Solubility, unit cell dimensions and crystallinity of fluoridated human dental enamel

The aim was to describe the dimensions of the fluorhydroxyapatite unit cell and the solubility of fluorhydroxyapatite as a function of the fluoride concentration in the apatitic lattice. Various amounts of powdered human dental enamel were suspended in a pH 4.5 acetate buffer containing fluoride. The suspensions were equilibrated for 365 days at 20 degrees C. After equilibration the fluoride concentration in the apatite varied from 0.003 to 0.89 (mole fraction). X-ray diffraction showed that the length of the alpha-axis decreased with increasing fluoride concentration, whilst a broadening of the (300) reflection indicated the presence of mixtures of various fluorhydroxyapatites. The solubility of the apatite after the long equilibration decreased slightly with increasing fluoride concentration in the solid. In a few samples the fluoride concentration in the aqueous phase was still discernible, indicating that even after 365 days, equilibrium had not been attained. In these samples the amount of dissolved apatite was extraordinarily low.