In-vitro wear simulation is a crucial step in testing artificial implants before their clinical application [1,2]. It allows for the prediction of their longevity and reliability, as well as the testing of new materials or geometries. Here, we investigate how representative in-vitro simulation is for a multilayer-coated knee implant, specifically regarding the oxide formation on the topmost ZrN layer. We examined four different locations on one in-vitro tested sample using scanning and transmission electron microscopy (SEM/TEM): unoxidized and oxidized locations within both the articulating and non-articulating areas (Figure 1). The surface oxides were compared with those on a recently investigated in-vivo explant, which was explanted after ~ 2 years due to early aseptic loosening [3]. As reference sample, an as-fabricated implant was used. The results revealed oxide formation on all investigated surfaces (Figure 1, panels 1-4) with thickness variations ranging from 15 to 250 nm. The oxide thickness, microstructure and composition were comparable between in-vitro and in-vivo samples. The coloured areas distribution was not comparable, which could be attributed to differences in exposure to the corrosive media, loading conditions and fluid composition.

Figure 1. Photograph: in-vitro sample; some visible features are caused by the reflections during imaging on the polished surface. For instance, black arrows mark the imaging light’s reflections. Green arrows indicate the locations where the TEM-foils were taken from. The two distinguished areas are the articulating area (1+2) and non-articulating area (3+4). Within these areas a golden region and a coloured (purplish/ yellowish/reddish) area was chosen. Panels 1-4 represent scanning transmission electron microscopy – high angle annular dark field (STEM-HAADF) images of the oxides for the locations marked in the photograph. The surfaces were protected by platinum which was applied during the SEM/FIB sample preparation.

In the long run, understanding the underlying mechanisms of oxide formation on ZrN under both in-vivo and in-vitro conditions will enable manufacturers to enhance their selection and design guidelines for materials and their microstructures.

References

[1] ISO 21536:2023 Non-active surgical implants — Joint replacement implants — Specific requirements for

      knee-joint replacement implants, 2023.

[2] C.K. Cheng, X.H. Wang, Y.C. Luan, N.Z. Zhang, B.L. Liu, X.Y. Ma, M.D. Nie, Med Eng Phys, 2019, 72, 49-54.

[3] J.S. Rau, G. Eriksson, P. Malmberg, A.L. Puente Reyna, J. Schwiesau, M. Andersson, M. Thuvander, Acta Biomaterialia, 2024, Journal Pre-proof, available online 23 October 2024, 1-12.