Atlanto-occipital dissociation injuries

Changed by Mostafa Elfeky, 8 May 2017

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Atlanto-occipital dissociation (AOD) injuries are severe and include both atlanto-occipital dislocations and atlanto-occipital subluxations.

Pathology

The tectorial membrane and alar ligaments provide the most the stability to the atlanto-occipital joint, and injury to these ligaments results in instability due to low inherent osseous stability ³.

Radiographic features

The key to the diagnosis, in addition to visualising gross disruption of the normal alignment of the atlanto-occipital joints, is by using a number of lines on the lateral horizontal shoot-through cervical spine film ¹:

basion-dens interval (BDI) >10 mm in adults ³
basion-axial interval (BAI) >12 mm in adults
Powers ratio >1 (insensitive to a vertical distraction injury or posterior dissociation)
atlantodental interval (ADI)
- >3 mm in adult males
- >2.5 mm in adult females

Differential diagnosis

occipital condyle fracture
Jefferson fracture: anterior and posterior c1 ring fracture, possible lateral masses displacement
odontoid fracture: type 2 will cause posterior dens displacement and will disrupt Powers ratio
atlanto-axial subluxation: atlantoaxial rotatory fixation (AARF) will cause C1 lateral mass asymmetry relative to the dens
Down syndrome: atlanto-occipital instability due to laxity of alar ligament
rheumatoid arthritis: CT/MRI will show atlantooccipital instability due to pannus destabilizesation of joints and ligaments, and x-ray will show erosions

-<a href="/articles/rheumatoid-arthritis-musculoskeletal-manifestations-1">rheumatoid arthritis</a>: CT/MRI will show atlantooccipital instability due to pannus destabilizesation of joints and ligaments, and x-ray will show erosions</li>
+<a href="/articles/rheumatoid-arthritis-musculoskeletal-manifestations-2">rheumatoid arthritis</a>: CT/MRI will show atlantooccipital instability due to pannus destabilizesation of joints and ligaments, and x-ray will show erosions</li>

Images Changes:

Image 3 Annotated image (Powers ratio) ( update )

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Case 6: base of skull lines

Image 9 CT (bone window) ( create )

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