Cracks in amalgam-filled teeth may be related to amalgam expansion. This study measured cuspal flexure and used finite element analysis to assess associated stress levels in amalgam-filled teeth.
External surfaces of 18 extracted molars were scanned in three dimensions. Nine molars were restored with mesio-occluso-distal amalgam fillings; the other teeth were left intact as controls. All teeth were stored in saline and scanned after two, four, and eight weeks. Cuspal flexure and restoration expansion were determined by calculating the difference between scanned surfaces. Stresses in a flexed tooth were calculated using finite element analysis.
Cusps of amalgam-filled teeth flexed outward approximately 3 μm, and restoration surfaces expanded 4 to 8 μm during storage. Cuspal flexure was significantly higher in the amalgam group (multivariate tests, p<0.05), but storage time had no significant effect (repeated measures, p>0.05). Expansion caused stress concentrations at the cavity line angles. These stress concentrations increased stresses due to mastication 44% to 178%.
Amalgam expansion pushed cavity walls outward, which created stress concentrations at the cavity line angles. Expansion stresses can raise stresses in amalgam-filled teeth and contribute to incidentally observed cracks.