Lateral atlantodental instability

Last revised by Daniel J Bell on 21 Mar 2021

Lateral atlantoaxial instability is a subtype of atlantoaxial subluxation, and is a poorly understood entity that may be encountered in post-traumatic and rheumatic patients with equal frequency to established sagittal plane atlantoaxial subluxation 1

There is limited information regarding clinical evaluation, imaging assessment, and treatment of lateral atlantoaxial instability. While it seems clear that upper cervical stabilizing ligamentous elements against lateral motion (such as the alar ligaments) are crucial for normal function, little is known about lateral stability assessment in clinical practice.

Radiographic features

Plain radiograph

Open mouth radiographs with right and left lateral bending may display excessive lateral translation of C1, however "normal" reference parameters have not been well-established. The paraodontoid space and/or lateral mass overhang distance may be measured, however lateral mass overhang is favored as it is less subject to projectional distortion from head rotation 2.

  • 1-2 mm of lateral mass overhang may be normal with lateral bending open mouth radiographs, thus measurements in excess of 2 mm should be considered suspicious for lateral atlantoaxial instability 3

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