Spondylolysis is best described as what on imaging?

• A. Vertebral slip
• B. Disc protrusion
• C. Pars interarticularis defect; Scottie dog fracture on oblique X-ray
• D. Spinal canal stenosis

Answer: (C)

Spondylolysis is a defect in the pars interarticularis, most clearly seen as a Scottie dog fracture on an oblique view of the lumbar spine. The pars interarticularis is the bony bridge between the superior and inferior articular facets; on oblique radiographs the normal posterior elements outline a Scottie dog, and a fracture in the pars appears as a collar around the neck. This imaging sign is distinct from a vertebral slip (spondylolisthesis), a disc protrusion, or spinal canal stenosis, which have different appearances and underlying issues. Bilateral pars defects can predispose to slippage, but the classic description on imaging is the pars interarticularis defect with the Scottie dog fracture.