Atlanto-occipital dissociation injuries are severe and include both atlanto-occipital dislocations and atlanto-occipital subluxations.
On this page:
Pathology
The tectorial membrane and alar ligaments provide most of the stability to the atlanto-occipital joint, and injury to these ligaments results in instability due to low inherent osseous stability 3.
Classification
The Traynelis classification describes injuries according to the displacement of the occipital condyles relative to the atlas.
The AO Spine classification of upper cervical injuries is another classification system split into location-specific patterns and then further subdivided according to injury type and presence of neurological signs and/or modifying factors.
Radiographic features
The key to the diagnosis, in addition to identifying gross disruption of the normal alignment of the atlanto-occipital joint, hinges on 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)
-
>3 mm in adult males
>2.5 mm in adult females
CT
For pediatric patients, the condyle-C1 interval (CCI) has been shown to provide the highest diagnostic accuracy 4.
condyle-C1 interval (CCI) >4 mm in children
Differential diagnosis
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 will cause C1 lateral mass asymmetry relative to the dens
Down syndrome: atlanto-occipital instability due to laxity of the alar ligament
rheumatoid arthritis: CT/MRI will show atlantooccipital instability due to pannus destabilisation of joints and ligaments, and x-ray will show erosions
