Oxidation of a zirconium nitride multilayer-covered knee implant after two years in clinical use

The surface composition and microstructure of an up to 5 µm thick multilayer on a knee implant were investigated. When the implant was explanted after approximately two years of clinical use due to failure from aseptic loosening, the topmost ZrN layer was found to be oxidized. Interestingly, only the non-articulating area was visibly oxidized (color change). Up until then, the formation and characteristics of the oxide and its influence on the tribological performance remained uncertain. The oxide was thoroughly analyzed using transmission electron microscopy (TEM) and atom probe tomography (APT). The articulating and non-articulating areas were compared with an as-fabricated implant, which served as a reference. The results show that a thin oxide was also present on the articulating surface. All measured oxides were thicker than expected from native oxidation. The oxygen content of the majority of the oxide, measured with energy dispersive X-ray spectroscopy (EDS) and APT, was lower than required for the stable ZrO₂ form. Underneath the oxide, the ZrN layer remained unaffected, demonstrating the oxide's effective passivating behavior against further oxidation. No cobalt from the substrate was detected within the ZrN layer, proving the multilayer's excellent barrier function against ion release from the base metal. STATEMENT OF SIGNIFICANCE: In our aging society, the use of artificial knee implants is widespread. Implant failure is not only costly, but often connected with pain for the patient and inevitably involves the implantation of a new joint. Hence, understanding the origin of joint failure is of high importance to extend the lifetime of the implant. In this research, we investigate the influence of a surface oxide, formed on an explant which failed after ∼ 2 years, on the multilayer stability. As a novelty, we focus not on the wear of the polyethylene gliding surface but on the formed oxide. The use of high-resolution analysis techniques allowed us to get a glimpse on the ongoing mechanisms at the explants surface.