PENETRATION, CREEP AND WEAR OF HIGHLY CROSSLINKED UHMWPE IN A HIP JOINT SIMULATOR

Abstract

Introduction Zero wear of highly crosslinked UHMWPE has been reported in hip simulators (1). In contrast 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 in a hip joint simulator and compare them to clinical wear rates.

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 PE 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 (gamma irradiated with 2.5MRad in air) and non-crosslinked PE were also tested. Wear measurements were taken every million cycles using a coordinate measuring machine. At each million cycles a 3D measurement was taken of the contact region of the acetabular cups.

Results and Discussion The wear rates decreased as crosslinking levels increased. The non-crosslinked material had an overall average wear rate (mm³/million cycles) of 45.6±1.35, the 2.5MRad material 46.9±9.4, the 7.5MRad 15.04±4.28 and the 10MRad 8.7±3.11. All four polyethylenes showed greater volume change in the first million cycles and this was associated with initial creep deformation. The results of the surface topography showed that the highly crosslinked materials became smoother than the other materials. This would benefit the crosslinked materials in aiding lubrication and could have contributed to the lower wear rate seen with these materials.

Conclusion Finite wear rates have been recorded for the first time in simulator studies with highly crosslinked polyethylene. The wear rates and wear surfaces compare with those found in clinical studies.