A general scheme for developing an accelerated aging method for irradiated biomaterials is proposed. Using UHMWPE implants as an example, an accelerated thermal diffusion oxidative aging (ATDOA) method has been developed. The method requires an optimum initial heating rate and an optimum aging temperature to accelerate oxidation reactions. Based upon oxidation-induced material property changes (crystallinity by DSC, tensile properties by ASTM D638 tensile test, oxidation index by FTIR, and low molecular weight fraction by GPC), correlations between accelerated aging time and post-radiation shelf aging time were obtained. The new ATDOA method allows a rapid evaluation of long-term irradiation effects on the material properties of UHMWPE implants. Ultra high molecular weight polyethylene (UHMWPE) has been used as an orthopaedic bearing material for more than two decades with satisfactory clinical results. The current research effort is aimed at prolonging its performance in vivo. As a virgin polymer, UHMWPE is chemically inert and stable. However, upon the forming and fabrication process (compression molding, ram extrusion, etc.) and high energy beam radiation ( such as γ-ray, x-ray, or electron beam irradiation, a step required for sterility), free radicals are formed causing the UHMWPE to become reactive.Oxidation can occur during irradiation, post-irradiation shelf-aging, or in vivo in the presence of oxidants. Accompanying the oxidation reaction are slow but progressive material property changes in UHMWPE which have been unobserved in the past, due to the lack of sensitive evaluation tools. In this study, we first evaluated shelf-aged UHMWPE specimens to obtain correlations between material properties and aging time. An accelerated aging method was then developed to simulate the long-term oxidation effects in a short period of time. Finally, material property changes of clinical retrievals reported in the literature were discussed.
Oxygen-containing chemical species may cause an artificially high level of measured oxidation in the infrared (IR) analysis of clinically retrieved or hip wear simulator tested ultra-high molecular weight polyethylene (UHMWPE) implants. Esterified fatty acids from the synovial fluid or calf serum commonly used in the hip or knee wear simulators are the most likely source of contamination. No such interference is encountered in the IR analysis of shelf-aged UHMWPE components that have never been implanted. When the contamination is observed, an additional IR peak at 1740 cm-1 appears along with the IR peak at 1717 cm-1 commonly assigned to the ketone group. A two-step decontamination procedure, with components first washed ultrasonically in a deionized water bath followed by an ultrasonic washing in heptane, was used in the study to remove possible contamination. This procedure is similar to one reported by James et al. Change in oxidation index before and after the two-step washing varied from 30 to 58% in the articulating region from a depth of 0 to 200 μm. From these results, the effect of contamination is significant and therefore a two-step washing procedure prior to IR analysis to remove water-soluble and organic contaminants is recommended.
Linear reciprocating pin-on-plate-type wear testing has been a standard technique for the screening of orthopaedic implant materials since the early 1980s. This investigation compares a wear screening technique based on linear motion with a modern hip joint simulator based on multi-axial motion. Two groups of differently sterilized UHMWPE samples were tested. The first group of samples was sterilized by ethylene oxide (EtO) gas that caused no structural changes in the UHMWPE. The second group of samples was sterilized in nitrogen by gamma-irradiation and then subjected to a stabilization treatment that resulted in a significant level of crosslinking in the UHMWPE. When tested on the linear reciprocating wear machine, the EtO sterilized specimens (non-crosslinked linear polyethylene) showed an approximately 30% lower wear rate than the gamma-irradiated and stabilized specimens (crosslinked polyethylene). When tested on the hip simulator, the EtO sterilized specimens exhibited two to three times higher wear rates than the gamma irradiated and stabilized specimens. The ranking of wear resistance obtained with the hip simulator was strikingly different than that obtained with the linear reciprocating wear machine. This study indicates that screening wear machines based on linear motion do not correlate with multi-axial joint simulators and may produce misleading results in the prediction of clinical wear performance of UHMWPE bearing materials.
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