Asymmetry of the odontoid-lateral mass interspaces: a radiographic finding of questionable clinical significance.

Asymmetry of the interspaces between the odontoid process and the lateral masses of the atlas is a finding occasionally noted on anteroposterior open-mouth radiographs obtained following trauma. Controversy exists as to the clinical significance of this finding in minimally symptomatic patients. Some believe this asymmetry is a normal variant, while others suggest that pathological rotation of the atlas on the axis may be present. This condition has been termed atlantoaxial rotatory subluxation (ARS), and when not corrected by 15 degrees head rotation, is said to be "fixed." Recent experience has shown that some patients with radiographic findings of ARS do not exhibit clinical signs or symptoms, leading us to question the significance of the radiographic diagnosis. A study was performed to evaluate the incidence of atlantoaxial asymmetry in normal subjects, the effect of rotation on the atlantoaxial relationship, and the ability to correct asymmetry by rotation. Eleven normal volunteers were evaluated with the following anteroposterior open-mouth views: without tilt or rotation (neutral); 15 degrees rotation in each direction; 15 degrees head and neck tilt to the right; right rotation with right tilt; and left rotation with right tilt. Six normals (54%) demonstrated atlantoaxial asymmetry in the neutral position despite proper positioning. No predictable change in the atlantoaxial relationship was produced by any of the manipulations described. Two normal subjects fulfilled the radiographic criterion for fixed atlantoaxial subluxation (asymmetry not corrected by rotation). We conclude that the radiographic finding of atlantoaxial asymmetry is common and not in itself abnormal.(ABSTRACT TRUNCATED AT 250 WORDS)