Epidemiological studies suggest that repetitive loading is of importance in the development of back injuriesleading to early disc degeneration. In vitro studies have confirmed the biological plausibility of injuriesoccurring from excess loading. However, in vitro studies are limited in their ability to improve ourunderstanding of the complex relationships that exist between the large numbers of parameters needed todescribe the effects of single and repetitive load applications and how disc tissues break down over thecourse of many years of exposure. We believe that an improved understanding of these complexrelationships, namely how failure progresses under cyclic loading in discs with different grades ofdegeneration and how facet degeneration changes the load path within the disc, can best be achieved bynumerical techniques such as the Finite-Element Model (FEM). Our studies to date have shown thepotential of FE models to study disc responses to a variety of loading conditions. At this time the FEM isone of a healthy disc, while in actual life disc degeneration occurs early and may alter the response of thedisc to loading. The already developed and validated poro-elastic FEM of a healthy disc will be modified toreflect various stages of disc degeneration and will be used to test the following hypotheses: (1) Discfailure will occur after fewer repetitions and at lower load levels and will propagate more rapidly indegenerative discs compared to healthy discs. (2) The existence of fissures, clefts and cracks in the discwill cause redistribution of the stresses and cause abnormal motions resulting in further enhancing thefailure of disc material. (3) As the loading pattern on the discs and the disc structure changes, areas of highstress concentrations will occur in the facet joints leading to failure of the facet joint cartilage.These hypotheses will be addressed with the help of the following specific aims: (1) Determine thefailure progression due to cyclic loads in a healthy disc and in discs of different grades of degenerationusing a more refined poro-elastic FEM which includes re-bar elements to represent annular fibers and uses'user-defined' material models to determine failure progression continuously as the cyclic loadingprogresses. (2) Determine the effect of including fissures in the outer and inner annulus and cracks in theendplates on the progression of failure in degenerated discs. Poro-elastic parameters such as porosity andpermeability of the disc components will also be modified to reflect these failures. (3) Determine the loadson facet cartilage in discs of different grades of degeneration.
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