Biological calcium phosphates and their role in the physiology of bone and dental tissues I. Composition and solubility of calcium phosphates

Variations in the composition of bone and tooth mineral are consistent with the model that the constituents are a mixed microcrystalline apatite (AP)-octocalcium phosphate (OCP) like phase and an amorphous or submicrocrystalline calcium phosphate (ACP) like phase whereby these phases can occur in different proportions. An appropriate model for a description of the variable composition and the solubility behavior of the apatite phase is given by the formula $$\begin{array}{*{20}c} {\begin{array}{*{20}c} {Ca_{5 - x - y - u} Na_{\frac{2}{3} y} } \\ {\{ (PO_4 )_{3 - x - y } (CO)_{x + y} \} (H_2 O)_{y + z} OH_{1 - x - \frac{1}{3} y - 2u} } \\\end{array}} \\\end{array}$$ in which the compositional parameters x, y, z, and u each account for one type of defect mechanism. Other point defects are formed as well by incorporation of minority amounts of ions such as Cl⁻, K⁺, and F⁻; a number of trace elements can substitute for Ca²⁺ ions under in vivo conditions. It is suggested that the incorporation of ions in or loss from the crystals in contact with aqueous solutions is reversible. Literature data are used to show the direction in which the solubility product of the apatite phase shifts by incorporation of the different physiologically relevant ions. A quantitative evaluation of the available literature data revealed that Na⁺ and CO₃ ⁼ incorporation is the main cause for shifts in the solubility product of biological apatites.