INFLUENCE OF AN AMORPHOUS PHASE ON THE MECHANISMS OF BRUSHITE DISSOLUTION

Brushite (DCPD) dissolution was studied by using unusual experimental conditions. It was carried out, at 37-degrees-C, in an unbuffered medium, with a low solid/solution ratio, equal to 0.4 mg/ml and without initial addition of ions such as F-, Mg2+, CO32-, Ca2+. Both solid and solution were studied as a function of time. The solid was examined at given dissolution times, by X-ray diffraction, IR spectroscopy, chemical analysis, S.E.M., and B.E.T. (measurement of specific surface area). The amounts of phosphate and calcium in the solution were determined as a function of time, as well as pH. These studies showed that DCPD dissolution occurs in several step and that the supersaturation of the solution with respect to hydroxyapatite is high-right from the beginning. In a first step, a calcium phosphate with a Ca/P ratio higher than DCPD appears onto the brushite surface.. This phosphate is amorphous. During the second stage, there is an important increase in the dissolution rate. This phenomenon was connected to the crystallisation of the amorphous phosphate to a deficient apatite. The third step corresponds to the achievement of DCPD transformation to deficient apatite. Later, the deficient apatite becomes slowly more stoichiometric.