Complete dislocation is fatal due to stretching injury of the brain stem. Subluxation is rare and can be missed on plain films. It should be suspected in patients with severe facial trauma, and the current methods used to determine subluxation are based on the distances between the occiput and axis which are as follows: Basion-axial interval (BAI) is the ­distance from the basion (inferior tip of the clivus) to a line drawn along the posterior body of C2, which should measure less than 12 mm. Vertical Basion-dens distance which should measure less than 12 mm (Fig. 22.2).

These fractures are discussed with spine trauma due to the mechanism and their association with other cervical spine fractures. They occur from high-energy blunt trauma due to axial loading and lateral bending. Fractures of the occipital condyles are often undiagnosed on plain radiography and prevertebral soft tissue swelling may be the only radiographic marker (Fig. 22.3). These fractures may extend into the hypoglossal canal or the jugular foramen; therefore, clinical signs of injury to cranial nerves IX to XII may be found. Anderson-Montesano divides occipital condyle fractures into three types (Table 22.5):

This occurs with axial loading injury with downward compression forces to C1, through the occipital condyles which can cause Jefferson fracture, Jefferson variants, or lateral mass fractures. The classic Jefferson fracture is a four-part fracture of the anterior and posterior arches (Fig. 22.4a). Jefferson variants are two- and three-part fractures (Fig. 22.4b) with the most common one being fracture of one anterior and two posterior arches. On frontal radiography, the presence of Jefferson fracture is implied by offset of the lateral masses of C1 relative to C2 and widening of the atlantodental interval. On MDCT with reformats, the fracture is easy to identify. These fractures can be unstable, if they are associated with disruption of the transverse atlantal ligament which fixes the dens to the anterior arch of C1 or when the lateral mass cumulative displacement is greater than 7 mm, if the atlanto-dens interval is greater than 5 mm or when they are associated with C2 fractures. Neurological compromise is infrequent as the fractures cause widening of the C1, thereby limiting cord compression. If the axial loading force is eccentric, a lateral mass fracture may result. This fracture is often associated with fracture of the occipital condyle or articular process, and they are regarded as unstable fractures because the transverse atlantal ligament is often detached.

Anderson and D’Alonso classified dens fracture based on the location as types 1, 2, and 3 (Table 22.6). Type 1 fracture (Fig. 22.5a) is rare but stable. Type 2 (either acute or ununited) (Fig. 22.5b) and Type 3 fractures (Fig. 22.6) are considered unstable because they can result in atlantoaxial instability (since the ligaments, proximal fragment, C1, and occiput act as a single unit during motion).

The os terminale is an ossification center at the tip of the dens, which normally fuses by 12 years of age but may remain unfused in adulthood and simulate a type 1 fracture. The os odontoideum formation is debatable as some authors think it is congenital due to lack of fusion with the base of the dens or posttraumatic fracture of the synchondrosis before closure at age 5–6 years resulting in a large ossicle that is separated from the hypoplastic dens by a wide gap. It may be fixed to the arch of C1 or the clivus and may simulate a nonunited dens fracture or rarely result in atlantoaxial instability [8].

Bilateral pars interarticularis fracture at C2 is known as hangman’s fracture and usually also involves the bilateral neural arches and the pedicles causing traumatic spondylolysis or spondylolisthesis (Fig. 22.7). They have been classified by Effindi and modified by Levine as follows (Table 22.7):