1. PRINCIPLES OF TOOTH PREPARATION (Lecture or Part-2)

2. 3. STRUCTURAL DURABILITY
A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion.The bulk must be confined to the space created by the tooth preparation ie.
Occlusal reduction
Functional cusp bevel
Adequate axial reduction
Marginal integrity
Marginal placement
Marginal Adaptation
Marginal geometry
Margin designs

3. Gold alloys – 1.5mm (FC) & 1mm(NFC)
OCCLUSAL REDUCTION:-
An important feature for providing adequate bulk of metal & strength to the restoration is occlusal clearance.
The occlusal reduction should reproduce the basic inclined plane pattern of the occlusal surface without overshortening or flat plane.
Gold alloys – 1.5mm (FC) & 1mm(NFC)
Metal-ceramic crowns mm(FC) & 1-1.5mm(NFC)
All-ceramic crowns – 1.5 (NFC) -2mm (FC) or incisal edge

4. FUNCTIONAL CUSP BEVEL:-
It is an integral part of the occlusal reduction.
A wide bevel on the lingual inclines of the maxillary palatal cusps & the buccal inclines of the mandibular buccal cusps provides space for an adequate bulk of metal or ceramic in an area of heavy occusal contact.

5. If a wide bevel is not placed on the functional cusp, several problems may occur :
- If the crown is waxed & cast to normal contour it can cause a thin area or perforation in the casting.
-To prevent this the crown may be waxed to optimal thickness resulting in overcontouring & poor occlusion.

6. AXIAL REDUCTION:-
It plays an important role in securing space for an adequate thickness of restorative material.
A. adequate axial reduction
B. inadequate axial reduction results in crown with thin , weak walls
C. Bulky crown results in promoting plaque
accumulation , jeopardising the health of
periodontium

7. Other features that will provide space for metal that will improve the rigidity & durability of the restoration are:
The offset, occlusal shoulder, isthmus, proximal groove & box.

8. 4. MARGINAL INTEGRITY
The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavosurface finish line of the preparation.
Should be close fitting- minimum cement exposure
Adequate strength
Locatable to the dentist and cleansable to patient
Types of Margins :
Knife edge Chamfer Heavy chamfer Shoulder
Radial shoulder Bevel shoulder

9. MARGIN PLACEMENT:- Whenever possible the margin of the preparation should be supragingival.
Advantages of supragingival margins are:-
Easily finished.
easily kept clean
Easy to make impressions with less damage to soft tissue.
Restorations –easily evaluated –at recall.

10. Subgingival margin is justified if any of the following pertains:-
Subgingival margins of cemented restorations are a major factor in periodontal disease.
Subgingival margin is justified if any of the following pertains:-
1)Dental caries, cervical erosion extend subgingivally.
2)Proximal contact area extends to the gingival crest
3)Additional retention is needed
4)Margin of a metal-ceramic crown is to be hidden behind the labiogingival crest.
5)Root sensitivity (not controlled by conservative means or dentin bonding agents).
6)Modification of axial contour is indicated.

11. MARGIN ADAPTATION:-
The junction between a cemented restoration and the tooth is always a potential site for recurrent caries because of the dissolution of luting agent and inherent roughness.
The more accurately the restoration is adapted to the tooth, the lesser the chance of recurrent caries or periodontal disease.

12. MARGIN GEOMETRY:-
For the evaluation of the margin the following guidelines for margin design should be considered.
1)  Ease of preparation without overextension or unsupported enamel.
   2) Readily identifiable in impression & on die.
   3) A distinct boundary to which the wax pattern can be finished.
   4) Provide sufficient bulk of material.
5) Conservation of tooth structure.

13. MARGIN DESIGNS:- Feather Edge Chisel Edge Chamfer Heavy or Deep
Feather Edge
Chisel Edge
Chamfer
Heavy or Deep
Shoulder
Sloped (Radial) Shoulder
Shoulder with
Bevel
Advantages
Conservation of
tooth structure
Conservation
Distinct margin, adequate bulk easier to control.
Bulk of material + advantage of round internal angles
Bulk of restorative Material
Bulk of material + advantage of bevel
Bulk of material + Adv. Of bevel
Disadvantages
Does not provide sufficient bulk
Location of margin
difficult to control
Care is needed to remove unsupported lip of enamel
Less conservative
Less conservative apically extends
Indications
Not recommended
Occasionally on tilted tooth
Cast metal
restoration
Facial margin of metal ceramic & all margin of Full ceramic restorations
Facial margin of metal ceramic & all margin of Full ceramic restorations with supraginigval margins
Bevel

14. 5. Preservation of periodontium
The placement of finish lines has a direct bearing on the ease of fabricating a restoration & on the ultimate success of the restoration.
Margins that are smooth & fully exposed to cleansing action give best results.
Finish lines should be placed in enamel when it is possible to do so.
Subgingival restorations are described as a major etiologic factor in periodontitis.
The deeper the restoration margin resides in the gingival sulcus the greater the inflammatory response.

15. REFERENCES
Fundamentals of Fixed Prosthodontics, Herbert T. Shillingburg
Contemporary Fixed Prosthodontics ,Stephen F.Rosensteil