Atlanto-axial subluxation

Last revised by Mostafa Elfeky on 10 Jul 2023

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.

Atlanto-axial subluxations are often associated with high energy traumas, with higher occurrence among children and young adults 6.

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 the anterior arch of C1 and the dens in the fully flexed position should be <3 mm in an adult (~5 mm in a child) 5.

Also, the anterior translocation of the atlas causes the posterior arch of C1 to become anterior to the spinolaminar line 7.

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:

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Cases and figures

  • Figure 1: Fielding and Hawkins classification
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  • Case 1: on sagittal CT
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  • Case 2
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  • Case 3: with Jefferson fracture
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  • Case 4: traumatic
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  • Case 5: with rheumatoid arthritis
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  • Case 6
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  • Case 7: os odontoideum
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  • Case 8: in Down syndrome
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