Spondylolysis is a developmental condition or a stress fracture phenomenon, in which a defect in the pars interarticularis leads to varying degrees of instability in the lumbar spine. A finite element model was used to study the effect of pars defect as well as pars defect combined with varying slip on the change in flexibility of the lumbar motion segments under mechanical loading. Compared to the intact model, a pars defect at L3–L4 level allowed a small increase in motion at L2 with respect to L5 under all loading modes except torsion. Motion of L2 with respect to L5 was further increased by 3% to 20% with 50% translation prior to loading in all loading modes except torsion. Fifty percent translation increased the torsion motion further by 50%. In every model, the increase in motion was greatest under torsion. This study showed that stability of a motion segment in spondylolysis depends on the amount of spondylolisthesis and the loading condition. There was more motion with increased slip in all-loading modes with largest increase with torsion moment load. The type of loading also affects the degree of motion, with torsion creating significant instability.

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