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Introduction
  • Occipitocervical instability may be
    • traumatic occipitocervical dislocation
      • severe injury and patients rarely survive
      • most patients die of brainstem destruction
    • acquired occipitocervical instability
      • may be seen in patients with Down's syndrome
      • occipital condyle hypoplasia
        • results in limited AOJ motion and basilar invagination
  • Epidemiology
    • traumatic
      • incidence
        • ~15-30% of cervical spine injuries occur at the occipitocervical junction
      • prevalence
        • identified in 19% fatal cervical injuries
    • acquired
      • most frequently seen in Down syndrome population
      • usually asymptomatic and identified in screen for surgery or special olympic participation
  • Pathophysiology
    • traumatic
      • mechanism of injury
        • high-energy trauma
        • translation or distraction injuries that destabilize the occipitocervical junction
      • pathoanatomy
        • head most often displaces anteriorly
    • acquired
      • pathoanatomy
        • due to bony dysplasia or ligament and soft-tissue laxity
  • Associated conditions
    • atlantoaxial instability
      • also seen in Down syndrome patients
    • neurologic deficits
    • vertebral or carotid artery injuries
    • Down Syndrome
Classification
 
Traynelis Classification (direction of displacement)
Type I Anterior occiput dislocation                                      
Type II Longitudinal dislocation
Type III Posterior occiput dislocation

Harbourview Classification System (degree of instability)
Stage I             Minimal or non-displaced, unilateral injury to craniocervical ligaments
Stable
Stage II                     Minimally displaced, but MRI demonstrates significant soft-tissue injuries. Stability may be based on traction test  Stable or Unstable 
Stage III  Gross craniocervical misaligment (BAI or BDI > 2mm beyond normal limits) Unstable

 

Imaging
  • Radiographs
    • recommended views
      • AP, lateral and odontoid views
    • findings
      • low sensitivity in detecting injury (57%)
    • measurements 
      • used to diagnosis occipitocervical dislocation 
        • Powers ratio = C-D/A-B  
          • C-D: distance from basion to posterior arch
          • A-B: distance from anterior arch to opisthion 
            • significance 
              • ratio ~ 1 is normal 
                • if > 1.0 concern for 
                  • anterior dislocation
                • ratio < 1.0 raises concern for 
                  • posterior atlanto-occipital dislocation
                  • odontoid fractures
                  • ring of atlas fractures
        • Harris rule of 12  
          •  basion-dens interval or basion-posterior axial interval 
            • >12mm suggest occipitocervical dissociation
  • CT
    • indications
      • considered gold standard for osseous injuries of the spine
    • views
      • midsaggital CT reconstruction
  • MRI
    • indications
      • suspected ligamentous injury with preserved alignment or occult injury 
      • neurological deficits
Treatment
  • Nonoperative
    • provisional stabilization while avoiding traction
      • indications
        • traumatic instability with distraction of the occipitoatlantal joint
      • techniques
        • halo vest
        • tongs
        • prolonged cervical orthosis is not recommended due to poor stabilization of the AOJ
      • outcomes
        • use of traction should be avoided in most cases
        • traction may be considered in stage 2 injuries when MRI demonstates soft-tissue injury with perserved aligment
  • Operative
    • posterior occipitocervical fusion (C0 - C2 or lower)
      • indications
        • most traumatic cases require stabilization
        • acquired cases when evidence of myelpathy or significant symptomatic neck pain
        • invagination and atlanto-axial impaction secondary to inflammatory arthropathy (e.g., rheumatoid arthritis)
        • tumor
Technique
  • Posterior occipitocervical fusion
    • approach
      • midline posterior approach to base of skull
    • instrumentation
      • rigid occipitocervical screw-rod or plate construct
        • aim for 3 bicortical occipital screws on each side of the midline (total 6 screws in occiput)
        • extend to C2 or lower with polyaxial pedical screws to achieve fixation
      • the safe zone for occipital screws is located within an area measuring 20mm lateral to the external occipital protuberance along the superior nuchal line  
      • the major dural venous sinuses are located just below the external occipital protuberance and are at risk of penetrative injury during occipitocervical fusion 
      • autogenous bone graft 
Complications
  • Nonunion
  • Bleeding  
 

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