Degradation potential of plasma-sprayed hydroxyapatite-coated titanium implants.

The purpose of this study was to develop an analytical method for evaluating the dissolution behavior of plasma-sprayed hydroxyapatite (HAP) coatings. Six commercially available and clinically used coatings applied to nonporous titanium coupons according to their respective specifications for orthopedic devices were used in this study. Dissolution behavior was monitored by first preconditioning the implant in 0.15 mol L-1 sodium chloride solution at 37 degrees C until either equilibrium or the desired change in solution hydrogen and calcium ion concentrations had been reached. In the second step, the implants were subjected to dual constant composition (DCC) dissolution under conditions of controlled undersaturation. Results indicate that the dissolution rates of the HAP coatings may differ by as much as a factor of 5 despite the fact that analytical techniques, including X-ray diffraction and FTIR, indicate no change in crystallinity or composition of the coatings before and after the solution treatment. These results indicate that HAP coatings from different sources react very differently when placed in the same local aqueous environment. However, more work will be necessary before these in vitro results can be used to predict in vivo behavior.