A Robotic Test Setup for Biotribological Investigations on Complex Shaped Cartilage Surfaces

Investigating the frictional properties of the articular cartilage (AC) contributes to the understanding of the knee joint's overall functionality [1]. However, a key challenge in tribological studies remains addressing the complex geometry of the tibial and femoral AC surfaces. Therefore, the aim of this study was to develop a robotic based tribological setup that allows for friction measurements on curved AC surfaces, without disrupting its structural integrity, under orthogonal contact conditions and controlled normal force application. A six-axis industrial robot was programmed to move a pin along trajectories orthogonal to a surface. First, for system validation the friction coefficients (µ=friction force/normal force) of polyoxymethylene (POM) pins on three different polyamide (PA) shapes were determined (Figure 1A). Second, the friction coefficients on six medial porcine tibial surfaces were assessed (Figure 1B). On each plateau, three trajectories were tested in intact conditions and after inducing an anteromedial defect with sandpaper on two trajectories. Phosphate-buffered saline was used as lubricant. In all tests, a normal force of 5 N and a sliding velocity of 1 mm/s was applied. Parametric statistical tests were used for data comparison with p<0.05 considered statistically significant. Friedman testing revealed no significant differences in the friction coefficients µ across the PA shapes (Figure 2A). Introducing an anteromedial AC defect significantly increased friction on the affected trajectories (µ +30%, t-testing: p < 0.05) The visualization of local friction coefficients along the trajectory demonstrated that the overall higher friction was attributable to higher µ values in the defect region The robotic tribometer was successfully validated, as the friction coefficients of the PA shapes showed no significant differences despite varying shape complexities. The robotic tribometer effectively measured friction on curved porcine tibial AC surfaces while maintaining the tissue integrity. Moreover, it was possible to detect localized friction changes caused by local structural damage. This methodological opportunity is particularly relevant for OA research, as localized AC damage is a defining characteristic of the disease [2]. Overall, the robotic tribometer offers a novel method for investigating the AC's frictional behaviour, which has the potential to enhance our understanding of the tissue's tribology in healthy and diseased conditions. For any figures or tables, please contact the authors directly.