Imaging of the Cervical Spine > Interpretation > Atlanto-occipital Alignment

Atlanto-occipital Alignment

The anterior margin of the foramen magnum should line up with the dens. A line projected downward from the dorsum sellae along the clivus to the basion should point to the dens.

The posterior margin of foramen magnum should line up with the C1 spinolaminar line.

The ratio of Basion - spinolaminar line of C1 to Opisthion - posterior cortex of C1 anterior arch normally ranges from 0.6 to 1.0, with the mean being 0.8. A ratio greater than 1.0 implies anterior cranio-cervical dislocation.

See an example of atlanto-occipital dislocation.