STP1307

    Analysis of Morphological Alteration Accompanying Post-Irradiation Aging in Montell 1900 Ultra-High Molecular Weight Polyethylene (UHMWPE)

    Published: Jan 1998


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    Abstract

    For many years, ultra-high molecular weight polyethylene (UHMWPE) components of joint replacement prostheses have been sterilized with a 25 to 37 kilograys dose of gamma radiation in an air environment. Gamma radiation is known to cause chemical and morphological changes in polyethylene that continue to occur with both in vivo and shelf-life age. Generation of free radicals leading to predominantly crosslinking and chain scission (to a lesser extent) are immediate chemical effects. Slow oxidation of the free radicals contributes to further chain scission, thereby lowering the overall molecular weight of UHMWPE components with aging. Since lower molecular weight polyethylenes have a lower resistance to wear, oxidation-induced chain scission is believed to lower the wear resistance of UHMWPE components used in joint replacement prostheses.

    We have studied mechanisms of morphological alterations in UHMWPE components (Montell 1900) that accompany radiation-induced chemical changes with shelf-life aging. Differential scanning calorimetry on aged and unirradiated Montell 1900 UHMWPE revealed that there was an increase in crystallinity in components of post-irradiation aged Montell 1900 UHMWPE, in agreement with previous studies. Small-angle X-ray scattering measurements on unirradiated Montell 1900 UHMWPE revealed that there was a low degree of morphological order. One-dimensional correlation functions obtained by Fourier transformation of the scattering curves were employed to characterize the morphology of Montell 1900. The long period or the most probable center-to-center distance between neighboring crystalline lamellae, was 940 ± 15 and 550 ± 15Å. In the case of post-irradiation aged Montell 1900 UHMWPE, a new long period of 480 ± 15 Å was present in addition to the original long period. The emergence of a new long period along with an increase in crystallinity showed that recrystallization with post-irradiation aging occurred by growth of new crystallites in the amorphous domains in-between crystallites rather than by thickening of original crystallites. Growth of new crystallites was attributed to oxidation in the amorphous domains that releases constraints on interlamellar tie molecules by chain scission, thereby allowing the chains to form new crystallites.

    Keywords:

    polyethylene, ultra-high molecular weight polyethylene, molding materials, orthopaedic medical devices, sterilization, morphology, wear testing, radiation exposure, joint replacement prostheses


    Author Information:

    Bellare, A
    Research fellow and director, Brigham & Women's Hospital, Harvard Medical School, Boston, MA

    Spector, M
    Research fellow and director, Brigham & Women's Hospital, Harvard Medical School, Boston, MA

    Cohen, RE
    St. Laurent Professor of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA


    Paper ID: STP11910S

    Committee/Subcommittee: F04.02

    DOI: 10.1520/STP11910S


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