Cervical Lateral Mass Fracture-Separation (Floating Lateral Mass)

• Mechanism. A high-energy extension–compression or extension–rotation load drives the inferior articular process of the level above into the lateral mass, splitting it; the same force may be applied in lateral bending with axial load. Because it takes that much energy, an isolated finding should prompt a search for associated cervical and non-cervical injuries rather than reassurance.
• Presentation. Axial neck pain with restricted, painful rotation (the fractured facet complex is the restraint), often torticollis-like posturing to the injured side. Radicular symptoms in the exiting root are common because the root sits immediately anterior to the fractured mass — document power in the myotome for that level before any intervention. A Horner syndrome, hemiparesis, ataxia or posterior circulation symptoms point at the vertebral artery, not the root.
• Non-operative selection. Where the injury is a genuinely isolated lateral mass fracture with no translation or kyphosis, an intact discoligamentous complex on MRI and normal neurology, a rigid collar for a period of weeks with upright radiographs at intervals is reasonable — the critical part is the interval imaging, because late subluxation is the failure mode that converts a conservative plan into a delayed fusion.
• What pushes toward surgery: translation or facet subluxation, progressive kyphosis, a positive discoligamentous injury on MRI, radiculopathy that does not settle, or the same injury in a multi-level or polytrauma context where a collar cannot be relied on.

If posterior fixation is chosen, the segment is usually held with lateral mass screws, and the examinable content is the trajectory — because the two structures at risk sit either side of the screw.

• Roy-Camille. Start at the midpoint of the lateral mass, aim straight lateral by about 10 degrees and perpendicular to the posterior cortex (no cephalad angulation).
• Magerl. Start 1–2 mm medial and caudal to the midpoint, aim about 25 degrees lateral and parallel to the facet joint surface (roughly 30 degrees cephalad). More lateral and cephalad angulation.
• An. Start at the midpoint, aim about 15 degrees lateral and 30 degrees cephalad.
• The principle behind all three: aim LATERAL and CEPHALAD. Medial angulation threatens the vertebral artery in the transverse foramen; insufficient cephalad angulation threatens the exiting nerve root below. Getting that single sentence out is worth more than reciting exact degrees.
• Where lateral mass screws stop. They are used C3–C6 reliably. C7 has a thin, small lateral mass, so pedicle screw fixation is usually preferred there, and C2 takes a pars, pedicle or translaminar screw instead.
• A fractured lateral mass cannot hold a screw at the injured level, so the construct must span it — instrumenting the intact levels above and below rather than the fracture itself.