Effects of creep value and occlusal force on marginal adaptation of amalgam filling

Creep of amalgam is thought to be a factor determining poor marginal adaptation. Clarification of the relationship between creep of amalgam in the cavity and creep value (American Dental Association specification no. 1; ADA #1) is essential to an understanding of the mechanism of marginal fracture and service life. However, only elastic stress analyses of amalgam in a cavity have been reported in the dental field. Therefore, the creep of amalgam in a class I cavity was simulated by an axisymmetric elastic creep finite-element method. The stress and time dependencies of the creep rate of amalgam were incorporated into this calculation. Creep values of 1.0 and 3.0% (ADA #1) were selected as amalgam properties. When the occlusal force on the amalgam filling was assumed in the vertical direction along the axis of the tooth, the rate of extrusion and rate of opening of the gap at the cavity margin were revealed by the simulation. The computed results show that the gap and extrusion were developed in direct proportion of the creep value and proportionally with the square of the occlusal force. Simulated results are in agreement with clinical findings of the service lives of restorations, i.e., a poor adaptation to the margin can be seen for 4-to-10-year-old amalgam fillings.