Reverse hamburger bun sign | Radiology Signs

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.” The term was coined alongside the companion “hamburger bun sign” by Daffner and Daffner in 2002.

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.

Alternative names: Naked facet sign, hamburger sign (the reverse hamburger bun sign is synonymous with the naked facet sign in the literature)

Other associated named signs: Headphones sign (indicates associated uncovertebral joint injury), hamburger bun sign (the normal, non-dislocated counterpart appearance)

References

1. Daffner SD, Daffner RH. Computed tomography diagnosis of facet dislocations: the “hamburger bun” and “reverse hamburger bun” signs. J Emerg Med. 2002.