Aseptic loosening due to osteolysis in total hip replacement has been related to wear debris released from prosthetic components. Retrospective longterm observations of patients with the metal-on-metal prosthesis has shown long-term survivorship and good mechanical performance. Thus, the new and modified metal-on-metal prosthesis has been introduced on the market. Historical clinical data from the 1st generation metal-on-metal hip prosthesis may not be relevant for the 2nd generation of metal-on-metal hip prosthesis. Therefore, preclinical testing of the prosthesis must be conducted before clinical evaluation. We assessed the tribological performance of the metal-on-metal prosthesis versus the metal-on-polyethylene prosthesis introduced on the market as Metasul and Protasul, respectively. In a 12-channel joint simulator, 6 metal-on-metal bearing and 3 metal on polyethylene prostheses were tested, with the same number of corresponding soak controls. The wear was assessed gravimetrically. The "steady-state" wear-rates from the metal-on-metal prosthesis were almost 100 times less than that from the metal-on-polyethylene prosthesis. The tribological wear performance of the metal-on-metal hip prosthetic system is promising.
Alumina-on-alumina hip implants with microseparation were run in a hip simulator for comparison of the nonseparation simulator mode and retrievals. The 28-, 32-, and 36-mm Biolox-forte implants were run to 5 million cycles with the use of 50% newborn calf serum. Howmedica Osteonics Trident cups with titanium backing were used in all sets. In standard (STD) and microseparation (MSX) mode, the typical biphasic wear trend was evident, but the MSX test mode had much higher magnitudes. There was a 5-fold increase for run-in wear and up to a 35-fold increase in steady-state wear. The stripe wear on the ball formed early, but did not progress in grade beyond 0.6 Mc. The locations of the stripes were similar in retrieved and simulator balls. However, the stripes from the simulator were narrower than short-term retrievals and much narrower than some long-term retrievals. The long-term retrieved balls had a grade of wear greater than the simulators. In vivo a broader range of motion occurs and this may lead to the wider stripe observed on the retrievals. These observations suggested that simulators could produce the loading and kinematics similar to a patient walking but not necessarily the variety of motions possible in the in vivo situation.
Wear rates of polytetrafluoroethylene (PTFE) and polyethylene cups were compared in 9-channel and 12-channel simulators, using serum lubrication and gravimetric techniques for wear assessment. Cobalt-chromium (CoCr) and alumina ceramic femoral heads in 22-42 mm diameter range were used to validate simulator wear rates against clinical data. This was also the first study of three femoral head sizes evaluated concurrently in a simulator (with three replicate specimens) and also the first report in which any wear experiments were repeated. Fluid absorption artefacts were within +/-1 per cent of wear magnitude for PTFE and +/-8 per cent for polyethylene and were corrected for. Wear rates were linear as a function of test duration. Precision within each set of three cups was within +/-6 per cent. The wear rates from experiments repeated over 15 months were reproducible to within +/-24 per cent. However, the magnitudes of the simulator wear rates were not clinically accurate, the PTFE wear rates (2843 mm3/10(6) cycles; 22 mm diameter) were over three times higher than in vivo, the polyethylene 30 to 50 per cent on the low side (23 mm3/10(6) cycles; 22 mm diameter). Volumetric wear rate increased with respect to size of femoral head and a linearly increasing relationship of 7 8 per cent/mm was evident with respect to femoral head diameter for both PTFE and polyethylene. These data compared well with the clinical data.
We carried out simulator studies on ceramic-polyethylene total-hip combinations to determine the volumetric wear-rates of 22 mm, 26 mm and 28 mm femoral-head sizes. Bovine-serum lubrication and 2 kN peak sinusoidal load-profile were used with polyethylene (UHMWPE) cups. Wear was assessed by gravimetric technique. Precision (9%) was ensured by the use of multiple specimens, multiple wear-events, and the linear-regression method of estimating the average wear trend, thereby reducing the inherent, unpredictable nature of each wear-event. Volumetric wear-rates for polyethylene averaged 23 mm3 per 10(6) cycles for the 22 mm ceramic head and up to 32 mm3 per 10(6) cycles for the 28 mm head. The difference between 22 mm and the larger head-sizes was significant. This may well be the first laboratory confirmation of Charnley's original clinical Low-Friction Arthroplasty concept with regard to wear rate. The wear penalty increased linearly at the rate of 6% to 9% per mm of diameter increase.
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