- The atlanto-axial joint (AAJ) is located between the atlas (C1) and the axis (C2).
- More specifically, there are three articulations between the atlas and the axis:
- Two paired (left and right) facet joints between the inferior articular processes of the atlas and the superior articular processes of the axis. These are synovial, diarthrotic, planar joints.
- An atlanto-odontoid joint between the anterior arch of the atlas and the odontoid process of the axis. This is a synovial, uniaxial, pivot joint.
- The atlanto-axial joint (complex) essentially allows motion in two cardinal planes:
- Right rotation and left rotation of the atlas upon the axis.
- Flexion and extension of the atlas upon the axis.
- Frontal plane lateral flexion is negligible at the atlanto-axial joint.
NOTES:
- The atlas is a ring-shaped bone because it has no body. What would have been the body of the atlas instead fused onto the axis, forming the odontoid process (aka the dens).
- The axis is called the axis because the odontoid process/dens forms a vertical axis around which the atlas rotates.
- The atlanto-axial joint is responsible for nearly 50% of all cervicocranial (head and neck) rotation.