• Spondylolysis is a unilateral or bilateral defect in the pars interarticularis portion of the vertebrae.
• It is a stress fracture mostly seen in the lumbar vertebrae, and most commonly L5.
• Pain is relieved with rest and worsened by lateral bending or extension (NOTE: most back pain is worsened by flexion).
• If neurologic symptoms and/or radiculopathy are present, an alternative diagnosis should be considered, because they are rarely associated with spondylolysis.
• Diagnostic imaging should start with plain radiographs with added oblique views.
• Classically, oblique views show the Scotty dog sign with a crack on the dog’s neck/collar, the pars.

http://www.gentili.net/signs/images/400/spinescottyparsdefectdrawing.JPG

The Scotty dog’s head (superior articular facet), nose (transverse process), eye (pedicle), neck (pars interarticularis), and body (lamina) should be easily identified on the oblique radiograph.
 

References