Spondylolysis and Spondylolisthesis
Spondylolysis is a defect in the pars interarticularis, commonly involving the L5 and occasionally the L4 vertebrae. Most spondylolysis is thought to be produced by repetitive stress. The gravitational and muscular loads acting across the steep incline of the upper surface of the sacrum can be resolved into a shearing component, which tends to displace the L5 vertebral body forward on S1, and a compressive component at right angles to the superior surface of S1 (Fig. 6-63). In accordance with…