Silver diamine fluoride promotes formation of fluorohydroxyapatite

Abstract

Objectives: This in vitro experiment was designed to investigate the effects of addition of silver diamine fluoride (SDF) on the nucleation and growth of apatite crystals of calcium phosphate. Methods: In this in vitro experiment different concentrations of SDF (0.38 mg/ml, 1.52 mg/ml, 2.66 mg/ml and 3.80 mg/ml) were added to a calcium phosphate solution (5.88 mM CaCl2+4.12 mM K2HPO4). Two control groups, namely calcium phosphate only (CaCl2+ K2HPO4 in buffer solution) and SDF only (AgF(NH3)2 in buffer solution) were also prepared. After incubation at 37oC for 24 hrs, the shape and organisation of the crystals were examined by bright field transmission electron microscopy (TEM) and electron diffraction. Unit cell parameters of the obtained crystals were determined with powder X-ray diffraction (P-XRD). The vibrational and rotational modes of the crystals in the various groups were analysed using Raman microscopy. Results: The TEM and selected-area electron diffraction confirmed that all solids precipitated within the SDF groups were crystalline and that there was a positive correlation between the crystal size and the concentration of SDF. The P-XRD patterns indicated that fluorohydroxyapatite and silver chloride were formed in all the SDF groups. Compared with calcium phosphate only control, a contraction of the unit cell in the a-direction but not the c-direction in the crystals of the SDF groups was revealed, which suggested that small, localised fluoride anions substituted the hydroxyl anions in the hydroxyapatite crystals. This was further supported by the Raman spectra, which displayed up-field shift of the phosphate band in all of the SDF groups and confirmed that the chemical environment of the phosphate functionalities indeed changed. As expected, no hydroxyapatite crystal was detected in the SDF control. These results suggested that SDF reacted with calcium and phosphate ions to produce fluorohydroxyapatite. Conclusions: Preferential precipitation of fluorohydroxyapatite with reduced solubility provides an explanation for the remineralisation of dental caries lesions treated with SDF.