## 4 Discussion  
Using the rolling-slide test bench described here, different knee implant materials with simple geometries can be tested under the physiological loading conditions of the knee joint. The applied force was adjusted in the test bench so that different tribological pairings could be tested at the same physiological contact pressure. Nevertheless, some limitations must be considered when using the test bench. The simplified geometries offer the advantage that new materials can initially be screened with an affordable cost and little effort. For further investigations, long-term tests in a wear simulator are required, which take into account realistic implant geometries. A further limitation is related to the simplification of the geometries, since there is a lack of anterior-posterior translation and rotational movements. Hence, no transverse motions can be generated, which are considered important for PE wear propagation [33; 34]. However, because flexion and extension are the main directions of movement in the knee [3], this is only a minor limitation. In order to ensure consistent loading on the tested samples, both test chambers with samples have to be perfectly aligned under the test wheels of each station. Otherwise, station-related differences in the wear patterns may appear. Since congruent worn surfaces of the polymer samples could be observed in both stations after loading, an adequate alignment of both test chambers can be assumed. Likewise, during articulation, approximately one-third of the testing wheel surface was in contact with the running surface of the polymer samples in the two stations. To prevent the results from being influenced by friction-related sample heating, various tribological situations (loading in a dry test chamber, loading in medium, static and dynamic loading, and loading at room temperature and with connected heating) were investigated in preliminary tests using a thermal imaging camera and temperature sensors. A maximum warming of 4 \({}^{\circ}\)C was detected only in the dry running test. Since no temperature differences of the PU sample were detected under lubricated test conditions with medium, the influence of friction on the temperature was considered to have a negligible effect. Temperature-dependent changes in sample volume during wear testing could be excluded. Furthermore, the rolling-sliding test bench allows only minor changes in the volume of samples. Force curves according to ISO 14243-1 were adapted to the tested bearing material to simulate the same surface pressure that is present in the natural knee joint. In addition, load curves other than those from the ISO 14243-1 can be created, and, if necessary, they can be adapted to specific material and load cases. Higher loading or other gait patterns, such as stair climbs, can be implemented in the rolling-sliding test bench based on data derived from biomechanical studies [35]. Since native cartilage is a material with nonlinear behavior, the use of the Hertzian contact model as a contact model for the transmission of  
Figure 8: Surface roughness of the running surfaces of the tested samples before (left bars) and after (right bars) the wear test. the physiologically acting contact pressure represents a limitation of our test bench. Since samples with linear material behavior are tested in this study and a complete simulation of the conditions in the native knee joint is not possible, we assume this simplification is suitable for the present study. [PERSON] et al. [36] has also used a Hertzian contact model to determine physiological surface pressure. An even more accurate determination of the surface contact pressure could be achieved using multibody models [37; 38]. Hence, nonlinear material models for cartilage or acting muscle forces may be considered in future studies. Since the developed soft PU materials showed high abrasive wear during preliminary tests, the PU samples were only tested over 1 million cycles instead of 5 million cycles, as recommended in ISO 14243-1.