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

本研究的目的是探索高性能、接近自然膝关节的膝关节假体的优化设计方法。我们的主要目标是在膝关节假体中建立液体膜的形成。首先，将基于CAD理论的三维几何分析应用于自然膝关节，因为我们的假体模型具有与自然关节相似的几何形状。利用该数学表达式，得到了弯曲和拉伸状态下关节表面的接触点和承载特性。其次，采用三维光弹性冻结法研究了关节曲面在拉伸状态下的接触应力分布。结果：1.关节软骨和半月板明显降低了软骨下骨的接触应力。膝关节模拟器为了评价行走状态下膝关节假体内液体膜的形成情况，研制了膝关节模拟器，用于测量股骨和胫骨假体之间的接触电阻和摩擦力。在具有弹性胫骨假体的膝关节假体中，即使在加载站立阶段也会形成相当大的弹流体膜，而在具有聚乙烯胫骨假体的假体中，只有在脚跟接触后立即才能观察到轻微的液体膜形成，但在摆动阶段除外。在前者中，薄膜厚度在站立阶段下降，通常在脚趾离开前的峰值载荷时变得最小，并在摆动阶段恢复到以前的水平。实验结果表明，使用柔顺人工软骨的膝关节假体，在有足够的润滑剂的情况下，能够改善液体膜的形成。

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.