Principles of tooth prep /certified fixed orthodontic courses by Indian dental academy

PRINCIPALS OF TOOTH
PREPARATION
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

PRINCIPALS OF TOOTH PREPARATION


BIOLOGIC CONSIDERATIONS
Prevention of Damage During Tooth Preparation
a) Adjacent teeth
b) Soft tissues
c) Pulp
causes of injury
temperature
bacterial action
chemical action

CONSERVATION OF TOOTH STRUCTURE
Partial coverage rather than complete coverage
Preparation with minimum convergence angle (taper)
between axial walls
Preparation of the occlusal surface so reduction follows the
anatomic planes to give uniform thickness in the restoration
Preparation of the axial surfaces so tooth structure is
removed evenly; if necessary, teeth should be orthodontically
repositioned
Selection of a conservative margin compatible with the
other principles of tooth preparation
Avoidance of unnecessary apical extension of preparation

MARGIN PLACEMENT
Whenever possible, the margin of the preparation
should be supragingival.
Advantages of supragingival margins include.
1. They can be easily finished with trauma to soft
tissues.
2. They are more easily kept clean.
3. Impressions are more easily made, with less
potential for soft tissue damage.
4. Restorations can be easily evaluated at recall
appointments. www.indiandentalacademy.com

Indication of subgingival margin
1. Dental caries, cervical erosion, or restorations extend
subgingivally and a crown-lengthening procedure is not
indicated.
2. The proximal contact area extends to the gingival crest.
3. Additional retention is needed.
4. The margin of a metal-ceramic crown is to be hidden
behind the labiogingival crest.
5. Root sensitivity cannot be controlled by more
conservative procedures.
6. Modification of thewww.indiandentalacademy.com indicated.
axial contour is

MARGIN ADAPTATION
The junction between a cemented restoration and the
tooth is always a potential site for recurrent caries because of
dissolution of the luting agent and inherent roughness. The
more accurately the restoration is adapted to the tooth, the
less will be the chance of recurrent caries or periodontal
disease. A well-designed preparation has a margin that is
smooth and even. Rough, irregular, or "stepped" junctions
greatly increase the length of the margin and substantially

reduce the adaptation of the restoration

MARGIN GEOMETRY
Guidelines for margin design
1. Ease of preparation without overextension.
2. Readily identifiable in the impression and on the die.
3. A distinct boundary to which the wax pattern can be
finished.
4. Sufficient bulk of material (to enable the wax pattern
to be handled without distortion as well as give the
restoration strength and, when porcelain is used,
esthetics).
5. Conservative of tooth structure (provided the other
criteria are met).

PROPOSED MARGIN DESIGN
FEATHER EDGE
Advantage
conservative
Disadvantage
does not provide sufficient bulk
Indication
not recommended

CHISEL EDGE
Advantage
conservative
Disadvantage
location of margin difficult to control
Indication
occasionaly on tilted teeth
BEVEL EDGE
Advantage
remove unsupported enamel, allows finishing of metal
Disadvantage
extends preparation into sulcus if used on apical margin
Indication
facial margin of maxillary partial coverage crowns

CHAMFER
Advantage
distinct margin, adequate bulk, easier to control
Disadvantage
care needed to avoid unsupported lip of enamel
Indication
cast metal restorations, lingual margin of metal ceramic crowns
SHOULDER
Advantage
bulk of restorative material,
Disadvantage
less conservative
Indication
facial margin of metal ceramic crowns, complete ceramic
crowns

SLOPED SHOULDER
Advantage
bulk of material, advantages of bevel
Disadvantage
less conservative
Indication
facial margin of metal ceramic crowns
SHOULDER WITH BEVEL
Advantage
bulk of material, advantages of bevel
Disadvantage
less conservative, extends preparation apically
Indication
facial margin of posterior metal ceramic crowns with
supragingival margins

MECHANICAL CONSIDERATIONS
Retention form
Resistance form


RETENTION FORM
The quality of a preparation that prevents the restoration from
becoming dislodged by such forces parallel to the path of
withdrawal is known as retention
Factors determining retention
I. Magnitude of the dislodging forces
The magnitude of the dislodging forces depends on the
stickiness of the food and the surface area and texture of the
restoration being pulled.

2. Geometry of the tooth preparation
a)Taper
As the taper increases, however, so does the free
movement of the restoration and retention will be
reduced.
Selection of the appropriate degree of taper for
tooth preparation involves compromise. Too small a taper
may lead to unwanted undercuts; too large will no longer
be retentive. The recommended convergence between
opposing walls is 6 degrees.

b)surface area
Retention is dependent on the length of path of withdrawal
or, more precisely, on the surface area in sliding contact.
Therefore, crowns with long axial walls are more retentive
than those with short axial walls
c)stress concentration
changes in the geometry of the preparation (e.g., rounding
the internal line angles) may reduce stress concentrations
and hence increase the retention of the restoration.

d)Type of preparation
Different types of preparation have different
retentive values and these correspond fairly closely to
the surface area of the axial walls, provided other factors
(e.g., taper) are kept constant. Thus the retention of a
complete crown is about double that of partial coverage
restorations.

3. Roughness of the fitting surface of the restoration.
The more the roughness of the internal surface of a
restoration, the more the retention
4. Type of luting agent
The type of luting agent chosen affects the retention
of a cemented restoration. Adhesive resin cements are the
more retentive than other cement.

RESISTANCE FORM
Resistance prevents dislodgement of the restoration by
forces directed in an apical or oblique direction and
prevents any movement of restoration under occlusal
forces.
Factors determining resistance
Leverage
Forces should pass with the margin of a crown


Length of the preparation
Longer preparation will have more surface area and
will be more retentive . Because the axial wall occlusal to
the finish line interferes with displacement, the length and
inclination of that wall become factors in resistance to
tipping forces.for a restoration to succeed, the length must
be great enough to interfere with the arch of the casting
pivoting about a point on the margin on the opposite side
of the restoration.Resistance can be increased by grooves.


Path of insertion
The path of insertion is an imaginary line along
which the restoration will be placed onto or removed
from the preparation. The path of all abutments should
be parallel to each other
The path of insertion must be considered in two
dimension: faciolingually and mesiodistally.
The faciolingual orientation of the path can affect the
esthetics of metal ceramic or partial veneer crowns.
The mesiodistal inclination of the path must parallel
the contact areas of the adjacent teeth.

STRUCTURAL DURABILITY
A restoration should have sufficient bulk of
material that is adequate to withstand the forces of
occlusion. This bulk must be confined to the space
created by the tooth preparation. Only by this way can the
occlusion on the restoration can be harmonious and the
axial contour normal, preventing periodontal problems
around the restoration.

OCCLUSAL REDUCTION

Sufficient occlusal clearance should be provided
to provide adequate bulk of metal and strength to the
restoration.
Recommended reduction
MATERIAL

Functional cusp

Non- functional
cusp

PFM
Metal

2mm
1.5- 2mm

1.5mm
1mm

Gold

1.5mm

1mm


FUNCTIONAL CUSPAL BEVEL
The Functional cuspal bevel is given on buccal
cusp of lower and palatal cusp of upper teeth.This
provides for adequate bulk of material in an area of heavy
occlusal contact.


If the bevel is not placed several problems may ariseThe casting will be extremely thin in the area
overlying the junction between the occlusal and axial
reduction.


An overcontoured restoration may result

If an attempt is made to obtain space for an adequate
bulk in a normally contoured casting without a bevel, the
result will be an overcut axial surface.


AXIAL REDUCTION
It is essential that a tooth preparation provide
sufficient space for the development of good axial contours.
This will enable the junction between the restoration and the
tooth to be smooth and free of any ledges or abrupt changes
in direction. Sufficient tooth structure must be removed to
allow the development of correctly formed axial contours,
particularly in the interproximal and furcation areas, where
periodontal disease often begins.

Sufficient axial reduction allows bulk of the
restorative material which prevents distortion of the
material under occlusal load. The features that will serve to
provide space for metal that will improve the rigidity and
durability of the restoration are the offset, the occlusal
shoulder, the isthmus, the proximal groove and the box.


ESTHETIC CONSIDERATION
Whenever possible, accomplishment of an
esthetically acceptable result without the use of metal
ceramic crowns is preferred, not only because tooth
structure is conserved but also because no restorative
material can approach the appearance of intact tooth
enamel.

THE COMPLETE CAST CROWN
INDICATION
Extensive destruction from caries or trauma
Endodontically treated teeth
Existing restoration
Necessity for maximum retention and strength
To provide contour to receive a removable appliance
CONTRAINDICATION
Less than maximum retention necessary
Esthetics


ADVANTAGE
Strong
High retentive qualities
Usually easy to obtain adequate resistance form
Option to modify form and occlusion
DISADVANTAGE
Adverse effects on metal
Vitality testing not readily feasible
Display of metal


CRITERIA


METAL CERAMIC CROWN
INDICATION
Esthetics
If porcelain jacket crown is contraindicated
Gingival involvement
CONTRAINDICATION
Large pulp chamber
Intact buccal wall

When more conservative retainer is technically feasible

ADVANTAGE
Superior esthetics as compared to cast gold restoration
DISADVANTAGE
Removal of substantial tooth structure
Subject to fracture because porcelain is brittle
Difficult to obtain accurate occlusion in glazed porcelain
Shade selection can be difficult
Inferior esthetics as compared to porcelain jacket crown
Expensive


ALL CERAMIC CROWN
INDICATION
High esthetic requirement
Considerable proximal caries
Incisal edge reasonably intact
Endodontically treated teeth with post and core
Favorable distribution of occlusal load
CONTRAINDICATION
When superior strength is required
Thin teeth faciolingually

Unfavorable distribution of occlusal load

ADVANTAGE
Esthetically unsurpassed
Good tissue response
DISADVANTAGE
Reduced strength compared to metal ceramic crown
Proper preparation extremely critical
Brittle nature of material

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