What causes reverse hamburger bun sign in cervical spine on axial CT imaging?
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The reverse hamburger bun sign is caused by facet joint dislocation in the cervical spine, typically due to traumatic flexion-distraction injuries. This sign reflects the abnormal orientation of the articular facets when the inferior articular process of the vertebra above dislocates and becomes locked posteriorly relative to the superior articular process of the vertebra below. This injury is often seen in unilateral or bilateral locked facet joints, which are associated with severe ligamentous disruption and spinal instability.
Why is it called so?
The normal appearance of a cervical facet joint on axial CT resembles a hamburger, where the superior articular facet forms the “top bun” and the inferior articular facet the “bottom bun,” with the joint space as the “patty.” When dislocation occurs, this orientation is reversed, causing the “buns” to flip, thus resembling a hamburger turned inside-out or reversed, hence the term “reverse hamburger bun sign.”
Pathophysiology
The sign develops because traumatic flexion forces cause the inferior articular process of the superior vertebra to slide and lock behind the superior articular process of the inferior vertebra, disrupting normal facet joint congruity. This displacement exposes or “uncovers” the facet surfaces (“naked facet”), reversing their expected anatomical alignment on axial images.
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