A comparison of friction in 28 mm conventional and 55 mm resurfacing metal-on-metal hip replacements

Total hip replacement has been a common, successful surgical intervention for many years. However, it has long been considered unsatisfactory for younger, more active patients due to the limited survivorship of conventional implants employing polyethylene. Larger resurfacing implants were developed to preserve bone stock and improve stability, however, early metal-on-polyethylene implants failed due to high wear. More recent developments, utilizing a metal-on-metal bearing, have encouraging short- to medium-term clinical performance. Concerns exist regarding the increased sliding distance and frictional torque generated within a larger diameter bearing. A large diameter metal-on-metal surface replacement was contrasted with a conventional 28 mm diameter implant with the same bearing material combination using a pendulum friction simulator. Studies were performed under different swing-phase load and lubrication conditions. The larger diameter bearing exhibited the lower friction factor under all test conditions, although the measured frictional torque was higher. Increasing swing-phase load was shown to cause an increase in friction factor in all tests. Increased serum concentration resulted in a reduction in friction factor for both bearings. Variation of the friction factor with the head diameter suggested that fluid-film lubrication contributed to the reduction of friction.