Microradiography and electron-microprobe analysis of some caries-like lesions of enamel prepared in vitro in human teeth.

Six lesions were made on the buccal surfaces of premolars. The volume percentage of mineral was determined as a function of depth by microradiography. Using the electron microprobe, the signals for Ca, Na, Mg, P and Cl were recorded as a function of depth both through the lesions and through the adjacent sound enamel. In the demineralized parts, there was a preferential loss of Na and a preferential retention of chloride. In the surface layers, the Na:Ca and Cl:Ca ratios were almost the same as in the adjacent sound enamel, indicating that the surface layers were not formed by gross dissolution of the original mineral followed by gross reprecipitation of another, less-soluble calcium phosphate, but remained probably because their microcrystals were protected by a thin layer of precipitated fluorapatite or fluoridated hydroxyapatite. The same had been found for surface layers in natural caries.