[A microscopical study on destructive process in human enamel--enamel destruction caused by lactic, propionic, and citric acid].

The object of the present experiment was to observe continuously the pattern of structural changes in the surface layer of enamel produced by the action of lactic, propionic, and citric acid, and also to compare these changes with those previously reported for acetic acid and ethylene diamine tetra-acetic acid (EDTA). According to the sandwich method devised by us, a plano-parallel thin section of tooth was cemented between the cover and slide glass using an adhesive material, with exposure of the outer area of the enamel surface alone. The decalcifying solution was then permited to act on the surface area in order to observe continuously the main changes under the microscope. Exposure times varied from approximately20 to 100 hours. After the decalcification, contact microradiographs of each decalcified section were prepared.The main findings are as follows. The ends of the striae of Retzius and some small defects including narrow channel-like defects appeared to be the point of the initiation of attack, as already reported in the study on acetic acid and EDTA decalcification. This phenomenon seems to be independent of the organic acid used. The spread and degree of enamel destruction owing to the action of citric acid were more rapid and pronounced than in acetic, lactic, and propionic acid decalcification. Citric acid attack on enamel was characterized by preferential dissolution of the peripheral regions of prisms and the striae of Retzius, whereas the etching pattern of other organic acids seems to be unrelated to prism morphology.