title: Os odontoideum. A case report of one identical twin.
abstract: METHODS
  This case report identifies one identical twin with an os odontoideum after trauma and the other twin with a normal cervical spine and no history of trauma.
  
  
  OBJECTIVE
  To highlight the etiology of os odontoideum as posttraumatic as opposed to congenital in nature.
  
  
  BACKGROUND
  The etiology of os odontoideum has been debated since its original description by Giacomini in 1886. Although proponents of the congenital origin are supported by association with various congenital anomalies, including Down's syndrome, Klippel-Feil syndrome, and multiple epiphyseal dysplasia, the majority of reports suggest a posttraumatic etiology.
  
  
  METHODS
  We reviewed the cervical radiographs of an identical twin with transient paresis and significant trauma to her neck as a child after a motor vehicle accident. The radiographs were compared with those of her twin sister, who had no history of trauma.
  
  
  RESULTS
  Os odontoideum was identified in one identical twin with a history of trauma. She was treated with a posterior C1-C2 arthrodesis using the Magerel transarticular screw technique supplemented by a modified Gallie's wiring. At a 12-month follow-up, she was asymptomatic and had a solid fusion based on flexion and extension views.
  
  
  CONCLUSIONS
  This case report of an os odontoideum occurring in one identical twin after a traumatic injury supports the possibility of a posttraumatic, as opposed to congenital, theory of origin.
authors.affiliation: Department of Orthopaedic Surgery, University of Washington, Seattle, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Verska
authors.forename: P A
authors.initials: PA
authors.lastname: Anderson
mesh_terms: D000293:Adolescent; D001174:Arthrodesis; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging* / Q000601:surgery; D001863:Bone Screws; D001864:Bone Wires; D005260:Female; D006801:Humans; D009333:Neck / Q000000981:diagnostic imaging; D019838:Neck Injuries; D011859:Radiography; D014430:Twins, Monozygotic