Cervical Lateral Mass Fracture-Separation (Floating Lateral Mass)

A floating lateral mass (FLM) fracture-separation is an uncommon, high-energy subtype of subaxial cervical facet injury. What the literature supports:

• How rare: the only dedicated FLM cohort (Prezelski et al, Spine 2024) identified 45 patients over a 10-year span at a single centre - a useful honest anchor for how infrequently the pattern is seen, rather than a population incidence.
• The trauma context: in a 4-year series of 690 blunt cervical spine injuries (Du et al, Spine J 2024), 66% were screened with CT angiography and BCVI was found in 30% of those screened; all 9 strokes in the series occurred in patients with BCVI.
• The vascular context: in the Denver screening experience (Biffl et al, Ann Surg 2000), blunt vertebral artery injury was diagnosed in 0.53% of blunt trauma admissions, motor vehicle crash was the most common mechanism, and 71% of patients with a vertebral artery injury had a cervical spine injury.
• Mechanism: high-energy extension-compression, extension-rotation or lateral-bending axial loads; the energy required means associated cervical and non-cervical injuries are the rule, not the exception.

• The lateral mass is the column of bone between the superior and inferior articular processes of a subaxial vertebra. Its bony tethers are the pedicle anteromedially and the lamina posteromedially - fracture both and the mass, carrying both articular processes, is free.
• The neighbours that get hurt: the vertebral artery runs in the transverse foramen immediately lateral to the mass and pedicle; the exiting nerve root passes directly anterior to the mass in the neuroforamen. These two structures define the injury's danger list.
• Why isolation means instability: the facet joints guide and restrain cervical rotation and flexion, and the lateral mass is the only bony link between the facet joint above and the facet joint below. Isolate it and both joints lose their anchor - the segment is free to rotate, sublux and kyphose, and the detached mass itself may rotate into a horizontal orientation on axial imaging.
• Load path: with the posterolateral column disrupted, loads that the facets would normally share are transferred to the disc and the remaining ligaments, which is why an associated discoligamentous injury so often decides the management.

• 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.

• CT (fine-cut) is definitive: it demonstrates the paired pedicle and lamina fractures, the alignment of both facet joints, and any rotation of the detached mass. Plain radiographs are insensitive - they may show malalignment but cannot exclude the injury.
• CT angiography screens the vertebral artery: the only fracture pattern independently associated with BCVI in the Du 2023 series was fracture with dislocation/subluxation (OR 3.8), with combined upper-cervical (OC-C3) and multilevel fractures also at increased risk - an FLM with any subluxation sits squarely in the screen-positive group.
• MRI is added when the discoligamentous complex must be assessed (any consideration of non-operative management) or when there is a neurological deficit to explain.
• Interval upright radiographs are not optional extras in non-operative care - they are the surveillance that catches the late subluxation which converts a collar plan into a delayed fusion.

• Approach-specific (Prezelski 2024 cohort): complications clustered in the posterior group (two hardware failures) and the combined group (two postoperative respiratory complications); none were observed in the anterior group.
• Vascular: an unrecognised vertebral artery injury is the catastrophic complication - in the Denver series the incidence of posterior circulation stroke was 24% and the BVI-attributable death rate 8% (Biffl 2000), which is the entire argument for CTA screening and antithrombotic therapy.
• Neurological: exiting nerve root injury at the fractured level; cord injury is uncommon unless there is subluxation or translation.
• Non-operative failure mode: late subluxation or progressive kyphosis - the reason interval upright imaging is mandatory rather than optional.
• General surgical risks: infection, pseudarthrosis, adjacent-segment degeneration, and screw-related vertebral artery or nerve root injury (medial or insufficiently cephalad trajectory).

• Non-operative (selected): in the Prezelski cohort, none of the 25 non-operatively treated patients crossed over to surgery for subluxation - evidence that careful selection works, from a single-centre retrospective series.
• Operative: all three approaches achieved stabilisation; complications were approach-specific (posterior hardware failure, combined respiratory) rather than failures of fusion.
• Vascular outcome dominates when BCVI is present: Biffl's series associated systemic anticoagulation with fewer poor neurological outcomes, fewer strokes and less injury progression - the basis of modern antithrombotic-first management.
• Overall prognosis is governed less by the fracture than by the associated injuries and the neurological status at presentation; an isolated FLM with intact neurology and no BCVI has a favourable outlook once stabilised.

• Classification language: the AOSpine subaxial system (Vaccaro 2016) is the international common language for reporting these injuries; SLIC (Vaccaro 2007) remains the widely taught severity-score framework for the operative/non-operative threshold.
• BCVI screening: trauma-society screening protocols derived from the Denver and Memphis experiences endorse CT angiography for high-risk cervical fracture patterns - fracture-dislocation/subluxation, transverse-foraminal involvement and upper-cervical or multilevel fractures; the Du 2023 data support exactly that selection.
• Antithrombotic-first management of BCVI (antiplatelet or anticoagulation for Biffl grades I-IV, with endovascular treatment reserved for enlarging pseudoaneurysm or transection) is the consistent international practice pattern.
• Practice variation: the operative-versus-non-operative threshold for a minimally displaced FLM varies between centres and health systems; the published FLM cohort is single-centre and retrospective, so local protocols and MDT discussion appropriately govern borderline cases.