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Updated: Apr 27 2023

Occipitocervical Instability

  • Summary
    • Occipitocervical instability can be traumatic or aquired through a degenerative process such as rheumatoid arthritis or Down's Syndrome.
    • Diagnosis is usually confirmed with a combination of CT scan, MRI, and lateral flexion-extension radiographs.
    • Traumatic instability is treated with occipitocervical fusion. Aquired instability is treated with observation or occipitocervical fusion depending on the presence of neurologic deficits.
  • Epidemiology
    • traumatic occipitocervical instability
      • incidence
        • ~15-30% of cervical spine injuries occur at the occipitocervical junction
      • prevalence
        • identified in 19% fatal cervical injuries
    • acquired occipitocervical instability
      • most frequently seen in Down syndrome population
      • usually asymptomatic and identified in screen for surgery or special olympic participation
  • Etiology 
    • Terminology
      • also called
        • atlanto-occipital dissociation (AOD)
        • occipitocervical dislocation
    • 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
  • Anatomy
    • Osteology
      • morphology
        • occipital condyles are paired prominences of the occipital bone
        • oval or bean shaped structures forming lateral aspects of the foramen magnum
      • joint articulations
        • intrinsic relationship between occiput, atlas and axis to form the occipitoatlantoaxial complex or CCJ
        • 6 main synovial articulations
          • anterior and posterior median atlanto-odontoid joints
          • paired occipitoatloid joints
          • paired atlantoaxial joints
    • Ligaments
      • intrinsic ligaments are located within the spinal canal, provide most of the ligamentous stability. They include
        • transverse ligament
          • primary stabilizer of atlantoaxial junction
          • connects the posterior odontoid to the anterior atlas arch, inserting laterally on bony tubercles.
        • paired alar ligaments
          • connect the odontoid to the occipital condyles
          • relatively strong and contributes to occipitalcervical stability
        • apical ligament
          • relatively weak midline structure
          • runs vertically between the odontoid and foramen magnum.
        • tectorial membrane
          • connects the posterior body of the axis to the anterior foramen magnum and is the cephalad continuation of the PLL
    • Vascular system
      • occipital condyles in proximity to vertebral arteries
    • Nervous system
      • proximity of the occipital condyles to:
        • medulla oblongata
        • spinal cord
        • lower cranial nerves (CN IX - CN XII)
  • Classification
      • Traynelis Classification (direction of displacement)
      • Type
      • 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
    • CT angio
      • indications
        • evaluate for injury to vertebral artery
        • identify anatomy of vertebral artery prior to occipitocervical fusion
    • 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
      • occipitocervical fusion
        • 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
    • Occipitocervical fusion
      • approach
        • posterior midline incision with patient in prone position
        • Mayfield retractor used to obtain proper craniocervical alignment
          • establish preoprative O-C2 angle with lateral fluoroscopy prior to draping
      • deep dissection
        • if performing C1 lateral mass screw fixation work within safe zone and do not disect above the posterior arch of C1 more than 1 cm lateral to midline to avoid injury to vertebral artery
      • instrumentation
        • length
          • posterior segental instrumented fusion is usually performed from the occiput to C3
        • occipital
          • occipital plates usually allow for 3 or 4 total screws with adjustable rod holders
          • occipital screws
            • usually unicortical to avoid injury to venous sinus
              • major dural venous sinuses are located just below the external occipital protuberance and are at risk of penetrative injury
              • some institutions prefer bicortical screws but they come at increase risk
            • occipital screw safe zone
              • the safe zone for occipital screws is located within an area measuring 2 cm lateral and 1 cm inferior to the external occipital protuberance along the superior nuchal line
        • C1 lateral mass screws
          • often skipped due to angle at base of skull making it more difficult to place a rod
          • may choose a unilateral screw to provide some rotational stability to C1 ring
        • C2 fixation
          • pars, pedicle screws, transarticular, or translaminar screws all options
        • C3 fixation
          • standard lateral mass screws aimed cephalad and lateral to avoid vertebral artery
      • arthrodesis
        • may require bone grafting or removal of boney fragments compressing neurovascular structures.
  • Complications
    • Nonunion
      • Internal Carotid Artery
      • Vertebral Artery
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