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Multiaxial Fatigue Behavior of Oxidized and Unoxidized Conventional UHMWPE
Villarraga, M. L., Edidin, A.A., Herr, M. P., Boatwright, J., Kurtz, S. M.
Abstract
We have previously evaluated the influence of radiation crosslinkingon the fatigue response of UHMWPE under cyclic loading usingthe small punch test and showed enhanced low cycle fatigueresistance with gamma radiation crosslinking. Fatigue failuresobserved under SEM were consistent with a single source ofinitiation. We hypothesize that oxidation would also influencethe resistance to fatigue crack initiation and propagation.To evaluate this, we subjected 10 mm tibial insert surrogatesmachined from extruded GUR 1050 to accelerated aging protocolsfollowing the ASTM F-2003-00 for 14, 21 and 28 days. Subsurfacedisc specimens from the control and aged materials at eachtime period were then subjected to cyclic small punch loadingto failure at a rate of 200N/s. A significant decrease infatigue loading was observed, relative to the unaged controls,starting at 3 weeks of aging in an oxygen bomb. Furthermore,SEM examination of the failed, aged specimens revealed a networkof multiple secondary initiation sites. This pattern was alsoconfirmed with endoscopic evaluation during testing, and withmicroCT after failure. Thus, in contrast to the non-oxidizedhighly crosslinked conventional materials evaluated previously,the oxidized materials failed by the initiation and propagationfrom numerous initiation sites and had a more brittle appearancewith increase in oxidation time. These sites appeared to beon or near particle boundaries, consistent with preferentialoxidation of the particle boundaries prior to the resin particlesthemselves. These results suggest that oxidized UHMWPE exhibitsa fundamentally different fatigue crack initiation and propagationbehavior as compared to unoxidized UHMWPE. Future studieswill be needed to determine the association between acceleratedaging conditions and the oxidation states of implanted tibialinserts. This is necessary to increase our understanding ofthe clinically acceptable fatigue properties for new tibial bearing materials, such as highly crosslinked UHMWPEs.
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