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Introduction
  • 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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