What causes the dagger sign on the AP spine radiograph?
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Answer:
The dagger sign is a single, dense, vertical radiopaque line running down the midline of the spine on a frontal radiograph, produced by ossification of the supraspinous and interspinous ligaments. It is a feature of advanced ankylosing spondylitis and reflects the enthesitis that characterises the disease. It should be distinguished from the trolley track sign, in which the central line is flanked by two further vertical lines from ossification of the apophyseal joint capsules, giving three parallel tracks; the dagger sign is the central line alone and generally precedes the trolley track appearance. Both are late findings and appear alongside the rest of the radiographic spectrum: bilateral symmetrical sacroiliitis, squaring of the vertebral bodies from anterior ligamentous ossification and erosion, the shiny corner (Romanus) lesion, the Andersson lesion, marginal syndesmophytes and, ultimately, the fused bamboo spine. Modern practice detects the disease far earlier with MRI of the sacroiliac joints, so the dagger sign is now largely a marker of long-standing disease.
Why is it called so?
The single dense vertical midline stripe resembles the blade of a dagger laid along the spine.
Pathophysiology
Ankylosing spondylitis is driven by inflammation at entheses, the sites where ligaments and tendons insert into bone. Repeated cycles of inflammation, granulation tissue formation and reparative new bone formation at the interspinous and supraspinous ligament attachments cause those ligaments to ossify progressively along their length. Because both lie in the sagittal midline plane, their ossified substance is projected end-on as a single continuous vertical line on the AP view. When the apophyseal joint capsules ossify as well, their more laterally placed ossification adds the two flanking lines of the trolley track sign.
Alternative names: None
Other associated named signs: Trolley track sign, bamboo spine, shiny corner (Romanus) sign, squaring of vertebral bodies, and the Andersson lesion
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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