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The Reduced Fracture Toughness that Accompanies Elevated Cross-Linking of Polyethylene is not associated with an Increase in Pitting and Delamination Type Wear
Maher, S. A.
Abstract
In vitro tests have shown that elevated cross-linked ultrahigh molecular weight polyethylene has lower wear rates anddecreased fracture toughness when compared with conventionalpolyethylene. Although decreasing wear rates is a goal towardsimproving implant longevity, the influence of reduced fracturetoughness on implant performance is unknown. Pitting and delaminationtype damage in tibial tray inserts is believed to be drivenby a fatigue-fracture process. This suggests that a connectionbetween reduced fracture toughness and increased damage mayexist. The hypothesis of this study was that the reduced fracturetoughness associated with elevated cross-linked polyethylenewould lead to an increase in pitting and delamination in samplestested in a knee wear apparatus.
Blocks of polyethylene were electron beam irradiated at twodifferent levels, 120 kGy and 65 kGy, while a third groupof blocks was not irradiated (control group). The 120 and65 kGy blocks were subjected to a post-irradiation heat treatment.Wear samples and J-integral samples were machined from theblocks. The 120 and 65 kGy samples were gas plasma sterilized.The control samples were gamma sterilized resulting in irradiationto 25 kGy. All samples were aged and tested. 65 kGy and 120kGy irradiated material had significantly lower fracture toughnesscompared to the control material. Despite the reduced fracturetoughness no evidence of pitting, delamination, or subsurfacedamage occurred in the corresponding wear samples after 2million cycles. 50% of the control samples exhibited extensivepitting. The elevated cross-linked samples had larger weartrack areas; however, this difference was not significantuntil 1,250,000 cycles, by which stage pitting was alreadyevident in the control samples. Our hypothesis was rejected:reduced fracture toughness associated with the elevated cross-linkedpolyethylene groups was not associated with an increase in pitting and delamination.
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