Modeling of the naked facet sign in the lumbar spine.

The study design is a computer visualization model that simulated flexion deformities about the lumbar spine for evaluation of the naked facet sign (NFS). The objectives were to ascertain the angles of rotation required for NFS to occur in the lumbar spine with various centers of rotation about the vertebral body and to assess whether NFS correlates with unstable flexion-distraction injuries in the lumbar spine. The presence of the NFS on axial computed tomography (CT) images occurs when the inferior articulating facet of the cephalad vertebra is not paired with an adjacent superior articulating facet of the caudal vertebra. This sign, when evidenced in the lumbar spine, is suggestive of significant injury secondary to a flexion-distraction force. A previous study using a computer-generated spine model challenged the utility of the NFS in the thoracolumbar spine. The NFS may prove to be more diagnostic of an unstable injury in the lumbar spine because of its normal lordotic resting position. A commercial spine computer visualization model was used to simulate various degrees of flexion injury in the lumbar spine. Lumbar functional spinal units (FSU) L2-L5 were each examined separately. The model simulated two CT scan slices (each 2 mm thick), which were created parallel to the inferior endplate of the cephalad vertebra of each FSU. The cephalad vertebra was rotated in 0.5 degrees increments until NFS was produced. The appearance of NFS required >/=11 degrees kyphotic angulation in more than two thirds of simulated centers of rotation about the lumbar vertebral bodies. The NFS was produced between a range of 8-24.5 degrees. For rotations about a point located 3 cm anterior to the vertebral body (to simulate seat-belt-type flexion-distraction injuries), the minimum angle required for NFS was 7.5 degrees. Our data correlate well with previously published results from in vitro and cadaveric studies. As opposed to the thoracolumbar spine, which normally rests in a neutral position, the lumbar spine normally rests in a lordotic position. Therefore, NFS in the lumbar spine may be more suggestive of an unstable injury and would warrant closer examination of the patient and additional radiographic studies.