Abstract
Introduction
The fatigue strength of ultrahigh molecular weight polyethylene (UHMWPE) in total joint implants is crucial to its long term success in high demand applications, such as in the knee, and is typically determined by measuring the crack propagation resistance in razor-notched specimens under cyclic load [1]. This only tells part of the story: that is, how well the material resists crack propagation once a crack is present. A second, equally important component of fatigue strength is how well the material resists crack formation. Previous studies cyclically loaded a cantilevered post until failure [2], postulating that the post would break very quickly after crack initiation. Parran et. al. proposed a novel method to measure the crack initiation time by holding a sample in constant tension until a crack was visually observed [3]. We hypothesize that the crack initiation times of various UHMWPEs will follow similar trends as the more omnipresent crack propagation resistance tests.
Materials and Methods
The following UHMWPE formulations were tested: (i) virgin, (ii) gamma sterilized in vacuum, (iii) 91 kGy gamma irradiated, and (iv) 91 kGy gamma irradiated and subsequently melted. GUR1020 and GUR1050 bar stock of varying irradiation doses were machined into compact tension specimens [4] with a notch depth of 17 mm and a blunt notch root radius of 0.25 mm, mimicking a geometry of a joint replacement component.
Specimens were held in constant tension until failure; 3 to 5 different loads between 1 kN and 2.25 kN (n=3 samples per load per material) were tested. A video camera was focused on the face of the notch and took a picture every 10 seconds. The photos were reviewed to manually determine the crack initiation time (Fig 1). The time it took for the sample to completely fail – that is, shear into two separate pieces – was also recorded.
Results
For all materials tested, the crack initiation time (Fig 2a,b) and the time to failure (Fig 2c,d) decreased as the applied load increased. The crack initiation time increased for the gamma sterilized materials when compared to the virgin materials while the time to failure decreased. The highly crosslinked, 91 kGy materials had crack initiation times and times to failure that were less than that of the virgin material. Post irradiation melting greatly diminished the fatigue strength of the material, yielding the lowest crack initiation time and time to failure.
Discussion
The test yielded results consistent with current knowledge: that is, high-dose irradiation yields a slight drop in fatigue strength, and post-irradiation melting greatly reduces strength. This test was simple to set up and run and can be a good tool to determine the relative fatigue strengths of UHMWPE formulations for orthopaedic applications.
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