is the top vertebrae of the spine. It is made up of the atlas (C1) and the axis (C2). These vertebrae are atypical and are different in shape and function from the rest of the cervical vertebrae.

Normally, the shadows created by both sides of C2 superimpose, creating a distinct ring on the radiograph. Disruption of the ring suggests a fracture to the axis, particularly type III odontoid ...