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OCCIPITOATLANTAL DISSOCIATION (OAD) In occipitoatlantal dissociation, the skull may be displaced anteriorly or posteriorly or distracted from the cervical spine. OAD frequently results in death. Severe OAD is easily detected on the lateral radiograph. Occipitoatlantal subluxation is more difficult to detect on radiographs. Harris has described a method for detecting occipitoatlantal injury. The basion-axial interval (BAI) can be measured in most lateral radiographs. This is the distance between the basion and a line extending from the posterior cortex of C2. The BAI should not exceed 12 mm (Fig, 264:16). The basion-dental interval (BDI) is the distance between the basion and the superior cortex of the dens. This distance should also be less than 12 mm ( Fig . ¿64-17). Atlantooccipital injuries are extremely unstable and mandate immediate specialist consultation.

FIG. 264-17. Basion-dental interval (BDI).

TRANSVERSE LIGAMENT DISRUPTION The transverse ligament is located anterior on the inside of the ring of C1 and runs along the posterior surface of the dens. The transverse ligament is crucial to maintaining the stability of the first and second vertebrae. Pure ligamentous rupture without an associated fracture can occur in older patients with a direct blow to the occiput, as would occur in a fall. Without a fracture present, radiographic diagnosis relies on identifying the atlanto-dens interval, also known as the predental space, which is viewed on the lateral x-ray. The space is between the posterior aspect of the anterior arch of C1 and the anterior border of the odontoid. The space should be 3 mm or less in adults. More than 3 mm of space implies damage to the transverse ligament; more than 5 mm implies rupture of the transverse ligament. Immediate specialist consultation is necessary for these injuries.

ODONTOID FRACTURES One-third of all fractures of the ring of C1 occur in combination with fractures of the odontoid. 22 Odontoid fractures make up 7 to 14 percent of cervical spine fractures.23 Fractures of the odontoid are usually due to major forces and frequently involve other injuries to the cervical spine as well as multisystem trauma.24 Awake patients will usually complain of immediate and severe high cervical pain with muscle spasm aggravated by movement. The pain may not be severe and can radiate to the occiput of the head. Neurologic injury presents in 18 to 25 percent of cases. This can range from minimal sensory or motor loss to quadriplegia.25 Classification of odontoid fractures relies on identifying the level of injury. Type I fractures are avulsions of the tip. The transverse ligament remains attached to the dens, the fracture is stable, and the injury carries a good prognosis. Type II fractures occur at the junction of the odontoid and the body of C2. This is the most common odontoid fracture. Type III odontoid fractures occur through the superior portion of C2 at the base of the dens ( Fig.,.264:18). Immediate specialist consultation is recommended.

FIG. 264-18. Odontoid fractures.

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