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Neuroradiology Case of
the Week
Case 221
Jared Christensen,
MD, and Sven Ekholm, MD, PhD
Clinical
Presentation:
A
55-year-old male post rollover motor vehicle collision with ejection.
Imaging Findings: CT of the cervical spine with sagittal and coronal reconstructions demonstrated an antegrade dislocation of C7 over T1 with reversal of the normal superior and inferior facet configuration bilaterally (see Fig. 1). There is anterolisthesis of C7 on T1 comprising approximately 50% of the vertebral body width. A compression fracture of the anterior superior endplate of the T1 vertebral body as well as the spinous process of C7 are also present. Posterior to the C7 vertebral body there are foci of mildly attenuating soft tissue density, likely representing portions of the disc, hemorrhage, and associated edema. There is narrowing of the spinal canal at this location, concerning for cord compression.
Dedicated MR of the cervical spine was performed which confirmed the CT findings. The cord was compressed on the left lateral aspect secondary to vertebral body subluxation consistent with cord compression; there was, however, no significant abnormal signal within the cord. Findings also confirmed ligamentous injury of the C7/T1 capsular complex.
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Figure 1: Noncontrast CT of the cervical spine. A. Right parasagittal reformat demonstrating the abnormal orientation of the superior facet (SF) of T1 lying posterior to the inferior facet (IF) of C7 resulting in a "locked facet" (arrow). The finding was bilateral. B. Axial image at the level of the C7/T1 facets again reveals the abnormal facet orientation. C. Normal orientation of the facet joints.
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Figure 2: Midline sagittal view demonstrating C7 on T1 anterolisthesis of approximately half the vertebral body width (arrowhead) resulting in cord compression. Associated T1 superior endplate compression fracture (open arrow) and C7 spinous process fracture (arrow) are present.
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Diagnosis: Bilateral locked facets at C7/T1 with associated C7 spinous process fracture, anterolisthesis, and cord compression
Discussion: The facet joints are normally maintained in a fixed orientation, with minimal physiologic movement in both flexion and extension. The supraspinous and interspinous ligaments, the ligamentum flavum, and the facet joint capsule maintain this anatomic relation. In severe flexion-distraction injury of the spine, disruption of these ligamentous structures occurs with or without fractures.
Excessive flexion 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. This has been termed the “naked facet sign” as the superior and inferior articulating facets lie uncovered or "naked."
The degree of facet uncovering may be partial (subluxed facets) or complete (perched facets). Further flexion forces can dislocate the superior facet of the lower vertebral body posteriorly, allowing the inferior facet to slide anterior and inferiorly, effectively transforming perched facets into locked facets. In this case 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. Lateral radiographs and sagittal CT reconstructions reveal the reversed relationship of the facet joints and an anterior subluxation of 50% or more of the superior vertebral body.
Unilateral locked facets (due to flexion and rotation) is considered a stable injury. However bilateral locked facets (flexion-distraction) is unstable as there is nothing to prevent further anterolisthesis of the upper vertebral body, compromising the central canal and intervertebral neural foramina in the process. Consequently, this injury pattern is invariably associated with neurologic deficits (75% of cases). MR is recommended to fully evaluate the extend of cord compression and ligamentous disruption. All patients with locked facets require stabilization and traction in hopes of spontaneous reduction. Surgery may be required emergently to restore the normal anatomic relationships depending upon the degree of neurologic compromise.
References:
- Berquist TH. Imaging of adult cervical spine trauma. Radiographics. 1988 Jul;8(4):667-94. [Medline]
- Lingawa SS. The naked facet sign. Radiology. 2001 May;219(2):366-7. [Medline]
- O'Callaghan JP, Ullrich CG, Yuan HA, Kieffer SA. CT of facet distraction in flexion injuries of the thoracolumbar spine: the "naked" facet. AJR Am J Roentgenol. 1980 Mar;134(3):563-8. [Medline]
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