Abstract
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.
Results & Discussion
The observed characteristic ESR peak for VE radicals was shown to decrease with time in the electron-beam-irradiated VE blended UHMWPE samples that were doped with ascorbic acid 6-palmitate (Figure 1 & 2). This particular spectrum was confirmed as that for VE radicals through g-value and line width analysis (Figure 3). These results showed that the number of VE radicals in electron-beam-irradiated VE blended UHMWPE was reduced by doping with ascorbic acid 6-palmitate at 100 MPa. The reduced VE radical may transform into a quinine, or react with other VE radicals to form dimmers/trimers. Due to the fact that VE radicals in UHMWPE have been shown to be stable at room temperature, it is thought that the observed reduction in number of VE radicals is a result of the direct action ascorbic acid 6-palmitate.
