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8/3/2019 Bio Mechanics of Tooth Preparation
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Department of Prosthodontics Including Crown &
Bridge, Maxillofacial Prosthodontics & Oral
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Functional Cusp Bevel
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
Functional Cusp Bevel
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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Margins should be 2mm away
from the alveolar crest
Margins should be 2mm away
from the alveolar crest Garguilo. J Periodontol 1961
Co td
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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ABUTMENT EVALUATION ABUTMENT EVALUATION
Evaluated for 3 factors:
CROWN-ROOT RATIO
Evaluated for 3 factors:
CROWN-ROOT RATIO
Klaffenbach. JADA 1936
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ROOT
CONFIGURATION
PERIODONTAL
LIGAMENT AREA
ROOT
CONFIGURATION
PERIODONTAL
LIGAMENT AREA
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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
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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
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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
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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
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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
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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
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STUDY MONTHS TO
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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
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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
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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
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MECHANICALMECHANICAL
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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
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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
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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
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GEOMETRY OF TOOTH
PREPARATION
GEOMETRY OF TOOTH
PREPARATION
Taper
Surface area
Stress concentration
Taper
Surface area
Stress concentration
Co td
TAPERTAPER
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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
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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
Co td
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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
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Co td
OPTIMUM DEGREE OFOPTIMUM DEGREE OF
TAPER
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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
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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
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TENSILE STRESS
SHEAR & COMPRESSIVE
STRESS
COMPRESSIVE STRESS
SHEAR STRESS
Co td
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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
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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
Co td
To obtain greatest area of cement To obtain greatest area of cement
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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
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A full veneer crown preparation has an A full veneer crown preparation has an
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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
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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
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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
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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
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Co td
ROUGHNESS OF SURFACES BEINGROUGHNESS OF SURFACES BEING
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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
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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
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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
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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 FORM
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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
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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
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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
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The permissible taper of The permissible taper of
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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
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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
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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
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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
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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
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The magnitude of the torque produced The magnitude of the torque produced
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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
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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
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The area encompassed by thistangent line is referred to as the
The area encompassed by thistangent line is referred to as the
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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
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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
preparations of equal length are subjected to
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preparations of equal length are subjected to
identical forces, the longer crown is more
likely to fail because the force on it actsthrough a longer lever arm
preparations of equal length are subjected to
identical forces, the longer crown is more
likely to fail because the force on it actsthrough a longer lever arm
Co td
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Here the arc of Here the arc of
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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
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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
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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
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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:
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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
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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
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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
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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
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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
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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
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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
Type I & Type II gold alloys are
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Type I & Type II gold alloys are
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
Type I & Type II gold alloys are
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
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A three quarter crown
without grooves has
A three quarter crown
without grooves has
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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
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PATH OF INSERTIONPATH OF INSERTION
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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
full & partial veneer crowns is
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full & partial veneer crowns is
usually parallel with the long axis of the tooth
full & partial veneer crowns is
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
for an anterior 3 – quarter crown should
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for an anterior 3 – quarter crown should
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
for an anterior 3 – quarter crown should
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
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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
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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
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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
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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%
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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
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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
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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
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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
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CONCLUSIONCONCLUSION
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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
8/3/2019 Bio Mechanics of Tooth Preparation
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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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T T
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H H
AA
N N
KK