Imaging of the atlas (C1) and axis (C2).

In imaging the atlantoaxial region in injured patients, the initial modality is plain radiography. The lateral C-spine as well as the open-mouth view are essential in this regard. On these views, it is not only important to examine the bony contour but also to look for indirect signs of injury such as prevertebral soft-tissue swelling, air in the prevertebral space, an increased width of the anterior atlantodental interval, and overriding of the C1-C2 joint on one side (the so-called wink sign of atlantoaxial rotatory subluxation). In patients in whom there is a high index of suspicion for occult trauma, but without fractures suggested or adequately visualized on routine films, or in those with severe cranial trauma, further studies should be pursued. CT scan is the modality of choice in optimally imaging the bony contours of the axis and atlas. It has limitations in visualizing transversely oriented fractures such as high dens fractures, transverse fractures of the facet joints (although widening of the facet joint is an indirect indication of facet fracture), or transverse arch fractures. Plain tomography may better demonstrate such transverse fractures but has several disadvantages. Plain tomography is often not as readily available as CT; it requires that the patient be placed in lateral decubitus position to obtain lateral tomograms, which may be contraindicated in such clinical circumstances; and it is not as easy to appreciate three-dimensional relationships on plain tomography as it is on CT. CT clearly defines the location of displaced bone fragments in relationship to the spinal canal as well as often demonstrating disc injuries. Ligamentous injury, though potentially visualized directly on MR imaging, is more commonly addressed with flexion-extension films. Flexion-extension studies should, obviously, be performed only in awake, oriented patients who are without neurologic deficit, and the studies should be done with close physician supervision and stopped at the first onset of pain. MR imaging may be helpful in demonstrating soft-tissue injuries such as hemorrhage, disc herniation, nerve root impingement, and direct spinal cord damage.