The atlanto-axial joint: Physiological range of rotation on MRI and CT

Aims: To measure the range of rotation and determine the instantaneous axis of rotation of the atlanto-axial joint in healthy volunteers using magnetic resonance imaging (MRI) and to highlight the appearances of the rotated atlanto-axial joint on computed tomography (CT) and MRI. Materials and Methods: Twenty-eight healthy volunteers were examined using MRI during maximal head rotation. In addition, an anatomical specimen of the atlanto-axial joint, fixed in varying degrees of rotation, was imaged using CT. Results: At the extremes of physiological rotation in healthy subjects there is striking but incomplete loss of contact between the articular surfaces of the atlas (C1) and the axis (C2). The range of rotation to the right was 20-48.5° (mean 32.4°) and to the left was 13-52.75° (mean 34.2°). There was no significant difference between rotation to the right and left (P = 0.455). Total rotation was 45-88.5° (mean 69.25°). The instantaneous axis of rotation was located within the odontoid peg. Conclusion: There is a wide range of atlanto-axial rotation in normal subjects. The instantaneous axis of rotation lies within the odontoid peg. The appearances of the rotated atlanto-axial joint are striking and may be misinterpreted as subluxation.