Why are partial coverage interim restorations weaker against occlusal forces?

Partial coverage interim restorations are designed to cover only a portion of a tooth. This design inherently means that they do not provide full encirclement of the tooth, which is a critical factor in distributing occlusal forces. When a restoration encircles the tooth completely, it can better support the tooth structure and withstand the pressures exerted during biting and chewing. Without full encirclement, there are weaker points in the structure, making it more susceptible to fracture or displacement under occlusal stress.

In contrast, the other factors mentioned may contribute to the overall function and longevity of a restoration but are not the primary reasons for the reduced strength of partial coverage interim restorations against occlusal forces. For example, while inferior marginal fit can affect the overall stability and health of the restoration, it is not the sole reason for weakness under occlusal forces. Supragingival margin locations primarily relate to periodontal health and retention rather than mechanical strength. Missing proximal contacts can influence the fit and function of the restoration but do not directly correlate with its ability to withstand occlusal forces as significantly as the lack of encirclement does.