The wear behaviour of total knee arthroplasty (TKA) is dominated by two wear mechanisms: the abrasive wear and the delamination of the gliding components, where the second is strongly linked to aging processes and stress concentration in the material. The addition of vitamin E to the bulk material is a potential way to reduce the aging processes. This study evaluates the wear behaviour and delamination susceptibility of the gliding components of a vitamin E blended, ultra-high molecular weight polyethylene (UHMWPE) cruciate retaining (CR) total knee arthroplasty. Daily activities such as level walking, ascending and descending stairs, bending of the knee, and sitting and rising from a chair were simulated with a data set received from an instrumented knee prosthesis. After 5 million test cycles no structural failure of the gliding components was observed. The wear rate was with5.62±0.53 mg/million cycles falling within the limit of previous reports for established wear test methods.

Introduction. Unicompartmental arthroplasty of the knee in patients with isolated medial osteoarthritis gives good results, but survival is inferior to that of total knee prosthesis. Knees may fail because positioning of the prosthesis has been suboptimal. The aim of this study was to investigate the influence of the tibial slope on the rate of wear of a medial fixed-bearing unicompartmental knee arthroplasty.Materials and Methods. We simulated wear on a medial fixed-bearing unicompartmental knee prosthesis (Univation) in vitro with a customised, four-station, and servohydraulic knee wear simulator, which exactly reproduced the walking cycle (International Organisation for Standardisation (ISO) 14243-1:2002(E)). The medial prostheses were inserted with 3 different posterior tibial slopes: 0°, 4°, and 8° (n= 3 in each group).Results. The wear rate decreased significantly between 0° and 4° slope from 10.4 (SD 0.62) mg/million cycles to 3.22 (SD 1.71) mg/million cycles. Increasing the tibial slope to 8° did not significantly change the wear rate.Discussion. As an increase in the tibial slope reduced the wear rate in a fixed-bearing prosthesis, a higher tibial slope should be recommended. However, other factors that are influenced by the tibial slope (e.g., the tension of the ligament) must also be considered.

Carbon fiber reinforced poly-ether-ether-ketone (CFR-PEEK) represents a promising alternative material for bushings in total knee replacements, after early clinical failures of polyethylene in this application. The objective of the present study was to evaluate the damage modes and the extent of damage observed on CFR-PEEK hinge mechanism articulation components after in vivo service in a rotating hinge knee (RHK) system and to compare the results with corresponding components subjected to in vitro wear tests. Key question was if there were any similarities or differences between in vivo and in vitro damage characteristics. Twelve retrieved RHK systems after an average of 34.9 months in vivo underwent wear damage analysis with focus on the four integrated CFR-PEEK components and distinction between different damage modes and classification with a scoring system. The analysis included visual examination, scanning electron microscopy, and energy dispersive X-ray spectroscopy, as well as surface roughness and profile measurements. The main wear damage modes were comparable between retrieved and in vitro specimens (n=3), whereby the size of affected area on the retrieved components showed a higher variation. Overall, the retrieved specimens seemed to be slightly heavier damaged which was probably attributable to the more complex loading and kinematic conditions in vivo.

Experimental wear simulation of a PEEK-OPTIMA™ polymer-on-UHMWPE total knee replacement has shown equivalent UHMWPE wear to conventional knee replacement materials (cobalt chrome-on-UHMWPE) when tested in a clean environment. The aim of this study was to experimentally investigate the wear of this all-polymer total knee replacement under third body wear conditions. Three PEEK-OPTIMA™ and three cobalt chrome femoral components articulating against all-polyethylene tibial components were tested in a knee simulator. One million cycles of wear simulation was carried out in clean lubricant under conditions replicating walking followed by one million cycles with the lubricant contaminated with porcine bone particles, then one million cycles with PMMA cement particles. UHMWPE wear was determined gravimetrically. Statistical analysis compared UHMWPE wear against PEEK femoral components to cobalt chrome. In clean lubricant and with bone particles in the lubricant, there was no significant difference (p > 0.05) in UHMWPE wear against the different femoral component materials, and wear rates were similar under both conditions. With PMMA particles, there was a dramatic increase in UHMWPE wear for both knee replacement systems but no significant difference (p > 0.05) in UHMWPE wear between the femoral component materials. This is the first study to investigate wear of an all-polymer knee under third body wear conditions.

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 scanning (transmission) electron microscopy (STEM) 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.