Cineradiographic motion analysis of atlantoaxial instability in os odontoideum.

Cineradiography was used to study six patients with os odontoideum. Atlantoaxial kinematics was analyzed with the relative motion of the atlas on the axis. In all cases, the tracing of the anterior arch was straight in the sagittal plane, whereas the posterior arch moved in two different configurations: straight and S-shaped. In cases of S-shaped configurations, the anterior translation occurred in the neutral position. Such translation could threaten the stability of the atlantoaxial complex fixed with posterior wiring because circumferential wires allow the laminas to rotate in the sagittal direction. In patients with os odontoideum, pathomechanics of the atlantoaxial joint should be examined with cineradiography in order biomechanically to determine the soundest fixation.