Dens Fracture

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.

Links to online textbooks

Wheeless' Orthopedics: discussion of dens fractures

References:

Blacksin MF, Lee HJ: Frequency and significance of fractures of the upper cervical spine detected by CT in patients with severe neck trauma AJR Am J Roentgenol. 1995 Nov;165(5):1201-4. [See related articles]
Marchesi DG: Management of odontoid fractures Orthopedics. 1997 Oct;20(10):911-6. [See related articles]

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.