Cyclic fatigue of hydroxyapatite-coated titanium alloy implant material--effect of crystallinity.

Titanium alloy (ASTM F-136) rods were coated with hydroxyapatite (HA) of 3 levels of crystallinity, which were determined by X-ray diffraction (XRD) analysis to be 60.5%, 52.8%, and 47.8%. Fourier Transform Infrared (FTIR) spectroscopy analysis showed the removal of the hydroxyl and carbonate groups as compared to the original HA powder. It appears that these changes are caused by the high temperature plasma spray coating process. Cyclic fatigue testing in a lactated Ringer's solution to 5 million cycles showed no statistical difference in calcium dissolution among the 3 crystalline levels, whereas phosphorus dissolution was lowest from the highest crystalline coating sample. The mechanical properties, however, did not change in response to fatigue loading.