ASSALAMU ALAIKUM WA RAHMATULLAHI WA BARAKATAHU
INTRODUCTION
Teeth require preparations to receive
restorations.
Teeth preparations must be based on
fundamental principles.
PRINCIPLES OF TOOTH PREPARATION
PRESERVATION OF TOOTH STRUCTURE.
RETENTION AND RESISTANCE.
STRUCTURAL DURABILITY.
MARGINAL INTEGRITY.
PRESERVATION OF PERIODONTIUM
PRINCIPLES OF TOOTH PREPARATION
1 Biologic consideration
• which effect health of oral tissue
2 Mechanical consideration
• which effect the integrity
&durability of restoration
3 Esthetic consideration
• Which effect patient’s appearance
Mechanical
Biologic
Esthetic
BIOLOGIC CONSIDERATIONS
During tooth preparation, the structures easily
damaged are
adjacent teeth,
soft tissues, and
pulp of tooth.
BIOLOGIC CONSIDERATIONS
A Prevention of damage during tooth preparation
1 Adjacent teeth
2 soft tissue
proximal surface of adjacent tooth may get
damaged;
Damage of;
tongue and
cheeks prevented
by retraction
with;
use matrix band around
the adjacent tooth.
matrix band may be
perforated and damage
the tooth.
Preferred method;
leave a slight lip or fin of
proximal enamel of
prepared tooth without
causing excessive tooth
reduction .
aspirator tip ,
mouth mirror,
or flanged
saliva ejector.
BIOLOGIC CONSIDERATIONS
A Prevention of damage during tooth preparation
3 Pulp
Factors affecting pulp can cause irreversible pulpitis.
(a) Temperature –
heat generated by
friction between
instrument and tooth
surface.
Coolant used with
high speed hand
piece.
Light pressure is
required during
preparation .
(b) Chemical action
(C) Bacterial action
Certain dental
material can cause
Pulpal damage.
bacteria may not be
completely removed
from tooth or due to
micro leakage.
Cavity varnish;
effective barrier
Remove all carious
part before
restorations
Recent dental
material have anti
microbial property
BIOLOGIC CONSIDERATIONS
B Conservation of tooth structure
use of partial
coverage rather than
complete coverage
restoration.
selection of a
conservative margin.
avoidance of
unnecessary apical
extension of the
preparation.
-Anatomically prepared occlusal surface result in adequate occlusal
clearance without excessive tooth reduction
1 insufficient reduction
2 excessive reduction
Excessive taper results in considerable loss of tooth
structure
BIOLOGIC CONSIDERATIONS
C Considerations affecting future dental health
Improper tooth preparation may have an adverse effect on long
term dental health, for example;
insufficient axial reduction results in over countered restoration,
hampers plaque control,
causes gingival inflammation around crown or abutment tooth .
inadequate occlusal reduction may result occlusal dysfunction &
may lead to chipped enamel or cusp fracture.
GINGIVAL MARGINS
Margin placement – whenever possible the margin of
preparation should be supra gingival , because sub gingival
margin may cause periodontal problem.
Advantage of supra gingival margin
easily finished
Easily clean
Impression is easily made
Restoration can be easily evaluated at recall appointment.
GINGIVAL MARGINS
Indications for sub gingival margin
anterior region where esthetics is of prime
consideration
Cervical caries, cervical erosion or restoration
extended sub gingival.
Additional retention is required.
GINGIVAL MARGINS
Margin geometry – Guide line for margin design
Margins must be smooth and even.
Ease of preparation without overextension .
Easily identifiable in the impression & on the die.
A distinct boundary to which the wax pattern can be finished.
Sufficient bulk of material .
Conservation of tooth structure.
GINGIVAL MARGINS
Different marginal design
1 Featheredge or Shoulderless
2 Chisel edge
3 Chamfer
4 Bevel
5 Shoulder
6 Sloped shoulder
7 Bevel Shoulder
1 Featheredge or Shoulderless
should be avoided because it fails to
provide adequate bulk to margin.
2 Chisel edge
same as Feather edge ,
there is larger angle between the axial
surface and the unprepared tooth surface.
Indication – occasionally on tilted teeth
3 Chamfer
suitable for cast metal restoration , and
metal portion of the metal ceramic
restoration.
instrument for making chamfer is tapered
diamond with rounded tip
Advantage
Distinct and readily identified.
Provide room for adequate bulk of material .
Can be placed with precision.
Disadvantage
Care needed to avoid unsupported lip of
enamel
4 Bevel
suitable for cast restoration , particularly when a
ledge or shoulder already exists possibly from
dental caries , cervical erosion or previous
restoration.
Objectives
to allow the cast metal margin to be burnished
against the prepared tooth.
to protect unprepared tooth structure from
chipping.
5 Shoulder
where bulk of the material is necessary like in
facial surface of metal ceramic restoration and
full ceramic restoration.
It should form 90 degree angle from axial
surface to unprepared tooth structure.
Shoulder margin is less conservative
then chamfer margin.
1 chamfer
2 shoulder
6 sloped shoulder
A 120 degree sloped shoulder used as an
alternative to 90 degree shoulder for facial
margin of metal ceramic crown.
Advantages of sloped shoulder is to reduce
the possibility of leaving unsupported
enamel.
7 Bevel Shoulder
Bevel shoulder is recommended for facial
surface of metal ceramic crown , where
metal collar is to be used .
Beveling remove unsupported enamel and
finished the metal .
Shoulder or Sloped Shoulder is preferred
over Bevel Shoulder because of biologic and
esthetic reason.
Mechanical consideration
Mechanical considerations divided in 3 parts
1 Retentive form
2 Resistance form
3 Preventing
deformation of the
restoration
Mechanical consideration
1 Retention form – The quality of preparation that prevents the
restoration from becoming dislodged by such forces parallel to the
path of withdrawal is known as retention.
1 Taper
Factors influencing the
retention of cemented
restoration
Retention of FPD depend on the
geometric form of preparation rather
than on adhesion for retention.
2 Surface area
3 Type of preparation
4 Surface texture
5 Film thickness
6 Luting agent
1 Taper –
Maximum retention is obtained
when preparation have parallel
walls.
A slight convergence is necessary
when preparing a tooth.
Less taper ------ more retention
More taper ----- less retention
The recommended convergence
angle is 6 degree .
2 Surface area
Retention depends on the surface area in sliding
contact.
So crown with long axial wall are more retentive
than shorter wall .
And molar crown are more retentive then
premolar crown.
Stress concentration –
If rounding the internal line angles may reduce
the stress concentration & increase retention.
3 Type of preparation
Different type of preparation have different
retentive value, mainly depend on surface area
Molar premolar
Partial crown intracoronal restoration
Where groove and boxes limits the path of
withdrawal, so retention is increase.
4 Roughness of the
surface being cemented
5 Material being
cemented
If the internal surface of restoration is
very smooth then retentive failure occur
at the cement restoration interface.
So the internal surface of restoration is
roughened or grooved retention will be
increase.
Base metal alloy are more retentive then
gold alloy
In general adhesive resin cement are the
most retentive cement
6 Type of luting agent
Adhesive resin GICzinc phosphate or
poly carboxilate ZNOE
2 Resistance form –
Mastication & Para functional activity may
transfer horizontal & oblique forces to the
restoration .
These lateral forces tend to displace the
prosthesis by causing rotation around the
gingival margin.
Rotation is prevented by any areas of the tooth
preparation that are placed in compression called
resistance area.
2 Resistance form –
Adequate resistance depends on –
1 Magnitude &
direction of
dislodging
forces –
The preparation must be able to
withstand considerable oblique
forces.
The tooth preparation must be so
shaped that particular area of
axial wall will prevent rotation of
the crown.
2 Geometry of
tooth
preparation –
Resistance form will also provided
with groove and boxes.
A complete crown have more
resistance than partial coverage
restoration.
3 Physical properties of luting agent –
Resistance to deformation is affected by physical
properties of the luting agent such as;
• compressive strength and module of elasticity.
Silicophosphate  zinc phosphate  gic 
poly carboxilate ZNOE
Increase temperature ---- less compressive strength
3 Preventing deformation of the restoration
A restoration must have sufficient strength to prevent it
from permanently deformed during function.
This may be the result of –
1 Inappropriate
alloy selection
2 Inadequate tooth
preparation
3 Poor metal
ceramic frame work
Alloy selection
Less harden ------------------------------- more harden
Type 1 & type 2 gold alloy type 3 & type 4 gold alloy  nickel
chromium
Adequate tooth reduction
It is recommended that there is minimum alloy
thickness of about 1.5mm at centric cusp & 1 mm on
non functional cusp.
Occlusal reduction is as uniform as possible .
Sufficient occlusal clearance is needed with
preservation of as much tooth structure as possible.
Anatomically prepared occlusal surface give rigidity
to crown.
Esthetic consideration
Esthetic is also a major factor which must
be consider.
All ceramic crowns preferred
Proximal margin –
placement of the proximal margin ( particularly the medial )
must be placed just buccal to the proximal contact area.
Where metal will be hidden by the distal line angle of
neighboring teeth.
References
Contemporary fixed prosthodontic – Stephen F. Roessential
Fundamentals of Fixed prosthodontics – Herbert T.
Shillingburg
Theory and practice of fixed prosthodontics - Tylman