This cervical spine trauma CT has both a peg (C2) and C1 fracture.

However, the C1 component initially was thought to be a Jefferson fracture (we all learn everyday), however, on retrospect is only a unilateral C1 anterior arch fracture.

The linear 'defect' through the posterior arches of the atlas (C1) are smooth and rather established-looking and are in fact thought to represent congenital non-union of the posterior arch lateral synchondroses.