Spondylolysis refers to a defect in one of the lumbar vertebra. There are five vertebrae in the low back area. The last one (L5) is affected most often. In spondylolysis there's a crack or fracture of the pars interarticularis. This area of the bone is a support structure between the main body of the vertebra and the spinal joints called facet joints.
Spondylolysis occurs in about six to seven percent of the adult population. This is just about the same in children and adults. In about 80 percent of patients with spondylolysis the body of the vertebra with the crack slips forward over the vertebra below it. This is called spondylolisthesis.