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The exceptional tribological properties of articular cartilage are still far from being fully understood. Articular cartilage is able to withstand high loads and provide exceptionally low friction. Although the regeneration abilities of the tissue are very limited, it can last for many decades. These biomechanical properties are realized by an interplay of different lubrication and wear protection mechanisms. The deterioration of cartilage due to aging or injury leads to the development of osteoarthritis. A current treatment strategy focuses on supplementing the intra-articular fluid with a saline solution containing hyaluronic acid. In the work presented here, we investigated how changing the lubricating fluid affects friction and wear of articular cartilage, focusing on the boundary and mixed lubrication as well as interstitial fluid pressurization mechanisms. Different length and time scales were probed by atomic force microscopy, tribology and profilometry. We compared aqueous solutions with different NaCl concentrations to a viscosupplement containing hyaluronic acid (HA). In particular, we found that the presence of ions changes the frictional behavior and the wear resistance. In contrast, hyaluronic acid showed no significant impact on the friction coefficient, but considerably reduced wear. This study confirms the previous notion that friction and wear are not necessarily correlated in articular cartilage tribology and that the main role of HA might be to provide wear protection for the articular surface., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

This paper presents a novel application of direct force control to test biological specimens using a serial manipulator with 6 degrees of freedom Direct force control compares actual force/moment values with desired values of load. The error is compensated by a proportional/integral controller (PI), a damping factor implemented with the velocity of the robot and acting in the direction of the force and a feedforward compensation. The controller works with a frequency of 0.5 kHz which enhances its performance due to the direct force feedback loop. A fresh porcine cervical spine C2-C4 was used. All muscle tissues were removed while leaving intact all ligaments and bony tissue. The specimen was loaded separately with ±3 Nm in every spatial axis. The mean errors in the unconstrained axes in the present study were less than 1.70 N and 0.32 Nm. Direct force control of 6 axes with a high controller frequency of 0.5 kHz developed in this methodology shows a successful procedure to perform biomechanical in-vitro tests. The controller demonstrated the ability to maintain zero load targets in the unconstrained axes. This control approach allows the application of pure moments in order to perform in vitro biomechanical experiments with spine segments., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

An analytical, dynamic model of the human knee joint has been developed to simulate the unloaded knee joint behaviour in 6 degrees of freedom. It is based on extensive robot-based measurements of the elastic properties of a human cadaver knee joint. The measured data are compared with data from the literature to ensure that a proper database for modelling is used. The analytical modelling of the passive elastic joint properties is done with Local Linear Model Trees. The deduced knee joint model incorporates passive elastic properties of the internal knee joint structures, passive elastic muscle forces, damping forces, gravitational forces, and external forces. There are two sets of parameters, one simulating the movement of the intact knee joint, and a second simulating the knee joint with ruptured anterior cruciate ligament. The dynamic model can be easily processed in real-time. It is implemented in the haptic display of the Munich Knee Joint Simulator (MKS), which enables a person to move a plastic leg driven by a robot manipulator and feel the simulated knee joint force. Orthopaedic physicians judged the performance of the dynamic knee joint model by executing physical knee joint tests at the MKS.