What is the fat C2 sign on the lateral cervical spine radiograph?
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Answer:
The fat C2 sign is an apparent increase in the anteroposterior width of the body of the axis on the lateral cervical radiograph, produced when a fracture through the C2 body or a displaced fragment is projected over the vertebra so that its anterior margin appears duplicated and the body looks abnormally deep. It is an alerting sign for a C2 fracture, in particular a type III odontoid fracture extending into the body and some variants of traumatic spondylolisthesis of the axis, the hangman fracture. Its practical role is limited to the plain film era and to the initial trauma resuscitation film, because the required investigation in any adult with suspected cervical spine injury and a positive clinical decision rule is multidetector CT with sagittal and coronal reformats, which shows the fracture plane, the degree of displacement and angulation, and involvement of the transverse foramen, which raises the possibility of vertebral artery injury and prompts CT angiography. MRI is added when there is a neurological deficit, when ligamentous injury is suspected or when the patient cannot be clinically cleared. The other lateral radiographic markers of injury that should be checked with it are prevertebral soft tissue widening, disruption of the anterior, posterior and spinolaminar lines, interspinous widening and the harmony of the facet joints.
Why is it called so?
The projected fracture makes the body of C2 look wider, or fatter, than the vertebrae above and below it.
Pathophysiology
The axis has a complex three-dimensional shape, with the dens rising from the body and the pars interarticularis carrying load posterolaterally, so fracture lines through it rarely lie in a single plane. When a fracture separates the body obliquely, the two fragments are superimposed on the lateral projection and their anterior cortices are seen as two distinct lines, giving the appearance of a widened vertebra. The same superimposition explains why the sign is subtle and why displacement is easily underestimated on radiographs. Type III odontoid fractures pass through the cancellous body of the axis, which has a good blood supply and generally unites with immobilisation, whereas type II fractures at the base of the dens have a high non-union rate.
Alternative names: Fat C2; wide axis sign
Other associated named signs: Prevertebral soft tissue swelling, the naked facet sign and the double spinous process sign
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