Naked facet sign (vertebral column)

The naked facet sign (also known as the hamburger sign or reverse hamburger bun sign) refers to the CT appearance of an uncovered vertebral articular facet when the facet joint is dislocated, most often in cases of locked facet.

This CT sign is characteristic of a flexion-distraction injury and indicates severe ligamentous disruption and spinal instability.

Normally, at axial CT, the vertebral facet (apophyseal) joint space looks like a hamburger: the superior articular process of the vertebra below forms the semicircular “bun” on top of the “meat patty,” and the inferior articular process of the vertebra above forms the bun beneath the patty.

When the facet joint is dislocated, the articular facets become uncovered, or naked. The top bun of the hamburger (the superior articular facet) now lies posteriorly. This is the hamburger sign/reverse hamburger bun sign, which may be either unilateral or bilateral depending on whether facet dislocation is unilateral or bilateral.