Principles of Tooth Preparation Punam Bishnoi

Dr. Punam BishnoiDr. Punam Bishnoi

PRINCIPLES PRINCIPLES OF OF TOOTH PREPARATION TOOTH PREPARATION

CONTENTSCONTENTS• INTRODUCTIONINTRODUCTION

• BROAD CATEGORIES OF PRINCIPLESBROAD CATEGORIES OF PRINCIPLES

• BIOLOGICAL CONSIDERATIONSBIOLOGICAL CONSIDERATIONS

• MECHANICAL CONSIDERATIONSMECHANICAL CONSIDERATIONS

• ESTHETIC CONSIDERATIONSESTHETIC CONSIDERATIONS

• SUMMARY AND CONCLUSIONSUMMARY AND CONCLUSION

• REFERENCESREFERENCES

INTRODUCTIONINTRODUCTION

““Tooth preparation is defined as Tooth preparation is defined as mechanical treatment of dental disease mechanical treatment of dental disease or injury to hard tissues that restores a or injury to hard tissues that restores a tooth to original form.”tooth to original form.”

As tooth has no regenerative ability. As tooth has no regenerative ability. • Teeth require preparation to receive Teeth require preparation to receive

restorationrestoration• These preparation must be based on These preparation must be based on fundamental principles.fundamental principles.• Careful attention to every detail is Careful attention to every detail is

imperative imperative during tooth preparation during tooth preparation

Acc. to Shillinberg the design of a Acc. to Shillinberg the design of a preparation for a cast restoration preparation for a cast restoration and the execution of that design and the execution of that design are governed by five principles:are governed by five principles:

• Preservation of tooth structurePreservation of tooth structure• Retention and resistanceRetention and resistance• Structural durabilityStructural durability• Marginal integrityMarginal integrity• Preservation of the periodontium.Preservation of the periodontium.

Acc to DCNA 2004 certain guidelines are Acc to DCNA 2004 certain guidelines are given for preparation of toothgiven for preparation of tooth

• TOC(total occlusal convergence TOC(total occlusal convergence • Occlusocervical /incisocervical lengthOcclusocervical /incisocervical length• Ratio of occlusocervical dimension and Ratio of occlusocervical dimension and

faciolingual dimensionfaciolingual dimension• Circumferntial form of the prepared toothCircumferntial form of the prepared tooth• Preferred location for auxillary retentionPreferred location for auxillary retention• Uniform reductionUniform reduction• Finish line locationFinish line location• Finish line formFinish line form• Reduction depthsReduction depths• Line angle forn Line angle forn • Surface textureSurface texture

Acc. To Rosenstiel the tooth preparation Acc. To Rosenstiel the tooth preparation can be studied undercan be studied under 3 Broad categories :3 Broad categories :

1.1. BIOLOGIC CONSIDERATIONSBIOLOGIC CONSIDERATIONS,,

which affect the health of oral tissueswhich affect the health of oral tissues

2.2. MECHANICAL CONSIDERATIONSMECHANICAL CONSIDERATIONS,,

which affect the integrity and durability which affect the integrity and durability of of

the restorationthe restoration

3.3. ESTHETIC CONSIDERATIONS,ESTHETIC CONSIDERATIONS,

which affect the appearance of the which affect the appearance of the patientpatient

BIOLOGIC

Conservation of tooth structure

Avoidance of overcontouring

Supragingival margins

Harmonious occlusion

Protection against tooth fracture

MECHANICAL

Retention form

Resistance form

Structural durability

ESTHETIC

Minimum display of metal

Maximum thickness of porcelain

Porcelain occlusal surfaces

Subgingival margins

Best combination of compromises

BIOLOGIC CONSIDERATIONSBIOLOGIC CONSIDERATIONS

1.1. Prevention of damage during tooth Prevention of damage during tooth preparationpreparation

-Adjacent teeth-Adjacent teeth -Soft tissue -Soft tissue -Pulp-Pulp2.2. Conservation of tooth structureConservation of tooth structure3.3. Avoidance of overcontouringAvoidance of overcontouring -Axial reduction-Axial reduction4.4. Marginal integrity and preservation of Marginal integrity and preservation of

periodontiumperiodontium -Margin location-Margin location -Margin adaptation-Margin adaptation -Margin geometry or Finish line -Margin geometry or Finish line

configurationconfiguration5.5. Occlusal considerationsOcclusal considerations6.6. Preventing tooth fracturePreventing tooth fracture

Prevention of damage during tooth Prevention of damage during tooth preparationpreparation

• Adjacent teeth - iatrogenic damageAdjacent teeth - iatrogenic damage

- damaged proximal contact- damaged proximal contact

- To avoid- To avoid• Soft tissues - tongue and cheeksSoft tissues - tongue and cheeks

- careful retraction - careful retraction

• Pulp - excessive temperaturePulp - excessive temperature

- chemical irritation- chemical irritation

- microorganisms- microorganisms

• Prevention of damage during Prevention of damage during tooth preparationtooth preparation

• Conservation of tooth structureConservation of tooth structure• Avoidance of overcontouring Avoidance of overcontouring

• Marginal integrity and Marginal integrity and

preservation of periodontiumpreservation of periodontium• Occlusal considerationsOcclusal considerations• Preventing tooth fracturePreventing tooth fracture

• P.D.Laforgia , Vito Milano , Camillo Morea & Apollonia Desiate in 1991recorded the temperature changes in the pulp chamber during tooth preparation.

• Zach &Cohen - rise of 5.5 0C - 15% necrosis - rise of 11.10C - 60% necrosis

- rise of 16.60 C - 100% necrosis





Conservation of tooth structureConservation of tooth structure

• Dowden stated that any damage to the odontoblastic processes will adversely effect the cell nucleus no matter how far from the nucleus it occurs

• More the remaining dentin – lesser the pulpal More the remaining dentin – lesser the pulpal damagedamage

• Must be considered relativeMust be considered relative

GUIDELINES:GUIDELINES:

1.1. Partial coveragePartial coverage

2.2. Minimum taperMinimum taper





33. . Planar occlusal reductionPlanar occlusal reduction

4. Even reduction of axial surfaces4. Even reduction of axial surfaces





5. Selection of conservative margin5. Selection of conservative margin

6. Avoidance of unnecessary apical extension of 6. Avoidance of unnecessary apical extension of preparationpreparation





Avoidance of overcontouringAvoidance of overcontouring

• A crown should duplicate the contours A crown should duplicate the contours and profile of the original toothand profile of the original tooth

• Undercontoured restoration is better Undercontoured restoration is better than overcontoured restorationthan overcontoured restoration

• Anterior teeth, increasing the proximal Anterior teeth, increasing the proximal contour is bettercontour is better


• Conservation of tooth structureConservation of tooth structure• Avoidance of overcontouringAvoidance of overcontouring



MarginalMarginal integrity and preservation integrity and preservation of periodontiumof periodontium

• Margin location-Margin location- Biologic widthBiologic width It’s the dimension of space that the healthy It’s the dimension of space that the healthy

gingival tissues occupy above the alveolar bone.gingival tissues occupy above the alveolar bone.• It refers to the combined connective tissue-It refers to the combined connective tissue-

epithelial attachment from the crest of the epithelial attachment from the crest of the alveolar bone to the base of the alveolar bone to the base of the sulcus(2mm;connective tissue-1.07mm and sulcus(2mm;connective tissue-1.07mm and epithelium-0.97mm).epithelium-0.97mm).

• essential for preservation of periodontal health essential for preservation of periodontal health and removal of irritation that might damage the and removal of irritation that might damage the periodontiumperiodontium





Violation of biological width:-Violation of biological width:-

• Nevins and Sukrow in 1984Nevins and Sukrow in 1984 recommended that recommended that for the maintenance of healthy periodontium, for the maintenance of healthy periodontium, no restoration should violate the no restoration should violate the attachment ,even though its not possible for a attachment ,even though its not possible for a clinician to identify the most coronal extent of clinician to identify the most coronal extent of the junctional epithelium.the junctional epithelium.

• Wilson and MajnardWilson and Majnard cautioned against cautioned against extending restorations so far subgingivally that extending restorations so far subgingivally that the attachment complex is damaged.They the attachment complex is damaged.They stated that “Some distance of unprepared tooth stated that “Some distance of unprepared tooth structure should remain between the finished structure should remain between the finished line and junctional epithelium and this distance line and junctional epithelium and this distance should be ideally 0.5mm.should be ideally 0.5mm.





Violation of biologic widthViolation of biologic width

• Plaque accumulationPlaque accumulation• Attachment lossAttachment loss• Periodontal pocketsPeriodontal pockets• Advanced periodontitis Advanced periodontitis • Bone lossBone loss





Location of restorative margins:-Location of restorative margins:-

SupragingivalSupragingival

EquigingivalEquigingival

SubgingivalSubgingival





Supragingival marginSupragingival margin:-:-

Least traumatic to the soft tissuesLeast traumatic to the soft tissues Margin placement…Margin placement… Easily finishedEasily finished Impressions are easily madeImpressions are easily made Most accessible for cleansingMost accessible for cleansing

Restorations can be easily evaluatedRestorations can be easily evaluated





Subgingival marginsSubgingival margins:-:-

Clinical crown is short…Clinical crown is short… Old restorations with subgingival marginOld restorations with subgingival margin Esthetic appearanceEsthetic appearance Caries, abrasion or erosion…Caries, abrasion or erosion… Root sensitivityRoot sensitivity

Disadvantages of subgingival marginsDisadvantages of subgingival margins Plaque retention…Plaque retention… Margin placement…Margin placement… Mechanical irritation…Mechanical irritation… Violation of biological widthViolation of biological width Soft tissue injurySoft tissue injury……





• Criteria for subgingival margin Criteria for subgingival margin placement:-placement:-

Emergence profileEmergence profile Margins are closed and properly finishedMargins are closed and properly finished Adequate band of attached gingivaAdequate band of attached gingiva Margin should not violate the biological widthMargin should not violate the biological width





Crown lengthening procedures:-Crown lengthening procedures:-

It’s a procedure similar to the apical repositioning of the It’s a procedure similar to the apical repositioning of the flap with concomitant osteoplasty.flap with concomitant osteoplasty.

Indications –Indications – Short clinical crownShort clinical crown Restoration impinge on the biological widthRestoration impinge on the biological width Hopless teeth with extensive subgingival caries, Hopless teeth with extensive subgingival caries,

subgingival fracture and root perforationsubgingival fracture and root perforation

Crown lengthening procedureCrown lengthening procedure - - Surgical methodsSurgical methods Orthodontic methodOrthodontic method

D.A.Felton in 1991D.A.Felton in 1991 conducted a study on “Effects conducted a study on “Effects of in vivo crown margin discrepancies on the of in vivo crown margin discrepancies on the periodontal health” in his study he strongly periodontal health” in his study he strongly supported the placement of supragingival supported the placement of supragingival margins for artificial crowns and FPD’s.margins for artificial crowns and FPD’s.

William.G.Reeves William.G.Reeves in his review article concluded in his review article concluded that more supragingivally a restorative margin that more supragingivally a restorative margin is placed,the less chance that the margin will is placed,the less chance that the margin will contribute to gingival inflammation.contribute to gingival inflammation.

MARGINAL GEOMETRYMARGINAL GEOMETRY OR FINISH LINE CONFIGURATION OR FINISH LINE CONFIGURATION

• shape and bulk of the restorative material in the margin shape and bulk of the restorative material in the margin of the restorationof the restoration

• Marginal adaptationMarginal adaptation• Degree of seating of restorationDegree of seating of restoration

Guidelines for evaluation for margin designGuidelines for evaluation for margin design : : Ease of preparation without overextension or Ease of preparation without overextension or

unsupported enamelunsupported enamel Ease of identification in the impression and on the dieEase of identification in the impression and on the die Sufficient bulk of the materialSufficient bulk of the material Conservation of the tooth structureConservation of the tooth structure A distinct boundary to which wax pattern can be A distinct boundary to which wax pattern can be

finishedfinished

Finish linesFinish lines The finish line is the peripherel extension of a tooth The finish line is the peripherel extension of a tooth

preparationpreparation The most important consideration in selecting a cervical The most important consideration in selecting a cervical

margin design is its ability to consistently and margin design is its ability to consistently and predictably provide excellent marginal integrity.predictably provide excellent marginal integrity.

Knife edgeKnife edge Chisel edgeChisel edge Chamfer Chamfer ShoulderShoulder Sloped shoulderSloped shoulder Shoulder with bevelShoulder with bevel Radial shoulderRadial shoulder Heavy chamferHeavy chamferA. Feather edge

B. ChiselC. ChamferD. BevelE. ShoulderF. Sloped shoulderG. Beveled shoulder

KNIFE EDGE OR FEATHER EDGEKNIFE EDGE OR FEATHER EDGE

AdvantagesAdvantages Conservation of the tooth structureConservation of the tooth structure Permits an acute margin of the metalPermits an acute margin of the metal

Disadvantages:Disadvantages: The axial reduction may fade outThe axial reduction may fade out Thin margin may be difficult to accurately wax and castThin margin may be difficult to accurately wax and cast More susceptible to distortionMore susceptible to distortion Results in overcontouring Results in overcontouring

Indications:Indications: Not recommendedNot recommended

CHISEL EDGECHISEL EDGE

• Variation of feather edgeVariation of feather edge• Larger angle between the axial surface and Larger angle between the axial surface and

unprepared tooth structureunprepared tooth structure• Associated with excessively tapered preparationAssociated with excessively tapered preparation• Historic advantage-impression making with rigid Historic advantage-impression making with rigid

impression compound in coper bandsimpression compound in coper bands

Indications:Indications:• Lingual surface of mandibular posterior teeth Lingual surface of mandibular posterior teeth • The surfaces towards which tooth has tiltedThe surfaces towards which tooth has tilted• CementumCementum

ChamferChamferIt’s a finish line design for tooth preparation in It’s a finish line design for tooth preparation in

which gingival aspect meets the external which gingival aspect meets the external axial surface at an obtuse angle axial surface at an obtuse angle

Advantages Advantages It provides distinct marginIt provides distinct margin Adequate bulk to the restorationAdequate bulk to the restoration Easier to controlEasier to control Exhibits least stressesExhibits least stresses Disadvantage -Care needed to avoid Disadvantage -Care needed to avoid

unsupported lip of enamelunsupported lip of enamel

Indications:Indications: Cast metal restorationsCast metal restorations Lingual margin of the metal ceramic crownsLingual margin of the metal ceramic crowns

ShoulderShoulder Bulk of the restorationBulk of the restoration Wide ledge provides resistance to occlusal Wide ledge provides resistance to occlusal

forces forces Provides space for healthy restorative Provides space for healthy restorative

contourscontours Maximum estheticsMaximum esthetics It offers resistance against distortion during It offers resistance against distortion during

processingprocessing

Disadvantages:Disadvantages: Less conservative of tooth structure Less conservative of tooth structure The sharp 90The sharp 9000 internal line angle… internal line angle…

Indications:Indications: All ceramic restorationsAll ceramic restorations Facial magins of metal-ceramic crownsFacial magins of metal-ceramic crowns

SLOPED SHOULDERSLOPED SHOULDER

• Cavosurface margin - 120Cavosurface margin - 120°°• Reduces the possibility of leaving unsupported Reduces the possibility of leaving unsupported

enamel and yet leaves sufficient bulk to allow enamel and yet leaves sufficient bulk to allow thinning of the metal framework to a knife thinning of the metal framework to a knife edge for acceptable aesthetics.edge for acceptable aesthetics.

• Indicated for facial margin of metal ceramic Indicated for facial margin of metal ceramic crowns crowns

SHOULDER WITH BEVELSHOULDER WITH BEVEL

• Removes unsupported enamel, allows Removes unsupported enamel, allows finishing of metalfinishing of metal

• Recommended for extremely short wallsRecommended for extremely short walls• Disadvantage – extends the preparation Disadvantage – extends the preparation

into the sulcus if used on apical margininto the sulcus if used on apical margin• Indications –Indications – -facial margins of maxillary partial -facial margins of maxillary partial

coverage restorations coverage restorations - Inlay and onlay margin- Inlay and onlay margin -Shoulder is already present because-Shoulder is already present because of destruction by caries or presence of destruction by caries or presence of previous restorationsof previous restorations

RADIAL SHOULDER:RADIAL SHOULDER:

• Shoulder with rounded internal line angleShoulder with rounded internal line angle• Stress concentration is less in the tooth Stress concentration is less in the tooth

structurestructure

HEAVY CHAMFERHEAVY CHAMFER

• Internal line angle is large radius roundedInternal line angle is large radius rounded• Provides better support for a ceramic crown Provides better support for a ceramic crown

than chamfer, but it is not as good as shoulderthan chamfer, but it is not as good as shoulder• Easier to prepare than shoulderEasier to prepare than shoulder

MARGINAL ADAPTABILITYMARGINAL ADAPTABILITY

• The restorative margins must fit as closely as The restorative margins must fit as closely as possible against the finish line of the possible against the finish line of the preparation to minimize the width of exposed preparation to minimize the width of exposed cement.cement.

• They must have sufficient strength to They must have sufficient strength to withstand the forces of masticationwithstand the forces of mastication

• More accurate the adaptation – lesser the More accurate the adaptation – lesser the chance of recurrent carieschance of recurrent caries

• A well designed preparation should have a A well designed preparation should have a smooth and even marginssmooth and even margins

• Whenever possible they should be located in Whenever possible they should be located in the areas where the dentist can finish and the areas where the dentist can finish and inspect them and the patient can clean theminspect them and the patient can clean them

TO BEVEL OR NOT TO BEVELTO BEVEL OR NOT TO BEVEL• Controversial by OstlundControversial by Ostlund• Trignometric analysis- Trignometric analysis- more acute the bevel-more acute the bevel- -lesser the marginal discrepancy-lesser the marginal discrepancy -lesser the seating with cement as D will -lesser the seating with cement as D will

increase.increase.• So, bevel improves the marginal adaptation So, bevel improves the marginal adaptation

but reduces the seating of restorationbut reduces the seating of restoration• Empirical clinical results dictate that acute Empirical clinical results dictate that acute

margin of metal should continue to be used margin of metal should continue to be used on metal restorations but that angle should on metal restorations but that angle should be in 30-45be in 30-45°°

• Less angles will lead to thin and weak Less angles will lead to thin and weak margins.margins.

• More angle will lead to more marginal gap.More angle will lead to more marginal gap.

d=D sin ud=D sin ud=D cos qd=D cos q

ororD=d/sin uD=d/sin uD=d/cos qD=d/cos q

Bevel of 45Bevel of 450 0 preventprevent seating byseating by 1.4 factor 1.4 factor151500 prevent by 3.9 prevent by 3.95500 prevent by11.5 prevent by11.5

OCCLUSAL CONSIDERATIONSOCCLUSAL CONSIDERATIONS

• Occlusal scheme should not be traumaticOcclusal scheme should not be traumatic• Tooth preparation should allow sufficient space Tooth preparation should allow sufficient space

for developing a functional occlusal scheme in for developing a functional occlusal scheme in the finished restorationthe finished restoration

• Supraerupted or tilted teeth – reduce the teethSupraerupted or tilted teeth – reduce the teeth• Sometimes even the endodontic treatment is Sometimes even the endodontic treatment is

necessary to make enough roomnecessary to make enough room• Careful judgment is needed and diagnostic Careful judgment is needed and diagnostic

tooth preparation and waxing procedures are tooth preparation and waxing procedures are essential to determining the exact amount of essential to determining the exact amount of reduction required to develop an optimum reduction required to develop an optimum occlusionocclusion





PREVENTING THE PREVENTING THE TOOTH FRACTURETOOTH FRACTURE

• The likelihood that a restored tooth will The likelihood that a restored tooth will fracture can be lessened if the tooth fracture can be lessened if the tooth preparation be designed to minimize the preparation be designed to minimize the potentially destructive stressespotentially destructive stresses

• Inlay- greater potential for fractureInlay- greater potential for fracture• Onlay – lessens the chance of fractureOnlay – lessens the chance of fracture• Complete crown –greatest protection against Complete crown –greatest protection against

fracturefracture





MECHANICAL CONSIDERATIONSMECHANICAL CONSIDERATIONS

1.1. RETENTION FORM RETENTION FORM

2.2. RESISTANCE FORMRESISTANCE FORM

3.3. STRUCTURAL DURABILITYSTRUCTURAL DURABILITY

RETENTION FORMRETENTION FORM

• DEFINITION-The feature of a tooth DEFINITION-The feature of a tooth

preparation that resists dislodgement of preparation that resists dislodgement of

a crown in a vertical direction or along a crown in a vertical direction or along

the path of placement.the path of placement.

• In practice retention and resistance are In practice retention and resistance are

closely related and they are not always closely related and they are not always

clearly distinguishable.clearly distinguishable.

RETENTION FORMRETENTION FORM RESISTANCE FORMRESISTANCE FORMSTRUCTURAL DURABILITYSTRUCTURAL DURABILITY

FACTORS AFFECTING RETENTIONFACTORS AFFECTING RETENTION• Magnitude of dislodging forcesMagnitude of dislodging forces• Geometry of tooth preparationGeometry of tooth preparation -Taper-Taper -Surface area-Surface area -stress concetration -stress concetration -Type of preparation and secondary retentive -Type of preparation and secondary retentive featuresfeatures

• Roughness of surfaces being cementedRoughness of surfaces being cemented• Materials being cementedMaterials being cemented• Type of luting agentType of luting agent• Film thickness of luting agentFilm thickness of luting agent


MAGNITUDE OF DISLODGING FORCESMAGNITUDE OF DISLODGING FORCES

• small compared to those that tend to seat small compared to those that tend to seat or tilt itor tilt it

• by pulling the FPD with floss under the by pulling the FPD with floss under the connectors, connectors,

• when exceptionally sticky food is eatenwhen exceptionally sticky food is eaten• Depends on the stickiness of food and the Depends on the stickiness of food and the

surface area and texture of the surface area and texture of the restoration being pulled restoration being pulled


GEOMETRY OF TOOTH PREPARATIONGEOMETRY OF TOOTH PREPARATIONEssential elements of retention:- Essential elements of retention:- • opposing vertical surfaces in same preparation.opposing vertical surfaces in same preparation.• Path of insertionPath of insertion• Area under shear stressArea under shear stress• Freedom of displacementFreedom of displacement

FACTORSFACTORS• TaperTaper• Surface areaSurface area Length of preparationLength of preparation Width of preparationWidth of preparation• stress concetration stress concetration • Type of preparation and secondary retentive Type of preparation and secondary retentive

featuresfeatures


Opposing vertical surfaces in same Opposing vertical surfaces in same preparation:preparation:

1) 1) External surfacesExternal surfaces:- eg. Buccal & lingual :- eg. Buccal & lingual walls ofwalls of

full veneer crown.full veneer crown.

An extracoronal restoration is an example An extracoronal restoration is an example ofof

veneer, or sleeve retention.veneer, or sleeve retention.

Scan0001.jpg


2) 2) Internal surfacesInternal surfaces:- eg. Buccal & lingual walls :- eg. Buccal & lingual walls of theof the

proximal box of a proximo-occlusal inlay. proximal box of a proximo-occlusal inlay.

An intracoronal restoration resists An intracoronal restoration resists displacement bydisplacement by

wedge retention. wedge retention.


PATH OF INSERTION:-PATH OF INSERTION:-• It is an imaginary line along which the It is an imaginary line along which the

restoration will be placed onto or removed restoration will be placed onto or removed from the preparation.from the preparation.

• It is of special importance when preparing It is of special importance when preparing teeth to be fixed partial denture abutments, teeth to be fixed partial denture abutments, since the paths of all the abutment since the paths of all the abutment preparations must parallel each other.preparations must parallel each other.

• Surveying visually, since it is the primary Surveying visually, since it is the primary means of ensuring that the preparation is means of ensuring that the preparation is neither undercut nor over-tapered.neither undercut nor over-tapered.


• The mesiodistal inclination of the path must The mesiodistal inclination of the path must parallel the contact areas of adjacent teeth.parallel the contact areas of adjacent teeth.

• So if path is inclined mesially or distally, the So if path is inclined mesially or distally, the restoration will be held up at the proximal restoration will be held up at the proximal contact areas & be “locked out”.contact areas & be “locked out”.


Area under Shear Stress:Area under Shear Stress:• Most important feature for retention is that the Most important feature for retention is that the

total surface area of cement which will total surface area of cement which will experience shearing rather than tensile stress.experience shearing rather than tensile stress.

• To achieve this the preparation must have To achieve this the preparation must have opposing walls nearly parallel to each other.opposing walls nearly parallel to each other.


• To obtain the greatest area of cement To obtain the greatest area of cement under shear stress, the direction in under shear stress, the direction in which a restoration can be removed which a restoration can be removed must be limited to one path.must be limited to one path.


• Full veneer crown has excellent retention. Full veneer crown has excellent retention. When compared to partial veneer crown When compared to partial veneer crown because reducing the path of insertion to a because reducing the path of insertion to a narrow range.narrow range.


• If axial wall left unveneered retention is achieved by If axial wall left unveneered retention is achieved by substituting a grooves, boxes or pinholes for the substituting a grooves, boxes or pinholes for the missing wall.missing wall.

• A grooved lingual wall must be distinct and A grooved lingual wall must be distinct and perpendicular to the axial wall. perpendicular to the axial wall.


FREEDOM OF DISPLACEMENTFREEDOM OF DISPLACEMENT:-:-• Retention is improved by geometrically Retention is improved by geometrically

limiting the numbers of paths along which a limiting the numbers of paths along which a restoration can be removed from the tooth restoration can be removed from the tooth preparation.preparation.

• Maximum retention is achieved when there is Maximum retention is achieved when there is only one path. only one path.


TAPERTAPER

The axial walls of the preparation must taper The axial walls of the preparation must taper

slightly to permit the restoration to seat.slightly to permit the restoration to seat.

i.e. 2 opposing external walls must graduallyi.e. 2 opposing external walls must gradually

converge – converge – ANGLE OF CONVERGENCE.ANGLE OF CONVERGENCE.

2 opposing internal surfaces of the tooth structure2 opposing internal surfaces of the tooth structure

must diverge occlusally – must diverge occlusally – ANGLE OF ANGLE OF

DIVERGENCE.DIVERGENCE.• The relationship of one wall of a preparation to theThe relationship of one wall of a preparation to the

long axis of that preparation is the long axis of that preparation is the INCLINATIONINCLINATION

of that wall.of that wall.


• The more nearly parallel the opposing The more nearly parallel the opposing walls of a preparation, the greater walls of a preparation, the greater should be the retention.should be the retention.

• Most retentive preparation should be Most retentive preparation should be one with parallel walls, but the parallel one with parallel walls, but the parallel walls are impossible to create in the walls are impossible to create in the mouth without producing certain mouth without producing certain degree of taper.degree of taper.


• Tooth preparation taper should be kept Tooth preparation taper should be kept minimal because of its adverse effects on minimal because of its adverse effects on retention.retention.

ed taper ed retentioned taper ed retention

ed taper ed retention.ed taper ed retention.• Jogensen said as taper increases retention Jogensen said as taper increases retention

decreases.decreases.• Retention of a crown with 10 degree of taper was Retention of a crown with 10 degree of taper was

approximately half that of a crown with 5 degree approximately half that of a crown with 5 degree taper.taper.

• Ideal taper should Ideal taper should be within the be within the range of 2-6.5°.range of 2-6.5°.

• A taper of 6° have A taper of 6° have been proposed as been proposed as being clinically being clinically achievable while achievable while affording affording adequate adequate retention.retention.


• Jeffrey Nodlander, Dennis Weir, Warren Stoffer and ShigaroJeffrey Nodlander, Dennis Weir, Warren Stoffer and Shigaro

Ochi, JPD, vol. 60, 1988Ochi, JPD, vol. 60, 1988.. – Conducted a study to measure the – Conducted a study to measure the

convergence angles of a full coverage preparations performed in a convergence angles of a full coverage preparations performed in a

clinical environment. Teeth were prepared by the participants to attempt clinical environment. Teeth were prepared by the participants to attempt

a 4-10° convergence angle. The convergence angles of all preparations a 4-10° convergence angle. The convergence angles of all preparations

were determined by projecting the faciolingual and mesiodistal were determined by projecting the faciolingual and mesiodistal

silhouttes of the dies with an overhead projector. They concluded that,silhouttes of the dies with an overhead projector. They concluded that,

• the ideal convergence angle of 4-10° is seldom achieved. the ideal convergence angle of 4-10° is seldom achieved.

• The convergence angle for mandibular preparations were greater than The convergence angle for mandibular preparations were greater than

maxillary. maxillary.

• Auxillary retention should be used in molar region because these Auxillary retention should be used in molar region because these

preparations were found to have larger convergence angle.preparations were found to have larger convergence angle.

SURFACE AREASURFACE AREA

• Provided the restoration has limited path of Provided the restoration has limited path of withdrawal, Greater the surface area of a withdrawal, Greater the surface area of a preparation, greater is its retention.preparation, greater is its retention.

• Length…..Length…..• Crowns with long axial walls are more Crowns with long axial walls are more

retentive…retentive…• Molar crowns are more retentive than Molar crowns are more retentive than

premolar crowns of same taper premolar crowns of same taper


STRESS CONCENTRATIONSTRESS CONCENTRATION

• If line angle between axial and occlusal If line angle between axial and occlusal surface is sharp, it leads to concentration of surface is sharp, it leads to concentration of stresses around that junctionstresses around that junction

• Induced stresses exceeds the strength of the Induced stresses exceeds the strength of the cementcement

• Leads to cohesive failure of cementLeads to cohesive failure of cement• Rounding the internal line angles reduces the Rounding the internal line angles reduces the

stress concentration and thus increases the stress concentration and thus increases the retention of restorationretention of restoration


TYPE OF RESTORATION AND TYPE OF RESTORATION AND SECONDARY RETENTIVE FEATURESSECONDARY RETENTIVE FEATURES

• Full veneer crown has excellent retention Full veneer crown has excellent retention when compared to partial veneer crown when compared to partial veneer crown because reducing the path of insertion to a because reducing the path of insertion to a narrow range.narrow range.


• If axial wall left unveneered, retention is If axial wall left unveneered, retention is achieved by substituting a grooves, boxes or achieved by substituting a grooves, boxes or pinholes for the missing wall.pinholes for the missing wall.

• Secondary retentive features doesnot significantly affect the retention because the surface area is not increased significantly.

• But where these features limits the path of withdrawal, retention is increased


• Surface roughness:Surface roughness:• Adhesion of dental cements depends primarily Adhesion of dental cements depends primarily

on projections of the cement into microscopic on projections of the cement into microscopic irregularities.irregularities.

• JorgensenJorgensen found retention of castings found retention of castings cemented with ZnPO4 cement on test dies with cemented with ZnPO4 cement on test dies with a 10° taper to be twice as great on a 10° taper to be twice as great on preparations with 40µm scratches than 10µm.preparations with 40µm scratches than 10µm.

• Retention increases when restoration is Retention increases when restoration is roughened or grooved.roughened or grooved.

• Failure rarely occurs at the cement tooth Failure rarely occurs at the cement tooth interface. So roughening tooth preparation interface. So roughening tooth preparation hardly influences retention.hardly influences retention.


• Materials being cemented:Materials being cemented:• Retention is affected by both the casting Retention is affected by both the casting

alloy and the core material.alloy and the core material.• More reactive the alloy is more adhesion.More reactive the alloy is more adhesion.• Base metal alloys are better retained than Base metal alloys are better retained than

less reactive high gold content metals.less reactive high gold content metals.• Type of luting agent:Type of luting agent:

• Studies show that adhesive resin cements Studies show that adhesive resin cements are more retentive than compared to are more retentive than compared to conventional ZnPO4 and GIC cements.conventional ZnPO4 and GIC cements.


• Film thickness of the luting Film thickness of the luting agentagent..

• Ideal dimension for luting agemt space Ideal dimension for luting agemt space is 20-40 microns for each wall.is 20-40 microns for each wall.


• Steven M. Carter, Peter R. Steven M. Carter, Peter R. Wilson, IJP, vol. 9, 1996Wilson, IJP, vol. 9, 1996.- .- Conducted a study to see the effect Conducted a study to see the effect on die spacing on pre and post on die spacing on pre and post cementation crown removal and cementation crown removal and crown elevation. Different layers of crown elevation. Different layers of die spacer was used. They observed die spacer was used. They observed that the force required to remove a that the force required to remove a crowns before cementation crowns before cementation decreased with increased layers of decreased with increased layers of die spacers. die spacers.


RESISTANCE FORMRESISTANCE FORM• The features of a tooth preparation that enhance the The features of a tooth preparation that enhance the

stability of restoration and resist dislodgement along stability of restoration and resist dislodgement along

an axis other than the path of placement (GPT).an axis other than the path of placement (GPT).

• It prevents dislodgement of a restoration by forces It prevents dislodgement of a restoration by forces

directed in an apical, oblique or horizontal direction. directed in an apical, oblique or horizontal direction.

• The geometric configuration of tooth structure must The geometric configuration of tooth structure must

place the cement in compression to provide the place the cement in compression to provide the

necessary resistance.necessary resistance.

• Lateral forces tend to displace the restoration by Lateral forces tend to displace the restoration by

causing rotation around gingival margincausing rotation around gingival margin

RETENTION FORM RETENTION FORM RESISTANCE FORMRESISTANCE FORMSTRUCTURAL DURABILITYSTRUCTURAL DURABILITY

Methods to analyse resistance formMethods to analyse resistance formRETENTION FORM RETENTION FORM RESISTANCE FORMRESISTANCE FORMSTRUCTURAL DURABILITYSTRUCTURAL DURABILITY

FACTORS AFFECTING RESISTANCE FACTORS AFFECTING RESISTANCE FORMFORM

• Magnitude and direction of dislodging forcesMagnitude and direction of dislodging forces• LeverageLeverage• Length of the preparation Length of the preparation • Width of the preparationWidth of the preparation• Taper Taper • Type of preparation Type of preparation • Rotation about vertical axisRotation about vertical axis• Physical properties of luting agentPhysical properties of luting agent


Magnitude and direction of Magnitude and direction of dislodging forcesdislodging forces

• Resistance decreases in following Resistance decreases in following order:order:

- - normal occlusion- axially directed normal occlusion- axially directed forcesforces

- habits (pipe smoking and bruxing) – - habits (pipe smoking and bruxing) – large oblique forces to restorationlarge oblique forces to restoration

- eccentric interferences- eccentric interferences

- anterior guidance- anterior guidance


LEVERAGE AND RESISTANCE:LEVERAGE AND RESISTANCE:

• Leverage occurs when the line of action of Leverage occurs when the line of action of a force passes out side the supporting a force passes out side the supporting tooth structure. tooth structure.

• If the force passes within the margin of a If the force passes within the margin of a crown no tipping of the restoration when crown no tipping of the restoration when compared to the line of action passing compared to the line of action passing outside the margins of the restorationoutside the margins of the restoration


• Forces are outside the margin in the Forces are outside the margin in the following cases:following cases:

- wide occlusal table of restoration- wide occlusal table of restoration

- crowns on tipped teeth- crowns on tipped teeth

- retainers for cantilever bridge- retainers for cantilever bridge

- force at an oblique angle- force at an oblique angle


• If a line drawn from the center of rotation If a line drawn from the center of rotation perpendicular to the cement film on the perpendicular to the cement film on the opposite wall of the preparation the point opposite wall of the preparation the point where the line intersects the cement film is where the line intersects the cement film is known as tangent point. known as tangent point.


• If the tangent points of all the arcs of rotation around a given If the tangent points of all the arcs of rotation around a given axis are connected they form the tangent line. The area axis are connected they form the tangent line. The area above the tangent line is resisting area.above the tangent line is resisting area.

• Rotation is prevented by any areas of the tooth preparation Rotation is prevented by any areas of the tooth preparation

that are placed in compression and are called as RESISTANCE that are placed in compression and are called as RESISTANCE

AREAAREA

• To have effective resistance the tangent line should extend at To have effective resistance the tangent line should extend at least halfway down the preparation. least halfway down the preparation.


• Preparation length andPreparation length and resistance:resistance:

-Shortening of preparation will reduce the resistance area.

-In short crown lifting force is small when compared to long crown.

-Small restoration is less likely to fail through tipping than long restoration (on preparation of equal length).


• Resistance and tooth widthResistance and tooth width::• A wider preparation has a greater A wider preparation has a greater

retention but a narrow tooth can have retention but a narrow tooth can have greater resistance to tipping.greater resistance to tipping.

Because of smaller diameter a tangent line falls low on the wall opposite to axis of rotation. Resulting in a large resisting area.

Weak resistance can be enhanced by placing vertical grooves/ boxes/ pinholes.

• A grooved lingual wall must be distinct A grooved lingual wall must be distinct and perpendicular to the axial wall.and perpendicular to the axial wall.

• So U- shaped grooves or flared boxes So U- shaped grooves or flared boxes provides more resistance than V-shaped provides more resistance than V-shaped ones ones


• Taper and Resistance:Taper and Resistance:• More tapered a preparation less is resistance.More tapered a preparation less is resistance.

No taper the resisting area cover half the axial wall.

Ideal taper < ½ the axial wall.

Over tapered small resisting area near the occlusal surface.


• Permissible taper of a preparation is directly proportional to height : width ratio.Permissible taper of a preparation is directly proportional to height : width ratio.

• Taper that permit an effective resisting area for a preparation in which height Taper that permit an effective resisting area for a preparation in which height equals width is double than in a preparation where height is only half width. equals width is double than in a preparation where height is only half width.


• Formula to calculate:Formula to calculate:

• Taper:Taper:

T = arc sin (2r/w),T = arc sin (2r/w),

- Height of tangency point.- Height of tangency point.

r = (w sin T) / 2r = (w sin T) / 2

- Preparation height- Preparation height

h = [w tan (90°- T/2)]/2. h = [w tan (90°- T/2)]/2.


• Rotation around a vertical Rotation around a vertical axis:axis:

A partial veneer crown which has no grooves offer little resistance to rotation.

Axial symmetry of a full veneer crown preparation may allow rotation of the restoration.


Enhancing the resistance formEnhancing the resistance form by placing groove by placing groove


TYPE OF PREPARATION TYPE OF PREPARATION

• Partial coverage restoration may have less Partial coverage restoration may have less resistance than a complete crown because it resistance than a complete crown because it has no buccal resistance areahas no buccal resistance area

PHYSICAL PROPERTIES OF LUTING AGENTPHYSICAL PROPERTIES OF LUTING AGENT • Resistance to deformation is affected by Resistance to deformation is affected by

physical properties of the luting agent, such as physical properties of the luting agent, such as compressive strength and modulus of compressive strength and modulus of elasticityelasticity

• Adhesive resin Adhesive resin >>GIC GIC >> ZnPO ZnPO4 4 >> Polycarboxylate Polycarboxylate >> ZOE ZOE


STRUCTURAL DURABILITYSTRUCTURAL DURABILITY• A restoration must have sufficient strength to A restoration must have sufficient strength to

prevent permanent deformation during prevent permanent deformation during functionfunction

• DEFINITION- “The ability of a restoration to DEFINITION- “The ability of a restoration to withstand destruction due to external forces is withstand destruction due to external forces is known as structural durability”.known as structural durability”.


Factors affecting structural durabilityFactors affecting structural durability

1.1. Adequate tooth reduction.Adequate tooth reduction.

- occlusal reductionocclusal reduction

- functional cusp bevelfunctional cusp bevel

- axial reductionaxial reduction

2. Alloy selection.2. Alloy selection.

3.3. Metal-ceramic framework design.Metal-ceramic framework design.

4.4. Margin designMargin design


ADEQUATE TOOTH REDUCTIONADEQUATE TOOTH REDUCTION OCCLUSAL REDUCTIONOCCLUSAL REDUCTION:-:-

• An important feature for providing adequate bulk of metal & An important feature for providing adequate bulk of metal & strength to the restoration is occlusal clearance.strength to the restoration is occlusal clearance.

- Occlusal thickness varies with different restorative materialsOcclusal thickness varies with different restorative materials

Gold alloys – 1.5mm (FC) & 1mm(NFC)Gold alloys – 1.5mm (FC) & 1mm(NFC) Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC)Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC) All-ceramic crowns – 2mm of clearance on preparationAll-ceramic crowns – 2mm of clearance on preparation


- Firstly, opposing occlusal equilibration is to be - Firstly, opposing occlusal equilibration is to be

achieved eg. Plunger cusps to be rounded.achieved eg. Plunger cusps to be rounded.

- Round line and point angles, avoid deep grooves Round line and point angles, avoid deep grooves

in the center of the occlusal surfaces to prevent in the center of the occlusal surfaces to prevent

stress concentration and to distribute the forces stress concentration and to distribute the forces

over a larger surface area.over a larger surface area.

- Uniform and planar occlusal reduction. Uniform and planar occlusal reduction.

• This ensures:This ensures:

-sufficient occlusal clearance-sufficient occlusal clearance

- preservation of tooth structurepreservation of tooth structure

• OCCLUSAL OFFSET can be given on OCCLUSAL OFFSET can be given on posterior partial veneer crown preparation posterior partial veneer crown preparation to provide space for a TRUSS of metal to to provide space for a TRUSS of metal to form reinforcing strapform reinforcing strap

• INCISAL OFFSET can be given on anterior INCISAL OFFSET can be given on anterior partial veneer crown preparation to partial veneer crown preparation to provide space for metal that helps to provide space for metal that helps to strengthen the lingual-incisal marginstrengthen the lingual-incisal margin

FUNCTIONAL CUSP BEVELFUNCTIONAL CUSP BEVEL:-:-• It is an integral part of the occlusal reduction. It is an integral part of the occlusal reduction.

• A wide bevel on the lingual inclines of the maxillary A wide bevel on the lingual inclines of the maxillary lingual cusps & the buccal inclines of the mandibular lingual cusps & the buccal inclines of the mandibular buccal cusps provides space for an adequate bulk of buccal cusps provides space for an adequate bulk of metal in an area of heavy occlusal contact.metal in an area of heavy occlusal contact.

• If a wide bevel is not placed on the functional If a wide bevel is not placed on the functional cusp, several problems may occur :cusp, several problems may occur :

- If the crown is waxed & cast to normal - If the crown is waxed & cast to normal contour it can cause a thin area or perforation contour it can cause a thin area or perforation in the casting. in the casting.

- To prevent this the crown may be waxed to - To prevent this the crown may be waxed to optimal thickness resulting in overcontouring & optimal thickness resulting in overcontouring & poor occlusion.poor occlusion.

- If an attempt is made to obtain space for an - If an attempt is made to obtain space for an adequate bulk in a normally contoured adequate bulk in a normally contoured casting without a bevel, it will result in over casting without a bevel, it will result in over inclination of the buccal surface which will inclination of the buccal surface which will destroy excessive tooth structure while destroy excessive tooth structure while lessening retention. lessening retention.

AXIAL REDUCTIONAXIAL REDUCTION:-:-

• It plays an important role in securing space It plays an important role in securing space for an adequate thickness of restorative for an adequate thickness of restorative material.material.

• Inadequate axial reduction can cause thin Inadequate axial reduction can cause thin walls & a weak restoration subjected to walls & a weak restoration subjected to distortion or a bulbous, overcontoured distortion or a bulbous, overcontoured restoration which will strengthen the restoration which will strengthen the restoration but may have a disastrous effect restoration but may have a disastrous effect on periodontium.on periodontium.

• Other features that provide space for metal Other features that provide space for metal and improve the rigidity & durability of the and improve the rigidity & durability of the restoration are: restoration are:

The offset, the occlusal shoulder, the The offset, the occlusal shoulder, the isthmus,isthmus,

the proximal groove & the box.the proximal groove & the box.

Selection of the alloy:Selection of the alloy:

--It is essential that there be sufficient clinical evidence of It is essential that there be sufficient clinical evidence of

superiority, before selecting a particular material.superiority, before selecting a particular material.

DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS:• Biocompatibility• Ease of melting• Ease of casting• Ease of brazing (soldering)• Ease of polishing• Little solidification shrinkage• Minimal reactivity with the mold material• Good wear resistance• High strength• Excellent corrosion resistance• Porcelain Bonding

Classification :Classification :

1.1.Intended use:Intended use:

Traditionally alloys for casting were classified on the basis of their Traditionally alloys for casting were classified on the basis of their intended use-intended use-

-Type I: Simple inlays-Type I: Simple inlays

-Type II: Complex inlays-Type II: Complex inlays

-Type III: Crowns and fixed partial dentures-Type III: Crowns and fixed partial dentures

-Type IV: Removable partial dentures and pin ledges.-Type IV: Removable partial dentures and pin ledges.

Porcelain: metal-ceramic alloys.Porcelain: metal-ceramic alloys.

2.2. Physical properties: Physical properties: FDI (1965) classified casting alloys FDI (1965) classified casting alloys according to their physical properties as:according to their physical properties as:

Type I: SoftType I: Soft

Type II: MediumType II: Medium

Type III: Hard Type III: Hard

Type IV: Extra-hardType IV: Extra-hard

Factors considered when selecting an alloy: Factors considered when selecting an alloy:

1. Color: The patients view on the subject should be 1. Color: The patients view on the subject should be

sought if the metal will be visible in the mouth; sought if the metal will be visible in the mouth;

otherwise the color is irrelevant.otherwise the color is irrelevant.

2. Composition: The percentage composition by 2. Composition: The percentage composition by

weight of the main ingredients must be mentioned.weight of the main ingredients must be mentioned.

3. Cost.3. Cost.4.4. Clinical performance: Clinical performance:A) A) Biologic properties Biologic properties -Gingival irritation.-Gingival irritation.-Recurrent caries.-Recurrent caries.-Plaque accumulation.-Plaque accumulation.-Allergies.-Allergies.

5. Laboratory performance: 5. Laboratory performance:

Factors like casting accuracy, surface roughness, Factors like casting accuracy, surface roughness,

strength, metal-ceramic bond strength should be strength, metal-ceramic bond strength should be

considered.considered.

B) B) Mechanical properties:Mechanical properties:-Wear resistance and -Wear resistance and

strength.strength.-Marginal fit.-Marginal fit.-Ceramic bond failure.-Ceramic bond failure.-Connector failure.-Connector failure.-Tarnish and corrosion.-Tarnish and corrosion.

Choice of material:Choice of material: Gold:Gold: Indications-Indications-1.In situations of severe occlusal stress.1.In situations of severe occlusal stress.2.Following endodontic treatment of posterior teeth.2.Following endodontic treatment of posterior teeth.3.Full or partial coverage of posterior teeth where there 3.Full or partial coverage of posterior teeth where there

has been significant loss of coronal dentin.has been significant loss of coronal dentin.4.For restoration of adjacent or opposing teeth to avoid 4.For restoration of adjacent or opposing teeth to avoid

problems arising from use of dissimilar metals.problems arising from use of dissimilar metals.

Contraindications:Contraindications:-Aesthetics-Aesthetics--Cost--Cost

Porcelain (Ceramic):Porcelain (Ceramic):

Indications:Indications:

- Large inadequate restorations on the- Large inadequate restorations on the

anterior teeth provided there is enough anterior teeth provided there is enough

tooth substance.tooth substance.

- Severely discolored teeth.- Severely discolored teeth.

- Over an existing post and core- Over an existing post and core

substructure.substructure.

Contraindications:Contraindications:

-Teeth with short clinical crown-Teeth with short clinical crown

-Edge to edge occlusion-Edge to edge occlusion

-Teeth which do not allow ideal preparation-Teeth which do not allow ideal preparation

form to support the porcelain.form to support the porcelain.

Metal ceramic:Metal ceramic:

Indications:Indications:- Esthetics- Esthetics- Failure of porcelain jacket crowns.- Failure of porcelain jacket crowns.-Posterior teeth where esthetics is necessary and partial Posterior teeth where esthetics is necessary and partial coverage gold crowns are contraindicatedcoverage gold crowns are contraindicated

Contraindications:Contraindications:

-Young patients at risk of pulp being exposed-Young patients at risk of pulp being exposed

-large pulp chamber-large pulp chamber

-Traumatic occlusion (heavy occlusal forces) , where -Traumatic occlusion (heavy occlusal forces) , where

wear of opposing occlusal surfaces is expected.wear of opposing occlusal surfaces is expected.

Metal ceramic framework:Metal ceramic framework:

-A metal occlusal contact requires 1-1.5mm of reduction-A metal occlusal contact requires 1-1.5mm of reduction-A porcelain contact requires 2mm of reduction-A porcelain contact requires 2mm of reduction-Occlusal contacts need to be 1.5-2.0mm from the -Occlusal contacts need to be 1.5-2.0mm from the porcelain-porcelain- metal junctionmetal junction-The substructure must support an even thickness of the -The substructure must support an even thickness of the porcelain veneer (1mm minimum and 2mm porcelain veneer (1mm minimum and 2mm maximum).maximum).-The minimum thickness of the metal is 0.2-0.3mm.-The minimum thickness of the metal is 0.2-0.3mm.-Cut-back: porcelain-metal junction should be 90-Cut-back: porcelain-metal junction should be 90 or or greater. greater. -Metal should preferably be in the area of the centric -Metal should preferably be in the area of the centric stop to stop to enhance the durability of the restoration.enhance the durability of the restoration.

MARGIN DESIGNMARGIN DESIGN

- distortion of restoration margin is distortion of restoration margin is prevented by designing the preparation prevented by designing the preparation outline to avoid occlusal contact in this outline to avoid occlusal contact in this area area

- tooth reduction should provide sufficient tooth reduction should provide sufficient room for bulk of metal at the margin to room for bulk of metal at the margin to prevent distortionprevent distortion

ESTHETIC CONSIDERATIONSESTHETIC CONSIDERATIONS

METAL-CERAMIC RESTORATIONMETAL-CERAMIC RESTORATION- Usually poor appearance is due to Usually poor appearance is due to

insufficient porcelain thicknessinsufficient porcelain thickness- In addition, the labial margin of In addition, the labial margin of

metal ceramic crown is not always metal ceramic crown is not always accurately placedaccurately placed

- To correct all these deficiencies, To correct all these deficiencies, certain principles are recommended certain principles are recommended during tooth preparation during tooth preparation

FACIAL REDUCTIONFACIAL REDUCTION- Adequate thickness of porcelain is needed to Adequate thickness of porcelain is needed to

create a sense of color depth and translucencycreate a sense of color depth and translucency- Adequate reduction Adequate reduction sufficient bulk of sufficient bulk of

porcelain for appearance and metal for porcelain for appearance and metal for strengthstrength

- Minimum reduction of 1.5 mm is requiredMinimum reduction of 1.5 mm is required- Shade problems Shade problems incisal and cervical 3incisal and cervical 3rdrd of of

restoration (here direct light reflection from restoration (here direct light reflection from the opaque layer can make the restoration the opaque layer can make the restoration appear very noticeableappear very noticeable

- Opaque porcelain generally have a Opaque porcelain generally have a different shade from body porcelain, they different shade from body porcelain, they often need to be modified with special often need to be modified with special stains in these areas.stains in these areas.

- with very thin teeth like mandibular with very thin teeth like mandibular incisors.incisors.

INCISAL REDUCTIONINCISAL REDUCTION

- - Incisal edge Incisal edge no metal backing. So, it no metal backing. So, it can be made with a translucency similar to can be made with a translucency similar to that of natural tooth structurethat of natural tooth structure

- reduction reduction 2 mm2 mm- Excessive reduction – reduces retention Excessive reduction – reduces retention

and resistance and resistance

PROXIMAL REDUCTIONPROXIMAL REDUCTION

- Esthetics depends on exact location Esthetics depends on exact location of the metal ceramic junction in of the metal ceramic junction in

complete restorationcomplete restoration

- Proximal surface of anterior tooth - Proximal surface of anterior tooth will look most natural if they are will look most natural if they are restored as the incisal edges without restored as the incisal edges without metal backing.metal backing.

FEATURES OF DIFFERENT TYPE OF PREPARATIONS

ENDODONTICALLY TREATED ENDODONTICALLY TREATED TEETHTEETH

• Conservation of tooth structureConservation of tooth structure• Retention formRetention form• Resistance formResistance form

Conservation of tooth structureConservation of tooth structure

PREPARATION OF CANALPREPARATION OF CANAL• Only minimum tooth structure should be Only minimum tooth structure should be

removed from the canalremoved from the canal• Root canal should be enlarged only enough to Root canal should be enlarged only enough to

enable the post to fit accurately yet passively enable the post to fit accurately yet passively while ensuring strength and retentionwhile ensuring strength and retention

PREPARATION OF CORONAL TISSUEPREPARATION OF CORONAL TISSUE• Coronal tooth structure should be saved as much Coronal tooth structure should be saved as much

as possible because…..as possible because…..

• Ferrule:Ferrule:

- Extension of the axial wall of the crown apical - Extension of the axial wall of the crown apical

to the to the

missing tooth structure provides what is missing tooth structure provides what is

known as a known as a

ferrule.ferrule.

- helps to bind the remaining tooth structure - helps to bind the remaining tooth structure

together, together,

preventing root fracture during functionpreventing root fracture during function

• Retention formRetention form• Preparation geometryPreparation geometry• Post lengthPost length• Post diameterPost diameter• Post surface texturePost surface texture• Luting agentLuting agent

• Resistance formResistance form• Stress distributionStress distribution• Rotational resistanceRotational resistance

Fibre reinforced Fibre reinforced composite prosthesescomposite prostheses

Posterior FRC tooth preparation

Occlusal isthmus (1.0 mm depth)

Proximal step(1.0 mm axial depth)

Shoulder/deep chamfer (1.2-1.5 mm)

Anterior FRC tooth preparationAnterior FRC tooth preparation

Linguoproximal step(1.0 mm axial depth)

Shoulder/deep chamfer (1.2-1.5 mm)

SUMMARY AND CONCLUSIONSUMMARY AND CONCLUSION• Proper attention should be given during margin Proper attention should be given during margin

placement and the principle of “Do no harm” to the placement and the principle of “Do no harm” to the soft tissues should be followed. soft tissues should be followed.

• All preparations require the incorporation of factors All preparations require the incorporation of factors to prevent the dislodgement of restoration by to prevent the dislodgement of restoration by functional stresses. functional stresses.

• If too much emphasis is given on any one of the If too much emphasis is given on any one of the principles then the success of the procedure may be principles then the success of the procedure may be limited by a lack of consideration of the other factors.limited by a lack of consideration of the other factors.

• An analysis of these principles and factors should An analysis of these principles and factors should enable the dentist to effectively apply them during enable the dentist to effectively apply them during the design of any preparation.the design of any preparation.

REFERENCESREFERENCES

• 1. Tylman SD, Malone WFP. Tylman's Theory and 1. Tylman SD, Malone WFP. Tylman's Theory and practice of fixed prosthodontics. 7th ed. practice of fixed prosthodontics. 7th ed. St. LouisSt. Louis: : MosbyMosby; 1978. ; 1978.

• 2. Shillingburg HT, Hobo S, Whit&t LD. Fundamentals 2. Shillingburg HT, Hobo S, Whit&t LD. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence of fixed prosthodontics. 3rd ed. Chicago: Quintessence Publishing Co, 1981.Publishing Co, 1981.

• 3. Blair FM, Wassell RW, Steele JG, Crowns and other 3. Blair FM, Wassell RW, Steele JG, Crowns and other extra-coronal restorations: Preparations for full veneer extra-coronal restorations: Preparations for full veneer crowns.crowns. British Dental Journal 2002;192:561-71. British Dental Journal 2002;192:561-71.

• 4.Rosenstiel SF, Land MF, Fujumoto J. Contemporary 4.Rosenstiel SF, Land MF, Fujumoto J. Contemporary Fixed Prosthodontics.4th ed. St. Louis: Mosby; 2007.Fixed Prosthodontics.4th ed. St. Louis: Mosby; 2007.

• 5.Langeland K, Langeland LK. Pulpal reactions to 5.Langeland K, Langeland LK. Pulpal reactions to crown preparation, impression, temporary crown crown preparation, impression, temporary crown fixation and permanent cementation. J Prosthet Dent fixation and permanent cementation. J Prosthet Dent 1965;15:129-43.1965;15:129-43.

• 6.Laforgia PD, Milano V, Morea C, Desiate A. 6.Laforgia PD, Milano V, Morea C, Desiate A. Temperature change in the pulp chamber during Temperature change in the pulp chamber during complete crown preparation. J Prosthet Dent complete crown preparation. J Prosthet Dent 1991;65:56-61.1991;65:56-61.

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• 7. Brannstrom M, Nyborg H. Cavity treatment 7. Brannstrom M, Nyborg H. Cavity treatment with a microbicidal fluoride solution: Growth of with a microbicidal fluoride solution: Growth of bacterial and effect on the pulp. J Prosthet bacterial and effect on the pulp. J Prosthet Dent 1973;30:303-10 Dent 1973;30:303-10

• 8. Dowden WE. Discussion of methods and 8. Dowden WE. Discussion of methods and criteria in evaluation of dentin and pulpal criteria in evaluation of dentin and pulpal responses. Int Dent J 1970;20:531-5.responses. Int Dent J 1970;20:531-5.

• 9. Perel ML. Axial crown contours. J Prosthet 9. Perel ML. Axial crown contours. J Prosthet Dent 1971;25:642-8.Dent 1971;25:642-8.

• 10. Black GV. The management of enamel 10. Black GV. The management of enamel margins. Dent Cosmos 1891;33: 85-100.margins. Dent Cosmos 1891;33: 85-100.

• 11. Bader JD et al. Effect of crown margins on 11. Bader JD et al. Effect of crown margins on periodontal conditions in regularly attending periodontal conditions in regularly attending pataients. J Prosthet Dent 1991;65:75-82.pataients. J Prosthet Dent 1991;65:75-82.

• 12. Hunter AJ, Hunter AR. Gingival crown 12. Hunter AJ, Hunter AR. Gingival crown margin configurations. A review and margin configurations. A review and Discussion : Terminology and widths. J Discussion : Terminology and widths. J Prosthet Dent 1990;64:548-52.Prosthet Dent 1990;64:548-52.

• 13. Bell CJ, Bowden JR, Saund P, Smith M, Stephenson 13. Bell CJ, Bowden JR, Saund P, Smith M, Stephenson RI. The DicerRI. The Dicer

castable ceramic crown. Dent Practice 1987;25:20-5.castable ceramic crown. Dent Practice 1987;25:20-5.• 14. Rosner D. Function,placement and reproduction of 14. Rosner D. Function,placement and reproduction of

bevels for gold. J Prosthet Dent 1963:10:1160-66. bevels for gold. J Prosthet Dent 1963:10:1160-66. • 15. Ostlund LE. Cavity design and mathematics: their 15. Ostlund LE. Cavity design and mathematics: their

effect on gaps at the margins of cast casting. J effect on gaps at the margins of cast casting. J Prosthet Dent restorations. Operative Dent Prosthet Dent restorations. Operative Dent 1985;10:122-37.1985;10:122-37.

• 16. Schweikert E . Feather-edged or knife-edged 16. Schweikert E . Feather-edged or knife-edged Dreparation and impression technique. J Prosthet Dent Dreparation and impression technique. J Prosthet Dent 1984;52:243-6.1984;52:243-6.

• 17. Goodacre CJ. Designing tooth preparatios for 17. Goodacre CJ. Designing tooth preparatios for optimal success. Dent Clin N Am 2004;48:359-385.optimal success. Dent Clin N Am 2004;48:359-385.

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Principles of Tooth Preparation Punam Bishnoi

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