J Am Acad Orthop Surg, Vol 16, No suppl_1, July 2008, S80-S85.
© 2008 the American Academy of Orthopaedic Surgeons
How have new sterilization techniques and new forms of polyethylene influenced wear in total joint replacement?
Roy D. Crowninshield, PhD and
Orhun K. Muratoglu, PhD
Dr. Crowninshield is Adjunct Professor, Department of Orthopedic Surgery, Rush Medical College, Chicago, IL. Dr. Muratoglu is Co-Director, Harris Orthopedic Biomechanics and Biomaterials Laboratory, Massachusetts General Hospital, and Associate Professor of Orthopedic Surgery (Biomaterials), Harvard Medical School, Boston, MA.
*The Implant Wear Symposium 2007 Engineering Work Group included Donald L. Bartel, PhD, Thomas D. Brown, PhD, Ian C. Clarke, PhD, Roy D. Crowninshield, PhD, Darryl D’Lima, MD, PhD, A. Seth Greenwald, DPhil(Oxon), Steven M. Kurtz, PhD, Jack Lemons, PhD, Michael T. Manley, PhD, Harry A. McKellop, PhD, Orhun K. Muratoglu, PhD, Ebru Oral, PhD, Lisa Pruitt, PhD, Clare Rimnac, PhD, Peter S. Walker, PhD, and Timothy Wright, PhD.
Dr. Crowninshield or a member of his immediate family has received research or institutional support and royalties from Zimmer. Dr. Muratoglu or a member of his immediate family has received research or institutional support and royalties from Zimmer and Biomet; has received miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from Zimmer; and is a consultant for Zimmer.
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Abstract
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Polyethylene has undergone many changes over the past several decades, including changes in consolidation processes, resin types, sterilization methods, packaging, and the extent of cross-linking. We believe that new sterilization techniques and forms of polyethylene have generally improved wear performance. Polyethylene sterilized without the use of radiation has been shown to have relatively high rates of wear in vivo. Ram-extruded polyethylene sterilized via gamma irradiation in air has been the most commonly used bearing material in the past several decades. Recently, components molded and gamma-sterilized without oxygen as well as highly cross-linked material have found increased clinical use. Exposure of polyethylene to radiation, either to sterilize it or to intentionally cross-link it, has been shown to improve the wear performance of the material. Newer second-generation methods of cross-linking polyethylene include the use of vitamin E, which quenches free radicals and demonstrates promise in providing low wear and desirable mechanical properties.
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Effect of Sterilization
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Alarge body of laboratory data exists reporting on total joint simulator wear performance of numerous forms of polyethylene. These data have provided the basis of clinical utilization of several new polyethylene materials in total hip arthroplasty (THA) and total knee arthroplasty (TKA). As many of these materials have now experienced significant clinical use, the primary source of evidence that these materials have markedly improved the wear behavior of polyethylene bearing surfaces should be documented directly through clinical experience, including wear measured by radiographic analysis, radiostereometric analysis, and implant retrieval.
Retrospective radiographically measured wear studies1,2 of polyethylene acetabular components sterilized either by exposure to gamma radiation or to gas plasma demonstrated that radiation-sterilized components experienced 0.085 mm of annual wear, half that experienced by gas plasma–sterilized components. This substantial decrease in wear rate was attributed to polyethylene cross-linking induced by the gamma irradiation. Shelf aging of the gamma-sterilized polyethylene components stored in air had slightly diminished wear performance.
A retrospective radiographic study3 of 201 THA patients reported the in vivo wear performance of both all-polyethylene and metal-backed acetabular components sterilized either by exposure to ethylene oxide gas or radiation in an oxygen-free environment. Sterilization by exposure to radiation in an oxygen-free environment reduced wear by 50% compared with the nonirradiated polyethylene. The effectiveness of barrier packaging to prevent the presence of ambient air around the component during sterilization and storage is specific to package type and a package’s ability to maintain a barrier to oxygen.4,5
In a radiographic review6 of 150 consecutive THAs, both polyethylene consolidation by molding and radiation sterilization in an inert gas contributed to reduced in vivo wear. The lowest wear rate was measured in molded components gamma-sterilized in an inert gas; greater wear was measured in ram-extruded components gamma-sterilized in an inert gas; and the highest wear was measured in the ram-extruded components gamma-sterilized in air. In a study comparing patients with Hylamer polyethylene components (DePuy, Warsaw, IN) sterilized by gamma radiation in air and patients with components sterilized in the absence of oxygen, the latter group experienced a 60% reduction in wear.7
These studies offer important insight into the relative clinical wear performance of different forms of polyethylene, but many compare polyethylene component designs and materials that differ from each other in more than one variable. To isolate the effect of material from that of implant design, an assessment of surface wear and damage on retrieved components was performed for three different polyethylene types used in a single THA implant system.8 The polyethylene types were representative of the sequentially available bearing materials that have dominated use over the last several decades. Forty-six components with implantation durations of 12 to 96 months were assessed for surface wear and damage and for volume changes resulting from socket wear and creep. Acetabular components made from highly cross-linked polyethylene had a 50% lower total damage score than did components made from polyethylene that was gamma-sterilized either in air or in nitrogen. The components made from polyethylene gamma-sterilized in nitrogen had a higher wear rate than did the highly cross-linked components and a lower rate than did the components gamma-sterilized in air. The measured wear/creep socket volume change was 80% and 90% lower for the highly cross-linked components compared with the groups gamma-sterilized in air and in nitrogen, respectively. These results of direct wear measurement on retrieved components are consistent with earlier predictions and clinical radiographic measurements, confirming that recent changes in polyethylene material processing can lead to clinically improved bearing performance.
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Effect of Cross-linking
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Highly cross-linked ultra-high–molecular-weight polyethylene (UHMWPE) has become the bearing material of choice for THA over the past decade, and its use in TKA is increasing. Clinical follow-up studies of cross-linked polyethylene have shown significant reduction in wear of acetabular liners in comparison with conventional, inert gas gamma-sterilized liners. These results offer corroboration of the findings of low wear in earlier laboratory hip simulator studies.9-14 Cross-linking is achieved by exposing the polymer to ionizing radiation, which creates free radicals. The recombination of these free radicals results in the formation of cross-links. Some free radicals reside in the crystalline domains of the polymer, which prevents them from recombining with each other. These free radicals can migrate, react with diffused oxygen, and lead to deterioration of the material’s mechanical properties.15,16
In THA applications, a radiation dose of 100 kGy (10 mrad) produces markedly reduced wear (Figure 1), although a polyethylene more moderately cross-linked at 50 kGy has also found use in total hip replacement. In TKA applications, lower radiation dose levels have been used in an attempt to balance reductions in mechanical properties with improved wear resistance.
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Figure 1 Pin-on-disk wear rate of ultra-high–molecular-weight polyethylene as a function of radiation dose. (Reproduced with permission from Muratoglu OK, Bragdon CR, O’Connor DO, et al: Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes [UHMWPE]. Biomaterials 1999;20:1463-1470.)
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To avoid long-term oxidation, first-generation highly cross-linked UHMWPEs are quenched by postirradiation melting or annealing to eliminate or reduce residual free radicals (Table 1). Annealing below the melt temperature can have less effect on the cross-linked polyethylene mechanical properties.17,18 However, analysis of surgically explanted components showed that postirradiation melting is more effective than postirradiation annealing in preventing in vivo oxidation.19,20 Surgically retrieved acetabular liners prepared with the latter method showed the presence of residual free radicals and rim oxidation,21,22 while the irradiated and melted polyethylene liners showed no detectable oxidation after as much as 5 years in vivo.19,20 The clinical significance of in vivo oxidation in THA remains unclear and is the subject of ongoing research.
Several second-generation technologies have recently been advanced to address the oxidation related to postirradiation annealing and the reduction in mechanical properties related to postirradiation melting (Table 2). The common feature of these technologies is that they avoid melting to stabilize the residual free radicals. Because they retain crystallinity, these technologies have the potential to produce cross-linked materials with mechanical properties closer to those of nonirradiated conventional polyethylene.
One second-generation material is sequentially irradiated and annealed (X3) and has been offered as an alternative to first-generation cross-linked UHMWPE. The hypothesis is that separating the irradiation into three steps with interspersed annealing steps improves oxidative stability compared to that which results from a single step of irradiation followed by annealing.23
Another second-generation technology is irradiation followed by mechanical deformation below the melting point. Deformation mobilizes the crystalline domains where most residual free radicals are trapped and allows their recombination. Deformation is followed by annealing to recover most of the anisotropy induced in the material by the deformation.24
Yet another second-generation technology is the stabilization of free radicals through impregnation of irradiated UHMWPE with vitamin E.25-31 Vitamin E protects the polymer against oxidation, rendering unnecessary melting with the accompanying decrease in mechanical and fatigue properties.30 Vitamin E quenches the free radicals by donating a hydrogen atom from its hydroxyl group. This mechanism also decreases the cross-linking efficiency of polyethylene if vitamin E is present during irradiation. Using vitamin E/polyethylene blends with lower vitamin E concentrations and higher radiation doses addresses this concern. However, because some of the vitamin E molecules are used up during irradiation, the long-term oxidative stability of irradiated blends is under investigation.
An alterative method is to diffuse vitamin E into an already radiation–cross-linked polyethylene by soaking the parts in vitamin E below the melting point of the polymer. Diffusion is followed by homogenization by heating to below the melting point to ensure that vitamin E penetrates through the entire thickness of the implant.
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Conclusions
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The clinical wear performance of UHMWPE in total joint arthroplasty is affected by changes in sterilization technique and material form. Radiation exposure of polyethylene is generally helpful in improving wear. This improved wear results from cross-linking induced in the radiation sterilization process. Radiation exposure and component storage in an oxygen-free environment can result in improved clinical wear performance in THA.8 Extensive cross-linking resulting from radiation exposure and postprocessing to stabilize free radicals can result in further improvements in wear performance.
Although further studies are needed to assess the longer-term performance of these changes in sterilization and material types, substantial short- to intermediate-term data demonstrate that the improved polyethylenes used in THA have led to better wear performance. These data support continued use of these new materials to prolong implant longevity.
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Future Directions for Research
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Clinical research must be directed to documenting the long-term outcome of the use of various forms of polyethylene in joint arthroplasty. Reliable, accurate methods for measuring clinical wear (eg, radiosterometric analysis), together with implant retrieval analysis and imaging assessment of osteolysis, should be used to document performance. No validated, reproducible methods of in vivo wear evaluation of tibial inserts in TKA currently exist, hampering efforts to assess the efficacy of new forms of polyethylene in TKA.
Although newer, highly cross-linked UHMWPEs can have improved strength and fatigue resistance compared with first-generation cross-linked materials, future research should focus on further increasing the strength and fatigue resistance of polyethylene without compromising wear and oxidation resistance. These improved materials may provide an opportunity to produce implant systems that are more forgiving of variations in surgical placement and orientation of implant components and to broaden implant design boundaries, allowing the design of more functional implants.
Clinical studies should be conducted to evaluate the effectiveness of second-generation highly cross-linked UHMWPEs that were recently introduced for clinical use. In addition, analysis of surgically retrieved implants should also continue to evaluate the performance of these newer materials.
Future research into the effects of antioxidants on polyethylene should be pursued. Vitamin E improves the oxidation resistance of irradiated polyethylene; however, the mechanism by which vitamin E in UHMWPE acts is unknown. The use of other antioxidants may have synergistic effects on the wear and mechanical properties of irradiated polyethylene.
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Figures
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Tables
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References
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Abstract
Effect of Sterilization
