Abstract
Introduction
Increased oxidative stability of orthopedic implants can be achieved by adding an antioxidant, such as Vitamin E (VE) to UHMWPE[1]. The effect of shelf live and accelaterated aging in combination with shelf live on antioxidative effectiveness of VE needs to be investigated to better understand the long-time behavior of VE-blended UHMWPE in an oxidative environment. Currently, IR techniques provide detection limits as low as 0.05 %w/w[2], also it is known that thermo analytical techniques can push the limit of detection down to 10 ppm[3]. The goal of this study was to quantify VE in UHMWPE powder and compression molded UHMWPE with 11 different VE concentrations using FTIR and DSC techniques and to establish respective regression curves.
Methods & Materials
GUR 1050 UHMWPE resin (Ticona, Germany) was blended with VE (DSM, Netherlands) to the following target concentrations (%w/w): 0.01, 0.02, 0.05, 0.1, 0.25, 0.35. Concentrations of 0.001, 0.002, 0.003, 0.004, 0.005 were obtained by mixing of the 0.1 powder with virgin GUR 1050 powder. VE-free GUR 1050 was used as reference. Samples of blended resin were compression molded in a low oxygen environment atmosphere at Zimmer Inc. (Warsaw, IN). Subsequently the blocks were microtomed, creating films with a thickness of 200µm for each concentration. The VE concentration was measured using infrared (IR) spectroscopy (BioRad FT6000). The ratios were calculated by normalizing the integrated ether C-O(R) signal (1232cm−1 to 1275cm−1) and the integrated hydroxyl C-O(H) signal (1190cm−1 to 1228cm−1) using the twisting CH bond (1980cm∗∗∗∗∗ to 2100cm−1) as reference peak area. Oxidative induction time (OIT) at 200°C was measured according to ASTM D 3895–98 using blended and mixed powder samples.
Results
Infrared spectroscopy
A strong positive linear correlation between the normalized peak areas and the effectively blended VE content was detected for both ether (Figure 1) and hydroxyl ratios.
Figure 1: Normalized ether signal plotted over blended VE sample (928 IR spectra / 7 VE concentrations).
Samples below 0.01 %w/w were not FTIR tested as they fell below the detection limits of 0.0074 and 0.0092 %w/w[4] based on ether and hydroxyl ratios.
Oxidative-induction time
A logarithmic correlation between oxidation time and the effectively blended VE content was detected (Figure 2).
Figure 2: OIT over effectively blended VE concentration (36 data points / 12 VE concentrations)
Using OIT, VE concentrations down to 0.001 and 0.002 %w/w VE were quantified with Dabsolute of below 0.0002 %w/w and Drelative of below 20% to the regression (Figure 3).
Figure 3: Relative differences of quantification of VE over blended VE to regression via OIT and IR
Discussion
This study proves detectability of VE concentrations of 0.01 %w/w via calibrated IR absorbance and 0.001 %w/w using a calibrated OIT method at 200°C. Thus, mapping of VE chemical moities within UHMWPE samples below 0.05 %w/w VE by IR and detection of antioxidative stabilization in UHMWPE samples containing less than 0.002 %w/w VE can be achieved. Based on these findings, highly accurate VE measurements for aged, unaged, retrieved and differently processed materials shall be enabled.
