Effects of Ammonium Hexafluorosilicate Concentration on Crystallinity of Hydroxyapatite Powder and Enamel

Ammonium hexafluorosilicate [SiF: (NH4)2SiF6] solution was prepared in order to overcome the tooth discoloration caused by diamine silver fluoride [AgF: (NH3)2AgF] application. However, it is still unclear how the concentration of SiF solution affects the crystallinity of enamel. The aim of this study was to evaluate the effects of changing concentrations of SiF solution on the crystallinity of hydroxyapatite powder and structure of human enamel. Hydroxyapatite powder was treated with several concentrations of SiF solution (from 10 to 19,400 ppm) for five min. The crystallinity of hydroxyapatite powder before and after SiF treatment was then measured by powder X-ray diffraction (XRD) analysis. The angular width (β) of the 002 diffraction peak was measured at 1/2 the height of the maximum intensity. Also, enamel specimens were prepared from a human extracted tooth. Several concentrations of SiF solution were applied to polished or phosphoric acid etched enamel specimens. The enamel surface was then observed using scanning electron microscopy (SEM). XRD peaks became sharper after SiF treatment indicating that the crystallinity of apatite powder was increased. The 1/β value was increased from 2.8±0.1 to 4.3±0.1 after treatment with 1,000 ppm SiF solution. The amount of CaF2 formed in hydroxyapatite powder was gradually increased with increasing concentrations of SiF solution. The XRD pattern was consistent with CaF2 in case of over 9,000 ppm SiF solution. SEM photographs demonstrated that exposed enamel rods with acid etching were filled with CaF2-like precipitate after SiF treatment regardless of the concentration of SiF solution. It was concluded that SiF treatment increased the crystallinity of apatite powder and repaired the demineralized enamel surface with the formation of CaF2 or CaF2-like precipitate, and thus, SiF treatment would be useful for the prevention of dental caries.