WEAR OF CROSSLINKED UHMWPE IN A HIP JOINT SIMULATOR (2006)

Introduction: Crosslinking has been extensively introduced to reduce the wear of UHMWPE. Zero wear of highly crosslinked UHMWPE has been reported by some groups (1) in hip simulators, clinical studies have reported finite wear rates (2). The aim of this study was to compare the wear rates produced by UHMWPE with different levels of crosslinking. Materials and Methods: Studies were carried out using 28mm diameter cobalt chrome femoral heads. These were articulated against UHMWPE in the Leeds ProSim hip joint simulator. The acetabular cups were manufactured from UHMWPE GUR 1050. The GUR 1050 was highly crosslinked with 10MRad or 7.5MRad of gamma irradiation in nitrogen followed by re-melting at a temperature above 150°C. Slightly crosslinked GUR 1050 was also tested (gamma irradiated with 2.5MRad in air). Non-crosslinked GUR 1050 UHMWPE was used as a control. Five cups of the materials were tested with one station from each set of five being used for creep data. Wear measurements were taken every million cycles using a coordinate measuring machine and tests were run to 5 million cycles. The tests were carried out in low serum concentrations of 25% (v/v) bovine serum diluted with 0.1% (w/v) sodium azide in water. At each million cycles a 3D measurement was taken of the contact region of the acetabular cups using a Form Talysurf profilometer. Results and Discussion: The wear rate decreased as crosslinking levels increased. The non-crosslinked material had an overall average wear (mm3/million cycles) determined by volume change of 45.6+/−1.35, the 2.5MRad material 46.9+/−9.4, the 7.5MRad 15.04+/−4.28 and the 10MRad material 8.7+/−3.11. The intentionally cross-linked materials showed a significantly lower volume change than the other two materials, with the 10MRad polyethylene having a slightly lower volume change than the 7.5MRad polyethylene. All four polyethylenes showed greater volume change in the first million cycles than the subsequent four and this was associated with initial creep deformation in the first million cycles. The individual creep deformation cups confirmed this with volume changes in the first million cycles followed by stability. Creep volumes of between 10 and 25 mm3 total were measured with the lowest value being for the 10MRad polyethylene. The steady state wear rates for the PE’s between one and five million cycles were 0MRad 36.9+/−1.92 mm3/million cycles, 2.5MRad 44.12+/−10.09, 7.5MRad 7.89+/−2.32 and 10MRad 4.62+/−2.73. The results of the surface topography of the acetabular cups showed that the highly crosslinked materials became smoother than the other materials as the test progressed. This would benefit the crosslinked materials in aiding lubrication and could have contributed to the lower wear rate seen with these materials. Conclusion: The highly crosslinked UHMWPE gave lower wear volumes than the noncrosslinked materials. This could have been due to the smoother surfaces of the cups as the study progressed which resulted in better lubrication of the components. Finite wear rates have been recorded for the first time with highly cross-linked polyethylene, that compare with clinical observations.