Determination of surface and subsurface fatigue damage in dental composites

Dental filling materials are subjected to cyclic compression in the mouth. Nine resin-based composite filling materials were subjected to 2000 compression cycles between either 0 and 12 kg, or 0 and 40 kg. Surface deformation was measured as the diameter of the compression scar and surface microhardness determined by a Vickers' microhardness test at 4 sites around the scar. Subsurface damage was stained with silver nitrate and the area of stain determined by image analysis software. Subsurface microhardness was measured at 4 sites around the stained zone. Surface deformation at 12 kg was inversely proportional to the surface microhardness at 12 kg. At both loads subsurface damage was directly proportional to the subsurface microhardness. Samples with small filler particles experienced less subsurface damage than those with larger particles. Silver nitrate staining was found to be a useful method for identifying subsurface damage.