Changes in the crystallinity of hydroxyapatite powder and structure of enamel treated with several concentrations of ammonium hexafluorosilicate

Purpose: To evaluate the effects of changing concentrations of ammonium hexafluorosilicate [SiF: (NH₄)₂SiF₆] solution on the crystallinity of hydroxyapatite powder (HAP) and structure of human enamel in order to overcome the tooth discoloration caused by diamine silver fluoride [AgF: (NH₃)₂AgF] application. Methods: HAP was treated with several concentrations of SiF solution (from 10 to 19,400 ppm) for 5 minutes. The crystallinity of the HAP 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). Results: XRD peaks became sharper after SiF treatment indicating that the crystallinity of apatite powder was increased. The 1/ß 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 HAP 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.