Hydroxyapatite coatings.

Significant interest for orthopedic surgical applications of hydroxyapatite compounds is based on a number of reasons including the elemental chemistry of the primarily calcium and phosphate ion substances, the controllability of the crystalline and physical structures to provide a wide range of in vivo interactions, the existing literature on biocompatibility and the possibility for use as a surface coating, and the opportunity for bonding to bone and enhancing mechanical force transfer. A wide range of crystalline and structural forms can be constituted from reactions between calcium oxide and phosphorous pentoxide. These ceramics can be made in at least eight crystallographic forms with multiple opportunities to introduce other elements or structural defects. Microstructures and densities also can be varied to provide properties from relatively stable to completely biodegradable. One limitation has been the relatively low shear and fatigue strengths of the calcium phosphate-based ceramics. Although experimental for human orthopedic applications at this time, coatings are now in clinical trials using a range of substances, and active research is continuing on combinations of ceramics with higher strengths, cermets, and composites.