Occupationally-related low back injury and the resulting disability represent national health and economic problems of crisis proportions. For instance, medical costs associated with low back disorders are estimated to exceed 50 billion dollars annually. In attempts to protect workers from back injury during manual materials handling, significant progress has been made in the development of engineering models witch predict the muscular and spinal forces associated with specific lifting task. Unfortunately, relatively little is known regarding how these spinal forces are, in turn, linked to injury. This is the gap the research is intended to fill. Current injury tolerance criteria are based, in part on in vitro human cadaveric testing which describes the compressive strength of lumbar vertebral bodies. However, it is apparent that repeated spinal stress can lead to disc degeneration, and increased risk of injury, through more subtle, biologic pathways. With cadaveric testing, the body's normal process of degeneration due to cumulative loading and repair are missed. To clarify these factors, animal models can provide an important adjunct to cadaveric testing. However, no animal data currently exists which links the biological and biomechanical response of the disc to various static and dynamic loading regimens. Therefore, we have developed a mouse tail model in which controlled compressive stress can be applied to the intervertebral disc, and the biologic and biomechanical consequences monitored. Using this animal model preliminary studies demonstrate that disc degeneration is proportional to the magnitude, frequency, and duration of spinal loading. The goal of this proposal is to use these preliminary results and collect biological and biomechanical data from animals subjected to various spinal loading regimens in vivo. These data will be used to develop and validate a mathematical model that quantifies the degenerative response of the disc to various durations of dynamic compressive loading. Final results will be in the form that is appropriate for future combination with existing biomechanical lifting models, which together, can be used to refine occupational lifting limits.
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