Isthmic spondylolisthesis is commonly observed in L4 or L5 of _ young people. This disease shows slippage of vertebra and is induced by spondylolysis that is caused by mechanical loading as discussed by Yamamoto et al.. Clinically, the frequencies of spondylolisthesis in L4 and L5 are not equal. It may be attributable to the differences of loading conditions between L4 spondylolytic and L5 spondylolytic lumbar spine. In spondylolisthesis, the facet joint cannot carry the body weight due to isthmic defect then the load on intervertebral disc increases. The effects of isthmic defect on the increase of the load and the change of stress distribution would depends on the level of the intervertebral disc because the boundary condition is different between L4 and L5 vertebra. Thus it is important to discuss the causes of spondylolisthesis from mechanical point of view. On the basis of the background, we evaluated the stress distributions in intervertebral discs in spondylolytic lumbar spine using a three-dimensional finite element model and discussed the mechanical causes of isthmic spondylolisthesis and disc degeneration.
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