Fig. 15

Bilateral versus unilateral facet joint dislocation. a Axial CT of the C5–C6 facet joint shows bilateral facet joint dislocation, suggested by reversal of the normal facet relationship, with convex surfaces opposing each other—called the “reverse hamburger sign” (white arrows). Loss of concentric relationship of the uncinate processes (black arrows) to the superior vertebral body—called the “positive headphone sign”. b Axial CT of the C4–C5 facet joint shows unilateral right-sided facet joint dislocation, suggested by a “naked facet” (white arrow)—due to absence of the opposing facet joint articular process because of dislocation. Loss of concentric arrangement of the right uncinate process (black arrows) due to rotation of vertebral body to the undislocated left side, resulting in unilateral positive “headphone sign”—suggesting rotational deformity