Effects of magnesium on the formation of calcium-deficient hydroxyapatite from CaHPO4.2H2O and Ca4(PO4)2O.

Calcium-deficient hydroxyapatite (HA) with a Ca/P molar ratio of 1.50 was synthesized in various concentrations (0.01-75 mM) of MgCl2 at 37.4 degrees C by reaction between particulate CaHPO4.2H2O and Ca4(PO4)2O. The effects of magnesium on the kinetics of HA formation were determined using isothermal calorimetry. All reactions completely consumed the precursor phases as indicated by X-ray diffraction analysis and a constant enthalpy of reaction (240 kJ/mol). Magnesium concentrations below 1 mM had no effect on the kinetics of HA formation. Magnesium concentrations between 1 and 2.5 mM affected the reaction path but did not affect the time required for complete reaction. Higher concentrations extended the times of complete reaction due to magnesium adsorption on the precursor phase(s) and HA nuclei, and stabilization of a noncrystalline calcium phosphate (NCP). HA formation in the presence of magnesium resulted in separation of the following two events: initial formation of HA nuclei and NCP, and consumption of CaHPO4.2H2O. This was indicated by the appearance of an additional calorimetric peak. Variations in calcium, magnesium, and phosphate concentrations and pH with time were determined. Increasing the magnesium concentration resulted in elevated calcium concentrations. After an initial decrease in magnesium owing to its adsorption onto HA nuclei and precursor(s), a period of slow reaction at constant magnesium concentration was observed. Both the magnesium concentration in solution and the proportions of precursors present decreased prior to any evidence of a crystalline product phase. This is attributed to the formation of NCP capable of incorporating magnesium. This noncrystalline phase persisted for more than 1 year for reactions in magnesium concentrations about 2.5 mM. Its conversion to HA resulted in the release of magnesium to the solution.