Introduction
Definition: A severe cervical spine injury involving the disruption of the posterior ligamentous complex (PLC) and the displacement of the zygapophyseal (facet) joints.
Mechanism: Typically high-energy flexion-distraction injury (e.g., motor vehicle accidents, diving into shallow water).
Stability: Highly unstable; by definition, involves disruption of the disc, the facet capsules, and the posterior ligaments.
Classification
1. Unilateral Facet Dislocation
Mechanism: Flexion + Rotation.
Imaging: Displacement of < 50% of the vertebral body width.
Stability: More stable than bilateral, but still considered unstable.
Neurology: Often presents with radiculopathy (nerve root compression at the level of the foramen).
2. Bilateral Facet Dislocation
Mechanism: Pure flexion-distraction.
Imaging: Displacement of > 50% of the vertebral body width.
Stability: Highly unstable (3-column injury).
Neurology: High incidence of complete or incomplete spinal cord injury (SCI).
Imaging Findings
X-Ray (Lateral):
"Bowtie" or "Butterfly" Sign: Seen in unilateral dislocations where the two facets no longer overlap perfectly.
Loss of spinous process alignment (AP view).
CT Scan: The gold standard for identifying "naked facets" (the facet joint is empty) and associated fractures (e.g., facet fractures).
MRI: Essential for evaluating:
Disc herniation: Crucial for surgical planning (identifying a herniated disc that could be pushed into the cord during reduction).
Spinal cord edema/hemorrhage.
PLC integrity.
Treatment Algorithm
1. Initial Management
Rigid cervical collar (Miami J or Aspen).
Neurological assessment (ASIA score).
2. Reduction (The "Great Debate")
Closed Reduction (Cranial Traction):
Indications: Awake, alert, and cooperative patient with a neurological deficit.
Goal: Immediate decompression of the spinal cord.
Risk: Potential for a herniated disc to cause worsening neurology during traction.
MRI Before Reduction?
If patient is awake/examining: Proceed with traction/reduction and monitor neuro exam.
If patient is obtunded/unconscious: Get an MRI first to rule out a herniated disc.
3. Surgical Management
Anterior Approach (ACDF): Usually preferred if there is a significant disc herniation anteriorly.
Posterior Approach (Post. Instrumented Fusion): Preferred for irreducible dislocations or when superior biomechanical stability is needed.
Combined (360°): Reserved for highly unstable, multi-level, or osteoporotic bone.
Allen-Ferguson Classification (Cervical Spine)
This classification is based on the mechanical mechanism of injury. Facet dislocations fall under the Distraction Flexion (DF) category.
Stage | Description & Findings | Stability |
DF Stage 1 | Facet subluxation and interspinous widening ("Blunted facets"). | Relatively Stable |
DF Stage 2 | Unilateral Facet Dislocation. (Classic "Bowtie sign" on lateral X-ray). | Unstable |
DF Stage 3 | Bilateral Facet Dislocation. (50% translation/displacement). | Highly Unstable |
DF Stage 4 | Complete displacement ("Floating vertebra" or Spondyloptosis). | Extremely Unstable |
CRITICAL CLINICAL DECISION: The MRI vs. Traction Controversy
This is a high-yield topic for board exams and real-world ER management.
The Dilemma:
Reducing a facet dislocation (via traction) when an undiagnosed traumatic disc herniation is present can displace disc material into the spinal canal, leading to sudden and permanent neurological catastrophic loss.
Protocol 1: The Awake and Cooperative Patient
Action: Proceed with immediate Closed Reduction (Cranial Traction) under serial neurological monitoring.
Rationale: The patient can provide real-time feedback. If their neurological exam changes or pain increases during weight increments, you stop immediately. In these cases, early reduction is often more beneficial than waiting for an MRI.
Protocol 2: The Obtunded or Unconscious Patient
Action: Pre-reduction MRI is MANDATORY.
Rationale: Because the patient cannot provide a neurological exam, you must visualize the disc space before applying traction to ensure you aren't pulling a herniated disc into the cord.
Protocol 3: If a Large Disc Herniation is Found
Action: Anterior Discectomy and Fusion (ACDF) FIRST.
Rationale: You must remove the "retropulsed" disc material before attempting to reduce the facets to prevent cord compression during the maneuver.
Pits & Pearls
Pearl: The "Bowtie Sign" on a lateral X-ray is your biggest clue for a unilateral dislocation.
Pitfall: Reducing a facet dislocation in an unconscious patient without an MRI. If a large disc herniation exists, you may cause permanent paralysis.
Tip: In unilateral dislocations, the nerve root is usually trapped on the side of the dislocation.