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Effects of Sterilization Methods On Ultra-High Molecular-Weight Polyethylene (UHMWPE)

Ultra-high molecular-weight polyethylene (UHMWPE) is the most commonly used bearing material in total joint replacement. Wear of UHMWPE is a serious clinical problem that limits the longevity of orthopaedic implants. Excessive wear may lead to gross mechanical failure such as fracture and disassociation(1-3) and the release of particulate wear debris may induce biologcal responses that cause implant loosening(4,5). Researchers have documented excessive wear in retrieved tibial, patellar and acetabular components of joint replacements. The degree of wear reported often depends upon factors unrelated to the polyethylene quality such as patient age(8,9), activity level(10,11), body weight(9,11-18), implantation time(14,15), component alignment, component thickness(16,17) and articular geometry(16-18). However, control of polyethylene quality is essential to the improvement of wear resistance. Maintaining the original properties of polyethylene is also very important to its in-vivo performance. Therefore, any potential adverse effects on polyethylene that may be caused by processing, manufacturing or sterilization should be avoided or minimized.

Gamma radiation has been a common sterilization method for UHMWPE used in total joint replacement, The products are sterilized by gamma radiation from a Cobalt 60 source. It is well known that gamma radiation induces various chemical changes in plastics which lead to oxidation, and several studies have recently discussed the oxidation of UHMWPE following radiation and aging(19-23). In 1984 and 1987, Eyerer and coworkers(24,25), reported oxidation in several hip joint cups. Li et al.(20), suggested that oxidation of UHMWPE continues for long periods of time after gamma radiation. Saum(21) reported subsurface oxidation which remained significant to 2mm below the surface. Oxidation as deep as 3 mm into the samples was also found by Rimnac and coworkers(22). Trieu and Paxson(26) have recently demonstrated an oxidized surface layer as thick as 6 mm in UHMWPE components.

The common understanding of the oxidation mechanism is as follows. UHMWPE initially consists of extremely long molecular chains which makes h an excellent abrasion-resistant material. Sterilization by gamma radiation causes chain scission by breaking chemical bonds and creates reactive free radicals. Oxygen diffuses into the material and reacts with free radicals to cause oxidation, which leads to much shorter molecular chains. As a result, the original properties of polyethylene, including abrasion resistance, change significantly. Preliminary data indicates that long-term oxidative degradation can alter the performance of polyethylene in total joint replacement, especially its resistance to fatigue wear which can cause pitting and delamination.

The two sterilization methods that are commonly used for medical plastics are gamma radiation and ethylene oxide. The first method sterilizes products using radiation of gamma rays from a Cobalt 60 source. As discussed earlier, gamma radiation can lead to extensive oxidation in polyethylene. The second method uses a gas sterilant, ethylene oxide (EtO), to sterilize the products. EtO sterilization is commonly used for products that are sensitive to the heat of steam sterilization or materials that may be deteriorated by radiation sterilization.

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