Introduction
A prepared cavity is a designed geometric form, not simply the hole left after caries is removed. This topic covers the components every cavity is built from – walls, floors, line and point angles and the cavosurface margin, with their standard names – and the requirements a cavity design must meet. The design decides whether the tooth or the restoration fractures, whether the margins seal and whether the pulp survives.
Cavity preparation and the cavity form
Cavity (tooth) preparation is the mechanical alteration of a defective, injured or diseased tooth so that the restorative material placed in it re-establishes healthy form and function, including esthetics where needed. It is the first principle of caries treatment; treatment of the prepared cavity (medicatio cavi dentis) and its filling (obturatio cavi dentis) follow as separate topics.
The final shape, the cavity form, is the sum of the forms G. V. Black described – outline, resistance, retention and convenience form – and is described with a fixed set of components. By the number of tooth surfaces involved a cavity is:
- Simple – one surface (e.g. an occlusal Class I);
- Compound – two surfaces (e.g. mesio-occlusal, MO);
- Complex – three or more surfaces (e.g. mesio-occluso-distal, MOD).
A compound (Class II) cavity has its own parts: the proximal box on the proximal surface, the occlusal step on the occlusal surface and the isthmus, the narrow part that joins them. The isthmus is the weakest part of the restoration, so its width and the rounding of its angles matter.