Modeling of the naked facet sign in the thoracolumbar spine

On transverse computed tomographic (CT) scan cuts of the thoracolumbar spine, the naked facet sign occurs when the inferior articular facets of the cephalad vertebra do not appear adjacent to the superior facets of the subjacent caudal vertebra. The objective of this study was to determine the angles of rotation required for the naked facet sign to occur in the thoracolumbar spine, with the center of rotation located at various points in or anterior to the vertebral body. A commercial spinal model and visualization software were used to simulate various flexion injuries. Each functional spinal unit (FSU; T11-T12, T12-L1, and L1-L2) was examined separately. In the model, two CT scan slices (each 2 mm thick) were created parallel to the inferior end plate of the cephalad vertebra of each FSU. The cephalad vertebra was rotated in 0.5 degrees increments, and after rotation both modeled CT slices were examined for the presence of the naked facet sign. If the sign did not occur, the process was repeated, rotating the cephalad vertebra an additional 0.5 degrees until the naked facet sign occurred. The angle of rotation necessary for the sign to occur increases as the point of rotation of the vertebra moves from anterior to posterior and from superior to inferior. The naked facet sign occurred at a minimum rotation angle of 5 degrees (with respect to the anterior-superior point on T11) and at a maximum rotation angle of 16.5 degrees (with respect to the posterior-inferior paint on L1). For rotations about a point located 3 cm anterior to the vertebral body, the minimum angles required for the sign decreased only 1 degrees for each FSU. These results suggest that the naked facet sign does not consistently imply the presence of posterior column vertebral instability. This will help clinicians to relate the mechanism of injury, radiographic findings (including the naked facet sign), and the implied injury pattern to the determination of stability, and ultimately the management options for the injury.