Introduction
Direct composite is now the usual material for Class I and Class II cavities in premolars and molars. Because it is bonded to enamel and dentin, the cavity no longer has to hold the filling: the preparation removes caries, gives access and is shaped to limit polymerisation-shrinkage stress. The classical (amalgam) designs are topics 6–11.
Principles of the adhesive preparation
Retention comes from micromechanical bonding – etched enamel and the hybrid layer in dentin – not from the shape of the cavity. The preparation for composite is dictated by the lesion: caries and weakened tooth are removed, and nothing more.
- No extension for prevention: the outline stops where caries and unsupported enamel end; sound fissures are sealed, not cut.
- No mechanical retention: no dovetail, undercuts or retention grooves. Walls need not converge occlusally; they may be parallel or slightly flared.
- Depth set by the caries: pulpal and axial walls follow the excavated lesion, not a fixed level, and need not be flat.
- Rounded internal angles: less stress concentration, better adaptation without voids. Use round-ended burs (pear-shaped No. 330 or 245, round-ended diamonds).
- Minimal width: isthmus and box as narrow as the lesion and the bur allow; cusps and marginal ridges kept wherever possible.