Resorbability reduction by the incorporation of zinc into tricalcium phosphate

The solubility of zinc-containing tricalcium phosphate (ZnTCP) in vitro was compared with that in vivo to clarify whether cell-mediated resorption of ZnTCP implants is regulated by zinc ion released from the implants. ZnTCP powders with various zinc contents were immersed in a 0.08M acetic acid and sodium acetate buffer solution at pH 5.5 for 12 months at room temperature to obtain the solubility product (Ksp) of ZnTCF. From the Ksp value, solubility of ZnTCP with a zinc content of 0.31 wt% was calculated in the pH range from 5.0 to 7.4. Cylindrical implants of pure TCP and ZnTCP with a zinc content of 0.31 wt% were implanted in the femora of adult female New Zealand white rabbits for 4 weeks. The area of resorbed implant in the undecalcified sections was determined by an image analysis. Resorbed weight was calculated from the resorbed area and the thickness of the sections. Equilibrium solubility of the ZnTCP with a zinc content of 0.31 wt% was found to be 0.78 +/- 0.01 times that of pure TCP in the pH range from 5.0 to 7.5. However, the resorbed weight of the ZnTCP in vivo was only 0.26 +/- 0.18 times that of pure TCP. The inertness of the ZnTCP implant in vivo is not explained by the decrease in Ksp. It is suggested that zinc released from the ZnTCP implant depresses the cell-mediated resorption of the implant.