Principles of tooth preparation/ orthodontic seminars

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com
www.indiandentalacademy.comwww.indiandentalacademy.com

PRINCIPLES OF TOOTHPRINCIPLES OF TOOTH
PREPARATIONS IN FPDPREPARATIONS IN FPD
Tooth preparation is defined as theTooth preparation is defined as the
mechanical treatment of dental diseasemechanical treatment of dental disease
or injury to hard tissues that restores aor injury to hard tissues that restores a
tooth to original form.tooth to original form.

PRINCIPLES OF TOOTHPRINCIPLES OF TOOTH
PREPARATIONPREPARATION
1.1. Preservation of toothPreservation of tooth
structurestructure
2.2. Retention and resistanceRetention and resistance
formform
3.3. Structural durability of theStructural durability of the
restorationrestoration
4.4. Marginal integrityMarginal integrity
5.5. Preservation of thePreservation of the
periodontiumperiodontium

PREPARATIONPREPARATION
OBJECTIVESOBJECTIVES
1.1. Reduction of the tooth in miniature to provideReduction of the tooth in miniature to provide
retainer support.retainer support.
2.2. Preservation of healthy tooth structure toPreservation of healthy tooth structure to
secure resistance formsecure resistance form
3.3. Provision for acceptable finish linesProvision for acceptable finish lines
4.4. Performing pragmatic axial tooth reduction toPerforming pragmatic axial tooth reduction to
encourage favorable tissue responses fromencourage favorable tissue responses from
artificial crown contours, i.e., fluting of molars.artificial crown contours, i.e., fluting of molars.

 Biologic considerationsBiologic considerations which affect thewhich affect the
health of the oral tissues.health of the oral tissues.
 Mechanical considerationsMechanical considerations which affect thewhich affect the
integrity and durability of the restorations.integrity and durability of the restorations.
 Esthetics considerationsEsthetics considerations which affect thewhich affect the
appearance of the patient.appearance of the patient...

 Biologic considerations:Biologic considerations:
Pulpal considerationsPulpal considerations
Periodontal considerationsPeriodontal considerations
Protection of adjacent teethProtection of adjacent teeth
Protection of lips, cheeks and gums.Protection of lips, cheeks and gums.

 Pulpal considerations:Pulpal considerations:

conservation of tooth structureconservation of tooth structure

protection of pulp from bacterial actionprotection of pulp from bacterial action

protection of pulp from thermal injuryprotection of pulp from thermal injury

 Conservation of tooth structureConservation of tooth structure::
Factors that influence tooth conservation:Factors that influence tooth conservation:
 Selection of retainerSelection of retainer: in terms of surface involvement, partial: in terms of surface involvement, partial
veneer crowns are better than complete crowns because ofveneer crowns are better than complete crowns because of
the less number of surfaces involved.the less number of surfaces involved.
 Taper:Taper: as taper is increased, more tooth structure has to beas taper is increased, more tooth structure has to be
removed.removed.
 Occlusal reductionOcclusal reduction: inclined plane reduction following the: inclined plane reduction following the
anatomic planes help rather than flat plane reduction.anatomic planes help rather than flat plane reduction.
 Finish line locationFinish line location: supragingival margins are more: supragingival margins are more
conservative than subgingival margins.conservative than subgingival margins.
 Finish line configurationFinish line configuration: shoulder preparation is the least: shoulder preparation is the least
conservative after which comes chamfer and then knife edgeconservative after which comes chamfer and then knife edge
being the most conservative.being the most conservative.
 Inclined teethInclined teeth: Donald has put forth four principle techniques: Donald has put forth four principle techniques
for dealing with tilted molars:for dealing with tilted molars:

 Protection of pulp from bacterialProtection of pulp from bacterial
actionaction::
Pulpal damage under restorations hasPulpal damage under restorations has
been attributed to the action of bacteriabeen attributed to the action of bacteria
that were left behind or gained access tothat were left behind or gained access to
the dentine because of micro leakage. It isthe dentine because of micro leakage. It is
necessary to remove carious dentinenecessary to remove carious dentine
before placing a restoration that serves asbefore placing a restoration that serves as
a foundation for fixed prosthesis.a foundation for fixed prosthesis.

 Protection of pulp from thermal and chemical injuryProtection of pulp from thermal and chemical injury::
During tooth preparation, considerable heat is generated byDuring tooth preparation, considerable heat is generated by
friction. The amount of heat generated depends upon:friction. The amount of heat generated depends upon:
 Pressure:Pressure: greater the pressure applied greater is the heatgreater the pressure applied greater is the heat
generated. A light contact with the instrument held firmly isgenerated. A light contact with the instrument held firmly is
indicated.indicated.
 Condition of the rotary instrumentCondition of the rotary instrument: dull instruments, burs: dull instruments, burs
etc need greater pressure application thereby producingetc need greater pressure application thereby producing
greater heat.greater heat.
 Speed of reductionSpeed of reduction: it is directly proportional to the heat: it is directly proportional to the heat
generated.generated.
 Method of reductionMethod of reduction: rapid reduction generates more heat.: rapid reduction generates more heat.
 Coolants:Coolants: either air, water or air-water sprays used helps toeither air, water or air-water sprays used helps to
remove debris and reduce heat generated. Care to be takenremove debris and reduce heat generated. Care to be taken
when preparing grooves or pin holes as coolants do notwhen preparing grooves or pin holes as coolants do not
reach these areas efficiently.reach these areas efficiently.

Periodontal considerations:Periodontal considerations:
In case of any prosthesis that restores esthetic appearance willIn case of any prosthesis that restores esthetic appearance will
takes its toll on tissue irritation.takes its toll on tissue irritation.
Factors affecting are:Factors affecting are:
Axial crown contours and gingival healthAxial crown contours and gingival health: over contoured: over contoured
crown produces gingival irritation.crown produces gingival irritation.
Finish linesFinish lines: four fundamental criteria for successful margins: four fundamental criteria for successful margins
are:are:
1. Acceptable marginal adaptation, so no plaque1. Acceptable marginal adaptation, so no plaque
accumulation, less recurrent caries at crown margins.accumulation, less recurrent caries at crown margins.
2. Tissue tolerant surfaces which means they2. Tissue tolerant surfaces which means they
should be smooth and well polished.should be smooth and well polished.
3. Adequate contouring: sufficient bulk should be3. Adequate contouring: sufficient bulk should be
provided to the restoration without over contouring it.provided to the restoration without over contouring it.
4. Sufficient strength.4. Sufficient strength.www.indiandentalacademy.comwww.indiandentalacademy.com

 Protection of adjacent tooth:Protection of adjacent tooth:
Use of matrix bands and by using proximal enamelUse of matrix bands and by using proximal enamel
of the teeth being prepared for protection ofof the teeth being prepared for protection of
adjacent teeth can be protected. Damagedadjacent teeth can be protected. Damaged
proximal contact areas are more susceptible toproximal contact areas are more susceptible to
caries and plaque accumulation.caries and plaque accumulation.
 Protection of soft tissuesProtection of soft tissues::
Damage to lips, cheek and tongue can be preventedDamage to lips, cheek and tongue can be prevented
by careful retraction with mouth mirror, aspirationby careful retraction with mouth mirror, aspiration
tip, rubber dam.tip, rubber dam.
Gingival retraction should be done with care.Gingival retraction should be done with care.
Suction tip and saliva ejectors should be usedSuction tip and saliva ejectors should be used
carefully.carefully. www.indiandentalacademy.comwww.indiandentalacademy.com

 Occlusal considerationsOcclusal considerations::
Occlusal surface is reduced following the basicOcclusal surface is reduced following the basic
geometric functional pattern of the occlusalgeometric functional pattern of the occlusal
surfaces.surfaces.
 Advantages:Advantages:
Uniform reductionUniform reduction
Tooth conservationTooth conservation
Minimal pulpal traumaMinimal pulpal trauma
Helps in development of proper anatomic andHelps in development of proper anatomic and
functional morphologies in the restoration.functional morphologies in the restoration.
Flat plane reduction results in excessive toothFlat plane reduction results in excessive tooth
removal and at the same time may beremoval and at the same time may be
inadequate in certain areas.inadequate in certain areas.

Mechanical principles:Mechanical principles:
 Retention formRetention form
 Resistance formResistance form
 Structural durabilityStructural durability
 Marginal integrity.Marginal integrity.

 Retention form:Retention form:
This property prevents removal of restoration alongThis property prevents removal of restoration along
path of insertion or long axis of preparation.path of insertion or long axis of preparation.
 Geometric form:Geometric form:
It is the most important factor under the operator’sIt is the most important factor under the operator’s
control which determines whether or not thecontrol which determines whether or not the
restoration will remain cemented to the preparation.restoration will remain cemented to the preparation.
All cements exhibit their greatest strength underAll cements exhibit their greatest strength under
compression and are weakest under tension. Socompression and are weakest under tension. So
retention or resistance can be maximized byretention or resistance can be maximized by
shaping the preparation so that as much of itsshaping the preparation so that as much of its
surface as possible will experience compression orsurface as possible will experience compression or
shear when the restoration is subject to anshear when the restoration is subject to an
unseating force.unseating force.

RETENTIONRETENTION
Taper and retentionTaper and retention
Surface areaSurface area
Area under shearArea under shear
Surface roughnessSurface roughness

 Taper:Taper:
Theoretically, the most retentive preparationTheoretically, the most retentive preparation
should be one with parallel walls. But due toshould be one with parallel walls. But due to
difficulties of preparing parallel walls anddifficulties of preparing parallel walls and
possibility of undercuts or incomplete seating, apossibility of undercuts or incomplete seating, a
range of 2 to 6 degrees taper has beenrange of 2 to 6 degrees taper has been
considered optimal. To produce an optimal taperconsidered optimal. To produce an optimal taper
of 6 degrees, each opposing axial wall shouldof 6 degrees, each opposing axial wall should
have an inclination of 3 degrees to the path ofhave an inclination of 3 degrees to the path of
insertion.insertion.

 Surface area:Surface area:
Greater the surface area of the preparation,Greater the surface area of the preparation,
greater is the area of cement film bound to thegreater is the area of cement film bound to the
preparation and so greater is the retention.preparation and so greater is the retention.
Total surface area of the preparation isTotal surface area of the preparation is
influenced byinfluenced by
 Size of the toothSize of the tooth
 Extent of coverage by restorationExtent of coverage by restoration
 Features such as grooves, boxes etc.Features such as grooves, boxes etc.
 Crowns with long axial walls are more retentiveCrowns with long axial walls are more retentive
than those with shorter ones. Molar crowns arethan those with shorter ones. Molar crowns are
more retentive than premolar crowns.more retentive than premolar crowns.

 Surface roughness:Surface roughness:
Based on the fact that adhesion ofBased on the fact that adhesion of
dental cements depends on projection ofdental cements depends on projection of
cements into microscopic irregularities oncements into microscopic irregularities on
the surfaces being prepared, morethe surfaces being prepared, more
roughness on the preparation meansroughness on the preparation means
more retention.more retention.

 Area under shear and path of withdrawalArea under shear and path of withdrawal::
To decrease the failure potential, it is essential toTo decrease the failure potential, it is essential to
minimize the area of the cement under tensile stress. For theminimize the area of the cement under tensile stress. For the
shear strength to be utilized, the preparation must have:shear strength to be utilized, the preparation must have:
 Opposing wallsOpposing walls – in separate planes must be almost parallel– in separate planes must be almost parallel
to each other.to each other.
 The preparation should have onlyThe preparation should have only one path of removalone path of removal.. FullFull
crowncrown has excellent restoration because the mesial, distal,has excellent restoration because the mesial, distal,
facial and lingual walls limit the path of insertion to a narrowfacial and lingual walls limit the path of insertion to a narrow
range.range.
 To create a more retentive form in partial veneerTo create a more retentive form in partial veneer
preparations,preparations, grooves, boxes and pinholesgrooves, boxes and pinholes are substitutedare substituted
for missing walls.for missing walls.
 Long preparationLong preparation due to its surface area has more retentiondue to its surface area has more retention
than shorter ones.than shorter ones.
 Preparation withPreparation with greater diametergreater diameter or circumference hasor circumference has
more retention than with lesser diameter of same length.more retention than with lesser diameter of same length.

 Materials being cemented:Materials being cemented:
Retention is affected by both the castingRetention is affected by both the casting
alloy and the core material. Base metalalloy and the core material. Base metal
alloys are more retentive than the lessalloys are more retentive than the less
reactive gold content metals.reactive gold content metals.
 Type of Luting agent used:Type of Luting agent used:
the adhesive resin cements are the mostthe adhesive resin cements are the most
retentive among luting agents. GIC was theretentive among luting agents. GIC was the
most retentive and zinc phosphate was themost retentive and zinc phosphate was the
weakest.weakest.
 Film thickness of luting agentFilm thickness of luting agent::
Use of spacer facilitated complete seatingUse of spacer facilitated complete seating
of the casting thereby increasing retention.of the casting thereby increasing retention.

Secondary factors:Secondary factors:
 GroovesGrooves
 BoxesBoxes
 Pin holesPin holes
 CombinationCombination
These should be parallel to the line ofThese should be parallel to the line of
withdrawal, also to be placed at a pointwithdrawal, also to be placed at a point
which permits maximum length, or shouldwhich permits maximum length, or should
have a taper of 2 to 6 degrees.have a taper of 2 to 6 degrees.

RESISTANCERESISTANCE
Factors influencing resistanceFactors influencing resistance
1.1. LeverageLeverage
2.2. LengthLength
3.3. WidthWidth
4.4. TaperTaper
5.5. Rotation around verticalRotation around vertical
axisaxis
6.6. Path of insertionPath of insertion

 Resistance form:Resistance form:
This property prevents dislodgement of aThis property prevents dislodgement of a
restoration by forces directed in an apical, obliquerestoration by forces directed in an apical, oblique
and horizontal direction.and horizontal direction.
The more parallelism maintained between axialThe more parallelism maintained between axial
surfaces, greater is the resistance as the cement willsurfaces, greater is the resistance as the cement will
be compressed and failures are less likely to occurbe compressed and failures are less likely to occur
from compression than shear.from compression than shear.
 Geometry of the tooth preparationGeometry of the tooth preparation: the tooth: the tooth
preparation should be so shaped that particularpreparation should be so shaped that particular
areas of the axial walls will prevent rotation of theareas of the axial walls will prevent rotation of the
crown. Increased preparation taper and rounding ofcrown. Increased preparation taper and rounding of
axial angles tend to reduce resistance.axial angles tend to reduce resistance.

Length of preparationLength of preparation: shortening a preparation: shortening a preparation
decreases resisting area. In case of two crowns ofdecreases resisting area. In case of two crowns of
unequal length cemented on prepared tooth of equalunequal length cemented on prepared tooth of equal
length, longer crown is more likely to fail becauselength, longer crown is more likely to fail because
force on it acts through longer lever arm.force on it acts through longer lever arm.
Width:Width: in some cases narrow tooth can have morein some cases narrow tooth can have more
resistance than a wider tooth because it has aresistance than a wider tooth because it has a
shorter radius of rotation and also tangent line isshorter radius of rotation and also tangent line is
lower resulting in a longer resisting area.lower resulting in a longer resisting area.
Taper:Taper: as taper increases, tangent line approachesas taper increases, tangent line approaches
occlusal surfaces thus decreasing the resisting area.occlusal surfaces thus decreasing the resisting area.
Permissible taper of a preparation is directlyPermissible taper of a preparation is directly
proportional to the height/width ratio.proportional to the height/width ratio.www.indiandentalacademy.comwww.indiandentalacademy.com

 Grooves, boxes and wingsGrooves, boxes and wings: resistance is: resistance is
increased by blocking rotation around theincreased by blocking rotation around the
central axis.central axis.
 RetainerRetainer: a partial coverage restoration has less: a partial coverage restoration has less
resistance than does a complete crown.resistance than does a complete crown.
Resistance must be provided by grooves andResistance must be provided by grooves and
boxesboxes
 Physical properties of luting agentsPhysical properties of luting agents::
resistance is affected by compressive strengthresistance is affected by compressive strength
and modulus of elasticity of cement. Silico-and modulus of elasticity of cement. Silico-
phosphate cements have greater compressivephosphate cements have greater compressive
strength whereas glass ionomer,strength whereas glass ionomer,
polycarboxylate and zinc oxide eugenol havepolycarboxylate and zinc oxide eugenol have
lower values.lower values.

 Structural durabilityStructural durability::
A restoration must have sufficient strength toA restoration must have sufficient strength to
prevent it from being permanently deformed duringprevent it from being permanently deformed during
function. It depends on:function. It depends on:
 Alloy selectionAlloy selection: Nickel chromium alloys are: Nickel chromium alloys are
considerably harder.considerably harder.
 Adequate tooth reductionAdequate tooth reduction: there should be: there should be
sufficientsufficient restoration bulkrestoration bulk to withstand occlusalto withstand occlusal
forces and this is provided by giving a minimumforces and this is provided by giving a minimum
alloy thickness of 1.5 mm at centric cusps and 1 mmalloy thickness of 1.5 mm at centric cusps and 1 mm
at non-functional cusps.at non-functional cusps.
 Occlusal reductionOcclusal reduction is done along the functionalis done along the functional
plane and not in a flat plane.plane and not in a flat plane.
 Inadequate axial reductionInadequate axial reduction will lead to thin walls ofwill lead to thin walls of
restoration and thus is susceptible to deformation.restoration and thus is susceptible to deformation.

 Marginal integrityMarginal integrity::
A restoration can survive in the biologic environment ofA restoration can survive in the biologic environment of
the oral cavity only if the margin of the restoration isthe oral cavity only if the margin of the restoration is
closely adapted to the cavosurface finish line of theclosely adapted to the cavosurface finish line of the
preparation.preparation.
 Factors affecting are:Factors affecting are:
Margin placementMargin placement::
Supra gingival margins are more favorable because:Supra gingival margins are more favorable because:
They can be easily finishedThey can be easily finished
They can be kept cleanThey can be kept clean
Impressions, die formation and waxing up can be easilyImpressions, die formation and waxing up can be easily
done.done.
Restorations can be easily examined at recall visits.Restorations can be easily examined at recall visits.

 Margin adaptation:Margin adaptation:
Rough, irregular or stepped junction may increase lengthRough, irregular or stepped junction may increase length
and decrease adaptation. So proper tooth preparationand decrease adaptation. So proper tooth preparation
will facilitate proper margin adaptation.will facilitate proper margin adaptation.
 Margin geometry:Margin geometry:
The following guidelines should be followed for marginThe following guidelines should be followed for margin
design:design:
Ease of preparationEase of preparation
It should be readily identifiable in the impression and onIt should be readily identifiable in the impression and on
the die.the die.
Distinct boundary so that wax pattern can be finishedDistinct boundary so that wax pattern can be finished
easilyeasily
Sufficient bulk of material at the marginsSufficient bulk of material at the margins
There should be conservation of tooth structure.There should be conservation of tooth structure.www.indiandentalacademy.comwww.indiandentalacademy.com

 Esthetic consideration:Esthetic consideration:
In case of metal-ceramic restoration:In case of metal-ceramic restoration:
 Facial reductionFacial reduction should give sufficient bulk of porcelain and maintainshould give sufficient bulk of porcelain and maintain
translucency and color depth. (1.5 mm)translucency and color depth. (1.5 mm)
 Incisal reductionIncisal reduction should be around 2mm for good esthetics.should be around 2mm for good esthetics.
 Proximal reductionProximal reduction allowing passage of light gives the effect ofallowing passage of light gives the effect of
translucency.translucency.
 Labial margin placement:Labial margin placement: subgingival margin for esthetic purposes ifsubgingival margin for esthetic purposes if
patient has high lip line and margins should follow gingival contours.patient has high lip line and margins should follow gingival contours.
 Minimal display of metalMinimal display of metal
 Maximum thickness of porcelainMaximum thickness of porcelain
 Porcelain occlusal surfacesPorcelain occlusal surfaces
 Sub gingival margins.Sub gingival margins.www.indiandentalacademy.comwww.indiandentalacademy.com

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