Fluoridization of Synthesized Hydroxyapatite in Contact with Fluoride-Containing Restorative Materials

It is said that the fluoride-containing restorative materials strengthen the dental hard tissues and prevent the occurrence of the secondary caries. This is due to the reaction of the fluoride ions with hydroxyapatite to form fluorapatite, calcium fluoride or others. Above all, the fluorapatite formation seems to be the most effective in decreasing the susceptibility to decalcification and raising the caries resistance of the teeth. However, it was so far difficult to analyse the quantity of the fluorapatite separately from other fluorides, and hence the efficiencies of fluoridization have been evaluated by the total uptake of the fluoride ions in the hydroxyapatite. In the present study, the quantitative determination of the fluorapatite was successfully made utilizing ESR spectra of oxovanadium ions, VO^<2+>, adsorbed on the apatite. By this method, the fluorapatite formation was examined when the synthesized hydroxyapatite was kept in contact under water with several cements and an amalgam containing fluorides. Effects of the environmental factors such as pH, concentration of the fluoride ion, and existence of other ions or molecules were also investigated. The fluorapatite formation became maximum at the pH around 6, while the total uptake of the fluoride ions markedly increased as the pH decreased within the range from 4 to 9. Higher concentration of the fluoride ion as demonstrated before was not always necessary for fluorapatite formation but a relatively low concentration around 64 ppm was the most favorable. If the other ions or molecules existed, however, the optimum concentration of the fluoride ion tended to become somewhat higher, indicating that in practice the various ions or molecules leached out together with fluoride ions from the restorative materials might inhibit the fluorapatite formation. A silicate cement, Lux Silit, exhibited the highest fluorapatite formation with significant differences from other materials used, although this cement did not show the highest fluoride content or the highest total uptake of the fluoride ions. These findings suggest that the effective fluorapatite formation would require some preferable conditions of pH and/or concentration of the fluoride ion with consideration of the existence of other ions or molecules.