Measurements of the solubilities and dissolution rates of several hydroxyapatites

Calcium phosphate based materials, such as apatites, are increasingly being developed and used in implants for orthopedic and dental applications. Previous investigation of various calcium phosphate ceramics has demonstrated great variability in the solubility characteristics in solution between materials with similar stoichiometric composition. Therefore, in this study, the solubility and rate of dissolution of three apatite sources, BoneSource, Norian cranial repair system (CRS), and a sintered hydroxyapatite (Calcitite) are evaluated in a thermodynamically closed system. The measured solubility under physiological conditions (tris buffer solution, pH 7.4, 37°C) of BoneSource, Norian CRS and Calcitite is 7.5, 7.4 and 1.4 ppm, respectively. Initial dissolution rates at 10 min of BoneSource, CRS, and Calcitite were 0.0465, 0.1589, and essentially 0 mg/min, respectively. Solubility product constants at 37°C were calculated to be 1.49×10−35 for CRS, 1.19×10−35 for BoneSource, and 2.92×10−42 for Calcitite. The increased solubility of the BoneSource and Norian CRS materials over that of Calcitite is related to their poor crystallinity compared to sintered hydroxyapatite.