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