Oxidation of a ZrN knee implant coating under in-vitro wear simulation

Journal article, 2025

Before knee implants are used in clinical applications, in-vitro wear simulation is essential for predicting their reliability and longevity. This study investigated the representativeness of in-vitro wear simulation concerning oxide formation on a ZrN multilayer-coated knee implant. Four different locations on one in-vitro tested sample were analyzed using scanning (transmission) electron microscopy (SEM/TEM). The sample was subjected to 5 million gait cycles according to ISO 14242–1:2002 in newborn calf serum at body temperature, equivalent to ∼ 3–5 years in-vivo. The formed surface oxides were compared with a previously investigated as-fabricated femoral component and an ‘in-vivo’ implant explanted after approximately two years due to early aseptic loosening. The results showed the presence of an oxide on all investigated surfaces with varying thicknesses (15–280 nm). Hence, all oxides were thicker than what would be expected from native oxidation of ZrN. Oxide composition, thickness and microstructure were comparable between in-vitro and in-vivo samples, except one outlier on the in-vitro sample (280 nm thick oxide). However, the distribution of the colored areas was not comparable: the in-vivo sample exhibited a non-articulating area completely purplish colored (art. area golden), whereas colored areas on the in-vitro sample were distributed inhomogenously. Differences in loading conditions, presence of embedded particles, exposure duration and fluids could explain these variations.