Multiaxial fatigue behavior of conventional and highly crosslinked UHMWPE during cyclic small punch testing

Abstract

Previous observations of reduced uniaxial elongation, fracture resistance, and crack propagation resistance of highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have contributed to concern that the technology may not be appropriate for systems undergoing cyclic fatigue loading. Using a "total life" approach, we examined the influence of radiation crosslinking on the fatigue response of UHMWPE under cyclic loading via the small punch test. Our goal in this study was to evaluate the suitability of the small punch test for conducting miniature-specimen, cyclic loading, and fatigue experiments of conventional and highly crosslinked UHMWPE. We subjected four types of conventional and highly crosslinked UHMWPE to cyclic loading at 200 N/s and at body temperature in a small punch test apparatus. After failure, the fracture surfaces were characterized with the use of field emission scanning electron microscopy to evaluate the fatigue mechanisms. Cyclic small punch testing under load control was found to be an effective and repeatable method for relative assessment of the fatigue resistance of conventional and highly crosslinked UHMWPE specimens under multiaxial loading conditions. For each of the four conventional and highly crosslinked UHMWPE materials evaluated in this study, fatigue failures were consistently produced according to a power law relationship in the low cycle regimen, corresponding to failures below 10000 cycles. The fatigue failures were all found to be consistent with a single source of initiation and propagation to failure. Our long-term goal in this research is to develop miniature-specimen fatigue testing techniques for characterization of retrieved UHMWPE inserts.

Development and Application of the Small Punch Test to UHMWPE

“…16. Adapted from Villarraga et al [27]. The specimen at left failed at 7289 cycles, that on the right failed at 7994 cycles.…”

“…Therefore, methodological changes were made to the tensile small punch apparatus, as described by Villarraga et al [27], to permit cyclic loading of the test specimen at body temperature. However, as previously discussed briefly in the review of the shear punch test development, bearing components fail not just from continual adhesive/abrasive wear mechanisms but also from fatigue mechanisms, such as delamination and frank failure associated with crack initiation and propagation.…”

“…In their work to develop the fatigue punch test, Villarraga et al [27] tested four conditions of GUR 1050 UHMWPE used in contemporaneous clinical applications. These conditions included virgin (representing EtO or gas plasma sterilized bearings), gamma sterilized (2.5 to 4.0 Mrad in nitrogen), annealed highly crosslinked UHMWPE (100 kGy and 110°C), and remelted highly crosslinked UHMWPE (100 kGy and 150°C).…”

Orthopaedic Prostheses

“…Currently, greatly enhanced wear resistance is claimed for hip prostheses but there is limited understanding of how the treatment process increases the wear resistance. The bulk material properties of the cross-linked UHMWPE also differs from the untreated UHMWPE with a less plastic deformation before failure under low cycle fatigue (less than 10,000 cycles) [Villarraga et al, 2003].…”

“…The critical step appears to be combine gamma irradiation with subsequent remelting and / or surface melting with an electron beam to induce a high level of cross-linking between the polyethylene molecules [Muratoglu et al, 2001;Hastings et al, 1999, Di Maio et al, 1998]. The level of radiation used for cross linking is typically 100 kGy, which is significantly greater than the 30 kGy radiation dose used for sterilization [Kurtz et al, 1999;Villarraga et al, 2003]. Currently, greatly enhanced wear resistance is claimed for hip prostheses but there is limited understanding of how the treatment process increases the wear resistance.…”