The document discusses five biomechanical principles of tooth preparation for crowns: 1) Preservation of tooth structure by removing only the minimal amount of tooth structure needed. Excessive reduction can cause sensitivity, inflammation or necrosis. 2) Providing adequate retention and resistance form through features like a 5-6 degree taper, increased surface area, length and texture. 3) Ensuring the restoration is durable. Factors like occlusal reduction and cement selection influence durability. 4) Preserving the periodontium by placing margins supragingivally and providing proper embrasure form.

1. Biomechanical principles of tooth preparation

The design of the preparation of a tooth limited by five principles:

1- preservation of tooth structure

2- retention and resistance form

3- structure durability of the restoration

4- preservation of periodontium

5- marginal integrity

2. Preservation of the tooth structure

The preparation of tooth must be conservative ,minimal amount of tooth
structure must be removed .. excessive reduction will lead to thermal
hypersensitivity ,pulpal inflammation and necrosis.

Excessive tooth reduction : the tooth is over tapered and shortened and
this will affect the retention and resistance of the prepared tooth.

3. Total axial tapered

5. Retention and resistance form

Retention : ability of the preparation to resist the dislodgement of the crown
restoration by forces directed along its path of insertion.

Resistance : ability of the preparation to resist the dislodgment of the
crown restoration by forces directed obliquely or horizontally to the
restoration.

Path of insertion : is an imaginary line along which the restoration can be
inserted and removed with out causing lateral forces on the abutment.

7. Factors affecting retention and
resistance

1- taper of the preparation

2- surface area of the preparation

3- length and height of the preparation

4- tooth width

5- texture of the preparation

8. Taper of the preparation

Convergence angle: the angle that is formed between each
two opposing axial walls of a tooth preparation to receive
crown restoration . the magnitude of retention depends on the
degree of this angle.

5-6 degrees convergence angle is mostly used to provide
need retention..

Parallel wall better retention but difficult to do inside the
patient mouth with out undercut ..so its better to get retention
5-6 degree tapered

9. Surface area of the preparation

Increase the surface area will increase the retention…factor that influence
the surface area:

A- size of the tooth: the large size of tooth large area of preparation

B- extent of tooth: coverage by the restoration : full metal crown on molar
tooth is more retentive than three quarter crown

C- length of the crown :increase the length of the preparation will increase
the retention and resistance.

10. Diameter of the tooth

Crown on a narrow tooth can have greater resistance to tipping than on
wider crown because the crown on the narrower tooth has a shorter radius
for rotation.

Texture of the preparation : depending on the type of luting cement ,
smooth surface are less retentive than rough .

12. 
Occlusal clearance : is the space between the occlusal surface of the
prepared tooth and that of opposing tooth.

13. 
Note: flat occlusal reduction will lead to too thin metal and this will lead to
perforation of the crown restoration in the future…its non conservative
preparation …which prefer anatomical occlusal reduction.

14. Preservation of periodontium

The margin of preparation should be supra-gingivally

The crown restoration should have proper contact embrasure form.

Margin placement (finishing line placement): supra-gingivally..sub-gingivally..or
equi-gingivally.

15. Supra-gingival finishing line

21. 
Thank you