The Diagnosis of Minimal Atlanto-axial Subluxation

In adults the normal distance between the posteroinferior margin of the anterior arch of the atlas and the anterior surface of the odontoid process does not exceed 2·5 mm. There is normally no forward movement of the atlas on the axis. 2. In children this distance may be as much as 4·5 mm. and there is often a slight forward movement of the atlas on the axis in flexion. The significance of such forward movement must be assessed in the light of the clinical findings. 3. The clinical syndrome of hyperæmic softening of the transverse ligament of the atlas can exist without radiological evidence of displacement.