Atlanto-axial subluxation

Changed by Jignesh Modi, 21 Feb 2015

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Atlanto-axial subluxation (AAS) is a disorder of C1-C2 causing impairment in rotation of the neck. The anterior facet of C1 is fixed on the facet of C2. It may be associated with dislocation of the lateral mass of C1 on C2.

There are several ways in which a subluxation can occur:

antero-posterior subluxation
rotatory subluxation: characterized four different types ³
- type I: the atlas is rotated on the odontoid with no anterior displacement
- type II: the atlas is rotated on one lateral articular process with 3 to 5 mm of anterior displacement
- type III: comprises a rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm
- type IV: characterised by rotation and posterior displacement of the atlas
vertical subluxation
lateral subluxation ⁴

Radiographic features

Plain film (C spine)

In a non traumatic setting flexion and extension views may be performed. The expected distance between anterior arch of C1 and the dens in the fully flexed position should be <3 mm in an adult (~5 mm in a child).

In a vertical subluxation the dens is often above the McGregor line by over 8 mm in men and 9.7 mm in women

CT

On CT, C1 is not oriented in line with the head. The head may be pointed anteriorly, C1 is turned. If this is a fixed defect, C2 is rotated in conjunction with C1.

Predisposing factors

Congenital

Arthritides

Acquired

trauma
retropharyngeal abscess/Grisel syndrome

Differential diagnosis

odontoid fracture

+<li>lateral subluxation <sup>4</sup>
+</li>

References changed:

4. Reynolds MD. Lateral subluxation of atlanto-axial joint. Ann. Rheum. Dis. 1980;38 (5): 499. <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1000408">Free text at pubmed</a> - <a href="http://www.ncbi.nlm.nih.gov/pubmed/518151">Pubmed citation</a><span class="auto"></span>

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