Ultra high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in joint replacement prostheses for over 40 years. Recently, highly crosslinked UHMWPE (HXPE) materials were introduced based on the finding that crosslinking reduces wear. These first generation HXPE materials were produced by irradiation followed by heating below the melting temperature (annealing) or above the melting temperature (remelting). Both classes of HXPE material have demonstrated greatly reduced wear. However, remelted HXPE materials have reduced fatigue strength while annealed HXPE materials may oxidize when exposed to oxygen. A second generation HXPE material was produced using a sequential irradiation and annealing process (SXL). SXL materials have crosslinking levels equivalent to those of first generation HXPE materials, have fatigue and mechanical strength characteristics of first generation annealed HXPE material and have an oxidation resistance equivalent to that of virgin (unprocessed) UHMWPE. This combination of properties makes SXL HXPE a preferred material for bearing surfaces of joint prostheses.
The resin and processing route have been identified as potential variables influencing the mechanical behavior, and hence the clinical performance, of ultra-high molecular weight polyethylene (UHMWPE) orthopedic components. Researchers have reported that components fabricated from 1900 resin may oxidize to a lesser extent than components fabricated from GUR resin during shelf aging after gamma sterilization in air. Conflicting reports on the oxidation resistance for 1900 raise the question of whether resin or manufacturing method, or an interaction between resin and manufacturing method, influences the mechanical behavior of UHMWPE. We conducted a series of accelerated aging studies (no aging, aging in oxygen or in nitrogen) to systematically examine the influence of resin (GUR or 1900), manufacturing method (bulk compression molding or extrusion), and sterilization method (none, in air, or in nitrogen) on the mechanical behavior of UHMWPE. The small punch testing technique was used to evaluate the mechanical behavior of the materials, and Fourier transform infrared spectroscopy was used to characterize the oxidation in selected samples. Our study showed that the sterilization environment, aging condition, and specimen location (surface or subsurface) significantly affected the mechanical behavior of UHMWPE. Each of the three polyethylenes evaluated seem to degrade according to a similar pathway after artificial aging in oxygen and gamma irradiation in air. The initial ability of the materials to exhibit post-yield strain hardening was significantly compromised by degradation. In general, there were only minor differences in the aging behavior of molded and extruded GUR 1050, whereas the molded 1900 material seemed to degrade slightly faster than either of the 1050 materials.
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