Atlantoaxial Instability Workup

Updated: Jun 06, 2022
  • Author: Daniel P Leas, MD; Chief Editor: Jeffrey A Goldstein, MD  more...
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Laboratory Studies

No laboratory studies are relevant. For children, referral to a geneticist or endocrinologist may be beneficial for special laboratory evaluation.


Plain Radiography

Most injuries of the C1-2 articulation can be identified by means of plain radiography. [28]  Standard views include open-mouth odontoid and lateral cervical spine radiograph. For the highest degree of diagnostic accuracy, measurements should be obtained in neutral, flexion, and extension positions. However, it has been demonstrated that these measurements are not always reliable or reproducible and are not predictive of existing neurologic deficits or later progression of asymptomatic atlantoaxial instability (AAI) to symptomatic AAI.

On the open-mouth odontoid view, the combined spread of the lateral masses of C1 on C2 should not exceed 6.9 mm. A number greater than 6.9 mm would indicate rupture of the transverse ligament.

The presence of prevertebral swelling on the lateral film is an important finding for cervical spine trauma and should raise concerns about airway stability in the acute setting.

An atlantoaxial distance greater than 4-5 mm, as demonstrated by lateral radiographs, is indicative of AAI. Occult instability can be identified on the flexion-extension view. Another marker of instability in the anteroposterior (AP) plane is displacement of 3.5 mm in flexion-extension films. The normal atlantodental (or atlantodens or atlas-dens) interval (ADI) in children is less than 4 mm on a neutral-position lateral cervical spine radiograph.

Another marker is the posterior ADI (PADI), measured from the posterior border of the dens to the anterior border of the posterior tubercle. This index may be more important because it more directly assesses the space available to the spinal cord. The degree of neurologic deficits has been demonstrated to correlate with the PADI. [29]


CT and MRI

Use of computed tomography (CT) can provide additional information regarding the stability of the atlantoaxial joint. Measuring displacement of defining rotation may be difficult on plain radiographs. Fine-cut CT with reformatting can be used for measuring the amount of displacement. In addition, in a person in whom rotation deformity is suspected, patient-directed maximum-rotation CT scanning can delineate true rotational deformities. [30]

Magnetic resonance imaging (MRI) can provide additional information regarding the stability of the atlantoaxial joint, as well as associated soft tissue changes not visible on conventional radiography. [31, 32]


Other Tests

In patients with RA in whom a thorough physical examination is difficult, somatosensory evoked potentials are being explored as a means of following myelopathy.



Fielding and Hawkins suggested a four-part classification scheme for evaluating rotatory displacement, [33]  as follows:

  • Type I - Simple rotatory displacement with an intact transverse ligament
  • Type II - Anterior displacement of C1 on C2 of 3-5 mm with one lateral mass serving as a pivot point and a deficiency of the transverse ligament
  • Type III - Anterior displacement exceeding 5 mm
  • Type IV - Posterior displacement of C1 on C2

Both type III and type IV are highly unstable.