Lubrication mechanisms and contact mechanics have been analysed in a new generation of 'cushion form' bearings for artificial hip joints, which comprise low elastic modulus layers on the articulating surfaces. Comparisons have been made with 'hard' bearings used in existing prostheses and also with the natural hip joint. Lubricating film thicknesses are enhanced by larger contact areas and lower contact pressures. For a fixed contact area, simultaneous changes in layer thickness and radial clearance have been shown to have a small effect on elastohydrodynamic film thickness. Hard bearings designed with the same contact area as the cushion bearings produced a similar film thickness, but lubricant film thickness is not optimized in current designs. The main advantage of using a cushion bearing with low elastic modulus layers was found to be associated with microelastohydrodynamic lubrication. Careful selection of the elastic modulus is important in order to ensure that this lubrication regime was effective. Low elastic modulus layers may also produce local deformations, which enhance squeeze film action. The elastic modulus of the material should not be lower than necessary to produce effective microelastohydrodynamic lubrication, as a further reduction in modulus only increases the strain distribution in the material. A lubricant film thickness of 0.3 microns has been predicted for a cushion hip prosthesis with a femoral head diameter of 32 mm and radius of contact zone of 16 mm, using a 2 mm thick layer with an elastic modulus of 20 MPa.

译文

润滑机制和接触力学已在新一代的 “缓冲形式” 轴承中进行了分析,该轴承用于人造髋关节,在铰接表面上包括低弹性模量层。已与现有假体中使用的 “硬” 轴承以及自然髋关节进行了比较。通过较大的接触面积和较低的接触压力来增强润滑膜的厚度。对于固定接触面积,层厚度和径向间隙的同时变化已显示出对弹性流体动力膜厚度的影响很小。与缓冲轴承具有相同接触面积的硬轴承产生了相似的膜厚度,但是在当前设计中润滑剂膜厚度并未得到优化。发现使用具有低弹性模量层的缓冲轴承的主要优点与微弹性流体动力润滑有关。为了确保这种润滑方式有效,仔细选择弹性模量很重要。低弹性模量层还可能产生局部变形,从而增强了挤压膜的作用。材料的弹性模量不应低于产生有效的微弹性流体动力润滑所必需的,因为模量的进一步降低只会增加材料中的应变分布。对于具有32毫米的股骨头直径和16毫米的接触区半径的缓冲髋关节假体,使用弹性模量为20 mpa的2毫米厚层,已经预测了0.3微米的润滑剂膜厚度。

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