Raman characterization of conventional and cross-linked polyethylene in acetabular cups run on a hip joint simulator.

Five sets of differently sterilised conventional ultra-high molecular weight polyethylene (UHMWPE) and cross-linked polyethylene (XLPE) acetabular cups were run for 5million cycles on a hip joint simulator in order to evaluate their wear behaviour in relation to material properties (PE grade, conventional or cross-linked) and sterilisation method (ethylene oxide (EtO) treatment or γ -irradiation). Gravimetric measurements revealed that conventional UHMWPE wore significantly more than XLPE. The differences in wear behaviour could be partly related to the orthorhombic contents obtained by Raman spectroscopy in the unworn areas of the cups: XLPE cups showed a significantly higher crystallinity degree than the UHMWPE specimens. Raman analysis showed thatwear testing did not significantlymodify the orthorhombic content of any of the tested acetabular cups. However, the set of cups that showed the highestweight loss, i.e. γ -sterilised PE GUR1020, appeared themost homogeneously polished upon wear testing; from amolecular point of view, only this set of cups showed a significant increase of the I1130/I1060 intensity ratio, suggesting the occurrence of chain orientation.Onthe other hand,XLPE cups, despite the lowest weight loss undergone, showed a decrease in the amorphous content upon wear testing as well as a limited orthorhombic→ monoclinic transformation,which did not appear detrimental.