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Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_15 | Pages 80 - 80
1 Mar 2013
Iwade H Kawasaki T Tajima K Sakurai Y Uetsuki K Turner A Tomita N
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Background

dl-α-Tocopherol (vitamin E) blended ultra-high molecular weight Polyethylene (UHMWPE) was originally developed as a bearing material for use in knee prostheses (1). The reduced biological response observed for vitamin E (VE) blended UHMWPE wear particles in in vitro experimentation (2) has also demonstrated the materials potential for use in other orthopedic applications, especially total hip arthroplasty (THR). However, due to the excellent results achieved by highly crosslinked UHMWPE in hip simulator testing (3), the use of VE blended UHMWPE in THR would similarly require crosslinking. It was previously reported that VE radicals are formed during radiation crosslinking of VE blended UHMWPE (4), and it is hypothesized that these VE radicals may negatively impact the materials biological activity. In this study, ascorbic acid 6-palmitate (lipophilic vitamin C) was applied to electron-beam-irradiated VE blended UHMWPE in an attempt to oxidatively reduce the VE radicals. Electron Spin Resonance (ESR) was used to measure the number of VE radicals within the material and evaluate the regenerating effect of ascorbic acid 6-palmitate.

Materials & Methods

UHMWPE resin powder (GUR 1050, Ticona, USA) was mixed with dl-a-Tocopherol (vitamin E) at 0.3 wt% and molded under direct compression at 25 MPa and 220°C. Virgin samples were produced by the same process, but without the addition of vitamin E (VE). Cylindrical pins (length: 40 mm, diameter: 3.5 mm) were then machined from these samples, packaged in a vacuum, and irradiated by electron-beam at 300 kGy. Samples were subsequently doped with either ascorbic acid 6-palmitate (Sigma, Japan) or ethanol (Ethanol 99.5%, Kishida, Japan) and subjected to a hydrostatic pressure of 100 MPa for 7, 14, and 21 days at room temperature. Radical measurements were made using ESR at 9.44 GHz and room temperature. All ESR spectra were recorded at 0.1 mW microwave power and 0.1 mT modulation amplitude.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XL | Pages 90 - 90
1 Sep 2012
Kawasaki T Hamada D Tajima K Sakurai Y Uetsuki K Tomita N
Full Access

INTRODUCTION

Electron-beam-irradiated dl-α-Tocopherol (Vitamin E)-blended UHMWPE is now being considered as a potential new bearing surface material for hip prosthesis [1]. However, Vitamin E stabilizes some of the primary free-radicals required for crosslinking, thereby reducing the material's crosslink density [2]. Additionally, some biological-stabilization effects of Vitamin E may also be reduced by oxidation. In this study, Vitamin E radicals in electron-beam-irradiated UHMWPE were measured and identified using Electron Spin Resonance (ESR), and the effects of annealing on radical stabilization and crosslink density were examined.

MATERIALS & METHODS

Both pure UHMWPE and Vitamin E added (0.3% w/w) resin was used to produce bulk specimens via vacuum direct compression molding at 220°C under 25 MPa for 30 min. Cylindrical pins (3.5 mm diameter, 40 mm length) for ESR measurement were then machined and placed in vacuum packaging. The pins were irradiated at 300 kGy, with half of each test group annealed at 80°C for 24 hours. Free radical measurements were made using a high-sensitive X-band ESR operating at 9.44 GHz. Detection of Vitamin E radicals was performed by comparing the characteristic symmetrical spectrum of oxidized Vitamin E to the spectra observed for the pins using both g-value and linewidth as references. Crosslink density was measured via gel fraction analysis and was performed in accordance with ASTM D2765. Thin sections (20 × 40 mm2, 200 μm) were machined from the bulk specimens, which were then placed in vacuum packaging, irradiated and annealed at the same conditions as those for the ESR measurements. Two of these thin sections were then placed in a stainless-steel cage (200 µm pore diameter) and were immersed in decahydronaphtalene at 200°C for 24 hours. These specimens were then extracted using soxhlet extractor at 100°C for 24 hours and dried in vacuum at 150°C for 12 hours.