Atlantoaxial rotatory fixation. Radiographic study of its mechanism.

The mechanism of atlantoaxial rotatory fixation was investigated by means of CT scanning. During the acute stage, there was a common rotatory displacement of the occiput and atlas complex in relation to the axis. As symptom subsided spontaneously or with treatment, the displacement was reduced and the occiput and atlas complex was in normal alignment with the axis. In a few cases where survey x-ray presented persistence of a typical displacement between C1/2, there was a persistent rotatory displacement of the atlas within the occiput-atlas-axis (C0-C1-C2) complex. Here, the occiput faced in nearly the same direction as the axis and the rest of the cervical spine. This meant an interlocking of the rotated atlas between C0 and C2. Either one of the lateral mass articulation of the rotated atlas was anteriorly dislocated and interlocked. Compensatory derotation of the occiput and a hypermobility of the C0/1 articulation, limited to younger children, presumably produced such a rotatory displacement of the atlas within the C0-C1-C2 complex. Restriction of rotation and a residual postural deformity resulted from unilateral dislocation of the lateral mass articulation between C1/2 and residual rotatory displacement between C0/2, respectively. Difficulty in reducing such a postural deformity can be attributed to the fact that any manipulative force often fails to unlock the atlas within C0-C1-C2 complex because of an excessive mobility between the C0/C1, and a ligamentocapsular contracture can be established in the lateral mass articulation of the interlocked atlas in an ignored case.