Is Spondylolysis a Fatigue Fracture?

The forces acting across the pars interarticularis of the fifth lumbar vertebra while in the fully flexed and partially flexed postures are calculated. These values are compared with the strength of the neural arch obtained experimentally. This comparison shows the neural arch to be a relatively strong structure. However, results are presented which indicate that a spondylolytic type fracture can be initiated by fatigue loading. The mechanics of spondylolysis and spondylolisthesis are shown to be related through the shearing properties of the intervertebral disc.