Principles of tooth preparation

Principles of tooth preparation
Objectives of tooth Preparation:-
The main objectives of tooth preparation
1. To eliminate undercuts from the axial surface of the tooth.
2. To provide enough space for the crown restoration to withstand the force of mastication, this space depends on the material used, so the metal material needs little space while the plastic material needs more space.
3. Not to enlarge the size of the tooth.
4. To provide good esthetic.
Biomechanical principles of preparations:
The designs and preparations of a tooth for a cast metal or porcelain restorations are limited by five principles:-
1. Preservation of tooth structure.
2. Retention and resistance from.
3. Structural durability of the restoration.
4. Preservation of periodontium.
5. Marginal integrity.
1. Preservation of the tooth structure:
The preparation of the tooth must be conservative, the minimal amount of the tooth structure must be removed. Excessive removal of the tooth structure has many harmful effects: -
• Excessive reduction lead to thermal hypersensitivity, pulpal inflammation and necrosis may result from approaching to the pulp closely.
• The tooth might be over tapered or shortened and this might affect the retention and resistance of the prepared tooth.
2. Retention and resistance form:
Retention: is the ability of the preparation to resist the crown restoration from removal along its path of insertion.
Resistance: is the ability of the preparation to resist the dislodgment of the restoration by forces directed obliquely or horizontally to the restoration.
Path of insertion: An imaginary line along which the restoration can be inserted and removed without causing lateral force on the abutment.
The crown restoration should have a single path of insertion to be retentive. Most of the time the path of insertion of crown restorations is parallel to the long axis of the tooth except in ¾ crown for anterior teeth where the path of insertion should be parallel to the incisal 2/3 of the tooth crown (not to the long axis). By limiting the path of withdrawal, retention is improved.

Factors affecting retention and resistance:
• Taper of the preparation.
• Surface area of the preparation,
• Length and height of the preparation.
• Diameter of the tooth (tooth width).
• Texture of the preparation.
• Accessary mean.
1- Taper of the preparation
(5-6) degree convergence angle is mostly used to provide the needed retention. The more nearly parallel the opposing walls of preparation the greater will be the retention. But parallel wall is difficult to be obtained in the patient mouth without undercuts, also parallel walls might lead to difficulty in seating of the crown restoration, thus (5-6) degree convergence angle is mostly used to provide the needed retention.

Taper and Resistance:
The more parallel the axial walls the more will be the resistance of crown restoration. The walls of a short wide preparation must be kept nearly parallel to achieve adequate resistance from.

2- Surface area of the preparation:
Increasing the surface area increase retention. Factors that influence surface area are:
a) Size of the tooth
The larger the size of the tooth the more will be the surface area of the preparation, the more will be the retention thus full metal crown on molar tooth definitely more retentive than that on premolar tooth.
B) Extend of coverage by restoration:
The more the area that will be covered by the crown restoration, the more will be the retention, thus full metal crown on molar is more retentive than 3/4 crown on the same tooth.
c) Accessory feature such as boxes, grooves and pin holes.
3. Length (height) of the preparation:
Increasing the length increase retention and resistance and vice versa.
4. Diameter of the tooth (tooth width):
Under some circumstances, crown on narrow tooth can have grater resistance to tipping than a crown on a wider tooth, this occur because the crown on the narrower tooth has shorter radius for rotation resulting in a lower tangent line and a larger resisting area.

5. Texture of the preparation.
Depending on the type of cementation agent, texture of the preparation might effect on the retention of cast crown. Smooth surfaces are less retentive than rough (mechanical interlocking).
6. Extra retention means.
The retention of a preparation can be greatly enhanced by the addition of grooves, pin holes or boxes.
3- Structural Durability (SD):
The preparation must be designed so that it provide S.D. to the restoration, i.e. the crown restoration must be rigid enough to not flex, perforate (metal) or even fracture (plastic). For a restoration to be rigid it need bulk, so to provide enough bulk to the crown restoration, sufficient tooth structure must be removed from the prepared tooth to create enough space. By doing so the restoration allowed to withstand the forces of occlusion, prevent wearing holes in the gold and allow proper contouring and carving of the restoration.
Preparation features related to Structural Durability:-
1- Occlusal reduction.
Enough tooth structure must be removed from occlusal surface so that the restoration can be built back to ideal occlusion & thick enough to prevent wearing or distortion (1—1.5mm).
Occlusal clearance: is the space between the occlusal surface of the prepared tooth and that of opposing tooth. It should be evaluated in centric and eccentric relation. Enough tooth structure must be removed occlusally so that when the restoration is built back to ideal occlusion it will be thick enough to prevent wearing through or distortion.
Functional cusps: - the cusps that give centric stops of occlusion. (Palatal of upper posterior teeth and buccal of lower posterior teeth).
• Occlusal reduction must reflect the geometric inclined planes of occlusal surface.
• Avoid creating steep planes with sharp angles, because it lead to stress.
• Flat occlusal reduction lead to thin metal, this will lead to perforation of the crown restoration in future.
• Lowering the entire occlusal surface in attempt to providing sufficient space might lead to tooth structure destruction (non-conservative preparation) &shortening of the axial wall of prepped tooth which defiantly will affect the retention-resistance potential of the preparation.


Functional cusp bevel (FCB)
Wide bevel should be placed on the functional cusps of posterior teeth to provide structural durability (it allow adequate thickness of restoration at this critical area without structure destruction).
If FCB is omitted, the restoration is likely to be thin in this stress bearing area. If the restoration thickness is achieved by over tapering, this will compromise the retention.
In the absence of FCB --- technician will overbuild the crown restoration in attempt to provide structural durability for the restoration this will lead to super-occlusion or premature contact with the opposing tooth.
2- Axial reduction.
Sufficient reduction is important to provide sufficient space so that the restoration can be built with sufficient thickness, this will prevent flexing of the crown restoration when occlusal force acting on.