Introduction. 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 air, dramatically increases the wear rate. ASTM regulations for orthopaedic UHMWPE forbids the addiction of any antioxidant to the polymer powder or to fabricated forms. Vitamin E is widely employed as a biocompatible stabiliser in the food and cosmetic industry. Aim of the present study is to evaluate the efficiency of Vitamin E as a stabiliser for prosthetic UHMWPE.
Materials. Virgin UHMWPE samples were obtained from compression moulded slabs (GUR 1020, Perplas). In addiction, compression moulded slabs of GUR 1020 mixed with 500 and 1000 ppm of Vitamin E respectively were also studied. Electron beam irradiation was performed with doses ranging from 50 to 225 kGy, in air, at room temperature. Slices of controlled thickness (0,1–0,3 mm) were microtomed from the blocks and accelerated ageing was carried out in a ventilated oven at 90°C. FTIR spectroscopy were used to monitor changes in the polymer structure after irradiation and ageing. Mechanical properties were evaluated using the small punch test, as described in ASTM F2183-02.
Results. FTIR measurements on the aged samples showed that the addiction of Vitamin E induces a substantial increase in the oxidative stability of UHMWPE. The overall work to failure of original UHMWPE irradiated at 100 kGy was halved after 160 hours of accelerated ageing, due to the developed oxidation. On the other hand, the work to failure of samples with Vitamin E was constant up to 1800 hours of ageing under the same conditions.
Discussion Irradiation of UHMWPE induces C-C and C-H bond scissions, leading to the formation of alkyl radicals. When irradiation is carried out in air, macroal-kyl radicals can react with oxygen to form hydroperox-ides, which in turn decompose giving other oxidation species, mainly ketones and acids, which decrease the molecular mass. Oxidation of the polymer has been found to cause a dramatic deterioration of its mechanical properties. 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.
