Pitfalls in the CT diagnosis of atlantoaxial rotary subluxation.

CT was used to examine six patients with clinically evident atlantoaxial rotary fixation, two patients with torticollis, and six normal subjects who had turned their heads to the side as far as voluntarily possible. The CT appearances of the atlantoaxial complex were identical in all three groups. To differentiate these groups, we propose a functional scan through C1-C2 in which patients are scanned initially as they present, with their heads fixed in lateral rotation. Subsequent scans are obtained with their heads turned to the maximum contralateral rotation. CT scans in patients with atlantoaxial rotary fixation demonstrate no motion at C1-C2 during this maneuver, while those in patients with transient torticollis show a reduction or reversal of the rotation of C1 on C2.