Grafting of α-tocopherol upon γ-irradiation in UHMWPE probed by model hydrocarbons

Abstract

Today, UHMWPE implants are stabilized with α-tocopherol and cross-linked by irradiation in order to reduce wear. Little is known about the structural transformation of the antioxidant α-tocopherol upon irradiation. In the present investigation, the major irradiation reaction products of α-tocopherol dissolved at 0.1 wt. % in liquid model hydrocarbons were characterized spectroscopically and by independent synthesis. We observed only a single product group, namely phenolic alkyl ethers formed by radical recombination of a phenoxyl radical with a secondary alkyl radical.The irradiation dose is the parameter which controls the amount of consumption of α-tocopherol. At a dose of 27.5 kGy, 31-34 % of α-tocopherol was transformed into the corresponding ether, while at 97.9 kGy, the degree of transformation was 68-76 %.The observed ether formation in the liquid model hydrocarbons explains two significant observations for the α-tocopherol stabilized polymers, namely depletion of the α-tocopherol"s phenol group upon irradiation and "grafting", i.e. formation of a chemical bond between the polymer and its antioxidant.

Polyphenol-Assisted Chemical Crosslinking: A New Strategy to Achieve Highly Crosslinked, Antioxidative, and Antibacterial Ultrahigh-Molecular-Weight Polyethylene for Total Joint Replacement

“…Although remelting can eliminate free radicals, employing antioxidants to stabilize UHMWPE has been receiving increasing levels of attention in recent years. − As the analysis of retrieved implants indicated that UHMWPE oxidized through humoral attack in vivo, antioxidants are strongly backed up to achieve the long-term oxidation stability of highly crosslinked UHMWPE. , The α-tocopherol, also dubbed vitamin E (VE), is an exemplary success to protect UHMWPE implants from oxidation. , As a typical hindered phenol antioxidant, it neutralizes the radical of UHMWPE by donating a proton from the phenolic hydroxyl group. The residual phenoxy radical could isomerize into a tocopherol-benzyl radical and then reacts with another free radical of UHMWPE, forming a grafting structure. , However, it points to the vulnerability that VE attacks the crosslink precursors to hinder the crosslinking of UHMWPE. The loss in the crosslink efficiency of 1 wt % VE/UHMWPE approached 80% even at an irradiation dose of 200 kGy. , Thus, it is formidable to stabilize UHMWPE in high antioxidant contents without compromising the crosslinking efficiency at present.…”

“…The residual phenoxy radical could isomerize into a tocopherol-benzyl radical and then reacts with another free radical of UHMWPE, forming a grafting structure. 14,15 However, it points to the vulnerability that VE attacks the crosslink precursors to hinder the crosslinking of UHMWPE. The loss in the crosslink efficiency of 1 wt % VE/UHMWPE approached 80% even at an irradiation dose of 200 kGy.…”

Vitamin E-stabilised UHMWPE for surgical orthopaedic implants: Quantification of vitamin E and characterisation of its transformation products

“…To our knowledge, there is no published work on the fate of vitamin E, or, the formation and nature of any of its transformation products that may form in UHMWPE orthopaedic implant components (manufactured by Vitamin E antioxidant infused technology) during the various stages of their production; both after the initial high γ-irradiation dose (typically 100 kGy) used for crosslinking (Tibia samples at this stage are referred to here as 'Tibia-VEPE') and after the second lower dose gamma irradiation (typically 25-40 kGy) used for sterilization (Tibia samples at this stage are referred to as 'Tibia-VEPE-Sterile'). Some literature findings have made the assumption, based on hexane reflux extraction at 70 o C, that vitamin E becomes chemically bonded (grafted) onto the polymer [26,27], this assumption will be addressed here.…”

Grafted Vitamin-E UHMWPE may increase the durability of posterior stabilized and constrained condylar total knee replacements

“…The grafting method produces molded bars with uniform vitamin E distribution, but the free radical scavenging ability of vitamin E competes with the crosslinking process during irradiation which must be compensated for with higher irradiation dose to achieve equivalent wear performance of contemporary HXPE materials . A potential benefit of blended VE‐HXPE includes irradiation‐induced permanent molecular grafting of vitamin E into the UHMWPE backbone to produce a conjugate from which vitamin E cannot be readily eluted . While some work on wear, oxidative stability, and mechanical property evaluation of infused VE‐HXPE and grafted VE‐HXPE has been reported, there is a lack of delamination specific testing and no reported testing that replicates observed clinical post fracture failure modes.…”

“…30 A potential benefit of blended VE-HXPE includes irradiation-induced permanent molecular grafting of vitamin E into the UHMWPE backbone to produce a conjugate from which vitamin E cannot be readily eluted. 31 While some work on wear, oxidative stability, and mechanical property evaluation of infused VE-HXPE 28,29 and grafted VE-HXPE has been reported, 32 there is a lack of delamination specific testing and no reported testing that replicates observed clinical post fracture failure modes. Therefore, a critical question remains: is grafted VE-HXPE a suitable alternative to CPE for use in PS and CCK implants?…”