Optimum Design of Knee Prostheses with High Performance Similar to Natural Knee Joints

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The object of this research is to develop the approaches for the optimum design for knee prostheses with high performance similar to natural knee joints. What we are aiming principally at is to establish fluid film formation in knee prostheses.The following methods were carried out.1. Three-dimensional analysis for the contact conditions in knee jointsFirsty, the three-dimensional geometric analysis based on the CAD theory was applied to natural knee joints, since our trial knee prosthesis model has similar geometry to the natural joint. The contact points and load-carrying characteristics in the flexed and extended conditions were obtained by using this mathematical expression for curved articular surfaces.Secondary, three-dimensional photoelastic freezing method was applied to investigate the contact stress distribution in the extended position. It was shown that the articular cartilage and meniscus remarkably reduced the contact stresses in the subchondral bones.2. Knee joint simulator testTo evaluate the fluid film formation in knee prostheses under walking condition, the knee joint simulator was developed and used for the measurement of electric contact resistance and friction between femoral and tibial components. In a knee prosthesis with an elastomeric tibial component, elastohydrodynamic film was considerably formed even during the loading stance phase, whereas in a prosthesis with a polyethylene tibial one, slight fluid film formation was observed only immediately after the heel strike except during the swing phase. In the former, the film thickness decreases during the stance phase and generally becomes minimum at the peak load just before the toe off, and recovers to the previous level during the swing phase. It is suggested from the experimental results that the knee prosthesis with complient artificial cartilage is capable of improving the fluid film formation with adequate lubricants.