The biomaterials challenge: A comparison of polyethylene wear using a hip joint simulator

Although hip arthroplasty is an established procedure that relieves pain and improves functions, problems remain with wear and osteolysis. Highly cross-linked polyethylene and Vitamin-E-stabilized polyethylene were introduced in the last years to solve these problems. In this study we compared the in vitro wear behaviour of cross-linked polyethylene (XLPE) versus Vitamin-E diffused XLPE (XLPE_VE) versus conventional ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. The test was performed using a hip joint simulator run for two millions cycles under bovine calf serum as lubricant. Mass loss was found to decrease along the series UHMWPE>XLPE_VE>XLPE, although statistically significant differences were found only between the mass losses of XLPE and UHMWPE at 1.2 and 2 million cycles. The mass loss data were explained in relation to the crystalline morphology of the control unworn cups, as investigated by non-destructive micro-Raman spectroscopy. This technique allowed to disclose a different wear behaviour of the three sets of cups. Wear testing produced a stress-induced crystallisation in UHMWPE, with increases in both amorphous (alpha(a)) and orthorhombic (alpha(o)) phases at the expense of the third phase (alpha(b)), which decreased upon wear. Moreover, the all-trans content decreased, while the ortho-trans content increased, contrarily to the trend observed for XLPE and XLPE_VE, for which no statistically significant changes in alpha(o), alpha(a) and alpha(b) contents were detected. The XLPE_VE specimens underwent the least significant changes in the spectroscopic markers of micromorphology upon mechanical stress, probably due to their lower starting amorphous content. (C) 2015 Elsevier Ltd. All rights reserved.