Histological features of artificial secondary caries adjacent to amalgam restorations

Artificially induced carious lesions on either side of in vitro un/restored amalgam cavities were examined to establish the degree of randomness of caries development. Class I cavities were cut in 132 extracted premolars: twelve teeth were not restored, and 120 teeth were restored with one of 20 different restoration combinations of silver amalgam, base and varnish. After ageing for periods of 3 months and 1 year, the 12 unrestored teeth and 80 of the restored teeth were subjected to an in vitro bacterial challenge for 36 days, while the other 40 specimens were challenged in acidified broth (pH 4.0). Sections were then prepared for polarized light microscopy. Carious lesion configuration on either side of the cavity was noted, and outer, wall and dentine lesions measured. Data underwent Fisher's exact test, a chi-squared goodness of fit test and a Student's paired t-test with P<0.05. Except for dentine lesions, acid broth and unrestored specimens showed carious lesions having similar size and occurrence on either side of the cavity. Two unrestored cavities showed caries resistance. Restored, bacterially challenged specimens were significantly different regarding total and wall lesion distribution and wall lesion width and area on either side of the cut cavity. Acid broth challenge will promote regular caries development at the tooth-restoration interface. The random caries pattern which developed in restored bacterially challenged specimens indicates that the tooth-restoration interface forms a diverse environment providing sites of varying susceptibility to caries.