title: Atlantoaxial instability in os odontoideum with myelopathy.
abstract: METHODS
  This study analyzed the correlation between the atlantoaxial instability (especially sagittal plane rotatory instability) and myelopathy. Thirty-four patients with os odontoideum were reviewed in clinical symptoms and radiographs.
  
  
  OBJECTIVE
  The results were correlated to provide a rationale for the genesis of myelopathy.
  
  
  BACKGROUND
  Instability of the atlantoaxial joint has been evaluated with atlantodental interval, space available for the spinal cord, instability index, and others. These are excellent indices for anteroposterior instability. However, they are insufficient for the evaluation of the multidirectional instability in os odontoideum.
  
  
  METHODS
  Thirty-four patients were analyzed in three parameters on the lateral radiographs: the minimum distance from the posterior border of the C2 body to the posterior arch of atlas, the difference of the atlanto-axial angle between the flexion and extension position (sagittal plane rotation angle), and the change of space available for the spinal cord from flexion to extension (Instability Index).
  
  
  RESULTS
  The correlation between the sagittal plane rotation angle and the instability index was low (r = 0.561). In patients with more than 20 degrees of the sagittal plane rotation angle, myelopathy was shown at a high rate (86%). Myelopathy was also shown at a high rate (90%) in the patients with more than 40% of the instability index.
  
  
  CONCLUSIONS
  It is important to check the sagittal plane rotation angle and the instability index to evaluate the instability in os odontoideum. If the patient has either a sagittal plane rotation angle of more than 20 degrees or an instability index of more than 40%, he or she is likely to have the cord signs.
authors.affiliation: Department of Orthopaedic Surgery, School of Medicine, Keio University, Tokyo, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Watanabe
authors.forename: Y
authors.initials: Y
authors.lastname: Toyama
authors.forename: Y
authors.initials: Y
authors.lastname: Fujimura
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging / Q000503:physiopathology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000150:complications* / Q000000981:diagnostic imaging / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D009809:Odontoid Process / Q000002:abnormalities* / Q000000981:diagnostic imaging; D012189:Retrospective Studies; D012399:Rotation; D013118:Spinal Cord Diseases / Q000000981:diagnostic imaging / Q000209:etiology* / Q000473:pathology; D014057:Tomography, X-Ray Computed