Abstract
Introduction: There is increasing interest in the coupling of highly cross-linked polyethylene with large diameter heads in the hip. The aim of this study was to determine the wear of large (size 36 mm) highly cross-linked polyethylene inserts against ceramic and cobalt chrome femoral heads using a physiological hip simulator.
Methods: Size 36 mm Biolox® Forte alumina and cobalt chrome femoral heads were coupled with highly cross-linked polyethylene inserts in the ten station Leeds ProSim Physiological Anatomical Hip Joint Simulator. The simulator was run for 10 million cycles and the change in volume of the polyethylene inserts was determined geometrically.
Results: The volume change of the ceramic/cross-linked polyethylene bearing combinations during the first two million cycles of the hip simulator test was twice that of the cobalt chrome/cross-linked polyethylene bearing combinations due to increased creep. After 2 million cycles a steady state wear rate was reached. In contrast the cobalt chrome/cross-linked polyethylene bearing combinations reached their steady state at 1 million cycles.
The steady state wear rate for the ceramic/cross-linked polyethylene bearing combinations was 4.7 mm3/million cycles. This was a significant 40% reduction compared to the wear rate of the cobalt chrome/cross-linked polyethylene bearing combinations at 8.1 mm3/million cycles (p< 0.01).
Discussion: The clinical implications of this study relate to the measurement of in vivo wear, which is routinely assessed using penetration measured from radiographs. However, penetration is a measure of both wear and creep. This means that although the penetration of polyethylene inserts coupled with metal and ceramic femoral heads may be similar, the actual wear is likely to be lower with the ceramic heads due to their elevated creep
Correspondence should be addressed to Mr John Hodgkinson, BHS, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
