Stability of hydroxyapatite coating of different crystallinities on a titanium alloy implant material after cyclic fatigue

Plasma-sprayed hydroxyapatite (HA) coatings of three different crystallinities on titanium alloy (Ti-6Al-1V) substrate were analyzed for fatigue-induced changes in their stability. The HA starting powder and the coatings were characterized using X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The implants were subjected to 5 million cycles of fatigue loading while immersed in a lactated Ringer's solution to simulate a physiological condition. Calcium and phosphate analyses were performed on the immersing solutions before and after fatigue to quantify any dissolution of coating. Mechanical strengths of the pre- and post-fatigued implants were measured to determine the fatigue-induced changes in strength. XRD analysis showed the coating crystallinities were around 61, 53, and 48%. FTIR analysis showed a decrease in the hydroxyl and carbonate ions, and transformation of HA to oxyhydroxyapatite in the coatings as compared to the starting powder. Higher crystallinity coatings exhibited lesser dissolution in the presence and absence of fatigue loading. Mechanical properties of the coated implants remained unchanged after fatigue loading.