New Polymer for Joint Implants Retains Superior Toughness When Produced at Scale
Key findings
- Researchers at the Massachusetts General Hospital previously created a vitamin E–blended, peroxide cross-linked, high-temperature melted ultrahigh molecular weight polyethylene (UHMWPE) for use on the weight-bearing surfaces of total joint implants
- When prepared by a large-scale manufacturing process, the material showed excellent wear behavior, oxidation resistance and mechanical properties in vitro
- The osteolytic potential and periprosthetic biological effects of particulate debris were similar to those of a commercially available cross-linked UHMWPE
For years, the preferred bearing surface material for total joint arthroplasty implants has been cross-linked ultrahigh molecular weight polyethylene (UHMWPE). Cross-linking reduces the wear rate, and the incidence of wear-induced periprosthetic osteolysis has decreased substantially, resulting in better implant function and longer life.
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Cross-linking of UHMWPE results from the reaction of free radicals, which can be generated by exposing the polymer to ionizing radiation or by incorporating chemical agents such as peroxides. However, free radicals reduce oxidative stability. Incorporating an antioxidant, such as vitamin E, into radiation cross-linked UHMWPEs improves strength and wear resistance, and implants made from such materials have performed well for more than a decade.
In previous research, Orhun K. Muratoglu, PhD, director of Massachusetts General Hospital's Harris Orthopaedics Laboratory, and Ebru Oral, PhD, associate director of biomaterials, and colleagues substantially improved the impact toughness of UHMWPE by combining peroxide cross-linking, stabilization with vitamin E and high-temperature melting. This process also improved oxidation resistance. Surprisingly, despite a decrease in cross-link density, wear resistance was not compromised.
That study was done with a small amount of material, but Drs. Muratoglu and Oral now report that the polymer is feasible for scaled-up production. In the Journal of Biomedical Materials Research, they note that a major manufacturing advantage is the potential to eliminate the extra step of radiation cross-linking.
The researchers produced their polymer by ram extrusion, one of the common methods for large-scale consolidation of the UHMWPE bar stock from which implants are machined. They designated it PRX HTM. They compared it with a commercially available 100-kGy irradiated and melted UHMWPE that they designated CISM 100.
Mechanical Properties and Wear Behavior
- Wear rate: In both a pin-on-disc test and a hip simulator test, the wear rates of PRX HTM were similar to those of CISM 100
- Ultimate tensile strength was similar in the two polymers
- Elongation at break and impact strength were much higher with PRX HTM than with CISM 100, by 30% to 37% for elongation at break, and by 32% to 50% for impact strength. The researchers attribute this difference to the effect of the high-temperature melting
- Resistance to fatigue crack propagation was higher with PRX HTM than with CISM 100
- Oxidative stability was high for PRX HTM, as measured by oxidation induction time and corroborated by the results of accelerated aging. According to the researchers, this means oxidative fatigue damage is not likely to occur in PRX HTM in vivo
Biocompatibility
After hip simulator testing, the size and shape of PRX HTM particles were similar to those published for other cross-linked UHMWPEs. Based on these findings, the researchers believe the wear particles of PRX HTM will not cause any unexpected immune responses. They corroborated that expectation by implanting PRX HTM particles into mouse skulls, where their effects on osseous growth and breakdown did not differ from the effects of CISM 100.
The researchers acknowledge another concern about additives in UHMWPE: the effect of any extractables on periprosthetic tissues. They implanted PRX HTM and CISM 100 into knee joints and dorsum subcutaneous tissue of rabbits. Results of a standardized fibroblast assay and histology of periprosthetic tissues were unremarkable, demonstrating that contact with PRX HTM had no detrimental effects.
The research team points out that in large-scale manufacture of their novel polymer, consolidation and cross-linking occur simultaneously, so they expect peroxide decomposition to be completed during the ram extrusion process. Any remaining byproducts of peroxide chemistry should be volatilized from the polymer during the subsequent high-temperature melting.
The authors acknowledge there is no established correlation between any of the tests they ran and the clinical longevity of joint implants. Nevertheless, they feel confident that under similar loading conditions, PRX HTM will be more durable than currently available highly cross-linked UHMWPEs.
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