What forms the Scottie dog on the oblique lumbar radiograph?
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Answer:
The Scottie dog is the outline of the posterior elements of a lumbar vertebra as projected on a 45-degree oblique radiograph. The parts map exactly onto anatomy: the transverse process is the nose, the pedicle seen end-on is the eye, the superior articular facet is the ear, the pars interarticularis is the neck, the inferior articular facet is the foreleg, and the lamina and spinous process form the body. A defect in the pars, that is spondylolysis, appears as a lucent collar across the dog’s neck, or in a displaced lesion as a broken neck; the contralateral inferior articular facet forms the hind leg of the neighbouring dog. Recognition of the collar remains a useful skill, but oblique views add radiation dose for limited yield and have largely been superseded: CT best demonstrates the bony defect and its chronicity, and MRI or bone SPECT detects the pre-lytic stress reaction with marrow oedema before a defect is visible, which is the point at which the lesion is still reversible.
Why is it called so?
The projected posterior elements outline the head, ear, eye, neck and body of a Scottish terrier in profile.
Pathophysiology
The pars interarticularis is the narrow isthmus of bone between the superior and inferior articular facets, and it carries the shear and extension loads transmitted between adjacent facet joints. Repetitive hyperextension and rotation, typical of gymnastics, cricket fast bowling, football and diving in adolescents, concentrate stress at this isthmus and produce a fatigue fracture. Because the oblique projection displays the pars side-on as the narrowest part of the dog’s outline, a lucent fracture line crossing it appears as a discrete band, the collar. Bilateral defects allow the vertebral body to translate forwards, giving spondylolisthesis.
Alternative names: Scotty dog sign, Scottish terrier sign, Lachapele’s dog
Other associated named signs: Collar sign and broken neck sign of spondylolysis, the inverted Napoleon hat sign of high-grade spondylolisthesis, and the step-off sign
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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