What causes the naked facet sign in the spine on CT?
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Answer:
The naked facet sign is an uncovered articular facet on axial CT, where the normal apposition of the superior and inferior articular processes is lost because the facet joint is dislocated or distracted. It is characteristic of flexion-distraction injury of the cervical or thoracolumbar spine and indicates severe posterior ligamentous disruption and instability. The same finding is called the reverse hamburger bun sign, because the normal facet joint on an axial slice looks like a hamburger, the two apposed articular surfaces forming the buns and the joint space the patty; when a facet dislocates and locks, the convex surfaces face each other the wrong way round and the bun appears reversed. Dislocation may be unilateral, usually with rotation and often stable, or bilateral, which is grossly unstable and carries a high risk of cord injury; both need MRI for the disc, ligaments and cord before reduction. Perched and locked facets, an increased interspinous distance and anterolisthesis of about 25 per cent for a unilateral and 50 per cent for a bilateral dislocation are the accompanying findings.
Why is it called so?
The articular facet appears bare or naked on the axial slice because the process that should overlie it has moved away, leaving it without its partner.
Pathophysiology
Hyperflexion with distraction disrupts the facet joint capsule and the interspinous and supraspinous ligaments, allowing the vertebra above to translate forwards so that its inferior articular process no longer overlies the superior articular process of the vertebra below. Modelling work shows only about 5 to 16.5 degrees of rotation is needed to uncover the facet on axial images, depending on where the centre of rotation lies, which is why the sign appears before frank dislocation is obvious. Once the inferior process passes over the tip of the superior process it becomes perched and then locked in front of it, and the disrupted posterior ligamentous complex leaves the segment unstable in flexion.
Alternative names: Reverse hamburger bun sign; naked facet
Other associated named signs: Hamburger bun sign of the normal apposed facet joint, the headphones sign of uncovertebral joint injury in cervical facet dislocation, and perched and locked facets
References
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