PubMed · 3649680
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Total hip replacement.. https://pubmed.ncbi.nlm.nih.gov/3649680/
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The purpose of this study was to evaluate, via experimental models, the effect of larger head sizes for total hip arthroplasty on the type of impingement, range of motion (ROM), and joint stability. Testing was conducted using an anatomic full-size hip model (anatomic goniometer) and a novel anatomic dislocation simulator with 28-, 32-, 38-, and 44-mm diameter femoral heads within a 61-mm acetabular shell. Femoral heads >32-mm provided greater ROM and virtually complete elimination of component-to-component impingement. A significant increase in both flexion before dislocation and displacement between the femoral head and acetabulum to produce dislocation occurred with femoral heads >32-mm in diameter. These data indicate that larger femoral heads offer potential in providing greater hip ROM and joint stability.
Data from a series of four recent studies showed that conventional polyethylene liners sterilized by gamma irradiation in air and coupled with second-generation acetabular components outperformed first-generation components with consistent average wear behavior below 0.1 mm/yr. It was found that liners sterilized by gamma irradiation in air outperformed liners sterilized by gas plasma, indicating that cross-linking induced by gamma irradiation sterilization, even if the component is radiated in an oxygen environment, is essential for keeping wear rates low. No negative clinical effects were found on wear for conventional polyethylene sterilized by gamma irradiation in air with a shelf life up to 3 years (r2 = 0.04, P = 0.60). Finally, 16-year temporal wear patterns for gamma irradiation in air sterilized components demonstrated no significant changes in wear rates over time, indicating that gamma irradiation sterilized liners with short shelf lives will not likely experience late increases in wear because of continued in vivo oxidation of the polyethylene. From these studies it was determined that standardized radiographic methods, optimal radiographs, serial measurements, and minimum 4-year follow-up are essential for reliable wear data. More specifically, it was concluded that a conventional ultra-high molecular weight polyethylene liner gamma irradiation sterilized in air with a shelf life less than 3 years, coupled with a second-generation cup, performs well in vivo and is unlikely to experience late increases in wear. Because no comparable in vivo data will be available for new cross-linked polyethylene materials for several years, conventional gamma irradiated polyethylene can be considered the industry's current gold standard for wear performance comparisons.
Ultra-high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in total hip arthroplasty for nearly 40 years. Early failures have been attributed to gamma irradiation in air sterilization, poor implant design, and high patient activity. Currently available implants address the problems of gamma irradiation in air by using sterilization methods that avoid oxidative degradation. Previous efforts to modify UHMWPE, including carbon reinforcement, hot isostatic pressure, and heat pressing, have not resulted in improved clinical performance. More recently, highly cross-linked UHMWPEs have been developed that markedly reduced wear in hip simulators. However, cross-linking also reduces the mechanical properties, including fatigue crack propagation resistance. Although early clinical results with highly cross-linked UHMWPE are favorable, longer follow-up will be necessary to determine whether the results of in vitro testing accurately reflect long-term in vivo behavior.