Interactive influences of crosslinking, counterface roughness and kinematics on the wear of crosslinked UHMWPE
Ultra high molecular weight polyethylene (UHMWPE) wear debris has been shown to be a major cause of long term failure of total joint replacements. In vitro studies have shown that not only is the size and shape of the particles important for cellular response but also the volumetric concentration of the particles. Recently, crosslinking has been extensively introduced to reduce the wear of UHMWPE. In this study the wear and wear debris of non-crosslinked and crosslinked UHMWPE were compared. The materials examined were UHMWPE GUR 1050, non crosslinked, moderately crosslinked - 5MRad and highly crosslinked - IOMRad. The wear was examined on a multidirectional pin on plate rig. The first test looked at the wear of the UHMWPE's with different serum concentrations. The effect of counterface roughness with high multidirectionality against different counterfaces was then examined. The second test examined the effect of different counterface surfaces at lower multidirectionality. The debris produced was then analysed. The wear results from the first pin on plate study showed that serum concentration had a significant effect on the wear rate of non-crosslinked U}IMWPE. The high serum concentration resulted in a much higher wear rate than the lower serum concentration for the non-crosslinked UIHMWPE. The crosslinked UHMWPE's did not show any significant difference in wear rate with serum concentration. The results from the different counterface conditions showed that highly crosslinked UHMWPE significantly reduced wear against smooth and scratched counterfaces. The second pin on plate study showed that all the U MWPE's had lower wear rates than the previous study because of reduced cross shear frictional energy. As with the previous study the highly crosslinked UHMWPE had significantly lower wear rates against all the counterfaces, and there was no significant difference between the OMRad and 5MRad UHMWPE. The reduction in wear rate found with the highly crosslinked UHMWPE was dependent on kinematics with less reduction with lower levels of cross shear frictional energy. The wear debris from the high multidirectional study on smooth and highly scratched plates was analysed. The use of field emission gun SEM revealed nanometre sized particles for the first time in all three materials under both counterface conditions. No significant difference was found in the percentage number and percentage area of the particles from any of the materials against any of the counterfaces although more larger wear particles were observed from the scratched counterface debris. The actual volumes of the particles showed that the highly crosslinked UHMWPE had a significantly lower volume in the 0.1-1.0μm size range. This was an important observation as this is the most biologically active size range. The specific biological activity did not show any significant difference between the materials. The functional biological activity which takes into account the wear volume showed that the highly crosslinked UHMWPE had a lower FBA for debris from both the smooth and scratched counterfaces. The debris from the scratched tests had a higher FBA for all the materials because of the increased wear rates with the scratched counterface.