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

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.