Bio Mechanics of Tooth Preparation

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Department of Prosthodontics Including Crown &

Bridge, Maxillofacial Prosthodontics & Oral



I N T R O D U C T I O N

B IO L O G IC C O N S ID E R A T IO N S

M E C H A N I C A L

C O N S I D E R A T I O N S

E S T H E T I C C O N S I D E RA T IO N S

C O N C L U S I O N

R E F E R E N C E S

I N T R O D U C T I O N

B IO L O G IC C O N S ID E R A T IO N S

M E C H A N I C A L

C O N S I D E R A T I O N S

E S T H E T I C C O N S ID E R A T IO N S

C O N C L U S I O N

R E F E R E N C E S



INTRODUCTIONINTRODUCTION

Teeth do not have the regenerating

capacity as most other tissues have,

once when enamel / dentin are lost as a

result of caries, trauma etc, it requires a

restorative material to restore the form& function

Teeth require preparation to receive

restorations and these preparations arebased on fundamental principles which

determines the success of

prosthodontic treatment

Teeth do not have the regenerating

capacity as most other tissues have,

once when enamel / dentin are lost as a

result of caries, trauma etc, it requires a

restorative material to restore the form& function

Teeth require preparation to receive

restorations and these preparations arebased on fundamental principles which

determines the success of

prosthodontic treatment



Retention Form

Resistance

Form

Deformation

Conservation Of Tooth

Structure

Avoidance Of Over

contouring

Supragingival Margins

Harmonious Occlusion

Protection Against Fracture

Minimal Metal Display Maximum Porcelain

Thickness

Porcelain Occlusal

Surface

Sub gingival Margins



BIOLOGIC CONSIDERATIONSBIOLOGIC CONSIDERATIONS

PREVENTION OF DAMAGE DURING

TOOTH PREPARATION

Adjacent Tooth

Iatrogenic damage or nicking of adjacent tooth removes fluoride rich

superficial enamel layer and creates

a rough surface, which has every

possibility to accumulate plaque andeventually leads to dental caries

PREVENTION OF DAMAGE DURING

TOOTH PREPARATION

Adjacent Tooth

Iatrogenic damage or nicking of adjacent tooth removes fluoride rich

superficial enamel layer and creates

a rough surface, which has every

possibility to accumulate plaque andeventually leads to dental caries

Contd



This can be

prevented

with the helpof a matrix

band placed

in theinterdental

region during

proximal

tooth

preparation

This can be

prevented

with the helpof a matrix

band placed

in theinterdental

region during

proximal

tooth

preparation

MATRIX BAND PLACED TO

PROTECT THE ADJACENT TOOTH

STRUCTURE

Contd



The best way to protect

adjacent teeth is by using thintapered diamond through

interproximal contact area to

leave a slight lip of enamel or fin of enamel without causing

excessive tooth reduction or

undesirable angulation of rotary instrument

The best way to protect

adjacent teeth is by using thintapered diamond through

interproximal contact area to

leave a slight lip of enamel or fin of enamel without causing

excessive tooth reduction or

undesirable angulation of rotary instrument

Contd



Soft Tissues

Movable soft tissues like lips,

cheeks and tongue are kept away

from the site of preparation with the

help of mouth mirror, aspirator tip

and saliva ejector

Soft Tissues

Movable soft tissues like lips,

cheeks and tongue are kept away

from the site of preparation with the

help of mouth mirror, aspirator tip

and saliva ejector

USE OF MOUTH MIRRORS & SALIVA EJECTORS FOR SOFT

TISSUE RETRACTION Contd



Pulp

Preventing

pulpal injury isvery vital to save

the tooth

Extremes of temperature,

chemicals and

microorganisms can cause

irreversible

pulpitis

Pulp

Preventing

pulpal injury isvery vital to save

the tooth

Extremes of temperature,

chemicals and

microorganisms can cause

irreversible

pulpitisIRREVERSIBLE PULPITIS

Contd



CAUSES OF INJURY

Temperature

o

Heat is generated bythe friction between

the rotary

instrument & the

tooth beingprepared

o Excessive pressure,

condition of the bur ,

higher rotational

speeds all increase

the heat generated

CAUSES OF INJURY

Temperature

o

Heat is generated bythe friction between

the rotary

instrument & the

tooth beingprepared

o Excessive pressure,

condition of the bur ,

higher rotational

speeds all increase

the heat generated

TOOTH PREPARATION

USING AN AIRROTOR

HANDPIECE

Contd



o Air-water spray when accurately

directed reduces the heat

generated, prevents clogging and increases the cutting

efficiency of the bur

o

Special care should be whilepreparing grooves or pin holes,

as the coolant cannot reach the

cutting edge of the bur, heat

generation can be prevented by

using low rotational speed

o Air-water spray when accurately

directed reduces the heat

generated, prevents clogging and increases the cutting

efficiency of the bur

o

Special care should be whilepreparing grooves or pin holes,

as the coolant cannot reach the

cutting edge of the bur, heat

generation can be prevented by

using low rotational speed

Contd



Chemical Actiono The chemical action of certain

dental materials like bases,restorative resins, solvents &luting agents when applied tofreshly cut dentin can cause

pulpal damageo Cavity varnish & dentin bonding

agents form an effective barrier

in most cases but they canaffect on the retention of cemented restorations

Chemical Actiono The chemical action of certain

dental materials like bases,restorative resins, solvents &luting agents when applied tofreshly cut dentin can cause

pulpal damageo Cavity varnish & dentin bonding

agents form an effective barrier

in most cases but they canaffect on the retention of cemented restorations

Contd



Bacterial Action

o Bacteria those are left behind or

those which gain access to the dentin because of microleakage

can lead to pulpal damage

o Dental materials like zinc phosphate cement have an

antibacterial effect but, because

of property of vital dentin toresist infection the routine use of

antimicrobials is not advocated

Bacterial Action

o Bacteria those are left behind or

those which gain access to the dentin because of microleakage

can lead to pulpal damage

o Dental materials like zinc phosphate cement have an

antibacterial effect but, because

of property of vital dentin toresist infection the routine use of

antimicrobials is not advocated

Contd



CONSERVATION OF TOOTH

STRUCTURE One of the basic tenets of

restorative dentistry is the

conservation of toothstructure as much as

possible within mechanical and esthetic principles of

tooth preparation

CONSERVATION OF TOOTH

STRUCTURE One of the basic tenets of

restorative dentistry is the

conservation of toothstructure as much as

possible within mechanical and esthetic principles of

tooth preparation

Contd



Tooth structure is conserved by

employing the following guidelines

Use of partial coverage rather thancomplete coverage

Preparation with minimal

convergence angleUniform occlusal reduction following

anatomical inclined planes

Even preparation of axial surfacesConservative margin selection

Avoid unnecessary apical extension

Tooth structure is conserved by

employing the following guidelines

Use of partial coverage rather thancomplete coverage

Preparation with minimal

convergence angleUniform occlusal reduction following

anatomical inclined planes

Even preparation of axial surfacesConservative margin selection

Avoid unnecessary apical extension

Contd



CONSIDERATIONS AFFECTING

FUTURE DENTAL HEALTH

Structural Durability

The restoration must contain

adequate bulk to withstandforces of mastication

Occlusal Reduction

Functional Cusp Bevel

Axial Reduction

CONSIDERATIONS AFFECTING

FUTURE DENTAL HEALTH

Structural Durability

The restoration must contain

adequate bulk to withstandforces of mastication

Occlusal Reduction

Functional Cusp Bevel Axial Reduction

Contd



Occlusal Reduction

Occlusal

clearance provides adequate

strength and bulk

of metal

For gold alloys

occlusal clearance

in non-functional

cusps – 1.0 mm and for functional

cusps – 1.5 mm

Occlusal Reduction

Occlusal

clearance provides adequate

strength and bulk

of metal

For gold alloys

occlusal clearance

in non-functional

cusps – 1.0 mm and for functional

cusps – 1.5 mm CORRECT OCCLUSAL

REDUCTION PARALLELS THE

PLANE OF OCCLUSION

Contd



For metal ceramic

crowns in non-

functional cusps 1.0

– 1.5 mm and infunctional cusps 1.5

– 2.0 mm

For all ceramic 2.0

mm throughoutThe basic inclined

planes of the

occlusal surface

should be followedfor adequate

clearance and

without over

shortening

For metal ceramic

crowns in non-

functional cusps 1.0

– 1.5 mm and infunctional cusps 1.5

– 2.0 mm

For all ceramic 2.0

mm throughoutThe basic inclined

planes of the

occlusal surface

should be followedfor adequate

clearance and

without over

shortening

UNDESIRABLE OVER SHORTENING

OF THE TOOTH STRUCTURE

Contd



The amount of

occlusal

reduction is not

always the same

as the clearance

needed

Often part of atipped tooth is

already short of

the ideal occlusal

plane & willrequire less

reduction than

would a tooth in

ideal occlusion

The amount of

occlusal

reduction is not

always the same

as the clearance

needed

Often part of atipped tooth is

already short of

the ideal occlusal

plane & willrequire less

reduction than

would a tooth in

ideal occlusion OCCLUSAL REDUCTION FOR A

TIPPED TOOTH

Co td



Avoid creating steep planes with

sharp angles, since these can

increase stress & hinder complete

seating of the casting

For diminishing stresses rounding of the angles & avoidance of deep

grooves in the centre of the occlusal

surface & keeping the angulation of the occlusal planes shallow

Avoid creating steep planes with

sharp angles, since these can

increase stress & hinder completeseating of the casting

For diminishing stresses rounding of the angles & avoidance of deep

grooves in the centre of the occlusal

surface & keeping the angulation of the occlusal planes shallow

Contd




A bevel increases the surface area

of metal covering the cusp andhence provides adequate bulk and

strength for the restoration

When adequate bevel is not given,the thin metal resulting over the

functional cusps is easily damaged

because of weakness and createsdeflective occlusal contacts


A bevel increases the surface area

of metal covering the cusp andhence provides adequate bulk and

strength for the restoration

When adequate bevel is not given,the thin metal resulting over the

functional cusps is easily damaged

because of weakness and createsdeflective occlusal contacts

Contd



The functional

cusp bevel is

usually placed

on the facial

cusps of the

mandibular

teeth & on thelingual cusps

of the maxillary

teeth,

paralleling theinclination of

the cusp plane

it opposes

The functional

cusp bevel is

usually placed

on the facial

cusps of the

mandibular

teeth & on thelingual cusps

of the maxillary

teeth,

paralleling theinclination of

the cusp plane

it opposes

FUNCTIONAL CUSP BEVEL

Contd



In a cross – bite

occlusal

relationship, thefunctional cusps

are reversed & the

functional cuspbevel is placed on

the facial cusps of

the maxillary teeth & lingual cusps of

mandibular teeth

In a cross – bite

occlusal

relationship, thefunctional cusps

are reversed & the

functional cuspbevel is placed on

the facial cusps of

the maxillary teeth & lingual cusps of

mandibular teeth FUNCTIONAL CUSP BEVELS IN

A CROSS BITE RELATIONSHIP

Contd



Axial Reduction

Inadequate axial reduction is

commonly associated with gingival inflammation, probably because it is

more difficult for the patient to

maintain plaque control around thegingival margin

Axial Reduction

Inadequate axial reduction is

commonly associated with gingival inflammation, probably because it is

more difficult for the patient to

maintain plaque control around thegingival margin

ADEQUATE REDUCTION

INADEQUATE REDUCTION –

OVER CONTOURED

RESTORATION

INADEQUATE REDUCTION –

CROWN WITH THIN WEAK

WALLS Contd



The crown should duplicate

the contours & profile of the

original tooth unless it is

malformed or malpositioned

If an error is made, a slightlyunder contoured flat

restoration is better as it is

easier to keep free of plaque

The crown should duplicate

the contours & profile of the

original tooth unless it is

malformed or malpositioned

If an error is made, a slightlyunder contoured flat

restoration is better as it is

easier to keep free of plaque

Contd



Margin Placement

Whenever possible the margin of a

preparation should be Supragingival as Subgingival margins of cemented

restorations have been identified as

a major factor in periodontal diseaseSupragingival margins are easier to

prepare accurately without trauma to

the soft tissues They can be situated on hard

enamel , whereas Subgingival

margins are often on dentin or

Margin Placement

Whenever possible the margin of a

preparation should be Supragingival as Subgingival margins of cemented

restorations have been identified as

a major factor in periodontal diseaseSupragingival margins are easier to

prepare accurately without trauma to

the soft tissues They can be situated on hard

enamel , whereas Subgingival

margins are often on dentin or Co td



Advantages of supragingival margins:-

They can be easily finished They are more easily kept clean

Impressions are more easily made, with less potential for soft tissue damage

Restorations can be easily evaluated at recallappointments

Advantages of supragingival margins:-

They can be easily finished They are more easily kept clean

Impressions are more easily made, with less potential for soft tissue damage

Restorations can be easily evaluated at recallappointments

Co td



Indications for subgingival margins:-

Dental caries, cervical erosion, or

restorations extending subgingivally , & a crown

lengthening procedure is not

indicatedThe proximal contact area extends

to the gingival crest

Additional retention is requiredThe margin of a metal – ceramic

crown is to be hidden behind the

labiogingival crest

Indications for subgingival margins:-

Dental caries, cervical erosion, or

restorations extending subgingivally , & a crown

lengthening procedure is not

indicated

The proximal contact area extends

to the gingival crest

Additional retention is requiredThe margin of a metal – ceramic

crown is to be hidden behind the

labiogingival crest Co td



Root sensitivity cannot be

controlled by moreconservative procedures,

such as dentin bonding

agents

Modification of the axial

contour is indicated

Root sensitivity cannot be

controlled by moreconservative procedures,

such as dentin bonding

agents

Modification of the axial

contour is indicated

Co td



MARGIN PLACEMENT

Subgingival margins of cementedrestorations are a major factor in

periodontal disease

No difference between

subgingival and supragingival margins

MARGIN PLACEMENT

Subgingival margins of cementedrestorations are a major factor in

periodontal disease

No difference between

subgingival and supragingival margins

Block. JPD 1987; Bader. JPD 1991

Richer & Uno. JPD 1973, Koth. JPD 1982

Co td



Margins should be 2mm away

from the alveolar crest

Margins should be 2mm away

from the alveolar crest Garguilo. J Periodontol 1961

Co td



If the margin intrudes into this

biologic width, the inflammation will result

If the margin intrudes into this

biologic width, the inflammation

will result

INFLAMMATION & OSTEOCLASTIC ACTIVITY WHEN MARGIN OF A RESTORATION

INTRUDES INTO THE BIOLOGIC WIDTH

Co td



The bone will recede until once

again it is at least 2 mm away from

the margin

The bone will recede until once

again it is at least 2 mm away from

the margin

CONTINUOUS BONE RESORPTION UNTIL THE ALVEOLAR CREST IS AT LEAST 2

mm FROM THE RESTORATION MARGIN

Co td



Margin Adaptation

The junction between a cemented

restoration and tooth is always a

potential site for recurrent caries

The more accurately the margins

are adapted to the tooth, the less

chance for recurrent caries

A well designed preparation has a

smooth & even margin

Margin Adaptation

The junction between a cemented

restoration and tooth is always a

potential site for recurrent caries

The more accurately the margins

are adapted to the tooth, the less

chance for recurrent caries

A well designed preparation has a

smooth & even margin

Co td



Rough, irregular or stepped

junctions greatly increase the

length of the margin &

substantially reduce the

adaptation of the restorationFor a properly prepared tooth a

skilled technician can make a

casting that fits to within 10 µm &a porcelain margin that fits to

within 50 µm

Rough, irregular or stepped

junctions greatly increase the

length of the margin &

substantially reduce the

adaptation of the restoration

For a properly prepared tooth a

skilled technician can make a

casting that fits to within 10 µm &a porcelain margin that fits to

within 50 µm

Co td



Marginal Integrity

For survival of a restoration in the

biological environment of the oral

cavity the margins should be closely

adapted to the cavosurface finish

line of the preparationThe configuration of the preparation

finish line dictates the shape & bulk

of restorative material in the marginof the restoration

Marginal Integrity

For survival of a restoration in the

biological environment of the oral

cavity the margins should be closely

adapted to the cavosurface finish

line of the preparationThe configuration of the preparation

finish line dictates the shape & bulk

of restorative material in the marginof the restoration

Co td



Historically, the bevel was used as a

device for compensating for the

solidification shrinking of alloysused in fabricating cast restorations

Margins should be acute in cross-

section rather than right – angled tofacilitate a closer fit

To accomplish this, preparation

finish lines should take forms that permit acute edges in the restoration

margins

Historically, the bevel was used as a

device for compensating for the

solidification shrinking of alloysused in fabricating cast restorations

Margins should be acute in cross-

section rather than right – angled tofacilitate a closer fit

To accomplish this, preparation

finish lines should take forms that permit acute edges in the restoration

margins

Co td



Even the best crowns fail to seat by

several microns

If the prepared surface that is adjacentto a finish line is perpendicular to the

path of insertion, as a shoulder is, the

marginal gap, d , will be as great as thedistance by which the crown fails to

seat, D

If the inner surface of the metal marginforms an angle, m, of less than 90 0 with

the path of insertion, as does a bevel

or a chamfer , d will be smaller than D

Even the best crowns fail to seat by

several microns

If the prepared surface that is adjacentto a finish line is perpendicular to the

path of insertion, as a shoulder is, the

marginal gap, d , will be as great as thedistance by which the crown fails to

seat, D

If the inner surface of the metal marginforms an angle, m, of less than 90 0 with

the path of insertion, as does a bevel

or a chamfer , d will be smaller than D

Co td

D = distance by



y

which a crown

fails to seat a

restoration

d = marginal opening (in the but

joint)

In the presence of

a bevel, the

shortest distance from the margin to

the tooth

structure is less

than D & is a

function of the sine of the acute

angle of the

margin m , or of

the cosine of the

obtuse angle of

the finish line



The shortest distance from the

casting margin to the tooth structure,

d , can be stated as a function of D &the sine of the angle m or the cosine

of angle p which is the angle between

the surface of the bevel & the path of insertion

The shortest distance from the

casting margin to the tooth structure,

d , can be stated as a function of D &the sine of the angle m or the cosine

of angle p which is the angle between

the surface of the bevel & the path of

insertion

d = D sin m, or

d = D cos p

Co td



As the angle m is reduced, its sine

becomes smaller & so does d

The more obtuse the angle of toothstructure at a horizontal finish line,

therefore more acute the restoration

margin, the shorter the distancebetween the restoration margin & the

tooth

The margin angle must be quiteacute before the actual distance is

diminished to a great extent

As the angle m is reduced, its sine

becomes smaller & so does d

The more obtuse the angle of toothstructure at a horizontal finish line,

therefore more acute the restoration

margin, the shorter the distancebetween the restoration margin & the

tooth

The margin angle must be quiteacute before the actual distance is

diminished to a great extent

Co td



An angel of 35 0 – 40 0 is considered optimal

If it is made much more acute then it becomes

weak

An angel of 35 0 – 40 0 is considered optimal

If it is made much more acute then it becomes

weak

THE SMALLER THE ANGLE BETWEEN THE PREPARED TOOTH SURFACE AT THE

FINISH LINE & THE PATH OF INSERTION, THE LESS THE MARGINAL OPENING

FOR THE SAME AMOUNT OF INCOMPLETE SEATING Co td



Margin Geometry

Cross-sectional configuration of the margin has

been the subject of controversy and debate

Different shapes have been described and

advocated

Margin Geometry

Cross-sectional configuration of the margin has

been the subject of controversy and debate

Different shapes have been described and

advocated

Rosner. JPD 1963; Hunter. JPD 1990

Dykema. 1986; Shillenberg 1997

Co td



Configurations

Chamfer o Heavy chamfer

Shoulder

o Radial shoulder

o Shoulder with bevel

Knife edge

Configurations

Chamfer o Heavy chamfer

Shoulder

o Radial shoulder

o Shoulder with bevel

Knife edge

Co td

CHAMFERCHAMFER



CHAMFERCHAMFER

Preferred for veneer

metal restorations Exhibits least stress,

so that cement under

it will have lesslikelihood of failure

Care needed to

remove unsupported

Preferred for veneer

metal restorations Exhibits least stress,

so that cement under

it will have lesslikelihood of failure

Care needed to

remove unsupported

Panno et al, JPD 1986

Co td



HEAVY CHAMFERHEAVY CHAMFER

Rounded internal angle

Provides better

support thanchamfer for

ceramic

restorations

Not as good as

shoulder

Rounded internal angle

Provides better

support thanchamfer for

ceramic

restorations

Not as good as

shoulder

Hoffman, DCNA 1965

Co td

SHOULDERSHOULDER



SHOULDERSHOULDER

Margin of choice for all ceramic

More destruction of tooth but space

minimizes stress thatmight lead to fracture

90 degree internalline angleconcentrates stressin the tooth and isconducive to coronal

fracture

Margin of choice for all ceramic

More destruction of tooth but space

minimizes stress thatmight lead to fracture

90 degree internalline angleconcentrates stressin the tooth and isconducive to coronal

fracture Shillingburg 1997, Rosenstiel 2001 Co td

RADIAL SHOULDERRADIAL SHOULDER



RADIAL SHOULDERRADIAL SHOULDER

Internal line angle

is rounded by

modified bin-

angle chisel

Stress

concentration is

less in the tooth

structure

Internal line angle

is rounded by

modified bin-

angle chisel

Stress

concentration is

less in the tooth

structureShillingburg 1997, Rosenstiel 2001

Co td



KNIFE EDGEKNIFE EDGE

Lingual surfaceof mandibular posterior teeth

Tilted teeth

Lingual surfaceof mandibular posterior teeth

Tilted teeth

Shillingburg 1997, Rosenstiel 2001

Co td

Effect Of Finish Line OnEffect Of Finish Line On



Effect Of Finish Line OnMarginal Seal & Marginal Seat

Of Full Crown Preparation

Effect Of Finish Line OnMarginal Seal & Marginal Seat

Of Full Crown PreparationFeather edge & parallel bevel

demonstrated the best marginalseal

90 0 demonstrated the best seating

of restoration

Feather edge & parallel bevel

demonstrated the best marginalseal

90 0 demonstrated the best seating

of restoration

JPD 2004; 92: 1 – 7 Co td

DISTORTION RELATED TO MARGINDISTORTION RELATED TO MARGIN



DISTORTION RELATED TO MARGIN

DESIGN IN PFM RESTORATIONS

DISTORTION RELATED TO MARGIN

DESIGN IN PFM RESTORATIONS

Three margin designs were

comparedChamfer

Shoulder

Shoulder with bevel

Three margin designs were

comparedChamfer

Shoulder

Shoulder with bevel

Faucher & Nicholas 1980 Co td



Chamfer exhibited more distortionthan shoulder or shoulder with bevel

Type of distortion of shoulder and shoulder with bevel were almost

similar

Study supports the theory that

placing of additional metal at the

gingival margin reinforces the

margin and inhibits marginal

distortion

Chamfer exhibited more distortionthan shoulder or shoulder with bevel

Type of distortion of shoulder and shoulder with bevel were almost

similar

Study supports the theory that

placing of additional metal at the

gingival margin reinforces the

margin and inhibits marginal

distortion

Co td

ABUTMENT EVALUATIONABUTMENT EVALUATION



ABUTMENT EVALUATION ABUTMENT EVALUATION

Evaluated for 3 factors:

CROWN-ROOT RATIO

Evaluated for 3 factors:

CROWN-ROOT RATIO

Klaffenbach. JADA 1936



ROOT

CONFIGURATION

PERIODONTAL

LIGAMENT AREA

ROOT

CONFIGURATION

PERIODONTAL

LIGAMENT AREA



ROOT CONFIGURATION

Roots broader labio-lingually arepreferred than they are mesio-distally

Irregular preferred than conical

ROOT CONFIGURATION

Roots broader labio-lingually arepreferred than they are mesio-distally

Irregular preferred than conical

ROOT SURFACE AREAROOT SURFACE AREA



ROOT SURFACE AREAROOT SURFACE AREA

ANTE’S LAW

Irvin H Ante.1928

Dykema et al

Tylman

Shillingberg

ANTE’S LAW

Irvin H Ante.1928

Dykema et al

Tylman

Shillingberg

} SUPPORTEDTHE ANTE’S

LAW



Nyman & Erikson, J Clin Periodontol 1982

Nyman & Lindhe. J Clin Periodontol 1976

Cast doubt on the validity of Ante’s

Law by demonstrating that teeth

with considerably reduced bone

support can be successfully used

as FPD abutments

Nyman & Erikson, J Clin Periodontol 1982

Nyman & Lindhe. J Clin Periodontol 1976

Cast doubt on the validity of Ante’s

Law by demonstrating that teeth

with considerably reduced bone

support can be successfully used

as FPD abutments

No loss of attachment afterNo loss of attachment after



No loss of attachment after 8 to 11 yrs because of

Meticulous root planing Proper plaquecontrol Occlusal design of prosthesis

No loss of attachment after 8 to 11 yrs because of

Meticulous root planing Proper plaquecontrol Occlusal design of prosthesis

FPD’S SUPPORTED BY PERIODONTALLYFPD’S SUPPORTED BY PERIODONTALLY



FPD’S SUPPORTED BY PERIODONTALLY

COMPROMISED TEETH

FPD’S SUPPORTED BY PERIODONTALLY

COMPROMISED TEETH

Criteria For Inclusion In Study: 50% loss of attachment

Probing depth of 5mm at 1 site

Grade II or III mobility

Minimum of 3mm osseous support

Attachment level , probing depth, bleeding upon

probing & gingival index scores were non

significant after 2 yrs when compared with

contralateral teeth

Criteria For Inclusion In Study: 50% loss of attachment

Probing depth of 5mm at 1 site

Grade II or III mobility

Minimum of 3mm osseous support

Attachment level , probing depth, bleeding upon

probing & gingival index scores were non

significant after 2 yrs when compared with

contralateral teeth

Freilich et al. JPD 1991..

CANTILEVER F P D’CANTILEVER F P D’s



CANTILEVER F.P.D’sCANTILEVER F.P.D’s

Long term prognosis is poor

Cantilever designs may be preferred since re-

adhesion after failure is greatly facilitated and

often leads to predictable long term success

Long term prognosis is poor

Cantilever designs may be preferred since re-

adhesion after failure is greatly facilitated and

often leads to predictable long term success

Cheung et al, J Oral Rehabil 1990

Brigs et al, Br Dent J 1996



STUDY MONTHS TO



UNIVERSITY OF IOWAPerforated design 110

Etched metal 250

UNIVERSITY OF MARYLANDEtched metal 190

UNIVERSITY OF IOWAPerforated design 110

Etched metal 250

UNIVERSITY OF MARYLANDEtched metal 190

STUDY MONTHS TO

50% FAILURE

Boyer et al, J Dent Res 1993

De Rijk et al, J Dent Res 1996

AN ANLYSIS OF CLINICAL STUDIES ONAN ANLYSIS OF CLINICAL STUDIES ON



AN ANLYSIS OF CLINICAL STUDIES ON

RESIN-BONDED BRIDGES

AN ANLYSIS OF CLINICAL STUDIES ON

RESIN-BONDED BRIDGES

60 publications

16 included in study

Survival rate1yr - 89 ± 1%

2yr - 84 ± 1%

3yr - 80 ± 1%

4yr - 74 ± 2%

60 publications

16 included in study

Survival rate1yr - 89 ± 1%

2yr - 84 ± 1%

3yr - 80 ± 1%

4yr - 74 ± 2%

Creugers & Hoffman, J Dent Res 1991

RESIN BONDED FPD RETENTION :ARESIN BONDED FPD RETENTION :A



RESIN-BONDED FPD RETENTION :A

RETROSPECTIVE 13-YR FOLLOW-UP

RESIN-BONDED FPD RETENTION :A

RETROSPECTIVE 13-YR FOLLOW-UP

51 resin-bonded FPDs were inserted andevaluated after 13 yrs

Three levels of survival were defined:

Complete survival (no debonding) - 85 months,13%

Functional survival (loss of retention on 1

occasion with rebonding) - 112 months,10%Multiple survival (loss of retention on several

occasions with rebonding of original RBFPD) -

131months, 8%

51 resin-bonded FPDs were inserted andevaluated after 13 yrs

Three levels of survival were defined:

Complete survival (no debonding) - 85 months,13%

Functional survival (loss of retention on 1

occasion with rebonding) - 112 months,10%Multiple survival (loss of retention on several

occasions with rebonding of original RBFPD) -

131months, 8%

Zalkind, Handani & Hochman, J Oral Rehabil 2003



MECHANICALMECHANICAL



CONSIDERATIONSCONSIDERATIONS

RETENTION FORM:Quality of preparation that prevents the

restoration from becoming dislodged by

forces parallel to the path of withdrawal

Dental caries and porcelain failure outrank

lack of retention as a cause of failure

RETENTION FORM:Quality of preparation that prevents the

restoration from becoming dislodged by

forces parallel to the path of withdrawal

Dental caries and porcelain failure outrank

lack of retention as a cause of failure

Walton et al JPD 1986

Karlsson & Lindquist, Int J Prostho 1998

Co td



RETENTION DEPENDS ON:

Magnitude of dislodging forceGeometry of tooth preparation

Roughness of fitting surfaceMaterials being cemented

Film thickness of luting

cement

RETENTION DEPENDS ON:

Magnitude of dislodging forceGeometry of tooth preparation

Roughness of fitting surfaceMaterials being cemented

Film thickness of luting

cement

Co td

MAGNITUDE OF DISLODGINGMAGNITUDE OF DISLODGING



FORCESFORCES

Greatest removal forcesgenerally arise when

exceptionally sticky food is taken

Also depends on surface area

and texture of restoration beingpulled

Greatest removal forcesgenerally arise when

exceptionally sticky food is taken

Also depends on surface area

and texture of restoration beingpulled

Co td

GEOMETRY OF TOOTHGEOMETRY OF TOOTH



GEOMETRY OF TOOTH

PREPARATION

GEOMETRY OF TOOTH

PREPARATION

Taper

Surface area

Stress concentration

Taper

Surface area

Stress concentration

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TAPERTAPER



TAPERTAPER

Parallel walls were advocated for inlay

restorations

3 to 5 degrees

6 degrees

10 to 14 degrees

Parallel walls were advocated for inlay

restorations

3 to 5 degrees

6 degrees

10 to 14 degrees

Conzett; Dent Cosmos 1910

Dykema

Shillingberg; Wilson, J Prosthod 1994

Tylman

Co td

Overall 2.5 to 6.5 Overall 2.5 to 6.5



degrees has been

suggested as optimum

This is based on aninclination of

approximately 30 being

produced on each

surface external or

internal , by the sides of

the tapered instrument

The result would be anoverall taper or an angle

of convergence of 6 0

degrees has been

suggested as optimum

This is based on aninclination of

approximately 30 being

produced on each

surface external or

internal , by the sides of

the tapered instrument

The result would be anoverall taper or an angle

of convergence of 6 0 AN OPTIMAL 6 0

TAPER

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Average of 16.5 0

A taper of 16 0 has been proposed asbeing achievable clinically while stillaffording retention

Average of 19.2 0

Average of 16.5 0

A taper of 16 0 has been proposed asbeing achievable clinically while stillaffording retention

Average of 19.2 0

Mack. J Oral Rehabil 1980

Dodge et al. Quint Int 1985

Nonan & Goldfoger. JPD 1991

Co td



Co td

OPTIMUM DEGREE OFOPTIMUM DEGREE OF

TAPER



TAPERTAPERARCH MESIODISTAL FACIOLINGUAL OVERALL

MAXILLARY

ANTERIOR 10 10 10

PREMOLAR 14 14 14

MOLAR 17 21 17

MANDIBULAR

ANTERIOR 10 10 10

PREMOLAR 16 12 14

MOLAR 24 20 22

SURFACE AREASURFACE AREA



SURFACE AREASURFACE AREA

More the surface area covered morethe retention

Long axial walls are more retentive

than short walls

More the surface area covered morethe retention

Long axial walls are more retentive

than short walls

Shillingberg, Calif Dent Assoc J 1975

Co td



TENSILE STRESS

SHEAR & COMPRESSIVE

STRESS

COMPRESSIVE STRESS

SHEAR STRESS

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Stress is more concentrated at

the junction of axial and occlusal surfaces

Rounding of margins will

decrease stress concentration

Stress is more concentrated at

the junction of axial and occlusal surfaces

Rounding of margins will

decrease stress concentration

Nicholls. JPD 1974

Co td

For utilization of the For utilization of the



shear strength of the

cement, the preparation

must have opposingwalls i.e. the 2 surfaces

must be nearly parallel

with each other

These surfaces may be

internal , as the facial &

lingual walls of the

proximal box of aninlay or external such

as axial walls of a full

veneer crown

shear strength of the

cement, the preparation

must have opposingwalls i.e. the 2 surfaces

must be nearly parallel

with each other

These surfaces may be

internal , as the facial &

lingual walls of the

proximal box of aninlay or external such

as axial walls of a full

veneer crown

AN INLAY

A FULL VENEER CROWN

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To obtain greatest area of cement To obtain greatest area of cement



To obtain greatest area of cement

under shear , the directions in which

a restoration can be removed mustbe restricted to essentially one path

If features are added to the

preparation so that only a force in

one direction can move a restoration

without compressing the cement film

against one or more surfaces, the

retention is enhanced

To obtain greatest area of cement

under shear , the directions in which

a restoration can be removed mustbe restricted to essentially one path

If features are added to the

preparation so that only a force in

one direction can move a restoration

without compressing the cement film

against one or more surfaces, the

retention is enhanced

Co td



A full veneer crown preparation has an A full veneer crown preparation has an



excellent retention because of mesial, distal, &

facial walls limit the path of insertions to a

narrow range However, when the facial surface is left

uncovered the crown on this preparation could

be removed towards the lingual , the incisal , or

any direction in between

excellent retention because of mesial, distal, &

facial walls limit the path of insertions to a

narrow range However, when the facial surface is left

uncovered the crown on this preparation could

be removed towards the lingual , the incisal , or

any direction in between

PATHS OF INSERTION FOR

FULL & PARTIAL VENEER

CROWNS

Co td

To create a more To create a more



To create a more

retentive form,

grooves, boxes, or pinholes are

substituted for the

missing axial wall

These features are

also useful for

augmenting retention

on severely damaged

teeth

To create a more

retentive form,

grooves, boxes, or pinholes are

substituted for the

missing axial wall

These features are

also useful for

augmenting retention

on severely damaged

teeth GROOVES & PIN HOLES

FOR RETENTION

Co td

For a groove to effectively For a groove to effectively



substitute for the

uncovered facial surface,

the lingual wall of thegroove must be distinct &

perpendicular to the

adjoining axial surface

Otherwise a lingually directed force would cause

the ribs of the metal to

slide along the inclined

planes of the lingual walls

of the grooves, spreading the axial walls & opening

the margins

substitute for the

uncovered facial surface,

the lingual wall of thegroove must be distinct &

perpendicular to the

adjoining axial surface

Otherwise a lingually

directed force would cause

the ribs of the metal to

slide along the inclined

planes of the lingual walls

of the grooves, spreading the axial walls & opening

the marginsLINGUALLY DIRECTED FORCES

ACTING ON A 3 – QUARTER CROWN

Co td

Length of a preparation is an important Length of a preparation is an important



g p p p

factor in retention

A long preparation has greater retention than does a short preparation

This is due to greater surface area of the

longer preparation & to the fact that mostof the additional area is under shear than

tension

This greater surface area would lead to apreparation with larger diameter , which

will have greater retention than will a

narrow preparation

g p p p

factor in retention

A long preparation has greater retention than does a short preparation

This is due to greater surface area of the

longer preparation & to the fact that mostof the additional area is under shear than

tension

This greater surface area would lead to apreparation with larger diameter , which

will have greater retention than will a

narrow preparationCo td



Co td

ROUGHNESS OF SURFACES BEINGROUGHNESS OF SURFACES BEING



ROUGHNESS OF SURFACES BEING

CEMENTED

ROUGHNESS OF SURFACES BEING

CEMENTED

The adhesion of the dental cements

depends primarily on the projections

of the cement into microscopic irregularities & recesses on the

surfaces being joined

The prepared tooth surfaces thereforeshould not be highly polished

The adhesion of the dental cements

depends primarily on the projections

of the cement into microscopic irregularities & recesses on the

surfaces being joined

The prepared tooth surfaces thereforeshould not be highly polished

Co td



Retention increases if surface is

roughened or grooved

Airborne particle abrasion has beenshown to increase retention by 64%

when cemented with zinc phosphate

Retention increases if surface is

roughened or grooved

Airborne particle abrasion has beenshown to increase retention by 64%

when cemented with zinc phosphate

O’Connor. JPD 1990

Co td

MATERIALS BEING CEMENTED MATERIALS BEING CEMENTED



Base metals are more retentive thanhigh gold metals

TYPE OF LUTING CEMENT

Adhesive resins are the most retentive

FILM THICKNESS

Conflicting evidence20 to 25 µm most cements40 to 50 µm resin cements

Base metals are more retentive thanhigh gold metals

TYPE OF LUTING CEMENT

Adhesive resins are the most retentive

FILM THICKNESS

Conflicting evidence20 to 25 µm most cements40 to 50 µm resin cements

Saito et al, JPD 1976

El Mowafy et al, JPD 1996; Ayed et al, JPD 1997

RESISTANCE FORMRESISTANCE FORM




Prevents dislodgement of the

restoration by forces directed in an

apical or oblique direction and

prevents any movement of

restoration under occlusal forces

Prevents dislodgement of the

restoration by forces directed in an

apical or oblique direction and

prevents any movement of

restoration under occlusal forces

Co td




RESISTANCE FORMRESISTANCE FORMDepends on:

Magnitude & direction of

dislodging forces

Geometry of tooth preparationPhysical properties of luting

agent Deformation of material

Depends on:

Magnitude & direction of

dislodging forces

Geometry of tooth preparationPhysical properties of luting

agent Deformation of material

Co td

GEOMETRY OF TOOTH

PREPARATION

GEOMETRY OF TOOTH

PREPARATION



PREPARATIONPREPARATION Increased preparation taper and rounding of

axial angles tend to reduce resistance

Molars require more parallel preparation than

Premolars or Anterior Teeth to achieveadequate resistance form

Relationship between preparation height ,diameter and resistance to displacement is

approx. linear

Increased preparation taper and rounding of

axial angles tend to reduce resistance

Molars require more parallel preparation than

Premolars or Anterior Teeth to achieveadequate resistance form

Relationship between preparation height ,diameter and resistance to displacement is

approx. linear

Hegdahl & Silness. J Oral Rehabil 1977

Parker. JPD 1993

Wiskott et al, Int J Prostho 1997

Co td

The resisting area of a cylindrical The resisting area of a cylindrical



preparation would include half of its axial

surface As the degree of taper increases, the

tangent line approaches the occlusal

surface & the resisting area decreases

preparation would include half of its axial

surface

As the degree of taper increases, the

tangent line approaches the occlusal

surface & the resisting area decreases

Co td



The permissible taper of The permissible taper of



a preparation is directly

proportional to the

height/width ratio

The preparation taper

that will still permit an

effective resisting area,for a preparation in

which the height equals

the width, is double than

permissible in apreparation in which the

height is only one half

the width

a preparation is directly

proportional to the

height/width ratio The preparation taper

that will still permit an

effective resisting area,for a preparation in

which the height equals

the width, is double than

permissible in apreparation in which the

height is only one half

the width

EFFECTIVE RESISTANCE FOR A

PREPARATION WITH 1:1

HEIGHT/WITH RATIO WITH 15 0 TAPER

& WITH 1:2 HEIGHT/WIDTH RATIO

WITH 7 0 TAPER

Co td

Formulae used to calculate allowable Formulae used to calculate allowable



preparation tapers (convergence angles) & for

determining the height of the tangency point &

preparation height are:

preparation tapers (convergence angles) & for

determining the height of the tangency point &

preparation height are:

T = arc sin (2r/w)

r = ( w sin T) / 2

h = [ w tan (90 0 – T/2] /2

T = Preparation Taper In

Degrees

r = Height Of Tangency Point Imm

w = Preparation Width In mmCo td

MAGNITUDE & DIRECTION OF

DISLODGING FORCES

MAGNITUDE & DIRECTION OF

DISLODGING FORCES



DISLODGING FORCESDISLODGING FORCES

The strongest forces encounteredduring function are apically directed &

can produce tension & shear in the

cement film only through leverage

This leverage, which is probably the

predominant factor in dislodgement of

cemented restorations, occurs when

the line of action of a force passes

outside the tooth structure, or when the

structures flex

The strongest forces encounteredduring function are apically directed &

can produce tension & shear in the

cement film only through leverage

This leverage, which is probably the

predominant factor in dislodgement of

cemented restorations, occurs when

the line of action of a force passesoutside the tooth structure, or when the

structures flex

Co td

If the force passes If the force passes



within the margin of a

crown, there will be no

tipping of therestoration

The margin on all

sides of therestoration is

supported by the

preparation

The torque produced merely tends to seat

the crown further

within the margin of a

crown, there will be no

tipping of therestoration

The margin on all

sides of therestoration is

supported by the

preparation

The torque produced merely tends to seat

the crown further

LINE OF ACTION OF FORCE

PASSING WITHIN THE

MARGINS OF THE

RESTORATION – NO

SECONDARY LIFTING FORCES

Co td

If the occlusal table

f

If the occlusal table

f th t ti i



of the restoration is

wide, even a vertical

force can passoutside the

supported margin &

produce a

destructive torque

This can also occur

in crowns on tipped

teeth & retainers for cantilever bridges

of the restoration is

wide, even a vertical

force can passoutside the

supported margin &

produce a

destructive torque

This can also occur

in crowns on tipped

teeth & retainers for cantilever bridges

LINE OF ACTION OF FORCE

PASSING OUTSIDE THE

MARGINS OF THE

RESTORATION – SECONDARY

LIFTING FORCES

Co td



The magnitude of the torque produced The magnitude of the torque produced



is equal to the applied force multiplied

by its lever arm, which is the closestdistance between the line of action & the

fulcrum

In equilibrium this torque is balanced bythe sum of all the resisting tensile, shear

& compressive stresses generated in

the cement film

The farther these resisting forces lie

from the fulcrum, the greater their

mechanical advantage

is equal to the applied force multiplied

by its lever arm, which is the closestdistance between the line of action & the

fulcrum

In equilibrium this torque is balanced bythe sum of all the resisting tensile, shear

& compressive stresses generated in

the cement film

The farther these resisting forces lie

from the fulcrum, the greater their

mechanical advantage

Co td

If a line is drawn If a line is drawn



If a line is drawn

from the centre of

rotation perpendicular to the

cement film on the

opposite wall of thepreparation, the

point where this line

intercepts the

cement film can be

referred to as the

tangent point

If a line is drawn

from the centre of

rotation perpendicular to the

cement film on the

opposite wall of thepreparation, the

point where this line

intercepts the

cement film can be

referred to as the

tangent point

TANGENT LINE & POINT

Co td



The area encompassed by thistangent line is referred to as the

The area encompassed by thistangent line is referred to as the



tangent line is referred to as the

“resisting area” by Hegdahl &

Silness

Within this area the luting material is

subjected to varying degrees of compression as well as shear , while

all other points on the surface of the

preparation will experience somedegree of tension & will contribute

little to the resistance of the

preparation

tangent line is referred to as the

“resisting area” by Hegdahl &

Silness

Within this area the luting material is

subjected to varying degrees of compression as well as shear , while

all other points on the surface of the

preparation will experience somedegree of tension & will contribute

little to the resistance of the

preparation Co td

LENGTH OF THE PREPARATIONLENGTH OF THE PREPARATION



Length of a preparation has a strong

influence on its resistance

Shortening a preparation will

produce a proportionately greater diminution of the resisting area

The ability of a restoration to resist

tipping depends not only on thepreparation, but also on the

magnitude of torque

Length of a preparation has a strong

influence on its resistance

Shortening a preparation will

produce a proportionately greater diminution of the resisting area

The ability of a restoration to resist

tipping depends not only on thepreparation, but also on the

magnitude of torque

Co td

If two crowns of unequal length on 2

preparations of equal length are subjected to

If two crowns of unequal length on 2





identical forces, the longer crown is more

likely to fail because the force on it actsthrough a longer lever arm


identical forces, the longer crown is more

likely to fail because the force on it actsthrough a longer lever arm

Co td



Here the arc of Here the arc of



Here the arc of

radius r 2 is

effectively blocked

by the resisting

area of the groove

walls, while the arcof radius r 1

encounters little or

no resistance onthe far axial wall

Here the arc of

radius r 2 is

effectively blocked

by the resisting

area of the groove

walls, while the arcof radius r 1

encounters little or

no resistance onthe far axial wall

GROOVES ENHANCING

RESISTANCE

Co td

PHYSICAL PROPERTIES OF LUTING

AGENT

PHYSICAL PROPERTIES OF LUTING

AGENT



AGENTAGENT

Mostly dependent on:Compressive Strength

Modulus Of Elasticity

Creep

Fatigue Resistance

Mostly dependent on:Compressive Strength

Modulus Of Elasticity

Creep

Fatigue Resistance

Co td

COMPARATIVE EVALUATIONCOMPARATIVE EVALUATION



CEMENTS

COMPRESSIVE STRENGTH

MODULOUS OF ELASTICITY

ZINC PHOSPHATE 104

13.5

RESIN

70 to 172 2.1 to 3.2

POLYCARBOXYLATE 55

5.1

GLASS IONOMER 86 7.3

Co td

BIOLOGIC PROPERTIESBIOLOGIC PROPERTIES



BIOCOMPATIBLE:

There is little association between the choice of Zinc phosphate & GIC and increased pulpal sensitivity,

provided manufacturers recommendations are

followed

Application of desensitizing agents reduces retention

Biocompatibility of resin cements is related to its

degree of polymerization

BIOCOMPATIBLE:

There is little association between the choice of Zinc phosphate & GIC and increased pulpal sensitivity,

provided manufacturers recommendations are

followed

Application of desensitizing agents reduces retention

Biocompatibility of resin cements is related to its

degree of polymerization

Kern et al. JPD 1996 Bebermeyer & Berg Quint Int 1994

Johnson et al.

1993

Swift et al. JADA 1997; Mausner et al. JPD 1996

Co td

Caries Inhibition:Caries Inhibition:



GIC has been shown to increase

the fluoride concentration in the

short term

MicroleakageNon adhesive resins have

increased microleakage as

compared to traditional cements,least is of modified glass ionomer

GIC has been shown to increase

the fluoride concentration in the

short term

MicroleakageNon adhesive resins have

increased microleakage as

compared to traditional cements,least is of modified glass ionomer

Rezk-lega. Scand J Dent Res1991

Mash et al. JPD 1991

Co td

MECHANICAL PROPERTIESMECHANICAL PROPERTIES



Compressive Strength:ADA Specification:

70 Mpa at 24hrsGlass Ionomer increases

over weeks to 200 Mpa

Compressive Strength:ADA Specification:

70 Mpa at 24hrsGlass Ionomer increases

over weeks to 200 Mpa

Co td

MECHANICAL PROPERTIESMECHANICAL PROPERTIES



Flexure strength, diametrical tensile

strength, mod. of elasticity , fracture

toughness, hardness:

Resins exhibits higher values Creep:Composites are similar to G.I.C

Zinc phosphate has little creep

Flexure strength, diametrical tensile

strength, mod. of elasticity , fracture

toughness, hardness:

Resins exhibits higher values Creep:Composites are similar to G.I.C

Zinc phosphate has little creep

Papadogiannis et al. Dent Mat 1991

Wilson & Lewis. J Biomed Mater Res I980

Co td

Water Sorption Water Sorption



Adhesive Resins & Resin Modified

Glass Ionomer exhibits greatestsorption

Retention Adhesive Resin > Resin > Glass

Ionomer > Zinc Phosphate >

Polycarboxylate

Adhesive Resins & Resin Modified

Glass Ionomer exhibits greatestsorption

Retention Adhesive Resin > Resin > Glass

Ionomer > Zinc Phosphate >

Polycarboxylate

Blaem et al. Dent Mater 1995

Co td

WORKING PROPERTIESWORKING PROPERTIES



Mixing technique greatlyinfluences film thickness &

viscosity

Increased risk of incomplete

seating is seen with resin

cementsAs film thickness increases,

tensile bond strength decreases

Mixing technique greatlyinfluences film thickness &

viscosity

Increased risk of incomplete

seating is seen with resin

cementsAs film thickness increases,

tensile bond strength decreasesCo td

ESTHETICSESTHETICS

C l St bilitC l St bilit



Color Stability

Amine accelerator necessary for dualpolymerization causes color to change

Changes are not visually perceptible

Radiopacity

Should have increased radioopacity thandentin

Color Stability

Amine accelerator necessary for dualpolymerization causes color to change

Changes are not visually perceptible

Radiopacity

Should have increased radioopacity thandentin

Braurer et al. J Dent Res 2000

Noil et al. Int J Prosth 1995

Goshima & Goshima. Oral Surg 1991

Co td

DEFORMATION OF MATERIALDEFORMATION OF MATERIAL



Depends on: Alloy selection

Adequate tooth reduction

Depends on: Alloy selection

Adequate tooth reduction

Geryer. JPD 1970

Co td

Alloy Selection

Type I & Type II gold alloys are

Alloy Selection





satisfactory for intracoronal cast

restorations, they are too soft for

crowns & fixed partial dentures for

which Type III or Type IV gold alloys

are chosenHigh-noble metal content metal-

ceramic alloys are considerably harder

They may be indicated when largeforces are anticipated, such as with a

long span FPD


satisfactory for intracoronal cast

restorations, they are too soft for

crowns & fixed partial dentures for

which Type III or Type IV gold alloys

are chosenHigh-noble metal content metal-

ceramic alloys are considerably harder

They may be indicated when largeforces are anticipated, such as with a

long span FPD

Co td



A three quarter crown

without grooves has

A three quarter crown

without grooves has



without grooves has

little resistance torotational

displacement

The addition of

grooves places a

resisting surface at

right angles to the arc

of rotation, effectivelyblocking it

without grooves has

little resistance torotational

displacement

The addition of

grooves places a

resisting surface at

right angles to the arc

of rotation, effectivelyblocking it

PARTIAL VENEER CROWN WITH

NO GROOVES – LITTLE RESISTANCE TO ROTATION

RESISTANCE PROVIDED BY LINGUAL

WALLS OF THE GROOVES Co td



PATH OF INSERTIONPATH OF INSERTION



Before any tooth structure is cut the path

of placement should be decided keepingin mind the principles of tooth preparation

A path must be selected that will allow the

margins of the retainers to fit against their respective preparation finish lines with the

removal of minimum of sound tooth

structure This path should not encroach upon the

pulp or the adjacent teeth

Before any tooth structure is cut the path

of placement should be decided keepingin mind the principles of tooth preparation

A path must be selected that will allow the

margins of the retainers to fit against their respective preparation finish lines with the

removal of minimum of sound tooth

structure

This path should not encroach upon the

pulp or the adjacent teeth

Co td

The path of insertion for posterior

full & partial veneer crowns is

The path of insertion for posterior





usually parallel with the long axis of the tooth


usually parallel with the long axis of the tooth

THE IDEAL PATH OF INSERTION PARALLEL TO THE LONG AXIS

OF THE TOOTH

Co td

On the other hand the path of insertion

for an anterior 3 – quarter crown should

On the other hand the path of insertion





be inclined to parallel the incisal 2/3rds of

the facial surface enabling the

restoration to have almost no metal

visible on the facial surface


be inclined to parallel the incisal 2/3rds of

the facial surface enabling the

restoration to have almost no metal

visible on the facial surface

MAKING THE PATH OF INSERTION PARALLEL

TO THE LONG AXIS OF THE TOOTH RESULTS

IN UNNECESSARY METAL DISPLAY

THE PREFERRED PATH OF INSERTION

PARALLEL TO THE INCISAL 2/3RDS OF THE

INCISAL SURFACE

Co td

For a full crown to have structural

durability with proper contours its path

For a full crown to have structural

durability with proper contours its path



durability , with proper contours, its path

of insertion should be parallel to the long

axis of the tooth

durability , with proper contours, its path

of insertion should be parallel to the long

axis of the tooth

PATH OF INSERTION FOR A

FULL VENEER CROWN ON A

POSTERIOR TOOTH IN NORMAL ALIGNMENT

PARALLELS LONG AXIS OF

THE TOOTH

Co td

In case of a tilted

tooth, a path of

In case of a tilted

tooth, a path of



tooth, a path of

insertion paralleling

the long axis of thetooth may be

blocked by the

proximal contours

of the adjacent

tooth

In such cases the

path of insertion ismade perpendicular

to the occlusal

plane

tooth, a path of

insertion paralleling

the long axis of thetooth may be

blocked by the

proximal contours

of the adjacent

tooth

In such cases the

path of insertion ismade perpendicular

to the occlusal

planeFOR A TILTED TOOTH CORRECT PATH OF PLACEMENT IS

PERPENDICULAR TO OCCLUSAL PLANE Co td

A long standing loss of

proximal contact is

A long standing loss of

proximal contact is



p

usually accompanied

by tipping of theadjacent tooth into the

space

In such cases the path

of insertion parallel

with the long axis of

the tooth might not

allow a crown to seateven if the its distal

wall is grossly under

contoured

p

usually accompanied

by tipping of theadjacent tooth into the

space

In such cases the path

of insertion parallel

with the long axis of

the tooth might not

allow a crown to seateven if the its distal

wall is grossly under

contoured

Co td

The space between the

adjacent tooth must be

The space between the

adjacent tooth must be



j

made greater than the

mesiodistal diameter of the prepared tooth at the

gingival finish line

This can be achieved by

inclining the path of

insertion so that removal

of equal amounts of

enamel from each of theadjacent teeth will allow

a crown to seat on the

prepared tooth

made greater than the

mesiodistal diameter of the prepared tooth at the

gingival finish line

This can be achieved by

inclining the path of

insertion so that removal

of equal amounts of

enamel from each of theadjacent teeth will allow

a crown to seat on the

prepared tooth

INCLINING THE PATH OF

INSERTION TO FACILITATE

SEATING OF THE CROWN

Co td

In cases where more than 50% In cases where more than 50%



of the enamel thickness has to

be removed from either

adjacent tooth, or if there isn’t

adequate space for gingival embrasures then, teeth should

be separated & uprighted

orthodontically

of the enamel thickness has to

be removed from either

adjacent tooth, or if there isn’t

adequate space for gingival embrasures then, teeth should

be separated & uprighted

orthodontically

Co td

A negative taper or

undercut must be

A negative taper or

undercut must be



undercut must be

eliminated or it willprevent the seating

of the restoration

Preparation taper can be evaluated

by viewing it with

one eye from a

distance of

approximately 30

cm or 12 inches

undercut must be

eliminated or it willprevent the seating

of the restoration

Preparation taper can be evaluated

by viewing it with

one eye from a

distance of approximately 30

cm or 12 inches VIEWING PREPARATION TAPER

Co td

In this way it is

possible to see

In this way it is

possible to see



possible to see

all the axial walls with an

ideal taper of 6 0

An undercut asgreat as 8 0 can

be overlooked if

both the eyesare used

possible to see

all the axial walls with an

ideal taper of 6 0

An undercut asgreat as 8 0 can

be overlooked if

both the eyesare used

BINOCULAR VISION SHOULD

NEVER BE EMPLOYED

Co td

A mouth mirror can be

used when it is difficult

A mouth mirror can be

used when it is difficult



to survey the preparation

under direct vision The entire finish line

should be visible to one

eye from one fixed

position with noobstruction by any part

of the prepared tooth

To verify the parallel paths of insertion the

image should be

centered in the mirror

to survey the preparation

under direct vision The entire finish line

should be visible to one

eye from one fixed

position with noobstruction by any part

of the prepared tooth

To verify the parallel paths of insertion the

image should be

centered in the mirror

MIRROR USED TO EVALUATE

THE PREPARATION WHERE

DIRECT VISION IS NOT

POSSIBLE

Co td



CONCLUSIONCONCLUSION



Preparations for restorations

should be based on fundamental

principles i.e a preparation must

satisfy all the three principles i.ebiologic, mechanical & esthetic

principles which inturn are

responsible for the success of the prosthodontic treatment

Preparations for restorations

should be based on fundamental

principles i.e a preparation must

satisfy all the three principles i.ebiologic, mechanical & esthetic

principles which inturn are

responsible for the success of the prosthodontic treatment

REFERENCESEFERENCES

Goodacre CJ Bernal G Rungcharassaeng K Kan YK Goodacre CJ Bernal G Rungcharassaeng K Kan YK



Goodacre CJ, Bernal G, Rungcharassaeng K, Kan YK.

Clinical complications in fixed prosthodontics. J

Prosthet Dent. 2003; 90: 31 – 41

Walton JN, Gardner FM, Agar JR. A survey of crown &

fixed partial denture failures: Length of service &

reason for replacement. J Prosthet Dent. 1986; 56: 416

– 19 Creugers NHJ, Van MA. An analysis of clinical studies

on resin bonded cements. J Dent Res. 1991; 70: 146 –

9

Rijk WG, Wood M, Thompson VP. Maximum likelihoodestimates for the lifetime of bonded dental

prostheses. J Dent Res. 1996; 75: 1700 – 05

Goodacre CJ, Bernal G, Rungcharassaeng K, Kan YK.

Clinical complications in fixed prosthodontics. J

Prosthet Dent. 2003; 90: 31 – 41

Walton JN, Gardner FM, Agar JR. A survey of crown &

fixed partial denture failures: Length of service &

reason for replacement. J Prosthet Dent. 1986; 56: 416

– 19 Creugers NHJ, Van MA. An analysis of clinical studies

on resin bonded cements. J Dent Res. 1991; 70: 146 –

9

Rijk WG, Wood M, Thompson VP. Maximum likelihoodestimates for the lifetime of bonded dental

prostheses. J Dent Res. 1996; 75: 1700 – 05



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Bio Mechanics of Tooth Preparation

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