The central portion of C2 develops from the centra of C1 and C2. The superior projection of the dens develops from the centrum of C1. A condition where a variable portion of the ossification center for the odontoid process is separate from the body of C2. Associated with Down's, Morquio's and Klippel-Feil's syndromes. Plane synovial joints with articulations between the 2 convex occipital condyles and the 2 concave superior facets of the atlas.
Most motion is in the sagittal plane on a frontal axis.
There is about 10 o of flexion and 25 o of extension with very little sidebending and rotation. This is a synovial pivot joint. There are 2 laterally located facet joints where the inferior facets of the atlas are almost planar and the superior articular processes of the axis are also only gently curved to allow sliding within the joint capsules during rotation. There is a small amount of curvature to make the articulation convex on convex. There is a small amount of caudal head movement with rotation and rostral motion with return to neutral.