Spondylolysis is a disruption in a part of the vertebrae known as the pars, usually involving the fourth or the fifth lumbar vertebra in the lower back. In young people, spondylolysis may be a result of a stress fracture most commonly seen in active athletes, such as gymnasts, weightlifters, football players and dancers. It may also be caused by trauma or may be congenital.
In older patients, it is caused by degeneration of the joints and discs. Sometimes, spondylolysis may allow the vertebral bones to slip, throwing the spine out of alignment. In extreme cases, this may lead to spondylolisthesis, which may result in severe localized pain in the back as well as pain in the legs. This is slippage of one vertebra relative to another.