Effects of Fluoride on Precipitation and Hydrolysis of Octacalcium Phosphate in an Experimental Model Simulating Enamel Mineralization during Amelogenesis

The present study was undertaken to investigate effects of fluoride at low concentrations on the precipitation of calcium phosphates on hydroxyapatite seeds in media resembling the enamel fluid composition. To this end, we used a new crystal growth model adopting a miniaturized reaction column. Experimental solutions had the composition similar to that found in the enamel fluid, except for calcium concentrations which were adjusted at 1.0 and 0.5 mM. Precipitation reactions taking place in the system at the higher supersaturation ([Ca] = 1 mM) were characterized by co-precipitation of platelike OCP and rod-like apatite crystals, while the reaction at the lower supersaturation ([Ca] = 0.5 mM) was characterized by the precipitation and growth of apatitic crystals. Concerning threshold fluoride concentrations for OCP precipitation, the observation by scanning electron microscopy showed that precipitation of plate-like crystals occurred in media containing fluoride up to 0.3 ppm. More interestingly, neither analysis by X-ray diffraction nor Fourier transform infrared spectroscopy yielded characteristics of OCP for plate-like crystals exposed to fluoride at 0.05 ppm or higher concentrations. The results obtained in vitro support the contention that precipitation of OCP-like precursors and their rapid hydrolysis take place in enamel fluid microenvironment containing low concentrations of fluoride