Aspects of Calcium Phosphate Crystallization under Urinary Conditions

Abstract

Two of the most frequently observed calcium-containing minerals in renal stones are the oxalates and phosphates. Formation of concretions of the calcium phosphates is complicated by the precipitation of precursors during the overall crystallization process. Although the calcium phosphate precipitate is usually referred to as hydroxy-apatite [Ca5(PO4)3OH, HAP], there is still considerable uncertainty as to the nature of the phases which form in the early stages of the precipitation reactions. In many studies aimed at understanding the spontaneous precipitation reactions, experiments have been carried out using solutions supersaturated with respect to at least five phases in order of increasing solubility: HAP; β-tricalcium phosphate [Ca3(PO4)2, TCP]; octacalcium phosphate [Ca4H(PO4)3 2.5H2O, OCP]; dicalcium phosphate anhydrous (CaH PO4, DCPA); and dicalcium phosphate dihydrate (CaH PO4 • 2H2 O, DCPD). Interpretations of the results of these experiments usually assume the attainment of equilibrium and involve thermodynamic solubility products of the phases. However, it is now well established that kinetic factors may be much more important in determining the nature of the solids which formed initially (1). The paricipation of these phases during the crystallization of calcium phosphates to a large extent accounts for the difficulties in elucidating the overall mechanism of the process. The problem is best illustrated by plotting the solubility isotherms as pH functions of the product of total calcium and total phosphate molar concentrations, as in Figure 1. It is important to note that the positions of the curves and the singular points in this figure are functions both of the ionic strength and of the background electrolyte.