The purpose of this study was to investigate the mechanism of surface layer formation in artificial white spots. Two nondestructive nuclear micro-analysis techniques were employed to analyze windows of human dental enamel at progressive stages of an artificial lesion. Fluoride concentrations were measured to a depth of 9 μm using resonant reaction profiling. Surface concentrations of Ca, P, Zn, Fe, and Sr were measured using proton-induced X-ray emission. The enamel windows were analyzed longitudinally after zero, four, and 16 hours' exposure to lactic acid gels of pH 3.7 and pH 4.5. During the first four hr of lesion formation, non-fluoridated apatite was preferentially removed from the surface with up to a 40% loss of Ca and P, depending on the gel pH. The next 12 hr saw a build-up of Ca and P in the surface to values approaching those of healthy enamel. In comparison, F and Sr concentrations changed very little, Fe tended to show a small increase at the end of each demineralization period, and Zn was less readily lost and gained than was Ca. These results and others suggest that the white spot surface layer phenomenon is a combined demineralization/remineralization process aided by, but not requiring, the presence of endogenous fluoride.
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