Elastohydrodynamic lubrication analysis of hip implants with ultra high molecular weight polyethylene cups under transient conditions

The transient variation of both the load and speed experienced during walking in an elastohydrodynamic lubrication (EHL) analysis for artificial hip joints employing an ultra high molecular weight polyethylene (UHMWPE) acetabular cup against either a metallic or ceramic femoral head was considered in this study. A general numerical procedure to solve the transient EHL problem in spherical ball-in-socket coordinates, developed in a previous study by Jalali-Vahid and Jin in 2002, was applied under three specific conditions experienced during typical gait cycles, including speed reversal, a sudden load increase and a sudden load decrease. The predicted minimum film thickness was found to stay remarkably constant and similar to that prior to the change in either the load or the angular velocity, despite a large change in these operating conditions. This was attributed to the remarkably effective squeeze-film action of preserving and maintaining the lubricating film developed before the transient variation in either the load or speed. It is therefore possible to neglect the effect of these specific transient variations of load and speed under physiological walking conditions considered in the present study on the predicted film thickness in hip implants with UHMWPE cups.