Vibrational spectroscopy of ultra-high molecular weight polyethylene hip prostheses: Influence of the sterilisation method on crystallinity and surface oxidation

Due to its high strength and low creep, ultra-high molecular weight polyethylene (UHMWPE) has been used for 30 years in the replacement of damaged articulating cartilage for total joint replacement surgery. In this study, micro-Raman spectroscopy was used to investigate the effects of the sterilisation method (gamma and ethylene oxide (EtO) treatment) on the crystallinity changes of UHMWPE acetabular cups. The crystallinity of the cups was evaluated by micro-Raman spectroscopy coupled to the partial least square (PLS) regression as a function of the inner surface position. Unworn gamma-sterilised cups were found to be significantly more crystalline than the EtO-sterilised ones. No significant differences were observed between the crystallinity values of worn (in in vitro tests) and unworn cups for each type of sterilisation, showing that the changes in surface crystallinity were mainly caused by irradiation rather than by mechanical friction during the in vitro wear tests. These results were discussed in relation to gravimetric measurements, which revealed at the end of the in vitro tests, a higher mean weight loss for the EtO-sterilised cups than for the gamma-sterilised ones. No significant amounts of oxidative degradation products were detected by IR spectroscopy in the inner surface of the EtO-sterilised worn and unworn cups. Regarding the gamma-sterilised cups, the oxidation level appeared to be slightly higher in the centre of the worn cups than on their borders.