Spondylolysis is a defect in the pars interarticularis – a stress fracture from repeated microtrauma.  Heredity is also believed to be a factor. Approximately 3-7% of the general population has spondylolysis. Spondylolysis can be seen in both children and adults and is a common cause of low back pain in adolescent athletes.  

Approximately 95% of occur at the L5 level.  The process may be unilateral or bilateral, and may be painful or asymptomatic.  Bilateral spondylolysis can progress to significant slippage (or spondylolisthesis).