CORRInsights®: A Novel Technique for Assessing Antioxidant Concentration in Retrieved UHMWPE

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Improving the wear resistance of load-bearing, articular implants with radiation crosslinked ultra-high-molecular-weight polyethylene (UHMWPE) has been one of the major advances in the field of total joint arthroplasty. Thermal treatments following irradiation are necessary to reduce the concentration of residual free radicals and improve the oxidation resistance of UHMWPE. While radiation crosslinking reduces the mechanical strength, postirradiation melting, the most-effective stabilization method, further decreases the strength of UHMWPE. Antioxidants such as vitamin E and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)] propionate (PBHP) were introduced in the mid-2000s as an alternative for stabilizing the free radicals, eliminating the need for melting. This improved the mechanical properties of the polymer. Antioxidant stabilization is also expected to prevent the oxidation of highly crosslinked UHMWPE components in vivo in the long-term due to the more recently unveiled mechanisms triggered by the absorption of pro-oxidant lipids and cyclic stress [3].
The long-term effectiveness of antioxidants would depend on their concentration and continued antioxidant properties. There is potential for elution of some fraction of the antioxidant out of the UHMWPE components, possibly compromising the long-term oxidative stability of the devices. Therefore, it is important to quantify the extent of elution and the amount of residual antioxidant in surgically explanted, antioxidant stabilized, and highly crosslinked UHMWPE components. In vitro studies have shown that vitamin E, the most commonly used antioxidant, will elute out of highly crosslinked UHMWPE in deionized water at body temperature when it is present in concentrations above its saturation limit [5]. This elution was shown to reach a plateau at a certain concentration, likely around the saturation limit of vitamin E in UHMWPE. We also know that radiation crosslinking or gamma sterilization after the incorporation of the vitamin E into UHMWPE components causes chemical grafting of the antioxidant molecules to the polymer backbone [4]. Grafting would also decrease the extent of elution of the antioxidant in the long-term.
Another potential concern is the local tissue reaction to the eluted antioxidant. Biocompatibility studies using animal models have shown no adverse effects to antioxidants, which may be present in the synovial fluid and the surrounding tissue because of elution out of UHMWPE components in vivo [2].
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