Otherwise known as isthmic spondylolysis, pars interarticularis defects are acquired, and thought to result from repetitive stress injury. Occurring most commonly at L5, they may also be seen at L4. There is an association with intensive physical training starting at a young age, including cricket and rugby. The image above demonstrates well the “incomplete ring sign”: disruption of the ring formed by anterior and posterior elements. Eighty percent of patients with this abnormality have no symptoms. There may be associated spondylolisthesis as anterior displacement of the upper vertebral body is no longer prevented by its inferior articular processes. Progressive spondylolisthesis occurs before skeletal maturity and is more common in females – the risk of this complication is 3-28%.

Reference: Ross JS, et al. Diagnostic Imaging: Spine. Amirsys 2004.

Credit: Dr Laughlin Dawes