title: Resolution of intraspinal retro-odontoid cyst associated with os odontoideum after posterior fixation.
abstract: METHODS
  A case report and review of previous literature.
  
  
  OBJECTIVE
  To describe the diagnosis and successful treatment of an intraspinal retro-odontoid cyst associated with an os odontoideum.
  
  
  BACKGROUND
  Retro-odontoid cysts associated with atlantoaxial instability are extremely rare. Direct excision of the cysts has commonly been performed for the surgical treatment of myelopathy.
  
  
  METHODS
  A retro-odontoid cyst in a 58-year-old woman with os odontoideum was treated surgically by posterior fixation between C1 and C3 without resection of the retro-odontoid mass. The patient's history, clinical examination, imaging findings, and treatment are reported.
  
  
  RESULTS
  The patient experienced walking difficulty, numbness in the upper and lower limbs, and hypesthesia in both hands. Congenital C2/3 fusion and atlantoaxial instability associated with an os odontoideum were found in the radiographs. Magnetic resonance imaging detected a round retro-odontoid mass, which compressed the spinal cord. The mass showed uniform low intensity on T1-weighted images and uniform high signal intensity on T2-weighted images. Gadolinium-diethylenetriaminepenta-acetic acid-enhanced T1-weighted images showed rim enhancement of the mass with no internal enhancement. Three months after the operation, the mass was no longer visible in a magnetic resonance imaging examination and the patient completely recovered motor function in her extremities.
  
  
  CONCLUSIONS
  Posterior fixation without resection was successfully used to eliminate a retro-odontoid cyst associated with atlantoaxial instability.
authors.affiliation: Department of Orthopaedic Surgery, Ehime University School of Medicine, Tohon-city, Ehime, Japan. ogata@m.ehime-u.ac.jp
authors.forename: Tadanori
authors.initials: T
authors.lastname: Ogata
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Kawatani
authors.forename: Tadao
authors.initials: T
authors.lastname: Morino
authors.forename: Haruyasu
authors.initials: H
authors.lastname: Yamamoto
mesh_terms: D001845:Bone Cysts / Q000000981:diagnostic imaging* / Q000601:surgery*; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery; D005260:Female; D006801:Humans; D008875:Middle Aged; D009809:Odontoid Process / Q000000981:diagnostic imaging* / Q000601:surgery*; D011859:Radiography; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods