Resorbability and solubility of zinc-containing tricalcium phosphate

Using zinc-containing tricalcium phosphate (ZnTCP) as the zinc carrier for zinc-releasing calcium phosphate ceramic implants promoted bone formation around the implants. Because no quantitative information was available on the equilibrium solubility and resorbability of ZnTCP, in vitro equilibrium solubility and in vivo resorbability of ZnTCP were determined and compared quantitatively in this study. The solubility of ZnTCP decreased with increasing zinc content. The negative logarithm of the solubility product (K(sp)) of ZnTCP was expressed as pK(sp) = 28.686 + 1.7414C - 0.42239C(2) + 0.063911C(3) - 0.0051037C(4) + 0.0001595C(5) in air, where C is the zinc content in ZnTCP (mol %). The solubility of ZnTCP containing a nontoxic level of zinc (<0.63 wt %) decreased to 52-92% of the solubility of pure tricalcium phosphate (TCP) in the pH range 5.0-7.4. However, the in vivo resorbed volume of ZnTCP containing the same amount of zinc was much lower than that expected from the in vitro solubility, becoming as low as 26-20% of that of TCP. Cellular resorption of TCP is substantially a process of dissolution in a fluid with an acidic pH that is maintained by the activities of cells. Therefore, the reduction of the resorbability of ZnTCP could be attributable principally to its lowered cellular activation property relative to that associated with pure TCP.