Introduction
Conventional Class II configurations are the classical amalgam designs for proximal caries of premolars and molars, opened from the occlusal surface through the marginal ridge: the two-surface MO (mesio-occlusal) and DO (disto-occlusal) cavities and the three-surface MOD (mesio-occluso-distal) cavity. The configuration, fitted to each tooth's anatomy, decides how much the tooth is weakened – a wide MOD is the classic cause of cusp fracture.
MO, DO and MOD: what differs
Each configuration is an occlusal step joined through the isthmus to one or two proximal boxes. The parts and their general rules are covered in topic 8, slot and tunnel designs in topic 9, extensions onto the buccal or lingual surface (MOB, DOL) in topic 11.
- Two-surface (MO, DO): one carious proximal surface. The opposite marginal ridge is kept – at least 1.6 mm in premolars and 2 mm in molars. The step follows only the carious fissures; if it includes the opposite pit, the slight widening there is the occlusal dovetail, which resists proximal displacement.
- Three-surface (MOD): both proximal surfaces carious, or the remaining marginal ridge undermined or thinner than the minimum. The step runs along the central groove from box to box; the two opposing boxes lock the filling, so no dovetail is needed.
An MOD is cut only when both proximal surfaces need it: a sound marginal ridge is never removed to simplify the outline.
Losing a marginal ridge costs far more strength than the occlusal cavity: in extracted premolars, cusp stiffness fell by about 20 % after an occlusal cavity, 46 % after an MO and 63 % after an MOD (Reeh and co-workers, 1989). In an MOD the cusps stand as free walls, so the isthmus must stay narrow; if the outline reaches 1/2 of the distance from the central groove to a cusp tip, cusp capping is considered, at 2/3 it is required.