## 4 Discussion
A high amount of wear is also reflected in the increased surface roughness of the PU samples. To exclude the influence of laser scanning on the sample material during the surface analysis, this was carried out only after wear testing of the samples was complete. Since PE is an established bearing material in total knee arthroplasty, it was used as the reference material. In contrast to PU, no significant abrasive wear could be detected when the PE samples were loaded with 15.5 N or 16.5 N. To ensure comparability between PE and PU, the same force curve was used for all materials tested. Therefore, for the algorithmic estimation of condyle radius and contact length, we examined three datasets consisting of only female, only male, or both male and female data. We found that the radius and contact lengths for females are smaller than males by a few millimeters (radius ~2.5 mm, contact length ~4 mm). Hence, we considered only female data for the final evaluation because a lower radius and contact lengths, together with constant force, as specified by ISO 14243-1, result in a higher surface contact pressure (cf. Equation (1)). Implant materials should be designed not only for an average load situation, but also for severe conditions. Thus, by selecting appropriate datasets, the adjustment of applied forces to specific patient groups (for example, by age or sex) can be easily accomplished. ISO 14243-1 specifies that the samples should be subjected to a load of more than 5 million cycles. Furthermore, due to running-in behavior, the abrasion rate should only be determined after 0.5 million cycles. The high wear rates of PU samples in the current study suggest that this material is not suitable as a direct bearing partner for a unicompartmental interpositional knee implant. Since no comparable wear studies on PU in the knee joint using physiological rolling-sliding movements have been carried out to date, it is not possible to classify the abrasive wear rates of PU materials. Our results were not consistent with the findings of [PERSON] et al. [39], who showed lower wear rates for PU than for PE materials. However, a direct comparison of both studies is not possible due to non-comparable methods. We assume that the high wear rates of PU materials observed in the current study are related to the different properties of the material, and not the geometry of the test bench. The observed high wear rates were partly caused by pronounced backside wear of the PU materials. The wear patterns of the surface and backside of the samples correspond to the determined wear rates of the samples. On the PE samples, only polishing and smoothing of the manufactured surface was observed. The test wheels were able to sink much deeper into the PU samples during loading, which led to massive nuts in the direction of movement. Partial delamination of the PU surface was observed. However, we cannot conclude that the high backside wear and the high wear rates of the PU materials are caused only by abrasive wear and not adhesive wear. Nevertheless, PU could potentially be used in other human joints with reduced loading and friction, such as finger joints. The rolling-sliding test bench was successfully established as a tribological testing method. In order to test even more samples in parallel, it would be useful to expand this test bench with additional stations. In contrast to a knee wear simulator according to ISO 14243-1, simplified implant geometries can be used in the rolling-sliding test bench. Hence, the effort required to produce test prototypes of new implant materials for the first abrasive wear screening is reduced. Due to the simplified implant geometry and design of the test bench, reduced lubricant volumes are required when compared to knee wear simulator tests. The reduction of the amount of lubricating fluid leads to additional cost savings. The wear data of the PU materials can be classified due to the direct comparison of their wear rates with a clinically established PE material.