Influence of cavity depth on marginal degradation of amalgam restorations

In a 5-year clinical trial it was observed that the marginal degradation of class-II amalgam restorations could be related to the bulk of the restoration. The association between the occlusal cavity depth and the marginal degradation was observed after 6 months and varied for the different types of alloy. Ridit scores of the marginal degradation were correlated to various indices of the cavity sizes, to assess the possible reason for this association. The possibilities of the relationship being an indirect effect caused by longer condensation times or by poorer condensation due to the use of larger condenser sizes were rejected. Furthermore, the possibility that the association was the result of potential buildup of stresses on the restoration margins caused by flexible cusps and axial walls was not apparent. A possible mechanism may be that marginal degradation is the result of short-term or long-term expansion, or even extrusion of amalgam. Expansion may theoretically be caused over short periods by temperature changes or over long periods by corrosion or phase shifts in the amalgam. The theory does not exclude the role of creep or corrosion and may furthermore explain the lack of correlation between in-vitro tests and in-vivo performance of amalgam restorations.