Spondylolysis is a defect or fracture of one or both of the wing-shaped parts of a vertebra. These "wings" help keep the vertebrae in place. When a "wing" is absent, defective from birth, or damaged, a vertebra can slide forward or backward over the bone below, sometimes pressing on the spinal cord or a nerve root. This slipping, called spondylolisthesis, usually happens at the base of the spine. Symptoms of spondylolisthesis can include back pain and numbness or weakness in one or both legs, sometimes leading to a loss of leg function.

Spondylolysis has various causes. A vertebra can be defective from birth (congenital). It can also be broken by trauma or a stress fracture, or broken down by infection or disease. Spondylolisthesis is most commonly seen in children and adolescents involved in athletics.

Treatment for spondylolisthesis focuses on strengthening the supportive abdominal and back muscles while improving body mechanics in daily life. If severe pain and disability continue after physical therapy, surgery is sometimes done. Intensive rehabilitation is necessary after surgery.