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Updated: Nov 22 2022

Atlantoaxial Instability

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  • etiology
    • The atlantoaxial joint is an important "transitional zone" in the cervical spine
      • prone to instability by both degenerative and traumatic processes.
    • Pathophysiology
      • adult causes
        • degenerative
          • Down's syndrome
          • Rheumatoid Arthritis
          • Os odontoideum
        • traumatic
          • Type I odontoid fracture (very rare)
          • Atlas fractures
          • Transverse ligament injuries
      • pediatric causes
        • degenerative
          • JRA
          • Morquio's Syndrome
            • lysosomal storage disorder
        • trauma/infection
          • rotatory atlantoaxial subluxation
  • Anatomy
    • Osteology
      • bony articulations
        • C1-C2 facet joints
    • Ligaments
      • transverse apical alar ligament complex
        • transverse ligament
          • most important stabilizer
        • apical ligament
          • single midline structure
        • alar ligaments
          • paired parasagittal ligament
    • Biomechanics
      • the atlantoaxial joint provides ~50% of rotation in the cervical spine
        • this is enabled by the peg (C2)-ring(C1) anatomy
  • Physical Exam
    • Symptoms
      • symptomatic
      • neck pain
      • neurologic symptoms
    • Physical exam
      • neurologic deficits
        • often appear late in disease process due to capacious nature of spinal canal at the C1 level
        • myelopathic symptoms
          • hyperreflexia (patellar tendon reflex)
          • muscles weakness
          • broad based gait
          • decreased hand dexterity
          • loss of motor milestones
          • bladder problems
  • Imaging
    • Radiographs
      • flexion-extension xrays
        • atlanto-dens interval (ADI)
          • measurement
            • distance between odontoid process and the posterior border of the anterior arch of the atlas
          • adult parameters
            • > 3.5mm considered unstable
            • > 10mm indicates surgery in RA
          • other
            • must get preoperative flexion-extension radiographs to clear all high-risk patients for any type of surgery
        • space-available-cord (SAC) = posterior atlanto-dens-interval (PADI)
          • measurement
            • distance from posterior surface of dens to anterior surface of posterior arch of atlas
          • adult parameters
            • in adults with RA < 14 mm associated with increased risk of neurologic injury and is an indication for surgery
      • open mouth odontoid
        • sum of lateral mass displacement
          • measurement
            • lateral mass are connect by ring of C1, and therefore can only be displaced relative to each other if
              • there is a bony fracture (disruption of the ring)
              • the transverse ligament is ruptured
                • transverse ligaments binds them together
          • adult parameters
            • if > 8.1 mm, then a transverse ligament rupture is assured and the injury pattern is considered unstable
  • Treatment
    • Determined by specific condition
      • adult atlantoaxial instability
        • Down's syndrome
        • Rheumatoid Arthritis
        • Os odontoideum
        • Odontoid fracture
        • Atlas fractures
        • Transverse ligament injuries
      • pediatric atlantoaxial instability
        • JRA
        • Morquio's Syndrome
        • Rotatory atlantoaxial subluxation
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