Abstract
Aim: Wear of the UHMWPE component is responsible for many TJR failures. It is now well known that oxidation of UHMWPE, induced by radiation sterilisation in the presence of oxygen, dramatically increases the wear rate. Vitamin E is already used as biocompatible antioxidant in a number of applications, thus it has been suggested as a suitable stabiliser for orthopaedic UHMWPE. This work investigates the role of the vitamin E on the oxidation process of a gamma-sterilized material.
Methods: GUR 1050 (Meditech, Fort Wayne, IN) resins were blended with 0.05 wt% to 0.5wt% vitamin E and compression molded into billets. Gamma irradiation to 30 and 100 kGy was carried out in an industrial plant. This material was then sectioned using a microtome into 180 micron-thick specimens in preparation for the accelerated ageing.
The samples were treated in an air circulating oven at 90°C. Every 20 hours they were analyzed with FTIR and the carbonyl concentration was recorded.
The CL-imaging measurements were run at 180°C under oxygen in a Differential Scanning Calorimetry (DSC) coupled to a CCD camera. The Oxidation Induction Time (OIT) has been measured as the starting time of oxidation, extrapolated from the CL curve in the function of time.
Results: The results obtained from both techniques clearly evidence that 0.05 wt % of Vitamin E it is enough to stabilize even the material irradiated to the highest dose (100 kGy). Irradiation of UHMWPE leads to the formation of alkyl radicals. When irradiation is carried out in air, macroalkyl radicals can react with oxygen to form hydroperoxides, which in turn decompose giving other oxidation species, mainly ketones and acids. The overall result of irradiation in air is the formation of oxidation products and a decrease in the molecular mass, due to chain fragmentation.
The ability of α-tocopherol as a free radicals scavenger during gamma irradiation prevents the reaction of polymer radicals with oxygen. While performing this role, α-tocopherol is consumed and transformed into a variety of by-products. Nevertheless, higher OIT for the doped and irradiated specimens compared to the control (0% Vit.E, 0 kGy) suggest even a stabilising effect of these by-products.
Conclusions: Vitamin E has been shown to be highly efficient against radiation-induced oxidation and therefore it should be recommended as biocompatible stabilizer for orthopaedic UHMWPE, in order to preserve good mechanical properties.
The abstracts were prepared by incoming Professor Elena Brach del Prever. Correspondence should be addressed to IORS – President office, Dipartimento di Traumatologia, Ortopedia e Mediciana del Lavoro, Centro Traumatologico Ortopedico - Via Zuretti, 29 I-10135 Torino, Italy.
