Degradation phenomena occurring in the conical taper of a short-term retrieved ZTA femoral head: A case study

Despite being the most used ceramic biomaterial in hip joint prosthesis, Zirconia Toughened Alumina (ZTA) is liable to undergo several degradation mechanisms in vivo, which might drastically affect its structural performance over time. Our standing point in this investigation was that neither in vitro tests nor finite elements simulations are yet capable to fully reproduce the complexity of the in vivo conditions. Through accurate analyses of retrievals, we attempt to add more pieces to the puzzle of in vivo loading history and to obtain better insight into the ZTA degradation mechanisms. For doing so, we applied a previously established automated scanning protocol to investigate the coupling between a ZTA femoral head and its metal stem counterpart in a short-term (20 months) retrieved hip implant. Metal contamination on the ZTA surface was severe and polymorphic transformation widespread on the whole surface of the taper, though reaching different values depending on the specific zone of the taper. Clear signs of third body abrasive wear and fatigue crack initiation could be found by means of laser and electron microscopies, and X-ray spectroscopy. Highly resolved mapping by Raman micro-spectroscopy revealed very high monoclinic fractions and pronounced residual stress magnitudes despite the relatively short in-vivo lifetime.