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Neuroradiology Case of the Week

Case 423

July 2009

Lisa Siripun, MD and P-L Westesson, MD, PhD, DDS

Clinical Presentation: Patient is a 37-year-old male hit by a car.

Imaging Findings: See below

Diagnosis: Left unilateral locked facet with right perched facet

Discussion: Ligamentous injury of the cervical spine after hyperflexion injury or flexion rotation injury can produce subluxation, perched facet or locked facet of the inferior articular facet of the above vertebra and the superior articular facet of the vertebra below.
     Anterior subluxation of vertebral bodies usually occurs as a result of an excessive flexion force that causes disruption of the ligamentous complex that stabilizes the facet joint. Consequently, the superior vertebra undergoes forward subluxation, with anterior displacement of the corresponding inferior articulating facet on the superior articulating facet of the vertebra below. This results in uncovering of the articulating facet surfaces or "The Naked Facet sign". The degree of facet uncovering could be partial (subluxed facets) or complete (perched facets). Further flexion forces can transform perched facets into a facet lock.
     Unilateral facet lock is produced by the combination of flexion and rotation with rupture of the interspinous ligament and the capsule of the apophyseal joint. Such injury allows the ipsilateral inferior facet of one vertebra to dislocate and come to rest in the neural foramen anterior to the superior facet of the vertebral body below most commonly seen in C4-C5 and C5-C6. Both the posterosuperior aspect of the body below and the lamina above impinge on the spinal cord but only 30% of the patients experienced neurological deficit. Although anterior subluxation of the superior vertebra is typically present, the degree of subluxation is less than that seen with bilateral facet locks. Displacement and rotation of the articular facets produces the classic "bow tie" appearance which is formed when the articular facet of the superior vertebral body is projected at the anterior margin of the more inferior articular facet. On anteroposterior views, rotation of the spinous processes at the site of the lock and lateral step off of the spinous processes above the site of the injury. On CT scans, a frank lock is characterized by contact of the convex portions of the joints rather than the flattened portions. The axial CT image will reveal the reversed relation between the facet joints where the inferior facets of the vertebra above are displaced anterior to the superior facet of the vertebra below (back-to-back apposition).

References:

1. Lingawi S. The naked facet sign. Radiology. 2001 May;219(2):366-7. PMID: 11323458 [PubMed]
2. Scher A. Unilateral locked facet in cervical spine injuries. AJR Am J Roentgenol. 1977 Jul;129(1):45-8. PMID: 409141 [PubMed]