What is the most common cause of fracture at the isthmus of Class II dental amalgam restoration?

The most common cause of a fracture at the isthmus of a Class II dental amalgam restoration is inadequate depth at the isthmus area. This is due to the fact that the isthmus, which connects the two cusps, requires sufficient thickness of amalgam material to withstand occlusal forces and masticatory stress. If the depth is inadequate, the restoration becomes more susceptible to fracture under load because there is not enough bulk to absorb and distribute the stresses efficiently.

Amalgam, though durable, has limited resistance when placed in a thinner format. When a restoration is too shallow at the isthmus, it compromises the structural integrity of the restoration, leading to a higher likelihood of failure when subjected to chewing forces. Therefore, ensuring appropriate depth during preparation is crucial for the longevity and stability of the amalgam restoration in that area.