Ionizing irradiation for sterilization and modification of orthopedic biomaterials

In general, metals and ceramic components of total hip- or knee-replacement arthroplasties are not affected by any of the commonly practiced sterilization methods: autoclave, gas plasma treatment (GPT), gamma irradiation, or ethylene oxide (EtO). However, synthetic polymers such as conventional ultra-high molecular weight polyethylene (CPE), used as liners in the articulating surfaces of the total joints, and the two-part self-polymerizing poly (methyl methacrylate) (PMMA), used as bone anchoring material or bone cement, are affected adversely when gamma or e-beam irradiation is used in presence of oxygen. In absence of oxygen, irradiation is known to improve or maintain the properties of the joint components by crosslinking molecular chains of the polymers. Although with increased crosslinks ultra-high molecular weight polyethylene (UHMWPE) becomes resistant to wear, its crystallinity decreases and becomes more permeable to oxygen. As a result, oxidative reaction of free radicals is found to proceed at a faster rate in the cross-linked (XLPE) than in the conventional (CPE) polyethylene. Furthermore, mechanical dispersion, γ region, appears at a lower temperature in XLPE (120-160 K) than in CPE (150-210 K) while β dispersion regions (240-300 K) remain unchanged