In vitro studies of fluoride uptake in human enamel

abstract — In a closed agitated system, which included an enamel surface and a continuously magnetically stirred buffer at pH 4.0, 5.0, 6.0 and 7.0, the reaction (Ca₅(PO₄)₃OH)_S+F−→(Ca₅(PC₄)₃F)_S+OH− was investigated kinetically by measuring the fluoride ion activity. The buffers initially contained 1 part/10⁶ fluoride. Six experiments were carried out at each pH. It was found that the reaction followed first order kinetics. The rate of the reaction was very slow at pH 7.0, but increased considerably when pH was lowered, and at pH 4.0 it was 100 times that at pH 7.0. In supplemental experiments it was found that reducing the area of the enamel surface reduced the rate of the reaction. The results are discussed with reference to equilibrium conditions in water-apatite systems, assuming that equilibrium cannot be attained when hydroxyapatite is dissolving and fluorapatite is being formed. The findings are considered to explain the increased amount of fluoride present in the residual apatite in developing carious lesions, and may initiate new principles in the local treatment of teeth with fluorides.