Wear of
polyethylene and the resulting wear debris-induced
osteolysis remains a major cause of long-term failure in artificial hip joints. There is interest in understanding engineering and clinical conditions that influence wear rates. Fluoroscopic studies have shown separation of the head and the cup during the swing phase of walking due to
joint laxity. In ceramic-on-ceramic hips,
joint laxity and microseparation, which leads to contact of the head on the superior rim of the cup, has led to localized damage and increased wear in vivo and in vitro. The aim of this study was to investigate the influence of
joint laxity and microseparation on the wear of ceramic on
polyethylene artificial hip joints in an in vitro simulator. Microseparation during the swing phase of the walking cycle produced contact of the ceramic head on the rim of the
polyethylene acetabular cup that deformed the softer
polyethylene cup. No damage to the
alumina ceramic femoral head was found. Under standard simulator conditions the volume change of the moderately crosslinked
polyethylene cups was 25.6 +/- 5.3 mm3/million cycles and this reduced to 5.6 +/- 4.2 mm3/million cycles under microseparation conditions. Testing under microseparation conditions caused the rim of the
polyethylene cup to deform locally, possibly due to creep, and the volume change of the
polyethylene cup when the head relocated was substantially reduced, possibly due to improved lubrication.
Joint laxity may be caused by poor soft tissue tension or migration and subsidence of components. In ceramic-on-
polyethylene acetabular cups wear was decreased with a small degree of
joint laxity, while in contrast in hard-on-hard
alumina bearings, microseparation accelerated wear. These findings may have significant implications for the choice of fixation systems to be used for different types of bearing couples.