AVS 53rd International Symposium
    Biomaterial Interfaces Tuesday Sessions
       Session BI-TuP

Paper BI-TuP1
Effect of Cross-linking Ultra-high Molecular Weight Polyethylene on Surface Molecular Orientation and Wear Characteristics

Tuesday, November 14, 2006, 6:00 pm, Room 3rd Floor Lobby

Session: Biomaterial Interfaces Poster Session
Presenter: D.A. Fischer, National Institute of Standards and Technology
Authors: S. Sambasivan, National Institute of Standards and Technology
D.A. Fischer, National Institute of Standards and Technology
S.M. Hsu, National Institute of Standards and Technology
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Cross-linked ultra-high molecular weight polyethylene (UHMWPE) has become the dominant material used in human artificial joints due to substantial increase in wear resistance and ability to form conformal contacts. Wear debris resulting from wear has been identified as a major contributor to the eventual loosening of acetabular prostheses and failure of implants. Cross-linking of UHMWPE by gamma irradiation has been developed but the detailed mechanism of how cross-linking increases wear resistance is not understood. Wear of the UHMWPE surface layer changes the hardness and brittleness of the surface, and this, in turn, affects the wear resistance. This study uses a soft x-ray spectroscopic technique Near edge X-ray absorption fine structure (NEXAFS) to examine the degree of molecular orientation of the worn UHMWPE surface layer (up to 10nm). NEXAFS measurements at Carbon-K edge of worn UHMWPE samples which were subjected to gamma, ethylene-oxide (EtO), and electron beam sterilization techniques was conducted. Results conclusively suggest that cross-linking resists orientation when the samples were mechanically pulled or worn in a knee simulator. The molecular orientation in the C-C alkyl chains showed a monotonic decrease with increase in gamma radiation dosage levels suggesting highly crosslinked samples resists molecular orientation. EtO sterilized samples showed more C-C chain orientation compared hence less resistance to wear as compared to electron beam radiated samples. Direct comparison of surface molecular orientation of C-C alkyl chains of UHMWPE may offer explanation which of the cross-linking methods are more effective to produce a wear resistant artificial joints.