Radiographic classification of Os odontoideum and its clinical significance.

STUDY
DESIGN: This study analyzed the relationship between the configuration of the atlantoaxial joint and the clinical characteristics of patients with os odontoideum.
OBJECTIVES: To compare three types of atlantoaxial configurations of os odontoideum from the viewpoint of clinical characteristics.
METHODS: Twelve patients were classified into the following three types according to their atlantoaxial configuration on an anteroposterior radiogram: round (n = 6), cone (n = 4), and blunt-tooth (n = 2). Clinical features in each group were compared in terms of mean age, gender distribution, severity and type of myelopathy, and degenerative findings of the atlantoaxial joint.
RESULTS: The patients in the round type group were predominantly women and had more severe myelopathy than that of patients in the other groups. According to Crandall's classification of myelopathy, all patients showed the transverse lesion syndrome in the cone and blunt-tooth types. Four of six patients with the round type of atlantoaxial configuration had the Brown-Séquard syndrome. With the round type, it appeared that the lateral mass of the axis tended to dislocate laterally, and this lateral instability associated with anteroposterior instability was believed to induced the Brown-Séquard syndrome.
CONCLUSIONS: Myelopathy in patients in the round type group was the most severe compared with that of patients in the other three groups. This classification is considered useful for estimating clinical findings in os odontoideum.