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  • 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 
  • 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
  • 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
  • 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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Questions (1)

(OBQ09.111) An 11-year-old male complains of one year duration of neck pain. He denies any recent trauma. He has noticed intermittent episodes of gait imbalance and difficulty with buttoning his shirt over the past 3 months. Physical exam shows normal strength in all four extremities and hyper-reflexic patellar tendons. Neutral and flexion radiographs are shown in Figures A and B. A sagittal CT scan is shown in Figure C with a coronal reconstruction shown in Figure D. What is the most appropriate treatment? Review Topic


PT to strengthen the dynamic stabilizers of the neck




Soft collar wear during any athletic activities




Cessation of all contact sports with no surgical intervention




Posterior C1-C2 fusion




Anterior C1-2 fusion



Select Answer to see Preferred Response


The clinical presentation and imaging studies are consistent with os odontoideum with neurologic deficits. A posterior C1-C2 fusion is the most appropriate treatment.

Whether os odontoideum is congenital or the residual of a traumatic process is controversial. Most authors support a post-traumatic etiology; however, some evidence exists to support a congenital origin. Asymptomatic patients may be managed with cessation of contact sports alone. Neurologic findings and widened ADI are both indications for a posterior C1-C2 fusion.

Reilly et al. performed a study to evaluate transarticular screw use in pediatric patients with C1-C2 instability. They used a transarticular constuct in 9 patients with os odontoideum and 3 patients with Down Syndrome. They found, although vertebral size and congenital anomalies may make screw positioning challenging, the technique allows adequate fixation and is especially beneficial in the absence of a complete posterior arch of C1. They found high fusion rate in mid-term followup.

Sankar et al. designed a study to address the controversy over the etiology of os odontoideum. Their data supports two separate etiologies for the os odontoideum: post-traumatic and congenital.

Figure A and B are flexion extension views that show os odontoideum with associated dynamic atlantoaxial instability. Figure C and D are CT images that should os odontoideum. Notice the rounded sclerotic edges that differentiate this condition from an acute fracture.

Incorrect Answers:
Answer 1,2,3: Nonoperative modalities would not be appropriate in this patient with neurologic deficits.
Answer 5: An anterior C1-2 fusion is not a viable procedure due to the osseous anatomy at this level.

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