Several modifications have been proposed for in class II cavity design for amalgam restorations . These modifications are changes from the classic preparation design . They are indicated for those situations which are not ideal. Modifications are based on following factors:
- Extent of caries
- Location of proximal caries and relationship of adjoining teeth
- Esthetic considerations
- Involvement of several surfaces on the same tooth
- Prosthetic considerations
- SIMPLE BOX PREPARATIONS: It consists of only a proximal box preparation without an occlusal step and minimum facial and lingual extensions . It is indicated for a small proximal carious lesions in the absence of occlusal caries , moderate proximal caries, when there is narrow inter-proximal contact. The facial and lingual walls converge occlusally to provide retention form. For added retention axiopulpal grooves are placed gingivally.
2. SLOT PREPARATION : It is similar to class V amalgam cavity preparation except that it is done on the proximal surface of the tooth. It is indicated in proximal root surface caries with gingival recession. The design is similar to that of the slot preparation except it is approached from the facial aspect without disturbing the contact area.
3. CONSERVATIVE DESIGNS : It is recommended in all posterior teeth where caries incidence is very low and all occlusal pits and fissures are not involved. The conservative design preserves all the remaining sound tooth structures such as transverse ridge or oblique ridge and thereby protects the cuspal strength .
4. MODIFICATIONS TO PRESERVE ESTHETICS : To preserve the esthetics in the critical area during a mesio-occlusal cavity preparation , the facial wall of the mesial box should be prepared straight , i.e parallel to the long axis of the tooth rather than gingivally divergent . Another modification is to avoid breaking the facial contact whenever caries is limited only to the mesio-lingual embrassure.
5. MODIFICATIONS OF ROTATED TEETH : Depending on the degree of rotation , the proximal box is displaced facially or lingually.
6. ADJOINING RESTORATIONS : The intersecting margins of the two restorations -should be at right angles . Care should be taken so as to prepare the outline of the new cavity without weakening the amalgam margin of the existing restoration .
7. MODIFICATIONS FOR ABUTMENT TOOTH: The facial and lingual walls of the proximal box must be extended more so that the entire rest seat can be prepared in the amalgam without encroaching on the occlusal margins.
The pulpal floor is deepend an extra 0.5 mm apical to the region of the rest seat so as to provide an adequate thickness of amalgam.
8. CUSP CAPPING : It is indicated where there are extensive caries underneath the cusps and facio-lingual extensions of the occlusal preparation is more than 2/3rd the distance between the facial and lingual cusp tips. For cusp capping 1.5 to 2 mm of cusp reduction is necessary . The reduced cusps must meet the adjacent unreduced cusp at 90 degree cavosurface angle to provide both adequate edge strength of amalgam.
References: STURDEVANTS 7TH EDITION