Effect of walking patterns on the elastohydrodynamic lubrication of metal-on-metal total hip replacements

The profiles of forces and kinematics can significantly affect the results of both hip simulators and computer simulations of hip implants, in terms of lubrication and wear. Most of the related studies have employed the simplified load and motion profiles to simulate walking. The purpose of this study is to investigate the lubrication of a typical metal-on-metal (MOM) total hip implant, under different loading and motions of walking; which included three patterns for hip simulators (Leeds Mk I, Leeds ProSim, and the ISO standard) and a combined walking-stopping-start-up-walking pattern. A general ball-in-socket lubrication model was solved and full numerical solutions of the elastohydrodynamic lubrication were presented. The multi-grid method was adopted to solve the Reynolds equation. A combined multi-level multi-integration and fast Fourier transform technique was used to obtain the elastic deformation of bearing surfaces. Complex three-dimensional loading and motion were found to result in large variations of the film thickness during one walking cycle, particularly for the physiological case considered in the Leeds Mk I simulator. Under such a physiological pattern, the squeeze film effect was generally greater than the other two hip simulator patterns, particularly during the swing phase, resulting in larger film thickness. The squeeze film effect was encouraged by the translation of the lubricating contact area in the entrainment direction. Small variations in the film thickness and friction torque were observed between the Leeds ProSim and ISO patterns. Intermittent breaks during steady walking were unlikely to cause complete depletion of lubrication in the MOM hip implants. At 5 s after stopping, the lubrication regime was shifted from full film lubrication to mixed lubrication. A lubricant pocket was developed quickly and the film profile finally achieved an equilibrium condition.