The effects of irradiation and artificial aging on the wear behaviour of UHMWPE.

Ultra-high-molecular-weight-polyethylene (UHMWPE) for hip implants can present serious clinical problems: the cyclic nature of the contact stresses at the articular surface can lead to pitting, delamination and crystallinity changes of the polymer with formation of PE debris, which may lead to extensive bone loss around the implant and consequently osteolysis and implant loosening. The effects of different sterilization methods have been studied by various authors, but no one produced a full conclusive verdict (1,2). To assess long term stability of polymers, various accelerated aging methods have been developed. In this study, an internal protocol was developed to evaluate the effects of the accelerated aging on wear. Three EtO-sterilised and six gamma-irradiated (standard 3 Mrads dose in air) UHMWPE acetabular cups (Chirulen GUR 1050, POLY HI SOLIDUR, France) were aged in air at 80°C for 4 weeks and thereafter tested in conjunction with twelve 28-mm CoCrMo femoral heads. Wear tests were carried out using a 12-station hip joint simulator (Shore Western, USA) run for 3 million cycles, with bovine calf serum as lubricant. A frequency of 1 Hz, according to the rotation test frequency, was applied with a sinusoidal load having a peak magnitude of about 2 kN. Wear behaviour was evaluated by gravimetric measurements and Raman analysis. Since Raman spectra reflect PE morphology, Raman spectroscopy was used to investigate the crystallinity changes induced by aging and wear testing. Micro-Raman spectra were obtained in a non-destructive way using a Jasco NRS-2000C instrument (exc=488 nm, 20× magnification). For each cup, 12 spectra were recorded in the inner surface within 1.5 mm from the centre, the most worn area. Crystallinity (C%) was determined from Raman spectra using the partial least-squares (PLS) regression. Significant differences were observed between the wear behaviours of the two sets of acetabular cups after 3 million cycles ( = 0.0001). The mean C% values obtained for untreated gamma and EtO-sterilised UHMWPE unworn cups were significantly different (62 and 60%, respectively). Upon ageing significant changes in C% were observed for all the cups and were more pronounced for gamma-sterilised cups (meanly from about 62% to 65%) than for EtO-sterilised cups (meanly from 60% to 61%). Upon wear testing of the gamma-sterilised cups, only three samples showed significant increases in C%. Conversely, all the EtO-sterilized cups showed significant C% changes: one cup showed a C% increase (from (60.9±0.2)% to (61.9±0.4)%), while the other two underwent a C% decrease (from (60.3±0.3)% to (56.4±0.7)% and from (60.9±0.3)% to (59.4±0.4)%). The higher crystallinity found for the gamma-sterilised cups can be explained by considering that the energy of the gamma rays is sufficiently high to break the UHMWPE polymeric chains, resulting in a reduction of molecular weight and in a corresponding increase in crystallinity. Raman spectroscopy proved a valid tool to non-destructively monitor the C% changes the samples underwent upon aging and wear tests. By Raman-PLS analysis it was confirmed that aging affected more significantly the morphology and wear behaviour of the gamma-sterilised cups; also the EtO-sterilised cups underwent detectable C% changes, which, however, did not affect so dramatically wear behaviour.