Solution Ca/P ratio affects calcium phosphate crystal phases.

Abstract

Previous studies on brushite formation and dissolution concluded that brushite was stable only in acidic solutions with pH<6.5. Francis in 1965 predicted that, at pH range 3.5–6.5, brushite would be the preferred phase at low solution Ca/P ratio and hydroxyapatite at high Ca/P ratio. This work presents room-temperature experimental evidence that solution Ca/P ratio affects calcium phosphate crystal phases and that brushite can be the preferred phase in certain supersaturated aqueous solutions containing Ca++ and Pi, with or without Mg++, even at pH 7.0. Solution mixtures were prepared containing initial [CaCl2]=0.1−10.0 mM, [Na2HPO4]=0.1−100.0 mM, [MgCl2]=0 or 3 mM. By addition of NaCl, the ionic strengths in terms of osmolarity were also varied from 100 to 800 mosM. Precipitates were isolated from solutions on the 7th, 14th and 21st days and identified by x-ray diffraction. Results indicated that in the presence of 3 mM Mg++ and relatively high initial apparent activity products (Ca++)(Pi), brushite, whitlockite and hydroxyapatite were obtained as transformation products of initially formed amorphous calcium phosphate when solution Ca/P ratios had low, medium and high values, respectively. In the absence of Mg++, whitlockite was not found. However, with or without Mg++, when (Ca++)(Pi) was relatively low, only brushite was formed by direct crystallization. We concluded that although solutions were mixed at 23°C, pH 7.0, the results were useful in explaining the in vivo calcium phosphate crystal phases observed in renal and dental calculi.