Oxidation behaviour in prosthetic UHMWPE components sterilised with high energy radiation in a low-oxygen environment P. Bracco a , E.M. Brach del Prever b , M. Cannas c , M.P. Luda a , L. Costa a, * a Dipartimento di Chimica Inorganica e Chimica Fisica dell’Universita ` di Torino and NIS Centre of Excellence, Via P. Giuria 7, 10125 Torino, Italy b Dipartimento di Traumatologia, Ortopedia e Medicina del Lavoro dell’Universita ` di Torino, Centro Traumatologico Ortopedico, Via Zuretti 29, 10126 Torino, Italy c Dipartimento di Scienze Mediche, Universita ` del Piemonte Orientale ‘‘A. Avogadro’’, Via Solaroli 17, 28100 Novara, Italy Received 17 January 2006; accepted 5 February 2006 Available online 4 April 2006 Abstract Oxidation of the UHMWPE component during sterilisation with high energy radiation has been recognised as one of the main problems affecting the durability of orthopaedic implants. Oxidation is always present in radiation-sterilised polymeric components due to the oxygen diffused into UHMWPE. Hydroperoxides are the first products of the oxidation cycle and they are also a convenient species to follow for mon- itoring the oxidation. In the present study, the early stages of oxidation were investigated by assessing the hydroperoxide levels and distribution in a selection of ready-to-use UHMWPE prosthetic components. The samples included in the study were chosen from a larger group, according to the following criteria: they were packaged in a low-oxygen environment, radiation-sterilised and carbonyl free. FTIR spectroscopy and deri- vatisation techniques were used to characterize the hydroperoxide concentration and distribution. Hydroperoxides were detected in all samples, despite the sterilisation in inert atmosphere. Three different behaviours were identified and discussed and a correlation with the packaging and sterilisation conditions is proposed. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: UHMWPE; Gamma irradiation; Electron beam; Packaging; Oxidation; FTIR 1. Introduction It has been recognised for a long time that oxidation in polymers, polyolefins in particular, occurs easily, severely decreasing the physical properties of the original polymer materials [1]. Application of polymers in many fields requires appropriate stabilisation to avoid the dramatic consequences of oxidation. Nowadays a large selection of antioxidants is avail- able to successfully protect polyolefins against oxidation and the market of such additives now engages very large compa- nies and a huge budget. Despite these considerations, concern regarding possible toxicity of additives and of their degradation products pushed international legislation to prohibit the use of any antioxidant in orthopaedic Ultra High Molecular Weight PolyEthylene (UHMWPE) [2]. This is of particular relevance as the neces- sary sterilisation of UHMWPE prosthetic components (Pes) has often been reported to induce oxidation [3e5]. In effect, sterilisation by high energy radiation was commonly carried out in air until the late 1990s; next, sterilisation by g radiation in a low-oxygen environment was introduced to reduce oxida- tion and increase long time preservation of the mechanical properties of Pes [3]. Nevertheless, this technique has already shown a limited efficiency in controlling oxidation [6]. In the present work, the oxidation process of UHMWPE Pes sterilised using high energy radiation in an inert or oxy- gen-depleted environment is evaluated and discussed. As is known [5], hydroperoxides (ROOH) are created in the early stages of oxidation while carbonyl groups form later. There- fore, the aim of this study is gaining knowledge of the first * Corresponding author. E-mail address: luigi.costa@unito.it (L. Costa). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.02.003 Polymer Degradation and Stability 91 (2006) 2030e2038 www.elsevier.com/locate/polydegstab