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

:已经在新一代用于人工髋关节的“缓冲形式”轴承中分析了润滑机理和接触机理,该轴承在关节表面上包括低弹性模量层。与现有假肢中使用的“硬”轴承以及自然髋关节进行了比较。较大的接触面积和较低的接触压力会增加润滑膜的厚度。对于固定的接触面积,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。设计为与缓冲轴承接触面积相同的硬轴承可产生相似的薄膜厚度,但目前的设计并未优化润滑剂薄膜的厚度。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。仔细选择弹性模量很重要,以确保这种润滑方式有效。低弹性模量层也可能产生局部变形,从而增强挤压膜的作用。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需的弹性模量,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2 mm厚的弹性模量为20 MPa的股骨头直径为32 mm,接触区域半径为16 mm的坐垫髋关节假体,预测的润滑膜厚度为0.3微米。

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