Pros 1 chapter 3

PRINCIPLES OF TOOTH
PREPARATION

3 PRINCIPLES OF TOOTH
PREPARATION
The principles of tooth preparation may be
divided into three broad categories:
1. Mechanical considerations, which affect
the integrity and durability of the
restoration
2. Biologic considerations, which affect the
health of the oral tissues
3. Esthetic considerations, which affect the
appearance of the patient

MECHANICAL PRINCIPLES
Forces can be directed against the prosthesis
during function.

Significance?


Types of Occlusal Forces
1. Tipping force
2. Twisting or rotational force
3. Path of insertion force


1. Tipping force
- Can occur in
buccolingual or
mesiodistal directions.


1. Tipping force

- May cause a restoration to start to move
circumferentially around the prepared tooth

1. Tipping force
3. Path of insertion force
- can be apically or
occlusally directed
- depending on whether
the mandible is closing
into a bolus of food
or opening with sticky
food interposed between
the prosthesis and
opposing teeth.

Factors Preventing Restoration
Dislodgement
1. Axial Wall Height
Factors that will affect the length of
occlusocervical height of the abutment
1.1. magnitude of occluding force
1.2. span length
1.3. type of preparation
1.4. length of the lever arm
1.5. bone support

Dislodgement
- MINIMAL ACCEPTABLE HEIGHT
Height which allows the tooth structure to
interfere with the arc of rotation as tipping
forces attempt to cause rotation a fulcrum
located at the finish line on the opposite side
of the tooth

Dislodgement
On short teeth, adequate axial wall height may
only be achieved by extending the finish line
- subgingivally
- onto the root surface
- not desirable or advanageous
Alternative
- prepare tooth with less taper

Dislodgement
2. Taper of the Preparation
Opposing walls must converge occlusally
Divergent walls produce undercuts and
prevents seating of restoration

Dislodgement
Increased taper reduces the
ability of the restoration to
- resist occlusally
directed dislodging forces

- lessens its ability to
interfere with the arc of
rotation as tipping forces
act to unseat the
restoration

Dislodgement
A total convergence of 3-5 degrees is
considered ideal

Dislodgement
3. Ratio of Preparation Diameter to Axial Wall
Height
It is often mistakenly assumed that a large
diameter tooth will yield a more retentive
preparation than a smaller diameter ones.

Dislodgement
3. Ratio of Preparation Diameter to Axial Wall
Height
If the axial wall height and taper are the same
for both teeth, the smaller diameter tooth
interferes more effectively with the arc of
rotation because the smaller radius of
curvature allows the preparation to better
resist the dislodgement.

Dislodgement
4. Circumferential Irregularity
Circumference of the tooth is usually irregular
therefore uniform reduction will create an
irregular shaped abutment.
- resists tipping and twisting forces.

Dislodgement
4. Circumferential Irregularity
In round, short and/or overtapered abutment
- intentional placement irregularities are
done
- forms: boxes
grooves
- placement: middle of proximal
surface

Dislodgement
5. Occlusal Irregularity
Occlusal reduction following the anatomic form
produces an irregular surface which aids in
retention.
Irregularities can be used to enhance resistance
to dislodgement
- example: pinholes
- occlusal, cingulum, incisal

Dislodgement
6. Rigidity
Prosthesis must be thick enough to
resist flexure and loosening.
Occlusal reduction – minimum of 1-
1.5 mm
Axial reduction
occlusal area- minimum of 1 mm
cervical area – minimum of 0.3-0.5
mm

Dislodgement
7. Adaptation
Small amount of space is required between a
restoration and the prepared tooth to allow
complete seating during cementation.
Excessive space reduces resistance to
dislodgement by placing to much
dependence on the physical properties of
the luting agent.

Dislodgement
8. Surface area
Increased surface area is most significant when
the additional area results in greater axial
wall length.

Finish Line

The point at which a preparation terminates on
the tooth.

Functions of a Finish Line

1. During visual evaluation of tooth
preparation, it is a measure of the amount
tooth structure already removed.


2. One of the features that can be used to
evaluate the accuracy of the impression
made for the indirect procedures.

3. On the die, a distinct finish line helps in the
evaluation of the quality of the die and aids
in trimming it accurately.


4. The correct
marginal
adaptation of
the wax pattern
depends on an
obvious finish
line.


5. The evaluation of the restoration is also aided
by the proper finish line.

6. At cementation, a sharp finish line aids in
determining whether the restoration is fully
seated.

Forms of Finish Line
1.Chamfer
- preferred cervical
finish line for fixed
prosthodontics.
- should be utilized
whenever possible
because it is easily
developed and visually
distinct.

2. Knife edge or chisel
edge finish
- not as well
defined as
chamfer.
- often used on
tipped teeth when
formation of a
chamfer would
result in excessive
tooth reduction.

3. Feather edge
- unacceptable because –
- not sufficiently distinct
- results in so little cervical tooth
reduction
- restoration must be over contoured
to possess adequate rigidity.
- since a feather edge is difficult to see
visually, rregularities in the finish line
are more likely to be present, making
it more difficult to fabricate a
restoration that fits accurately.

4. Shoulder and beveled shoulder
- difficult to form
- produces a greatest depth of
tooth reduction
- required with ceramic
restorations because proper color
is achievable only through
material thickness.

INSTRUMENTATION

Instrumentation
Rotary instruments must be selected that allow
the tooth to be reduced
1. according to the requirements of proper
retention and resistance form and
2. finish line development.

Instrumentation
Kinds of Rotary Instruments
1. Diamond cutting
instruments.
- Have diamond particles
attached to a concentric
metal shaft.
- Available in coarse,
medium and fine grit
2. Dental burs possessing
carbide cutting blades.

Visibility
Ways on achieving good visual access

1. Use of fiberoptic handpiece lights and
numerous lights aimed at the oral cavity from
different directions greatly aids in the visibility.

2. Removing of excess oral fluids.

3. Retraction of soft issues that interfere with
vision.

BIOLOGIC PRINCIPLES

Teeth must be prepared in a manner that creates
the least amount of trauma to the pulp.
- retain as much tooth structure as possible
- proper use of rotary instruments and the
application of surface coolants

BIOLOGIC PRINCIPLES
Pulpal Considerations
1. Conservation of tooth structure.
2. Depth of reduction.
3. Speed of reduction.
4. Instrument age and use of pressure.use of
coolants.

BIOLOGIC PRINCIPLES

Retain as much as tooth structure as
practicable

BIOLOGIC PRINCIPLES


WAYS ON CONTROLLING THE AMOUNT OF
REDUCTION
1.Best provided by placing strategically
located depth cuts in the unprepared tooth
surfaces and placed to the desired depth.

BIOLOGIC PRINCIPLES

REDUCTION
2. The intervening tooth structure is
removed by using the base of the depth cut
as a guide to proper reduction.

BIOLOGIC PRINCIPLES
1. Conservation of tooth structure .
REDUCTION
3. If caries removal will make the preparation
excessively deep, place an insulating base material
(minimum of 0.5mm) over the area in proximity to
the pulp.
* For adequate protection against thermal shock

BIOLOGIC PRINCIPLES

Rapid continuous removal of tooth structure causes rapid heat
build-up that may cause irreversible pulpitis.
- reduction must be performed INTERMITTENTLY in a STEADY
and CONTROLLED MANNER to avoid EXCESSIVE HEAT
BUILD-UP.
1. Reducing the tooth for a period of 5-10 seconds
2. Then remove the instrument from the surface for a few
seconds

BIOLOGIC PRINCIPLES
1. Pulpal Considerations
Conservation of tooth structure.

4. Instrument age and use of pressure.
Only sharp instruments should be used for bulk tooth reduction.
- Dull instruments create more friction , thus more heat is
produced.
- Use of excessive pressure should be avoided because this will
cause undue heat generation.
- Accentuated if worn instruments are used.

BIOLOGIC PRINCIPLES

4. Instrument age and use of pressure .
5. Use of coolants.
Delivery of water stream from handpieces during reduction.
Disadvantage: interferes with vision

Periodontal Consideration
 Supragingival location for the finish line:
- allows good visual access for evaluating finish line
forms.
- facilitates accurate impression of prepared tooth.
- allows more accurate assessment of prosthesis fit
and contour.
- Provides access for marginal refinement and
polishing.
- permits more accurate long term post insertion
evaluation of marginal integrity

Periodontal Consideration

- MOST important reason relates to the preservation of
periodontal health.
NOTE: There is no junction of any restorative
material and the tooth that is as smooth as intact tooth
structure.
- marginal plaque acculmulation is
inevitable and when this occurs subjingivally, it is not
easy for the patient to remove it and the likelihood of
adverse periodontal changes increases.
- if tooth preparation extended
beyond the gingival margin for some reasons, care must
be exercised to avoid excessive tissue trauma.

Achieving a color that matches the surrounding teeth
necessitates a certain minimal thickness in the
ceramic material.

This is accomplished by adequate and uniform
reduction of the facial surface.

Use of Depth Guides
1. Prevents excessive depth reduction.
2. Ensures adequate uniform reduction.

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