This is usually the result of a vertical compression force ('blow out' fracture). The classic Jefferson fracture involves fractures of both the anterior and posterior arches bilaterally. Usual radiographic features are displacement of the lateral masses of C1 beyond the margins of the body of C2 (Figs 4.36 and 4.37). There is approximately a 41% chance of an associated C2 fracture, thus CT including C1-C3 is recommended  (Figs 4.38 and 4.39). One important caveat; the lack of fusion of the posterior arch may be seen in adults as a congenital anomaly defined by smooth margins. This fracture is usually unstable. Usually no neurological deficit is isolated, due to fragments being forced outwards.
Another important caveat is the rule of Spence. On an AP open mouth odontoid view, if the sum total of the overhang of both the C1 lateral masses relative to the body of C2 is greater than or equal to 7 mm, then the inference is that the transverse ligament is probably disrupted (which requires rigid immobilisation).
Describe the type of fracture (Figs 4.40 and 4.41).
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