Possibilities and limitations of novel in-vitro knee simulator.

The ex-vivo evaluation of knee kinematics remains vital to understand the impact of surgical treatments such as total knee arthroplasty (TKA). To that extent, knee simulators have been developed. However, these simulators have mainly focused on the simulation of a squatting motion. The relevance of this motion pattern for patients' activities of daily living is however questionable as squatting is difficult for elderly patients. Walking, stairs and cycling are more relevant motion patterns. This paper presents the design and control of a simulator that allows to independently control the applied kinematic and kinetic boundary conditions to simulate these daily life activities. Thereby, the knee is left with five degrees of freedom; only the knee flexion is actively controlled. From a kinetic point of view, the quadriceps and hamstring muscles are loaded. Optionally, a varus/valgus moment can be applied, facilitating a dynamic evaluation of the knee's stability. The simulator is based on three control loops, whose synchronization appears satisfactory. The input for these control loops can be determined from either musculoskeletal simulations or in accordance to literature data for traditional knee simulators. This opens the door towards an improved understanding of the knee biomechanics and comparison between different applied motion and force patterns.