Which radiographic findings are characteristic of atlantoaxial subluxation (AAS) in rheumatoid arthritis (RA)?

Updated: Nov 01, 2018
  • Author: Michele Calleja, MD, FRCR, MRCP; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Only half of patients with radiographic evidence of atlantoaxial subluxation (AAS) are actually symptomatic. The role of plain radiography is to establish whether there are risk factors for cord compression. AAS is defined as an anterior atlantodental interval (AADI) greater than 2.5 mm in adults. This distance is measured as the interosseous distance between the posterior aspect of the arch of the atlas and the anterior aspect of the odontoid process. The point of measurement of the joint is a subject of debate; the inferior point is the most popular.

Plain lateral radiograph of the normal cervical sp Plain lateral radiograph of the normal cervical spine taken in extension shows measurement of anterior atlantodental interval (yellow line) and posterior atlantodental interval (red line).
Lateral flexion view of the cervical spine shows a Lateral flexion view of the cervical spine shows atlantoaxial subluxation.

There may be a slight variation in normal measurements between men and women. More importantly, it should be noted that an atlantodental interval of less than 2.5 mm, which changes considerably on flexion and extension, may also be abnormal. An AADI of 3-6 mm indicates early instability and implies transverse ligament damage. [11]  An AADI greater than 6 mm indicates that the alar ligaments are also damaged. Some authors consider an AADI greater than 9 mm to be an indication for surgical stabilization.

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