Effect of C1-C2 rotation on canal size.

An anthropometric study of the atlas and axis was undertaken to determine spinal canal diameters, the degree of narrowing with rotation, and odontoid tilt. The mean sagittal diameter of the atlas was 30.1 mm and the mean coronal diameter was 28.8 mm. The mean sagittal diameter of the axis was 19.0 mm and the mean coronal diameter was 22.8 mm. Approximately 64 degrees of rotation occurred at the atlantoaxial complex before sufficient narrowing caused spinal cord compression in the average specimen. An average of 63 degrees of rotation was required to cause facet dislocation. Most specimens examined had a posterior odontoid tilt up to 45 degrees and a facet angle between 1 degree and 29 degrees. Bilateral facet dislocation occurred at 63 degrees. Spinal canal narrowing to 1 cm occurred at 64 degrees. This narrowing would probably not damage the cord, which averages 1 cm in diameter; however, if the rotary motion of C1 and C2 progressed, cord damage could occur. Facet angles and posterior odontoid tilts that are not within the normal range indicate pathological changes.