Stewart T, Jin Z M, Fisher J
School of Mechanical Engineering, University of Leeds.
Proc Inst Mech Eng H. 1998;212(1):1-10. doi: 10.1243/0954411981533773.
Recent research in the area of cushion form bearings for total joint replacements has primarily used thin, soft, elastomeric layers with similar elastic modulus to articular cartilage, bonded to rigid substrates. These are designed to promote the body's natural lubricants to separate the articulating surfaces and prevent wear. Applications to joint replacements have revealed that the abrupt change in stiffness between the soft layer and the rigid substrate and the relatively low strength of the interface resulted in high shear stresses and debonding of the soft layer from the substrate. The approach adopted in this study is to use components with a graded modulus or composite construction. The composite construction consists of a soft compliant layer of polyurethane and a second stiffer polyurethane layer thought to be rigid enough to mechanically interlock to a metallic tibial tray. In this composite structure the deformation of the more rigid polyurethane underlay may generally influence the stress distribution and deformation in the softer upper layer and at the interface between the two materials. A simple analysis technique is presented in the present study where the composite double layer was approximated as an equivalent modulus single layer. Single layer theory, which is readily available in the literature, can then be used to determine the contact parameters, including the maximum contact pressure and the contact radius, for the composite structure. By varying the layer thicknesses and material properties of both the soft surface layer and the stiffer structural support layer, the magnitudes and locations of stresses can be controlled. Results of a parametric stress analysis are presented to assist in the selection of the most appropriate composite layers for cushion knee designs. A 4 mm thick surface layer with an elastic modulus of 20 MPa and a 4 mm thick structural support layer with an elastic modulus of 1000 MPa were considered suitable for this application.
近期在全关节置换用缓冲型轴承领域的研究主要采用了薄的、柔软的、弹性模量与关节软骨相似的弹性体层，并将其粘结到刚性基底上。这些设计旨在促进人体天然润滑剂分离关节表面并防止磨损。关节置换应用表明，软层与刚性基底之间刚度的突然变化以及界面相对较低的强度导致了高剪切应力以及软层与基底的脱粘。本研究采用的方法是使用具有渐变模量或复合结构的部件。复合结构由一层柔软的聚氨酯柔顺层和第二层较硬的聚氨酯层组成，后者被认为足够坚硬，能够与金属胫骨托实现机械互锁。在这种复合结构中，较硬的聚氨酯底层的变形通常会影响较软上层以及两种材料界面处的应力分布和变形。本研究提出了一种简单的分析技术，即将复合双层近似为等效模量单层。然后可以使用文献中现成的单层理论来确定复合结构的接触参数，包括最大接触压力和接触半径。通过改变软表面层和较硬结构支撑层的层厚及材料特性，可以控制应力的大小和位置。给出了参数化应力分析结果，以协助选择最适合缓冲膝关节设计的复合层。认为厚度为4毫米、弹性模量为20兆帕的表面层以及厚度为4毫米、弹性模量为1000兆帕的结构支撑层适合此应用。
