title: Significance of occipitoaxial angle in subaxial lesion after occipitocervical fusion.
abstract: METHODS
  The significance of occipitoaxial angle in the development of subaxial subluxation after occipitocervical fusion was determined in a minimum 5-year follow-up study performed retrospectively.
  
  
  OBJECTIVE
  To clarify the association between the position of the fixed occipital bone and axis and the development of subaxial subluxation.
  
  
  BACKGROUND
  There have been few reports describing the association between the position of fixation of the occipital bone and axis and subaxial lesion in occipitocervical fusion.
  
  
  METHODS
  Thirty-eight patients with rheumatoid arthritis who underwent occipitocervical fusion for irreducible atlantoaxial dislocation were reviewed. The angle between the McGregor line and the inferior surface of the axis (O-C2) was measured in healthy volunteers and patients who had undergone occipitocervical fusion. The association between any changes in the alignment of the cervical vertebrae and the development of subaxial subluxation during follow-up periods was studied.
  
  
  RESULTS
  The number of the patients in whom postoperative kyphosis and swan neck deformity developed was only five, but in four (80%) of them, retroversion of the occipital bone was used to increase the O-C2 angle. In 14 patients, in whom anteversion of the occipital bone against the axis was excessive, 12 (86%) patients experienced subaxial subluxation after surgery. In the patients in whom fixed O-C2 angles were in normal range, only one patient developed such abnormal changes in the middle and lower cervical vertebrae.
  
  
  CONCLUSIONS
  It is necessary to give attention to the position of the fixed occipital bone and axis during procedures of occipitoaxial fusion for patients with rheumatoid arthritis.
authors.affiliation: Department of Orthopaedic Surgery, Faculty of Medicine, Kagoshima University, Kagoshima, Japan. shunji@m.kufm.kagoshima-u.ac.jp
authors.forename: S
authors.initials: S
authors.lastname: Matsunaga
authors.forename: T
authors.initials: T
authors.lastname: Onishi
authors.forename: T
authors.initials: T
authors.lastname: Sakou
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000150:complications / Q000473:pathology / Q000601:surgery*; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D001269:Atlanto-Occipital Joint / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D001696:Biomechanical Phenomena; D015984:Causality; D005260:Female; D006801:Humans; D016268:Internal Fixators / Q000009:adverse effects; D004204:Joint Dislocations / Q000000981:diagnostic imaging / Q000209:etiology* / Q000517:prevention & control; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000000981:diagnostic imaging / Q000209:etiology* / Q000517:prevention & control; D011859:Radiography; D013123:Spinal Fusion / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods*; D016896:Treatment Outcome