Publications & Reports

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Book Excerpt: Vitamin E-Blended UHMWPE Biomaterials

During the past decade, there has been an explosion of international interest in the development of Vitamin E as an antioxidant for medical grade UHMWPE. Vitamin E blended UHMWPE has been internationally recognized and commercially released as a new international standard for orthopedic implants. This chapter excerpt outlines the scientific foundation for the acceptance of Vitamin E blended UHMWPE biomaterials. The complete chapter, to be published in 2009, summarizes the development of ideas in the scientific and patent literature that led to the commercialization of Vitamin E blended UHMWPE for use in orthopedics.

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Review of Wear in Highly Crosslinked UHMWPEs

This review summarizes the applications of crosslinked polyethylene in the hip. The first part of the article reviews the basic science concepts and terminology surrounding crosslinked polyethylene. The second part of the review is a critical assessment of the peer-reviewed literature on the subject of femoral head penetration and wear in crosslinked polyethylenes measured in clinical studies.

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Diffusion of Vitamin E in UHMWPE

Researchers from MGH have analyzed the diffusion behavior of vitamin E through UHMWPE and predicted penetration depth following doping with vitamin E. Crosslinked UHMWPE (65- and 100-kGy irradiation) had higher activation energy and lower diffusion coefficients than uncrosslinked UHMWPE, but there were only slight differences in vitamin E profiles and penetration depth between the two doses. By using homogenization in inert gas below the melting point of the polymer following doping in pure vitamin E, the surface concentration of vitamin E was decreased and vitamin E stabilization was achieved throughout a desired thickness.

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Effects of High Dose Irradiation on Cross-Linking of Vitamin E-Blended UHMWPE

Vitamin E-stabilized, highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is a promising oxidation and wear resistant UHMWPE with improved mechanical strength in comparison with the first generation, irradiated and melted UHMWPE. However, radiation cross-linking efficiency of UHMWPE decreases in the presence of vitamin E. Therefore an optimum vitamin E concentration and radiation dose level need to be determined to achieve a cross-link density comparable to 100-kGy irradiated and melted UHMWPE, which has shown excellent wear properties in vivo. Vitamin E concentrations equal to or above 0.3wt% (3000 ppm) in blended materials are not recommended for subsequent irradiation to achieve a wear resistant cross-linked UHMWPE.

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Stabilization of Irradiated UHMWPE by High Pressure Crystalization

Researchers from MGH have developed a method of eliminating the free radicals in irradiated UHMWPE without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. Reseachers tested the hypothesis that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE.

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Trace Concentrations of Vitamin E Protect Radiation Crosslinked UHMWPE from Oxidative Degradation

This research tested the hypothesis that the oxidation resistance of Vitamin E-blended UHMWPE would be influenced by trace doses of antioxidant, resin, and radiation treatment. Trace concentrations (<=500 ppm w/w%) of alpha-tocopherol (Vitamin E) were blended separately with GUR 1020 and 1050 resins, molded into disks, and irradiated according to different protocols. The results of this experiment supported the hypotheses that trace concentrations of Vitamin E, coupled with radiation treatment-but not resin grade-influence the mechanical and oxidative degradation behavior of UHMWPE.

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Effects of Packaging on Free Radicals in Vitamin E Blended UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) powder (GUR 1020) was blended with high concentration (20%) of vitamin E (alpha-Tocopherol (alpha-T)) for direct detection of alpha-T radicals in presence of PE radicals. When irradiated in air, alpha-T-resin produced ESR signals characteristics of tochopheroxyl radicals (alpha-T-O(*)), suggesting that PE radicals are quenched by alpha-T. When alpha-T-resin was irradiated in N(2), presence of both PE and alpha-T radicals were evident in the ESR spectra.

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Assessment of Oxidative Changes after In Vivo Degradation of UHMWPE Knee Components

This study reports on the suitability of different experimental techniques to evaluate chemical, microstructural, and mechanical changes associated with in vivo oxidation encountered in historical polyethylene components. The chemical, physical, and mechanical properties data confirmed the occurrence of in vivo degradation in the long-term implanted knee components following gamma irradiation in air. Furthermore, infrared spectroscopy alone appeared to provide excellent insight into the oxidation and crystallization state of the in vivo oxidized polyethylene.

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2008 AAOS Scientific Exhibit: Gamma Inert Sterilization: A Solution to Polyethylene Oxidation?

This study explores the hypothesis that implants sterilized in a low oxygen environment undergo similar in vivo oxidative mechanisms as implants sterilized in air. Over the past seven years, a multi-institutional retrieval program has traced and analyzed 264 conventional polyethylene hip and knee components. Measurable oxidation was observed in all cohorts. The oxidation was regional. Surfaces with access to bodily fluids were more heavily oxidized than protected, bearing surfaces. This variation appeared to be greater in gamma-air sterilized implants. The research therefore demonstrates that gamma inert sterilization may have improved, but not completely solved, the problem of polyethylene oxidation for hip and knee arthroplasty.

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Correlation of In Vivo Wear and Motion Patterns in TDR

This study combined the evaluation of retrieved total disc replacements (TDRs) with a biomechanical study using human lumbar spines. Thirty-eight CHARITE TDRs were retrieved from 32 patients after 7.3 years average implantation. In parallel, 20 new implants were evaluated at L4-L5 and L5-S1 in an in vitro lumbar spine model. This is the first study to directly compare the long-term PE wear and damage mechanisms in TDR retrievals with the motion patterns generated by a validated in vitro cadaveric testing model. The retrievals exhibited wear patterns consistent with the in vitro testing.

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