The mechanism of action of solution fluoride upon the demineralization rate of human enamel.

Equations have been derived and computations carried out for the demineralization rates of tooth enamel employing the “Fluorapatite” model. The model assumes that when hydroxyapatite is exposed to a low fluoride concentration, a layer of fluorapatite is formed around the apatite crystal and this fluorapatite is assumed to be the thermodynamic governing phase during dissolution. The demineralixation rates have been obtained as a function of the fluoride ion concentration, the hydrogen ion concentration, the total buffer concentration, the diffusion coefficients of all the species involved, the dissociation constant of buffer acid (K.), and the solubility products of fluorapatite and hydroxyapatite. The “Fluorapatite” model has been critically examined with experimental data obtained with synthetic apatites and enamel powder. Experiments were carried out as a function of solution fluoride concentration, the common ions (calcium and phosphate) and pH. In the case of synthetic sample, TVA (P.D.), all of the experiments were in quantitative agreement with the model when a fluorapatite solubility product value of 1 x 10-‘30 to 1 x 10-132 was used in conjunction with a hydroxyapatite solubility product value of 1 x 10-lao to 1 x 10-13’. In the case of the powdered enamel studies values of Z&, = 1 x 1O-119 and K h*p 2: 1 x lo-“s gave good consistency with experiments. It was found necessary to consider the influence of fluoride initially present in the enamel samples to achieve the best agreement of the data with model. The “Fluorapatite” phase has been interpreted as that formed by the isomorphous substitution of the surface hydroxyls by the fluoride rather than by the reprecipitation of a “true” fluorapatite phase upon the original hydroxyapatite crystals. These studies have been found to be consistent with those reported earlier (HIGUCHI et al., 1969, 1965). Thus it may be concluded that a single a priori physical model has thus far described all of the experimental data on enamel and synthetic apatite dissolution rates in a self-consistent manner.