The effect of cross-shear on the wear of virgin and highly-crosslinked polyethylene

The phenomenon of orientation softening suggests that during multi-directional motion the principal direction experiences hardening due to molecular orientation, while the secondary direction experiences softening. Multi-directional motion occurs during gait for both hips and knees, and its effect on the wear of ultra-high molecular weight polyethylene (UHMWPE) is still not fully understood. The purpose of this study was to evaluate the phenomenon of orientation softening using a highly crosslinked polyethylene (HXPE). Both virgin polyethylene and HXPE pins were articulated against cobalt-chromium discs using an OrthoPod™ wear tester. Six different motion paths were used, ranging from a square pattern to a linear track. Gravimetric weight measurements were periodically collected and scanning electron micrographs were taken after the completion of each motion path. The results show the motion path has a significant effect on the wear rate of virgin polyethylene, but no significant effect on the wear of HXPE.