Atlanto-axial subluxation

Atlanto-axial subluxation 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:

  • anteroposterior subluxation
  • rotatory subluxation, known as atlantoaxial rotatory fixation (AARF) is characterized into four different types according to the Fielding and Hawkins classification 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 rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm
    • type IV: rotation and posterior displacement of the atlas
  • vertical subluxation
  • lateral subluxation 4

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.

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.

Possible differential considerations on imaging include:

Article information

rID: 942
Synonyms or Alternate Spellings:
  • atlantoaxial subluxation
  • Atlantoaxial subluxation
  • Atlanto-axial instability
  • AAS
  • AARF
  • atlantoaxial rotatory fixation

ADVERTISEMENT: Supporters see fewer/no ads

Cases and figures

  • Figure 1: Fielding and Hawkins classification
    Drag here to reorder.
  • Case 1: on sagittal CT
    Drag here to reorder.
  • Case 2
    Drag here to reorder.
  • Case 3: with Jefferson fracture
    Drag here to reorder.
  • Case 4: traumatic
    Drag here to reorder.
  • Case 5: with rheumatoid arthritis
    Drag here to reorder.
  • Case 6
    Drag here to reorder.
  • Updating… Please wait.

     Unable to process the form. Check for errors and try again.

     Thank you for updating your details.