparationwouldbe one withparallel walls.
 To allowthe complete seatingof restorationduringcementation,the range of 2 to 6.5
degreestaperhasbeenconsideredtobe optimal.
As the degree of taperof a preparation increases,itsabilitytoretainarestorationdecreases
To produce an optimal 6-degree taper,orconvergence angle,eachopposingaxial wall shouldhave
an inclinationof 3degreestothe path of insertion

4. 2)Surface area
 The greaterthe area of the cementfilmboundtothe preparationandto the internal detail
of the casting,the greaterthe retentionof the castingwill be.
 Therefore,the greaterthe surface areaof the preparation,the greaterthe retentionof its
restoration.
The total surface areais influencedby:
 The size of the tooth
 The extentof the coverage bythe restoration
 Featuressuchas groovesandboxesthatare placedinthe preparation.
3)Area under shear
 Most importantforretentionthanthe total surface area isthe area of cementthatwill
experience shearingratherthantensilestress.
 So,to decrease the failure potential ,itisessential tominimizetensilestress.
 To utilizedthe shearstrengthof the cement,the preparationmusthave opposing
walls,i.e.,twosurfacesof the preparationinseparate planesmustbe nearlyparallel with
each otherandthe line of draw.
 The opposingsurfacesmaybe internal orexternal.
(A)Aninlaydependsoninternalretentiontoholditwithinitspreparation.(B)Internalretentionis
createdby the close adaptationof a restorationto2 or more opposing,slightlydivergentinternal
walls.
(A) A CROWN DEPENDSPRIMARILY ON EXTERNAL RETENTION TO RESIST REMOVAL.(B) EXTERNAL
RETENTION IS PROVIDEDBY APPROXIMATION OFTHE RESTORATION TO THE OPPOSINGEXTERNAL
AXIALWALLS OFTHE PREPARATION.
(B)The additionof parallel-sidedgrooveslimitsthe pathof withdrawal toone direction
 Path of insertionandretention
 A severelyovertaperedpreparationhasmanypathsalong whicha tensile force could
remove acrown.
 Retentionisenhancedbyrestrictingthe possible pathsof withdrawal orpathsof insertion.
However,if one of the fouraxial wallsismissingorleftuncovered,the potential pathsof withdrawal
are greatlyincreasedandretentioniscompromised
 A full veneercrownpreparationhas excellentretentionbecausethe mesial,distal,lingual,
and facial wallslimitthe possiblepathsof insertiontoa narrow range.

5. A)Makingthe pathof insertionof ananteriorthree-quartercrownparallel withthe longaxisof the
toothwouldresultinan unnecessarydisplayof metal.
B)The preferredpathof insertionforananteriorpreparationparallelsthe incisal 2/3rd of
the labial
surface.Thiswill minimizethe displayof metal andallow the groovestobe longerandmore
retentive
Whenan axial wall isleftunveneered,retentionisachievedbysubstitutinggrooves(A),Boxes(B),or
Pinholes(C) forthe missingwall.
 Lengthof the preparationisanimportantfactor inretention.
 A longpreparationhasgreaterretentionthandoesashortpreparation.Thisisdue to ,
greaterthe surface area of cementfilm, greaterthe retention.
A restorationonlongpreparation(A) canwithstandaforce that couldremove arestorationfroma
shorterpreparationof equal diameter(B)
A preparationwithalargerdiameter(andcircumference ) will have greaterretentionthanwitha
narrow preparationof the same length.
Surface roughness
a)The adhesionof dental cementsdependsprimarilyonprojectionsof the cementintomicroscopic
irregularitiesandrecessesonthe surface beingjoined,the preparedtoothsurface shouldnotbe
highlypolished.
b)OiloandJorgensenfoundretentionof castings cementedwithzincphosphate cementontestdies
with10degreestaperto be twice as greaton preparationswith40micrometerscratchesasonthose
with10micrometerscratches.
Resistance form
 The featuresof a tooth preparationthatenhance the stability of arestorationandresists
dislodgementalonganaxisotherthanthe path of placement(GPT9)
 Resistance preventsdislodgementof arestorationbyforcesdirectedinanapical,oblique,or
horizontal direction.
 Leverage and resistance
 The strongestforcesencounteredinfunctionare apicallydirectedandcanproduce tension
and shearinthe cementedfilmonlythroughleverage.
(A) Whenthe line of actionof an appliedforce passeswithinthe marginsof the restoration,no
secondaryliftingforcesare producedbecausethe marginonall sidesof the restorationissupported
by the preparation.
(B)Whenthe line of actionpassesoutsidethe marginof the restoration,a torque isproducedthat
will tendtotipor rotate the crownaround a pointon the margin.

6. Preparationlengthandresistance
 Shorteningapreparationwill produce aproportionallygreaterdiminutionof the resisting
area.
 Decreasingthe lengthof ataperedpreparationcausesadisproportionatedecreaseinthe
resistingarea.
 A crownwith relativelylongaxial wallscanresistastrong tippingforce.
 If two crownsof unequal lengthontwopreparationsof equal lengthare subjectedto
identical forces,the longercrownismore likelytofail because the force onitacts througha
longerleverarm.
(A) The resistance of thispreparationisadequate topreventthe crownfromtippingunderthe
appliedforce.
(B) Althoughthe preparationandthe appliedforce in(B) are identicaltothose in(A) ,the crown
failsbecause of the greaterheightof the restoration.
The lengthof the primary leverarmisthe shortestdistance fromthe line of actionof the force to
the nearestmargin.
(A) On the short crownthe liftingforce issmall because the primaryleverarmisshort.
(B) Witha longcrown, the same force producesa greatertorque because itsline of action
passesfartherfromthe pointof rotation.
Resistance and tooth width
 A crownon the narrow toothcan have greaterresistance totippingthanone onthe wider
tooth.
 Thisoccurs because the crownon the narrowertooth has a shorterradiusof rotation
resultinginalowertangentline andalarger resistingarea.
A)Because of the small diameter,the tangentlineof thisnarrow preparationfallslow onthe wall
opposite the axisof rotation,resulting inalarge resistingarea.
B)Because of the muchgreaterradiusof the arc of rotation,itsresistingareaissmallerthanthatof
the narrowerpreparation.
The weakresistance of a short,wide preparation(A) canbe enhancedbythe additionof vertical
grooves(B).Fromthe side(C),itcanbe seenthatthe arc of radiusr2 iseffectively blockedbythe
resistingareaof the groove walls,whilethe arcof radiusr1 encounterslittle ornoresistance onthe
far axial wall
 Taper and resistance
The resistingareadecreasesasthe preparationtaperincreases.

7. A) For a cylinderwithnotaper,the resistingareawouldcoverhalf the axial walls
B) For a ideally- taperedtoothpreparationthe resistingareacoverssomewhatlessthanhalf
the axial walls
C) An over-tapered(20degrees) preparationhasonlyasmall resistingareanearthe occlusal
surface.
 The inclinationof the wallsof apreparationthatwill still provide effectiveresistance is
dependentonthe height/widthratio.
 A) A preparationwithheight/width ratioof 1:1 can have a wall inclinationof 15 degreesand
still have effectiveresistance.
 B) A preparationwitharatioof 1:2 can have a indivisual wallinclinationnogreaterthan
7degreeswithoutseriouslycompromising resistance
Rotationaroundvertical axis
 Whena crown issubjectedtoan eccentrichorizontal force,momentsof torque occur
arounda vertical aswell ashorizontal axis
A) A partial veneercrownwhichhasnogrooveshas little resistance torotationarounda
vertical axis
B) Whengrooves are present,theirlingual wallsprovide resistance byblockingthe arcof
rotation
A)The axial symmetryof afull veneercrownpreparationmayallow rotationof the restoration
aroundthe preparation
B)Resistance canbe gainedbyformingvertical planes(wings) whichare perpendiculartothe arc of
rotation.
PATH OFINSERTION
The specificdirectioninwhichprosthesisisplacedonthe abutmentteeth(GPT8)
The specificdirectioninwhichaprosthesisisplacedonthe residual alveolarridge,abutmentteeth,
dental implantabutment(s),orattatchments.(GPT9)
AlsoknownasPath of placement.
 A pathmust be selectedthatwill allowthe marginsof the retainerstofitagainsttheir
respective preparationfinishlineswiththe removal of minimumsoundtooth structure.
 The ideal pathof insertionforaposteriorfull orpartial veneercrownisparallel withthe long
axisof the tooth.
A)Fora full crownto have structural durabilitywithpropercontours,itspathof insertionwould
parallel the longaxisof the tooth.

8. 
B) If the tooth istilted
C) A path of insertionparallelingthe longaxisof the toothmaybe blockedbythe proximal contours
of the adjacentteeth.
D)Inthiscase , the path of insertionismade perpendiculartothe occlusal plane
Whenviewedwithone eye fromadistance of 30cm or 12 inch,all the axial surfacesof a
preparationwithanideal taperor angle of convergence of 6degeescanbe seen.
 All negative taper,orundercutmustbe eliminatedoritwill preventthe restorationfrom
seating.
 Binocularvisionshouldneverbe employedtoevaluate apreparationforcorrecttaper.With
botheyesopen,apreparationthatis undercutcan appearto have an acceptable degree of
taper
A mirror isusedto evaluate apreparationwhere directvisionisnotpossible.Anobstructedview of
the entire finishline barelyoutsidethe circumference of the occlusal surface indicatescorrecttaper.
Structural durabilityof the restoration
 It isthe abilityof the restorationtolastlongwithoutdamage, underocclusal forces.
 There are three preparationfeaturesthatcontribute tothe durabilityof the restoration:
 A)occlusal reduction
 B)axial reduction
 C)provisionforreinforcingstruts.
1) Occlusal reduction
 Enough toothstructure mustbe removedfrom the occlusal surface of the preparationso
that whenthe restorationisbuiltbacktoideal occlusionitwill be thickenoughtoprevent
wearingthroughor distorting.
 A goldcrownrequiresapproximately1.5mmclearance overthe functional cuspsand1.0mm
overthe non-functional cusps.
 If a porcelainveneerisextendedontothe occlusal surface,anadditional 0.5mmof space is
needed.
 Occlusal reductionshouldreflectthe geometricinclinedplanesunderlyingthe morphology
of the finishedcrownandfollowthe majorplanesof the opposingfacial andlingual cuspsas
well.
 Avoidcreatingsteepplaneswithsharpangles,since these canincrease stressandhinder
complete seatingof the casting.

9.  To diminishstress,roundthe anglesandavoiddeepgroovesinthe centerof the occlusal
surface.
A)Correctocclusal reductionparallelsthe majorplanesof the tooth
B)Flat,single-plane occlusalreductionmayresultininsufficientthicknessof the restorationoverthe
groovesandfossae.
C)Anattemptto avoidthisproblembyloweringthe entire occlusaltable will cause excessive
destructionof toothstructure,andthe axial wallswill be overshortenedwtharesultantlossof
retentionandresistance
 Functional cuspbevel :
A wide bevel shouldbe placedonthe functionalcuspsof posteriorteethtoprovide structural
durabilityinthiscritical area.
A) A functional cuspbevel inaplane parallelingthatof the opposingcuspallowsforadequate
restorationthicknesswithoutunduesacrificeof toothstructure.
B)If the functional cuspbevel isomitted,the restoratonislikelytobe toothinin thisstressbearing
area
C)If restorationthicknessisachievedbyovertapering the axial wall,retentionwillbe compromised
D)Frequently,inthe absence of a functional cuspbevel,the technicianwill overbulkthe crown.
Thiscan resultinsuperocclusionof the restoration,whichcouldonlybe correctedbyocclusal
reductionof the opposingtooth
Whenteethare in a crossbite relationship,the functional cuspbevelisplacedonthe buccal cuspsof
maxillaryteethandthe lingual cuspsof mandibularteeth.
2) Axial reduction
A)Whenaxial reductionissuffucient,restorationwalls canhave satisfactorythicknesseswithout
overcontouring.
B)If axial reductionisinadequate,arestorationwithnormal contourswouldbe thinandflexible.
C)The more commonresultof inadequate axial reductionisanovercontouredcrown
Marginal integrity
Margin: The outeredge of a crown,inlay,onlay,orotherrestoration(GPT9)
Finishline:Terminal portionorperipheral extensionof the preparedtooth(GPT8)
A boundarysurface of a tooth preparationistermedthe finishlineorfinishcurve.(GPT9)
There are 3 requirementsforsuccessful restorationmargins:

10. a)Theymustfitas closelyaspossible againstthe finishline of the preparationtominimizethe width
of exposedcement.
b)Theymusthave sufficientstrengthtowithstandthe forcesof masticationand
c)Wheneverpossible theyshouldbe locatedinareaswhere the dentistcanfinishandinspectthem
and the patientcan cleanthem.
Margin can be placedby:
1. Supragingival (atorabove the gingival crest)
2. Subgingival (belowthe gingival crest)
Margins shouldbe placedsupragingivallywheneverpossible.
Supragingival margins:
Advantages-
A) Placedonenamel
B) Easy to prepare withouttraumatosoft tissue
C) Can be easilyfinished
D) Impressionmakingisalsolesstraumatictosofttissuesandeasyto reproduce.
E) Fitof the restorationscanbe easilyevaluated
F) Can be maintainedeasilybythe patient
Subgingival margins:
Placedat least2mm above the alveolarcrestsothat the biologicwidthisnotencroached.
Indications:
a)caries,erosionorrestorationsextendingsubgingivally
b)Aesthetics-whenmetal-ceramicrestorationsare used.
c)Additionalretention- incase of shortcrown
d)Rootsensitivity
e)Proximalcontactextendingtogingival crest
bevel
 The processof slantingthe finishline andcurve of a toothpreparation.(GPT9)

11.  Roughnessof the toothsurface undermarginscan preventclose adaptaion,therefore all
bevelsandflaresshouldbe givenasmoothfinishline.
 Metal marginsshouldbe acute in cross-sectionratherthanright-angledtofacilitateacloser
fit.
A bevel allowscloserapproximationof crownmargintothe tooth.
 The distance,Dbywhicha crown failstoseat(A),isreflectedinthe marginal opening,d,in
the butt joint(B).However,inthe presence of abevel , the shortestdistance fromthe margin
to toothstructure is lessthanD.
 Andis a functionof the sine of the acute angle of the margin,mor the cosine of the obtuse
angle of the finishline,p(C)
It isexplainedbythe formula
d=D sinm, or
d=D cos p.
As the angle mis reduceditssine becomessmallerandsodoesd.The more obtuse the angle of
toothstructure at a horizontal finishline,andtherefore,the more acute the restorationmargin,the
shorterthe distance betweenthe restorationmarginandthe tooth.
d =the shortestdistance fromthe castingmargintotooth structure
D=the distance bywhichthe crownfailstoseat.
The smallerthe angle betweenthe prepared toothsurface atthe finishline andthe pathof
insertion,the lessthe marginalopeningforthe same amountof incomplete seating
The margin angle mustbecome quite acute before the actual distance isdiminishedtoa great
extent.
An angle of 30 to 45 degreesisconsideredoptimal.
If it is made muchmore acute it becomesweak.
 To impartstrengthand rigiditytoa margin, there shouldbe a nearbybulkof metal.Thiscan
be providedbyan:
 occlusal offsetorshoulder
 an isthmus
 a vertical groove
 a box or
 a gingival shoulder.

12. Occlusal bevels
The cavosurface line anglesof the preparationinthe occlusal portionof the toothshouldbe finished
witha bevel toavoida rightangle “butt joint”whichwouldleaveabrittle,easilyfracturededge of
toothstructure.
(left)Impactsstrikingenamel nexttoabevel canbe withstoodwithoutdamage.
(right)Where thereisnobevel,the unprotectedenamelrodsnearthe restorationcanbe more easily
fractured
Flares
The vertical finishlineof the inlay,onlayorpartial veneercrownpreparationisfinishedwithaflare,
whichformsan acute edge of metal inthe castingand extendsitintoanaccessible area.
A flare differsfromabevel inthata flare isa geometricplane inclinedslightlytothe pathof
insertionandcuttingthroughthe contourof the toothwhereasbevel followsthe contourof the
tooth.
Finishline configurations
1.chamfer
 It isan obtuse angledfinishline.
 It isdistinct.
 Exhibitsleaststress.
 Most conservative.
 Indications:Castmetal restorations,lingual marginsof metal ceramiccrowns.
 Roundendtaperingdiamond.
2.Heavychamfer
 Similartochamferbut preparedwithadiamondof greaterdiameter
 Indicatedforall ceramiccrowns
 Distinctmargin,adequate bulk,easytocontrol
 Can produce an unsupportedlipof enamel.
3.Shoulder
 It isa rightangledfinishline.
 Produce a wide ledge whichresistscompressive occlusal forces.
 Notconservative

13.  Indicatedforceramicrestorations- all ceramicandmetal ceramic
 Flatendtaperingdiamond.
a.Shoulderwithbevel
b.facial marginhiddeninmetal ceramicrestoration
4.Shoulderwithbevel
 Protectsthe edge of finishline preventingchipping
 Recommendedforextremelyshortaxial wallstocreate maximumretentionandresistance
form.
 Primarilyindicatedtohide the supragingival facial metalmarginof metal-ceramic
restorations.
5.Radial shoulder
 It isa shoulderfinishlinewithroundedlineangle.
 All ceramiccrowns
6.Slopedshoulder
 Similartoshoulderbutwithobtuse angle
 Indicatedforfacial marginof metal ceramiccrown
7.Knife edge
 It isa thinfinishline
 Advantages:Highlyconservative
 Disadvantages:Notdistinct
 May be usedformetal restoration.
Preservationof the periodontium
 Several studieshave determinedthatthe deeperthe restorationmarginextendsintothe
gingival sulcus,the more severe the inflammatoryresponse.
 A crownmargin shouldnotbe placedanycloserthan 2.0 mmaway fromthe alveolarcrest,
or bone resorptionwill occur.
The distance fromthe epithelial attatchmenttothe crestof the alveolar bone hasbeendescribedas
biologicwidth.Itisnormallyabout2mmwidth,includingthe epithelialattatchmentandconnective
tissue attatchment

14. Whenthe margin of a restorationintrudesintothe biologicwidth,inflammationandosteoclastic
activityare stimulated
Bone resorptionwill continue until the alveolarcrestisatleast 2.0mm fromthe restoration
margin.Thiscan resultinan infrabonypocket.
CONCLUSION
Biomechanicsis of prime consideration in achieving an aesthetic,functionaland durablefixed
prosthesisso every dentist should havea thorough knowledgeabouttheboimechanicsof tooth
preparation asevery step in this playsa majorrole in the fixed prosthesissuccess.
REFERENCES
Thankyou