19 These data support the importance of high strength and significant ductility in the polyethylene-bearing surfaces to resist fatigue. 7 The frequency of cracking or delamination in retrieved acetabular bearings after 4 years in vivo is 20%. Sixty-five percent of 600 retrieved gamma-in-air sterilized knee bearings with duration in vivo greater than 4 years had evidence of fatigue damage. 15 This oxidation reduces cross-linking and degrades mechanical properties to such an extent that cracking and delamination can occur. These free radicals are long lived, and react in the presence of oxygen to cause oxidation. Gamma irradiation sterilization produces free radicals in polyethylene. However, gamma sterilization in air also begins a process of oxidation in polyethylene. Gamma-sterilized polyethylenes, implanted as hip and knee bearings since the 1970s, are cross-linked to a certain extent by the sterilization process. The goal of the current study, in collaboration with orthopaedic implant manufacturers, was to compare and contrast the tradeoffs in mechanical properties that each company has made in developing their specific cross-linked polyethylene. Although all claim dramatic reduction in wear, other material characteristics may prove important in the overall performance of these materials for long-term clinical applications, particularly if the materials are used in total knee replacement. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.Ĭross-linked polyethylenes are described by their developers as a major advance against wear in total hip replacement, and most of the major orthopaedic manufacturers provide some form of radiation cross-linked polyethylene for clinical use. ![]() Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. Cross-linked materials were evaluated as received and after an accelerated aging protocol. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. ![]() To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. However, this reduction in wear rate is not without cost. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris.
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