One-year biodegradation study of UHMWPE as artificial joint materials: Variation of chemical structure and effect on friction and wear behavior

Ultra-high molecular weight polyethylene (UHMWPE) has been the most commonly used bearing materials in the total joint replacement. The biodegradation of implanted UHMWPE components has notable influence on wear and fatigue resistance. Although accelerated aging protocols in vitro have been developed to evaluate the oxidation behavior of the UHMWPE materials, the mechanism still remains not accurately understood. Thus, the biodegradation of UHMWPE in simulated body fluid (SBF) was performed for up to 12 months and the chemical structure ( i.e. oxidation and crystallinity) and scratch resistance, punch shearing strength, friction and wear behavior of biodegraded UHMWPE samples were investigated in this work. The results provided strong evidence that oxidation degradation significantly reduced the crystallinity and in consequent decreased the wear and scratch resistance of UHMWPE. It was found that the oxygen content on UHMWPE surface increased by almost two times and the crystallinity degree decreased by 16%, resulting in the reduction of scratch coefficient and peak breaking load by 30% and 15%, respectively, after one-year biodegradation. Also, significant wear rate increase up to 6 times for the one-year degraded UHMWPE sample was noticed in this study, and an exponential decay relation for the wear rate of UHMWPE to the crystallinity was identified. A bi-linear behavior of the crystallinity and oxidation index on UHMWPE sample was discovered, which corresponded to the two-stage wearing volume growth of degraded UHMWPE. The preliminary study suggested the necessity of a full-scale biodegradation test for UHMWPE materials including oxidative, hydrolysis and biodeterioration and even dynamic rubbing process. Moreover, it was indicated scratch test as an effective method to evaluate the superficial properties of UHMWPE samples which may be used as an important tool on comparing friction and wear behavior of this material.