Introduction
Class II cavities are, in Black's classification, cavities on the proximal (mesial and distal) surfaces of premolars and molars. The neighbouring tooth blocks direct access, so the lesion is reached through the marginal ridge, and the classical preparation joins two parts: an occlusal step and a proximal box. It is one of the most frequent preparations, and its typical errors – a damaged neighbour, a weak isthmus, a leaking gingival margin – lead directly to fracture or secondary caries.
Class II lesions and the classical preparation
Proximal caries starts just gingival (cervical) to the contact area, where plaque stagnates and is not removed by chewing or brushing. It is rarely seen directly: it shows on a bitewing radiograph, as a grey shadow under the marginal ridge, on transillumination, or after temporary separation of the teeth.
The classical (occluso-proximal) preparation is indicated for:
- proximal caries that has reached dentin or is cavitated, in a premolar or molar with a contacting neighbour;
- an undermined or broken marginal ridge, or proximal and occlusal caries in the same tooth;
- replacement of a failed proximal restoration.
It is not the right choice for:
- a non-cavitated enamel lesion – it is arrested without drilling (fluoride, flossing, sealing or resin infiltration);
- a small dentin lesion under a strong marginal ridge – a slot or tunnel preparation saves more tooth (topic 9).
Black designed the preparation for amalgam, which does not bond to tooth, needs bulk (at least about 1.5 mm) and breaks at thin edges. His extension for prevention carried the proximal walls into the self-cleansing embrasures and the gingival floor below the gingiva; today the walls only just clear the neighbour and the gingival margin stays supragingival where the caries allows. The individual outlines (MO, DO, MOD, buccal or lingual extensions) are topics 10 and 11.