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The aim of this experiment was to determine the acid susceptibility of fissures of human premolars at different depths and the effect of exposure to the oral environment on the acid susceptibility. A batch of 40 caries-free premolars with equal numbers of incomplete (A) and complete roots (B) was selected. Incomplete root formation is indicative of teeth which have been exposed to the oral environment for a short time. The premolars were broken in mesio-distal direction in two halves to allow exposure of the fissure wall to acids. After one week of demineralisation in a gel system with 0.1 M lactic acid (pH 4.60), both halves of the premolars were serial-sectioned in buccolingual direction with a wire saw. The sections were radiographed to determine the integrated mineral loss ('lesion size') at two depth levels in the fissure. The average lesion size in the premolars with incomplete and complete roots was 2,404 vol%WÌm (SD 1,093) and 1,690 vol%WÌm (SD 283), respectively. These differences were statistically significant (p ! 0.05) with an independent t-test. Within the fissures, the differences in lesion size were not statistically significant. There was a trend, however, that the lesions in the upper part of the fissure wall of the premolars in group A were larger than in the lower part, while in the premolars of group B the lesions were smaller in the upper part of the fissure wall than in the lower part. This would indicate that exposure of the human enamel fissure walls to the oral environment reduces its acid susceptibility. This reduction tended to be larger in the upper part of the fissure walls than in the lower part. |