BIOMECHANICAL EFFECTS OF REPETITIVE LOADING ON THE LUMBAR MOTION SEGMENT

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Epidemiological studies suggest that repetitive loading is of importance in the development of back injuries leading to early disc degeneration. In vitro studies have confirmed the biological plausibility of injuries occurring from excess loading. However, in vitro studies are limited in their ability to improve our understanding of the complex relationships that exist between the large numbers of parameters needed to describe the effects of single and repetitive load applications and how disc tissues break down over the course of many years of exposure. We believe that an improved understanding of these complex relationships, namely how failure progresses under cyclic loading in discs with different grades of degeneration and how facet degeneration changes the load path within the disc, can best be achieved by numerical techniques such as the Finite-Element Model (FEM). Our studies to date have shown the potential of FE models to study disc responses to a variety of loading conditions. At this time the FEM is one of a healthy disc, while in actual life disc degeneration occurs early and may alter the response of the disc to loading. The already developed and validated poro-elastic FEM of a healthy disc will be modified to reflect various stages of disc degeneration and will be used to test the following hypotheses: (1) Disc failure will occur after fewer repetitions and at lower load levels and will propagate more rapidly in degenerative discs compared to healthy discs. (2) The existence of fissures, clefts and cracks in the disc will cause redistribution of the stresses and cause abnormal motions resulting in further enhancing the failure of disc material. (3) As the loading pattern on the discs and the disc structure changes, areas of high stress 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 the failure progression due to cyclic loads in a healthy disc and in discs of different grades of degeneration using 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 loading progresses. (2) Determine the effect of including fissures in the outer and inner annulus and cracks in the endplates on the progression of failure in degenerated discs. Poro-elastic parameters such as porosity and permeability of the disc components will also be modified to reflect these failures. (3) Determine the loads on facet cartilage in discs of different grades of degeneration.