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Cross-linked polyethylene currently is being introduced in knee prostheses. The wear rates, wear debris, and biologic reactivity of non cross-linked, moderately cross-linked, and highly cross-linked polyethylene have been compared in multidirectional wear tests and knee simulators. Multidirectional pin-on-plate wear studies of noncross-linked, moderately cross-linked (5 Mrad), and highly cross-linked (10 Mrad) polyethylene showed a 75% reduction in wear with the highly cross-linked material under kinematics found in the hip, but only a 33% reduction under wear in kinematics representative of the knee. In knee simulator studies, with the fixed-bearing press-fit, condylar Sigma cruciateretaining knee under high kinematic input conditions, the wear of 5 Mrad moderately cross-linked polyethylene was 13 ± 4 mm 3 per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 ± 6 mm 3 /1 million cycles). For the lowcontact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 ± 1 mm 3 per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 ± 2 mm 3 /1 million cycles). The wear debris isolated from the fixed-bearing knees showed the moderately crosslinked material had a larger percentage volume of particles smaller than 1 µm in size, compared with GVF GUR 1020 polyethylene. Direct cell culture studies of wear debris generated in sterile wear simulators using multidirectional motion showed a increase (p < 0.05) in tumor necrosis factoralpha levels and reactivity for GUR 1050 cross-linked polyethylene debris compared with an equivalent volume of noncross-linked GUR 1050 polyethylene. The use of crosslinked polyethylene in the knee reduces the volumetric wear rate. However, the clinical significance of reduced fracture toughness, elevated wear in abrasive conditions, and the elevated tumor necrosis factor-alpha release from smaller more reactive particles warrant further investigation.Intentionally cross-linked polyethylene (PE) has been introduced extensively into hip prostheses during the last 5 years. Initial laboratory studies in hip simulators indicated that with high levels of cross-linking produced by 10 Mrad of irradiation, virtually 0 wear was produced. 2,13,14 More recently, both clinical and simulator studies of highly cross-linked PE done in more physiologically relevant conditions 5 showed a reduction (p < 0.05) in wear compared with conventional materials, with finite wear rates in the range of 5 to 10 mm 3 per 1 million cycles. Investigation of the wear debris from hip simulator studies has shown that moderately cross-linked PE has a larger percentage number and volume of smaller particles, compared with PE that has not been cross-linked. 4 These differences can only been shown in studies that analyze the whole size range of PE debris, and this is shown more clearly when the volume of wear debris rather than number of particles is analyzed. These observations are particularly important, as i...

There is currently considerable interest in the wear debris and osteolytic potential of different types of bearings used in total joint replacements. The biological activity of the wear debris is dependent on the size and volume of the particles produced. Wear volume also plays an important role in the functional biological activity of a joint replacement. In vitro studies have shown that crosslinking of ultra high molecular weight polyethylene (UHMWPE) acetabular cups and tibial trays produces a reduction in wear volume, and crosslinking has now been introduced clinically for both types of prostheses. Previous studies have identified both micron and submicron-sized polyethylene wear particles. The aim of this study was to characterize the wear and wear particles generated from moderately crosslinked GUR 1,020 GVF UHMWPE acetabular cups and tibial trays in hip and knee joint wear simulators down to 10 nanometers in size. The wear rates of the two prosthesis types were very similar at 25.6 +/- 5.3 mm(3) per million cycles for the hip prostheses and 22.75 +/- 5.95 mm(3) per million cycles for the knee prostheses. Nanometer-sized wear particles were isolated and characterized from both hip and knee simulator lubricants for the first time. Significantly higher numbers (p < 0.05) of particles in the nanometer (<0.1 microm) size range were produced by the hip prostheses compared to the knee prostheses. The knee prostheses produced larger particles, with the mode of particle size in the 0.1-1.0 microm size range, compared to <0.1 microm size range for the hip prostheses. In addition, the knee prostheses produced a greater volumetric concentration of wear particles in the 1.0-10 microm size range, and consequently lower specific biological activity and functional biological activity indices. These results indicated that the knee prostheses had a lower osteolytic potential compared to the hip prostheses.