Lubrication analysis of metal-on-metal hip joint prostheses in a free pendulum machine

Total joint arthroplasty is probably the most effective treatment for severe degenerative bone diseases such as osteo-arthritis. There has been renewed interest in the use of hard bearings, particularly the metal-on-metal combination for hip joint replacements due to the osteolysis problem associated with current metal-on-polyethylene prostheses. The potential for full fluid film lubrication has been shown to exist during part of the walking cycle in these hard bearing joints. In this study, theoretical and numerical analyses were carried out to characterise the loading and velocity and the corresponding lubrication regime in a typical metal-on-metal hip joint replacement tested in a free pendulum machine. It was shown that the loading in the joint remains relatively constant throughout the duration of swing. The maximum angular velocity achieved under frictionless conditions with an initial angular displacement of 5o, was found to be about 0.387 rad/s, corresponding to a sliding velocity of 5.412 mm/s for a femoral head radius of 14mm. The lubricating film thickness due to the entraining action is only 0.011 micro m, which implies at best a mixed lubrication regime is experienced for the metal-on-metal hip joint tested in a pendulum machine with an average surface roughness of about 0.01 micro m. Under squeeze-film motion, the lubricating film thickness decreases to only about 0.01 micro m after the first few swings, assuming an initial separation between the head and the cup by the radial clearance. The theoretical predictions are consistent with experimental observations reported in literature. The lubrication regime under more realistic walking conditions cannot be deduced in the pendulum machine, largely due to the inappropriate loading and velocity conditions experienced in the pendulum.