Spondylolysis is a defect in the pars interarticularis of a vertebra. The great majority of cases occur in the lowest of the lumbar vertebrae (L5), but spondylolysis may also occur in the other lumbar vertebrae, as well as in the thoracic vertebrae. Spondylolysis occurs in three to six percent of the population. Causes It is typically caused by stress fracture of the bone, and is especially common in adolescents who overtrain in activities such as tennis, diving, martial arts and gymnastics. Li… (More on Spondylolysis)