In vitro evaluation of amorphous calcium phosphate and poorly crystallized hydroxyapatite coatings on titanium implants.

Studies of various apatite coatings on metal orthopaedic prostheses suggest that coating dissolution may promote enhanced bone bonding. Little is known concerning the effects of crystallinity and the underlying roughness on calcium phosphate (Ca/P) coating dissolution rate. To address these issues, the surface chemistry of amorphous Ca/P and poorly crystallized hydroxyapatite (HA) coatings on "smooth" and "rough" titanium (Ti) alloy (Ti-6A1-4V) implants was studied following immersion in Hank's physiologic solution at pH 7.2 and 5.2 for 0-, 4-, and 12-week periods. Changes in Calcium (Ca) ion concentrations in the solutions, coating chemistry, and surface morphology were studied by ion selective electrode, x-ray diffraction (XRD), and scanning electron microscopy (SEM) respectively. The amount of Ca dissolved from Ca/P-coated implants was strongly dependent on the chemistry of the coating and less dependent on pH or time of incubation. The effect of the underlying surface (smooth vs. rough) was not significant. The poorly crystallized HA coating underwent the most degradation, greatest crystallographic alteration, and greatest surface film formation. The amorphous coating was more stable in the saline environment, and may be more suitable in vivo if coating longevity is desired. These results suggest that this in vitro method is an effective way of determining differences in HA coating integrity.