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 (αa) and orthorhombic (αo) phases at the expense of the third phase (αb), 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 αo, αa and αb 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.