Effects of cyclic bending and physiological solution on plasma-sprayed hydroxylapatite coatings of varying crystallinity

This study investigated the effects of cyclic bending stress levels and testing in simulated physiological solutions or air on the integrity of plasma-sprayed hydroxylapatite (HA) coatings of two different crystallinities. Hydroxylapatite-coated commercially pure (CP) Ti rods were evaluated by immersion testing in Hank's Balanced Salt Solution (HBSS) and by rotating bending in air and HBSS. Static immersion testing of nonstressed specimens resulted in significant microcracking of coating surfaces after 42 days. Specimens cyclically tested at bending stresses above the yield strength of Ti experienced low cycle fatigue failure of the Ti substrates prior to spallation of the HA coatings. Coatings tested at 1 x 10⁶ cycles with interface bending stresses of 180 MPa displayed increased surface microcracking, but no bulk coating spallation. Coatings cycled in HBSS displayed greater amounts of microcracking and surface alteration than samples cycled in air. There was no apparent relation between HA crystallinity and mechanical integrity under cyclic bending stresses.