1. Scope 1.1 It is the intent of this practice to permit an investigator to incorporate lipids found in the synovial environment into polymeric specimens. This can be used as a pre-conditioning step to evaluate the oxidative stability of Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) materials. This practice describes a laboratory procedure for pre-conditioning of UHMWPE specimens. 1.2 The pre-conditioned UHMWPE can be aged at elevated temperature and at elevated oxygen pressure following methods of accelerated aging described in ASTM F2003, to accelerate oxidation of the material and thereby allow for the evaluation of its long-term chemical stability. 1.3 The pre-conditioned UHMWPE can be tested without further aging using a method to evaluate oxidative stability such as oxidation induction time described in ASTM D3895. 1.4 Although the pre-conditioning method followed by accelerated aging described by this practice will permit an investigator to compare the oxidative stability of different UHMWPE materials, it is recognized that this method may not precisely simulate the degradative mechanisms for an implant during implantation. 1.5 The pre-conditioning and accelerated aging methods specified herein are intended to simulate the absorption of components, which may occur in UHMWPE following implantation and to determine susceptibility to oxidative changes. The methods have not been validated for use in pre-conditioning of UHMWPE components for subsequent testing of mechanical or wear properties. 1.6 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are for information only and are not considered standard. 1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.
Keywordsoxidation; pre-conditioning; stability; ultra-high-molecular-weight-polyethylene
Post-irradiation aging results in degradative changes to the physical, chemical, and mechanical properties of UHMWPE. Even under ambient conditions, oxidation of irradiated UHMWPE evolves at a slow pace, with a degradation rate measured in years. For irradiated UHMWPEs, the main factor determining oxidation levels and rates in vivo was thought to be detectable radiation-induced residual free radicals, but in recent years it has been shown that even irradiated (and melted) UHMWPEs without detectable free radicals at the time of implantation are prone to oxidative changes. The effect of synovial fluid lipids in initiating and accelerating oxidation in these materials was shown to be a previously unknown factor. As a result, pre-conditioning and accelerated aging test methods have been developed to accelerate oxidation in UHMWPE with the aid of synovial fluid lipids. Although research is still needed to elucidate the differences between in vitro conditioning/aging and long-term implant aging, this practice is intended to provide a specific procedure for incorporating lipids for evaluating the oxidative stability of UHMWPE specimens.
The title and scope are in draft form and are under development within this ASTM Committee.Back to Top
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