- 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.
Oblique view of the atlanto-axial joint between the atlas (C1) and the axis (C2). This oblique view is superior and posterolateral.
Right lateral view of the cervical spine, demonstrating the cervical spinal joints, including the articulation between the atlas and axis (atlanto-axial joint).
