Determining the optimal mechanical requirements for early intervention devices in the knee
Interpositional arthroplasty is a treatment option for isolated medial compartment osteoarthritis of the knee. No bone resection and no mechanical fixation are the main advantages of this procedure. However post-operative problems, such as implant dislocation, swelling and severe knee pain have been generally observed. Since these problems are related to the motions and loads occurring at the knee and probably to design factors of the implant, there is a need to investigate the kinematics and contact mechanics of the knee implanted with an interpositional spacer and its differences with respect to the normal knee. Although the knee joint is routinely involved in dynamic activities, to date, no experimental or numerical studies have investigated the kinematics and contact mechanics of the normal knee joint during dynamic loading conditions, most of the experimental and finite element studies are static or quasi-static in nature. Hence the purpose of this study was to investigate: 1) the behaviour of the normal knee joint under dynamic loading conditions, 2) the performance of a knee implanted with an interpositional spacer and 3) the effect of several implant design parameters on the behaviour of the implanted knee. The kinematics and contact pressures of the normal knee joint during the activities of walking, stair ascent and squatting were obtained using dynamic finite element analysis. Higher contact pressures were predicted when applying more demanding loads, during the simulation of the squatting and stair ascent activities. Similar ranges of motion were predicted for the three activities simulated, despite the difference in the magnitude of the loads applied, due to the restraining function of the ligaments and the geometry of the surfaces into contact. In the second study, the kinematics and contact pressures of a knee implanted with a polyurethane interpositional spacer were obtained and compared to the normal knee. In general, for the three activities simulated, the implanted knee was able to follow the kinematics of the normal knee, however higher contact pressures were predicted in the medial side of the tibial and femoral articular cartilages, which could increase the propensity for articular cartilage degeneration. In the third study, the influence of implant material, size, thickness and radii of the bearing surface in anterior/posterior and medial/lateral directions was assessed. A hard material compared to the menisci, such as a cobalt-chromium alloy, caused higher contact pressures in both the medial and lateral side of the knee. Implant size did not affect the kinematics or contact pressures in the knee, however anterior dislocation of the implant was observed for a large spacer during the squatting activity. Thickness and radii of the bearing surface did not show any significant influence on the kinematics or contact pressures.