title: Retro-dental reactive lesions related to development of myelopathy in patients with atlantoaxial instability secondary to Os odontoideum.
abstract: METHODS
  A retrospective analysis of 13 patients with atlantoaxial instability secondary to Os odontoideum who underwent posterior atlantoaxial fusion.
  
  
  OBJECTIVE
  To assess the relationships between the development of myelopathy and plain radiographic parameters in patients with atlantoaxial instability secondary to Os odontoideum and to determine whether the pathologic structures, which compress the spinal cord, are visualized using magnetic resonance imaging.
  
  
  BACKGROUND
  The development of myelopathy, which is the most serious complication associated with Os odontoideum, was thought to be related to either the degree of instability or direction of instability, or a decrease in the space available for the cord. However, such indirect radiographic parameters measured using plain radiographs cannot provide direct information concerning the causes of myelopathy in patients with atlantoaxial instability secondary to Os odontoideum.
  
  
  METHODS
  Thirteen patients who underwent posterior atlantoaxial fusion for clinical symptoms due to Os odontoideum were classified into two groups depending on whether they had (n = 9) or did not have (n = 4) myelopathy. Four radiographic parameters were measured using flexion and extension lateral radiographs; the degree of instability, the direction of instability, and the space available for the cord in flexion and extension. MRI was performed on all patients in the myelopathy group. The radiologic and clinical data were compared for the two groups.
  
  
  RESULTS
  There were no significant statistical differences in the degree of instability (6.83 vs. 7.38, P = 0.816), space available for the cord in flexion (6.94 vs. 7.13, P = 0.938), and space available for cord in extension (7.56 vs. 5.75, P = 0.434) between the two groups. There was a poor agreement between the direction of instability and the development of myelopathy (kappa = 0.268, P = 0.308). Magnetic resonance imaging did demonstrate, however, cord compression caused by retro-dental reactive lesions in the myelopathy; cystic masses were present in two patients; and fibrocartilaginous masses were present in seven.
  
  
  CONCLUSIONS
  The current study suggests that the value of plain radiographic parameters should be reevaluated as a means of evaluating myelopathy in patients with atlantoaxial instability secondary to Os odontoideum, and that retro-dental reactive lesions should be considered as the potential cause of myelopathy.
authors.affiliation: Department of Orthopaedic Surgery, Uijongbu St. Mary's Hospital, The Catholic University of Korea.
authors.forename: H
authors.initials: H
authors.lastname: Chang
authors.forename: J B
authors.initials: JB
authors.lastname: Park
authors.forename: K W
authors.initials: KW
authors.lastname: Kim
authors.forename: W S
authors.initials: WS
authors.lastname: Choi
mesh_terms: D000328:Adult; D000368:Aged; D001268:Atlanto-Axial Joint / Q000473:pathology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000000981:diagnostic imaging / Q000473:pathology*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009809:Odontoid Process / Q000473:pathology*; D011859:Radiography; D012189:Retrospective Studies; D013117:Spinal Cord Compression / Q000000981:diagnostic imaging / Q000473:pathology*; D013122:Spinal Diseases / Q000000981:diagnostic imaging / Q000473:pathology*