AVS 46th International Symposium
    Biomaterial Interfaces Group Thursday Sessions
       Session BI-ThM

Paper BI-ThM10
Molecular Orientation in Artificial Joint Polymers: Characterizing the Precursors of Wear with Soft X-ray Absorption

Thursday, October 28, 1999, 11:20 am, Room 613/614

Session: Biomineralization
Presenter: D.A. Fischer, National Institute of Standards and Technology
Authors: D.A. Fischer, National Institute of Standards and Technology
S. Sambasivan, Brookhaven National Laboratory
M. Shen, University of Maryland, College Park
S. Hsu, National Institute of Standards and Technology
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Over half a million patients receive artificial joint replacements annually and practically all the replacements consist of a sliding pair represented by a polymer (ultra-high molecular weight polyethylene -UHMWPE) and a hard counterface (metal or ceramic). For the past 30 years UHMWPE has remained the dominant polymer in artificial joints due to its outstanding wear resistance properties. It has been recognized that wear of UHMWPE contributes to the loosening of the implants and is the main cause for the failure of long-term implants. Hence there is an urgent need to understand the mechanism and the surface morphology leading to wear and failure of the artificial joint. Molecular orientation in biomaterials is thought to be critical in characterizing the precursors of wear and the production of debris during the wear process. Current methods of inferring or deducing orientation are not accurate and often rely on staining and cutting specimens. In this study we use the electric field polarization dependence of soft x-ray absorption to directly determine molecular orientation in UHMWPE and evaluate the utility of this technique for evaluating artificial joint materials. We have measured the change in molecular orientation of ultra high molecular weight polyethylene (UHMWPE) samples subjected to various wear motions and duration. Two motions were used: a unidirectional and a cross-shear (motion to form figure-eight) motion. The observed orientations of the UHMWPE molecular chains using soft x-ray absorption are discussed and contrasted with the current understanding of the wear process in UHMWPE.