isthmic spondylolisthesis AKA spondylolysis: a failure of the neural arch as a result of a defect in the pars interarticularis (identifiable as a discontinuity in the neck of the “Scotty dog” on oblique LS-spine x-ray). May be seen in 5–20% of spine x-rays ¹⁾. Rarely produces central spinal canal stenosis since only the anterior part of the vertebral body shifts forward. May cause narrowing of the neural foramen.