Structural studies on calcium fluoride formation and uptake of fluoride in surface enamel in vitro.

The structural changes in human enamel during exposure to an acetate buffer, pH 4, containing 150 parts/10(6) fluoride have been studied using scanning electron microscopy and polarized light microscopy. After exposure for 2 h the enamel surface was covered by a fine-granular layer of calcium fluoride. The underlying enamel was highly eroded with an increased pore volume in the outer layer. Following an equilibration period of 3 months the uptake of fluoride in the apatite lattice had resulted in a highly mineralized, 100-micrometer-thick surface layer rich in fluoride covering a subsurface porous zone with an unchanged fluoride content. The deeply located, caries-like porous zone may have provided calcium and phosphate for saturation of the liquid.