Naked facet sign | Radiology Signs

Answer:

Naked facet sign represents an uncovered vertebral articular facet due to facet joint dislocation, characteristically seen in flexion-distraction injuries of the cervical or thoracolumbar spine, indicating severe ligamentous disruption and spinal instability, which may be unilateral (often stable) or bilateral (unstable) and is associated with forced flexion, sometimes with rotation.

Why is it called so?

The sign is named for the “naked” or uncovered appearance of the articular facet on axial CT when the normal apposition of superior and inferior articular processes is lost due to dislocation, resembling an exposed facet without its overlying partner.

Pathophysiology

Facet dislocation occurs from hyperflexion with distraction, disrupting the facet joint capsule, interspinous, and supraspinous ligaments; this causes anterior displacement of the superior vertebra, rotating it such that the inferior articular facet of the cephalad vertebra no longer overlies the superior articular facet of the caudal vertebra, exposing the latter on axial CT slices (requiring 5-16.5 degrees of rotation depending on the center of rotation).

Alternative names: Reverse hamburger bun sign

Other associated named signs: Headphones sign (loss of the normal concentric uncovertebral joint appearance in cervical facet dislocation); hamburger bun sign describes the normal, non-dislocated facet joint apposition for comparison.

References

1. O’Callaghan JP et al. CT of facet distraction in flexion injuries of the thoracolumbar spine: the “naked” facet. AJR Am J Roentgenol. 1980;134(3):563-568.
2. Palmieri F et al. Uncovertebral joint injury in cervical facet dislocation: the headphones sign. Eur Radiol. 2006;16(6):1312-1315.