Os Terminale - Os Odontoideum Complex (3)

A sagittal-reformated CT image shows a characteristic appearance of os odontoideum (O). Note a wide gap between the os and the hypoplastic dens (D). The anterior arch of C1 (A) is hypertrophied.

Facts: Os Odontoideum

• Ossicle either in a normal odontoid tip (orthotopic) or near basi-occiput (dystrophic)
• Often fixed to the anterior C1 ring and the two move as a single unit
• Often asymptomatic (found incidentally) but some patients may have symptoms of C1-2 instability or at risk of developing cord injury following severe trauma

Imaging Features

• Smooth, small or large ossicle, can be rounded or oval or very bizarre and irregular in shape
• Hypoplastic dens. Wide gap between the os and dens
• Anterior arch of C1 is hypertrophied
• Jigsaw sign (a narrow joint space between the anterior C1 arch and the os, and an interdigitating joint line)

Differentiation of Unstable Os Odontoideum from Ununited Dens Fracture

• In ununited dens fracture, the dens is normal in size and configuration but there is nonunion through the base of the dens. Nonunited fragment becomes hypermobile and behave like an os odontoideum. However, either of this would need surgical stabilization

Clinical Implication

• The whole complex is variably unstable; therefore superimposed cervical spine trauma can make it more unstable and even can lead to acute cord injury
• Suggest instability if 1) forward flexion of C1 on C2 more than 2 mm on flexion view, or 2) the os is posterior to its normal location

References:

1. Fagan AB, Askin GN, Earwaker JWS. The jigsaw sign. A reliable indicator of congenital aetiology in os odontoideum. Eur Spine J 2004;13:295-300.

2. Truumees E. Os odontoideum. E-medicine, updated September 12, 2008.

3. Swischuk L. Imaging of cervical spine in children, 2004.