Study Design. This study analyzed the correlation between the atlantoaxial instability (especially sagittal plane rotatory instability) and myelopathy. Thirty-four patients with os odontoideum were reviewed in clinical symptoms and radiographs. Objectives. The results were correlated to provide a rationale for the genesis of myelopathy. Summary of Background Data. Instability of the atlantoaxial joint has been evaluated with atlantodental interval, space available for the spinal cord, instability index, and others. These are excellent indices for anteroposterior instability. However, they are insufficient for the evaluation of the multidirectional instability in os odontoideum. Methods. Thirty-four patients were analyzed in three parameters on the lateral radiographs: the minimum distance from the posterior border of the C2 body to the posterior arch of atlas, the difference of the atlantoaxial angle between the flexion and extension position (sagittal plane rotation angle), and the change of space available for the spinal cord from flexion to extension (instability index). Results. The correlation between the sagittal plane rotation angle and the instability index was low (r = 0.561). In patients with more than 20° of the sagittal plane rotation angle, myelopathy was shown at a high rate (86%). Myelopathy was also shown at a high rate (90%) in the patients with more than 40% of the instability index. Conclusions. It is important to check the sagittal plane rotation angle and the Instability Index to evaluate the instability in os odontoideum. If the patient has either a sagittal plane rotation angle of more than 20° or an Instability Index of more than 40%, he or she is likely to have the cord signs.
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