Abstract
Introduction
Vitamin-E (VE)-blended UHMWPE has been developed as a bearing-surface material due to the antioxidant ability of VE and has demonstrated a low wear rate in knee simulator [1]. Additionally, in vitro biological response testing has revealed that wear particles from VE blended UHMWPE induce the secretion of inflammatory cytokines at significantly lower levels compared to conventional UHMWPE [2]. However, as the joint kinematics are different between the knee and the hip, it is not guaranteed that these improvements will be repeated in the hip. In this study, the wear resistance of VE-blended UHMWPE was evaluated in knee and hip simulator tests and the effects of VE concentration and electron-beam irradiation were investigated.
Materials and Methods
VE blended samples (GUR_VE xx%) were manufactured via direct compression molding following the blending of UHMWPE resin powder with VE at several concentrations (0, 0.1, 0.3, 1.0%). Cross-linking for the VE samples was achieved by 10 MeV electron beam at several irradiance doses (30, 90, 300 kGy) and annealed below the melting point of UHMWPE.
Knee and hip simulator testing were carried out according to ISO 14243 and ISO 14242, respectively, and the volumetric wear was calculated. The gel fraction was determined by measuring the weight of the samples before and after soaking in decahydronaphthalene at 150°C. The oxidative resistance of the material was determined by measuring the Oxidation Index (OI) following ASTM F2102 before and after compulsory aging (ASTM2003). Radical measurements were made using high-sensitivity X-band ESR.
Results & Discussion
In knee simulator testing, the volumetric wear for GUR_VE was lower than that for non-blended UHMWPE (GUR), with this difference amplified by aging. Conversely, in hip simulator testing, the wear rates were the same for the GUR and GUR_VE samples, while the GUR_XL samples showed almost no wear both before and after aging. Additionally, the wear for the non-XL samples (GUR and GUR_VE) actually decreased after aging. Treatment utilizing electron-beam irradiation and annealing below the melting point achieved a high degree of cross-linking in VE-blended UHMWPE, while also preserving the antioxidant ability of VE. These results suggest that electron-beam irradiated VE blended UHMWPE can be used as a bearing surface material for hip prostheses. However, the longevity of prosthesis is determined not only by its wear performance, but also by its biocompatibility. ESR measurements have revealed that VE radicals are formed during radiation crosslinking of VE blended UHMWPE, and it is thought that the VE radicals may reduce the anti-inflammatory effects of UHMWPE particles containing VE. We are now developing a multidirectional lift-off type aseptic wear simulation device and procedures to measure the biological response to wear particles produced therein [3].
Acknowledgement
A part of this study was supported by the “Super special consortia” for supporting the development of cutting edge medical care (Cabinet office, Government of Japan) and Innovation promotion program (NEDO).
