Pars interarticularis defect
Defect in the pars interarticularis of a vertebra. Causes spondylothesis, but can have spondylothesis without spondylolysis. The defect is usually seen at L5 (82%) or L4 (15%), and rarely at L3 (1%). Most commonly bilateral. The pathogenesis is controversial. According to the most widely accepted theory the defect is in fact a fracture resulting from repeated asymptomatic microtrauma. On imaging with X-rays the finding is often visible in the lateral projection but it is best depicted in the oblique projections in which the upper and lower articular processes are degaged. An image of little dog (the "Scottie dog" sign) is produced in this projection by the structures of the vertebral arch in which the pars interarticularis represents the neck.