What causes the trolley track sign on the AP spine radiograph?
Let me know in the comments.
Click to reveal the answer
Answer:
The trolley track sign is three parallel vertical radiodense lines running down the spine on a frontal radiograph. The central line is the dagger sign, ossification of the supraspinous and interspinous ligaments; the two lateral lines are ossification of the apophyseal (facet) joint capsules. It is a late finding in ankylosing spondylitis and represents more extensive ossification than the dagger sign alone, marking progression towards complete spinal fusion. It is not a sign of rheumatoid arthritis and has nothing to do with the hands. It appears alongside the rest of the late radiographic spectrum: bilateral symmetrical sacroiliitis progressing to ankylosis, squared vertebral bodies, shiny corner (Romanus) lesions, thin marginal syndesmophytes and, finally, the bamboo spine. Because these changes take years to develop, current practice diagnoses axial spondyloarthritis far earlier with MRI of the sacroiliac joints, where bone marrow oedema at the subchondral bone identifies active inflammation long before any radiographic ossification.
Why is it called so?
The three parallel dense lines running the length of the spine resemble the rails and central conductor of a tram or trolley track.
Pathophysiology
Ankylosing spondylitis is an enthesitis-driven disease in which repeated cycles of inflammation and reparative new bone formation ossify the ligamentous and capsular attachments of the spine. The interspinous and supraspinous ligaments lie in the midline sagittal plane and ossify first, projecting end-on as the single central line. The apophyseal joint capsules lie symmetrically on either side, some distance lateral to the midline, so when they ossify in turn their new bone projects as two further vertical lines flanking the first. The three lines together therefore record the sequence and the anatomy of ossification.
Alternative names: Tram track sign of the spine (avoid: tram track is used for other entities)
Other associated named signs: Dagger sign, bamboo spine, shiny corner (Romanus) sign, Andersson lesion, and squaring of the vertebral bodies
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
Improve Content And Help Your Colleagues!
Found an error in the post? Please let us know using our contact page and we will update it with due credits!
If you wish to contribute radiological images for this case (or any other article on the website), you can submit them here and you will be duly credited: Submit radiology case