Dens fractures are classified according to the scheme of Anderson and D'Alonzo. They are the result of severe forces including rotation across the cervical spine. Type I is an avulsion fracture arising from the tip of the dens at the site of the alar ligaments. Type II (high) is a fracture at the base without extension to the body of C2. Type III (low) is a fracture through the body of C2, not actually involving the dens. Os odontoideum (either congenital or post-traumatic) is a type I fracture, and may be unstable.
| A||Reformatted CT. This shows a fracture at the base of the odontoid, a type II injury.|
| B||Axial CT. This shows a fracture of the base of the odontoid (Type II).|
| C||Axial CT. The fracture at the base of the dens is visible.|
| D||Lateral radiograph of the cervical spine. This demonstrates posterior translation of C1.|
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