Abstract
Ceramic on ceramic hip-joint replacements (THR) are known for their excellent wear resistance. Such rigid-rigid bearings generally exhibit a biphasic wear-performance, i.e. a rapid run-in phase decreasing into a steady-state phase. However, due to the ultra-low wear of ceramics, few studies have adequately characterized these wear phases. Since this behavior was not well defined for modern alumina-on-alumina hips, we studied this phenomenon using hip simulator techniques. We also compared all-ceramic THR to UHMWPE wear-rates for exact comparison. Run-in wear was measured at 200,000 cycle intervals to 1 million cycles (1 Mc) followed by 500,000 cycle intervals to 14 Mc.
Alumina heads started off with high wear but then demonstrated a curvilinear run-in phase that smoothly transited into steady-state wear. The alumina liners had linear run-in to 0.6 Mc and then abruptly transited into steady-state by 0.8 Mc. During run-in, the liner wear was 40% greater than for the mating heads. Steady-state liner wear varied from 0.002 to 0.007 mm3/Mc. It was also clear that at least 10 million cycles were required to define the steady-state wear for alumina implants due to their ultra-low wear magnitudes. Combined head and cup run-in wear averaged 0.33 mm3/Mc and was completed within 0.8 Mc while steady-state wear was < 0.01 mm3/Mc up to 20 Mc. This was a remarkable 30-fold reduction from run-in.
The run-in phase would probably be completed by the first year of follow-up. Compared to UHMWPE cups, the alumina implants demonstrated a 9,000-fold wear-reduction over 20 million cycles in the simulator. This may correspond to 20 years in the typical patient. In addition, the alumina/UHMWPE combination has been favored historically because using CoCr/ UHMWPE bearings resulted in a doubling of the wear-rates in comparative clinical studies. Clearly the all-ceramic THR offers a much superior alternative with its massive reduction in wear-debris volume.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
