Degradation of mechanical behavior in UHMWPE after natural and accelerated aging
Introduction
Degradation of ultra-high molecular weight polyethylene (UHMWPE) following gamma sterilization in air has been associated with decreased longevity of some total joint replacement components [1], [2]. It has been estimated that in the United States alone, between 1980 and 1995, up to 4 million patients may have been implanted with UHMWPE acetabular and tibial components that were gamma irradiated in air [1]. While contemporary sterilization practices minimize the likelihood of degradation of UHMWPE implants, the sequelae of gamma irradiation in air will remain clinically relevant throughout the first decade of the 21st century.
Because oxidation of UHMWPE takes months or years to reach appreciable levels at ambient or body temperature, thermal aging techniques have been developed to accelerate the oxidation of UHMWPE, with the expectation that the mechanical behavior after accelerated aging will be comparable to naturally aged material [3], [4], [5]. Predicated upon this assumption, accelerated thermal aging of UHMWPE has been widely used by members of the orthopaedic community to test the resistance to aging of modified UHMWPE materials [6], [7], as well as to precondition components prior to fatigue [8] or hip simulator wear testing [4], [9], [10]. The physical and chemical properties of UHMWPE following gamma sterilization have been studied extensively after natural and accelerated aging [1], [2]. However, the effect of aging on UHMWPE mechanical behavior as derived from tests on clinically relevant hip or knee components has not been well documented, mainly due to the difficulty with using conventional, relatively large mechanical test specimens for such characterization. The development and validation of miniature specimen testing techniques, such as the small punch test, now permits direct measurement of UHMWPE mechanical properties as a function of aging in clinically relevant components.
Thermally accelerated oxidation of UHMWPE was initially validated by comparing the density and infrared absorbance spectra through the thickness of thermally aged and naturally aged components [3], [4], [5]. Sun et al. have argued that for gamma air-irradiated UHMWPE, 23 days of thermal aging at 80°C, with an initial heating rate of 0.6°C/min or slower, is equivalent to 10 years of natural shelf aging [11]. However, recent research has highlighted certain limitations of thermal aging techniques. For example, Greer and colleagues have referred to thermal aging of UHMWPE as a `severe oxidative challenge’ to distinguish the process from natural aging [9]. Another limitation is that there are substantial differences between the crystalline microstructure of thermally aged and naturally aged air-irradiated UHMWPE [12]. For example, the stacking of crystalline lamellae observed in thermally aged UHMWPE may be consistent with an annealing process, and differs from the twisted lamellae observed in naturally aged UHMWPE [12].
The mechanical behavior of UHMWPE evolves during natural (shelf) aging after gamma irradiation in air, but the kinetics and characteristics of mechanical degradation remain poorly understood, due largely to previous emphasis on indirect measurement techniques. Furthermore, while it is recognized that aging at elevated temperatures will accelerate the oxidation of air-irradiated UHMWPE, the clinical relevance of such a thermally degraded material remains uncertain, particularly if fatigue or joint simulator testing is to be performed after aging. Recent advances in miniature specimen testing methods [13], [14], [15], [16], [17], [18], [19], [20], such as the small punch test [13], [14], [15], [16], have enabled direct mechanical characterization of orthopaedic component sized samples. Consequently, the purpose of this study was to compare the mechanical behavior of UHMWPE tibial components after 5 and 10 years of natural shelf aging with tibial components that had been subjected to accelerated aging. This study also addressed the following research questions: (a) what quantifiable changes in mechanical behavior are associated with the degradation of UHMWPE after gamma irradiation in air? and (b) how does the mechanical behavior of UHMWPE after accelerated aging compare with clinically relevant UHMWPE that has been naturally aged for 5–10 years? To address these research questions, miniature disk-shaped specimens 0.5 mm thick were prepared from the surface and subsurface regions of naturally aged and thermally aged components. The specimens were then tested in equibiaxial tension using the small punch (disk-bend) testing technique to characterize the linear elastic and nonlinear large deformation mechanical behavior of the UHMWPE. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were also conducted to complement the mechanical testing results. The long-term goal of this research is to elucidate the mechanisms of mechanical degradation in UHMWPE and the impact on long-term clinical performance of orthopaedic joint replacement bearings.
Section snippets
Methods and materials
We studied eight knee implants of the same commercially available design and manufacture (Omnifit; Stryker Howmedica Osteonics, Allendale, NJ). All of the implants studied were manufactured from compression molded GUR 1120 (RCH 1000 grade) sheet stock using the same machinery and programming normally used to produce commercially available inserts. The implants were sterilized with a standard dose (25–40 kGy) of gamma radiation (in air) in 1988 (n=2), 1993 (n=2), and 1997 (n=4), respectively.
Results
The small punch load–displacement (mechanical) behavior for the UHMWPE from the control group displayed an initial peak load (during initial bending of the disk-shaped specimen), followed by a drawing phase under equibiaxial tension (Fig. 3a). Both the surface and subsurface locations of the control implants displayed very similar behavior (Fig. 3a). Compared with the control material response, accelerated aging resulted in a change in the material small punch load–displacement behavior at the
Discussion
Post-irradiation aging increases the elastic modulus and decreases the ductility, ultimate strength and toughness of UHMWPE after gamma irradiation in air, consistent with a progressive embrittlement process. After 10 years of natural aging, the elastic modulus did not vary significantly as a function of depth, but the small punch test metrics of initial peak load, ultimate load, ultimate displacement, and work to failure displayed prominent depth dependence. The accelerated aging employed in
Acknowledgements
This work is supported by research grants from Stryker Howmedica Osteonics. Special thanks to Jude Foulds, Exponent Inc., for many helpful discussions.
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