A 28-year-old male presents to the emergency department following a diving accident with axial loading injury to the cervical spine. An open-mouth odontoid radiograph is obtained. The structure marked **A** in the diagram appears centered and symmetric beneath the C1 atlas, with no lateral mass offset noted. Which of the following fracture patterns would be MOST LIKELY to show bilateral lateral mass overhang of C1 on this same view, representing the pathognomonic sign of instability?

Explanation

## Why Jefferson fracture (C1 burst fracture from axial loading) is right The Jefferson fracture is a burst fracture of the C1 atlas caused by axial loading (diving accidents, roof strikes in MVAs). On the open-mouth odontoid view, the pathognomonic sign is **bilateral lateral mass overhang** — the lateral masses of C1 project laterally beyond the articular masses of C2. This offset indicates disruption of the ring of C1 and is frequently associated with transverse ligament rupture, making it unstable. The normal centered odontoid (structure **A**) shown in the diagram helps establish the baseline; when lateral mass offset is present, it signals Jefferson fracture. Per Maheshwari Orthopedics, the open-mouth view is the KEY radiograph for assessing upper cervical spine pathology and recognizing this critical unstable injury. ## Why each distractor is wrong - **Odontoid fracture Type II (base of dens)**: Type II odontoid fractures are best visualized on lateral cervical spine views, not the open-mouth odontoid view. They do not produce lateral mass offset of C1; instead, they show a fracture line at the base of structure **A** (the dens). Type II is the most common odontoid fracture but is not characterized by the bilateral lateral mass overhang sign. - **Hangman fracture (bilateral C2 pars interarticularis)**: Hangman fractures involve bilateral fractures through the pars interarticularis of C2 (not C1). They result from hyperextension and are best seen on lateral views. They do not produce the characteristic bilateral lateral mass offset of C1 on the open-mouth view. - **Basilar invagination (cranial migration of odontoid)**: Basilar invagination is a vertical migration of the odontoid process through the foramen magnum, seen in conditions like Paget disease, rheumatoid arthritis, and osteogenesis imperfecta. It does not produce lateral mass offset on the open-mouth view and is a chronic process, not an acute traumatic injury. **High-Yield:** Jefferson fracture = C1 burst from axial load → bilateral lateral mass overhang on open-mouth odontoid view = pathognomonic unstable sign. [cite: Maheshwari Orthopedics 10e — Normal Cervical Spine, Open Mouth Odontoid View; Upper Cervical Fractures section]