Influence of Cross-linking and Oxidation on the Microstructural Mechanical Properties of UHMWPE
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Influence of Cross-linking and Oxidation on the Microstructural Mechanical Properties of UHMWPE Marcel E. Roy Biomedical Engineering, Saint Louis University 3507 Lindell Blvd, St. Louis, MO 63103 ABSTRACT Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as a bearing surface in total joint replacements. This study examined the effects of cross-linking and oxidation on the mechanical properties of UHMWPE at the microstructural level, rather than at the bulk level. Nanoindentation tests were performed on the articulating surfaces of two freshly machined UHMWPE acetabular inserts, one of which was cross-linked, and also performed along a freshly cut cross-section of a UHMWPE tibial insert that had been sealed in its original packaging and shelf-aged for at least five years. The reduced modulus of cross-linked UHMWPE was significantly lower than native UHMWPE. Tests on the cross-section of the shelf-aged specimen revealed a reduced modulus profile with a subsurface peak that was similar to the oxidation profiles of aged UHMWPE observed by others. Future work should examine the microstructural mechanical properties of UHMWPE using methods that more accurately account for its viscoelastic nature, and should directly correlate microstructural properties with oxidation. INTRODUCTION Worldwide, over a million total joint replacements are performed every year. To reduce the likelihood revision surgery will be required, medical implants must be designed to last as long as possible. The primary cause of joint replacement failure is osteolysis due to accumulated microparticles of ultra-high molecular weight polyethylene (UHMWPE) from normal wear and the body's subsequent foreign body response. The improved wear resistance of cross-linked UHMWPE has been well documented in the literature [1-4]. Cross-linked UHMWPE is also more brittle and mechanically inferior to intact polyethylene [3, 5], but cross-linking followed by re-melting quenches the radiation-induced free radical accumulation. Re-melting decreases yield strength but improves long-term resistance to oxidation while retaining wear resistance [2, 6-8]. The effects of aging/oxidation on UHMWPE implants have also been widely reported. Sterilization of UHMWPE by gamma radiation in the presence of oxygen causes an increase in free radical concentration, leading to scission of the polymer chains and increased oxidation [3, 9-12], which in turn reduces its bulk mechanical properties [3, 13-14]. The amount of oxidation due to free radical accumulation peaks at about 0.5-2 mm below the surface of aged components [10, 12, 15-16], especially below areas in contact with bodily fluids [17]. This oxidation peak coincides with the brittle "white banding" region observed in cross-sections of aged and retrieved implants [15]. While unworn surfaces tend to be more heavily oxidized [17], the degradation of mechanical properties as oxidation increases in vivo [18] reduces the useful life of UHMWPE. EXPERIMENTAL DETAILS Nanoindentation methods, based on Sneddon's sol
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