Adaptation of amalgams to cavity walls

The adaptation of amalgam fillings to the cavity walls has been studied. Cavities were made in human teeth and filled with four different amalgams: (1) amalgam with an expansion of approx. 10 mum/cm after 48 h; (2) amalgam with no dimensional change after 48 h (balanced); (3) amalgam with a contraction of approx. 10 mum/cm after 48 h; and (4) amalgam with a contraction of approx. 20 mum/cm after 48 h. The filling surface and parts of the surrounding enamel were planed, polished and studied under the microscope. The degree of contact between the amalgam and the cavity wall and the number and dimensions of the marginal defects were observed. Fillings made from expanding or balanced amalgams mostly had good contact with the cavity wall, and the marginal defects were small and scattered. Around fillings made from contracting material, a slit formation was observed, and in these areas fractures of both the amalgam and the enamel margins had regularly changed the slit to a V-shaped gap 10-20 mum wide. Greater defects exceeding 50 mum were also frequently observed. The results showed a gradual decrease in the quality of adaptation from fillings made of expanding material to those made of contracting material. It was further shown that the polishing procedure caused fracturing of both the unsupported enamel and amalgam margins thus widening the defects considerably.