Spondylolisthesis, acquired: What is a classic radiographic sign of pars interarticularis defect on oblique views?

• A. Evidence of pars defect on sagittal MRI
• B. The collar sign of the pars interarticularis on oblique radiographs indicating defect
• C. Widening of the vertebral canal
• D. Sclerosis of the pars interarticularis on CT

Answer: (B)

On oblique lumbar radiographs, the classic sign of a pars interarticularis defect is the collar sign, which appears as a radiolucent line across the neck of the Scotty dog. This collar represents a fracture through the pars interarticularis and is the definitive radiographic clue for a pars defect (spondylolysis). While MRI or CT can show the defect or chronic changes, the oblique view’s collar sign is the hallmark finding. Widening of the vertebral canal or sclerosis on CT are not the characteristic oblique-view indicators of a pars defect.