What causes the hamburger bun sign in the vertebral column on axial CT imaging?
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The hamburger bun sign occurs due to the normal anatomical appearance of healthy vertebral facet joints seen on axial CT scans. It is disrupted when there is a facet joint dislocation, most commonly from a traumatic flexion-distraction injury causing severe ligamentous disruption and spinal instability. Such injuries lead to uncovering or dislocation of the articular facets, producing the so-called naked facet or reverse hamburger bun sign. This finding is frequently associated with locked facets in cervical spine trauma.
Why is it called so?
The name derives from the normal axial CT appearance of the vertebral facet joint resembling a hamburger, where the superior and inferior articular processes form the curved “buns” and the joint space represents the “meat patty.” When the facet joint is dislocated, this configuration is disrupted, and the uncovered facet resembles a “naked” or “reverse hamburger bun,” explaining the terminology.
Pathophysiology
Traumatic flexion-distraction forces stretch and rupture the ligamentous structures stabilizing the facet joints, resulting in facet dislocation. This dislocation causes displacement and uncovering of the articular processes on imaging, deforming the usual hamburger bun sign configuration. The superior articular process is displaced posteriorly relative to the inferior one, leading to the characteristic sign that signifies severe mechanical instability of the spinal segment.
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