Physiology of hard tissues in comparison with the solubility of synthetic calcium phosphates.

On the basis of the composition of the mineral in bone and the saturation level of blood plasma, with respect to slightly soluble calcium phosphates, it is shown that normal tissue fluids are not supersaturated with whole bone but with the minerals of dead bone. The phase controlling solubility in living bone is octocalcium phosphate, and the presence of this phase could regulate the calcium phosphate concentrations of the body fluids by a simple physicochemical equilibrium. It is explained how parathyroid hormone and calcitonine can act on osteocytes in such a way that shifts occur in this physicochemical equilibrium. It is also explained how the mineral of bone is affected by the activity of osteoclasts and osteoblasts during bone turnover. From this knowledge about the physiology of bone mineral, it is possible to predict the behavior of calcium phosphate bioceramics implanted in the body once their true solubility limits are known. Solubility studies have been carried out on hydroxyapatite, oxyhydroxyapatite, octocalcium phosphate, brushite, beta-tertiary calcium phosphate, and magnesium whitlockite. These studies indicate that the resorbabilities for implants of these materials are in the following order: magnesium whitlockite less than hydroxyapatite less than oxyhydroxyapatite less than beta-tertiary calcium phosphate. This order is consistent with data from implant studies now available in the literature.