COMPARISON OF SECOND GENERATION HIGHLY CROSSLINKED POLYETHYLENES UNDER ADVERSE AGING CONDITIONS

Abstract

Introduction: Two second generation highly crosslinked UHMWPEs have been cleared by the FDA for clinical use in the United States: sequentially crosslinked UHMWPE (X3™ UHMWPE, Stryker Inc., Mahwah, NJ, USA) and α-tocopherol stabilized UHMWPE (E-Poly™ UHMWPE, Biomet, Inc., Warsaw, IN, USA). Both have been shown to be oxidatively stable under standardized aging methods (ASTM F2003); however, these conventional aging methods did not consider the effect of mechanical loading on the oxidative behavior of the materials. By coupling the adverse effects of thermal aging and mechanical stress, we sought to investigate if either material was prone to environmental stress cracking (ESC). We hypothesize that the residual free radicals remaining in sequentially crosslinked PE will lead to oxidative degradation in this adverse test; furthermore, we hypothesized that the α-tocopherol infused in E-Poly™ will continue to protect the highly crosslinked PE even under such unfavorable conditions.

Materials and Methods: Three materials were tested:

1. Conventional: UHMWPE gamma sterilized in inert,

2. SXL: sequentially irradiated and annealed UHMWPE irradiated to a cumulative dose of 100kGy (33 kGy irradiation + 8 hour annealing in air, repeated 3 times) and gas plasma sterilized, and

3. E-Poly: UHMWPE irradiated to 100kGy, stabilized with α-tocopherol, and gamma sterilized in inert.

Four specimens from each group were subjected to a reciprocating mechanical stress of 10 MPa at a frequency of 0.5 Hz in an environmental chamber maintained at 80°C. Control samples were placed in the chamber but not subjected to cyclic mechanical stress. When a visible crack was observed on a sample’s surface or when a sample fractured, it and its corresponding control sample were analyzed by FTIR to quantify oxidation.

Results: All conventional specimens, half (2 of 4) of the SXL specimens, and none of the E-Poly specimens failed prior to the completion of 1,530,000 cycles (5 weeks of testing at 0.5 Hz). Cyclic loading had an adverse effect on the oxidation of the conventional and the SXL groups; the peak oxidation levels were higher in the cyclically loaded samples as compared to the control samples removed at the same time which were not loaded, likely due to an increase in chain scission induced by the mechanical load. The E-Poly specimens did not fail during the 5 weeks of testing, and FTIR did not reveal detectable oxidation in either control or loaded samples.

Discusssion and conclusion: Though the sequential processing of SXL creates a material with a lower free radical content compared to once-annealed material, it still yields a material prone to oxidation under extreme conditions, raising questions about its long-term oxidative stability. E-Poly™, protected by α-tocopherol, continues to exhibit high oxidation resistance even under adverse conditions.