Atlanto-Axial Subluxation

General Considerations

• Distance between the anterior surface of the dens and the posterior surface of the tubercle of C1 is usually 3 mm or less in adults and 5 mm or less in children
• This space is called by many names: predentate space, predental space, atlantodental distance
• The distance may increase slightly on flexion in children but is usually unchanged between flexion and extension in adults
• Forward movement of the atlas on the axis is normally restricted by the transverse ligament
• The transverse ligament is the primary restraint against atlantoaxial, anteroposterior  movement
• Atlantoaxial instability is defined by an increase in the predentate space of greater than 3 mm in adults and 5 mm in children
• Symptoms will be present when the atlas moves far enough forward on the atlas to narrow the spinal canal and impinge on the spinal cord
• The spinal canal is typically widest at the level of C2 and should not be less than 18 mm in ins widest AP dimension

• While chronic atlantoaxial dislocations which occur in the above diseases may be severe yet asymptomatic, acute atlantoaxial dislocations are more often symptomatic and can be life-threatening
• Traumatic atlantoaxial subluxation/dislocation usually results from a motor vehicle collision in which an unrestrained occupant’s head strikes the windshield or dashboard
• The pathologic mechanism involves hyperflexion of the neck
• Almost all atlantoaxial dislocations involve forward movement of C1 on C2; posterior dislocation is extremely rare
• Anterior atlantoaxial dislocations may be, but are not necessarily, associated with a fracture of the dens (~50% at autopsy)
• Associated fractures of the skull and/or facial bones are common
• This injury is unstable
• Neurologic injury occurs from cord compression between the odontoid and posterior arch of C1

Imaging findings

• Widening of the predentate space
• Disruption in the smooth curve of the imaginary line connecting the spinolaminar white lines of the vertebral bodies
• Soft tissue swelling
• C2 ─  retropharyngeal space should be < 7 mm
• C3 and C4 ─ retropharyngeal space should  be < 5 mm, or
• Less than half the AP diameter of C3 or C4
• C6 ─ retrotracheal space should be < 22 mm in adults and < 14 mm in children (under15 years)

Treatment

• These injuries are usually treated with some form of surgical fusion of C1 and C2

Atlantoaxial Subluxation. Lateral radiograph of the cervical spine. There is an increased distance of the predentate space between the anterior surface of the dens and the posterior surface of the anterior tubercle of C1 (white arrow) which should be less than 3 mm in an adult (blue line). The spinolaminar white line of C1 (blue arrow) is displaced forward of the smooth curve which should connect all of the spinolaminar white lines (yellow line). There is soft tissue swelling anterior to C1 (red line).
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Neck Injuries: II. Atlantoaxial Dislocation—A Pathologic Study of 14 Traffic Fatalities

Adams, VI Journal of Forensic Sciences 37:2, 1992