Atlanto-axial Joint
The atlas (C1) sits on the axis (C2) and the wear is borne through the lateral antlanto-axial joints. Apart from its weight-bearing function, it also allows a large range of axial rotation.
Structure
There are three atlanto-axial joints, one median, and two lateral.
- The median joint is between the posterior surface of the anterior arch of the atlas and the front of the odontoid process
- The lateral atlantoaxial joints are between the lateral mass of the atlas and axis on both sides.
The articular surface of both the lateral atlantial and axial facets are convex, hence the joint is biconvex. The spaces in the anterior and posterior aspects are filled with intra-articular meniscoids.
Movement
The atlanto-axial joint provides axial rotation of about 47°. During rotation the anterior arch of the atlas pivots around the odontoid process coupled with sliding.
At neutral, the biconvex joint peaks approximate each other. With atlas rotation the ipsilateral facet slides down the posterior slope of its axial facet, while the contralateral facet slides down the anterior slope of its axial facet. The results in the atlas descending into the axis. With rotation back to neutral the atlas rises back up.
Flexion-extension is only about 10°, and lateral flexion about 5°.
Muscles of the atlas:
- Levator scapulae: this inserts on the transverse process of the atlas, but this acts on the scapula not the atlas.
- Obliquus superior and rectus capitis posterior minor: acts on the occiput
- Rectus anterior and rectus lateralis: acts on the occiput
- Longus cervicis (longus colli): attaches to the anterior tubercle of the atlas and is a weak flexor of the head. There is no antagonist muscle.
The atlas is essentially a passive washer between the head and rest of the cervical spine. The movements are essentially passive. The principal movements are directed by the muscles that act on the head. For rotation of the atlas, it is splenius capitis and sternocleidomastoid acting on the head that cause this movement.
With this passive motion, the atlas can often extend during full cervical flexion depending on whether the compression load is directed anterior or posterior to the atlanto-axial joint contact point.
Stability and Restraints
There are no true ligaments providing restraint to flexion and extension. There are fascial membranes that don't provide any significant ligamentous restraint. The atlas can flex until the posterior arch hits the occiput, or extend until it hits the neural arch of C2.
For axial rotation, the capsules of the lateral atlanto-axial joints and the alar ligaments provide some restraints. The capsules have a minor contribution. The alar ligaments are very important, and dislocation in rotation doesn't occur while this is intact. The alar ligament binds the head of the odontoid process of the axis.
For sliding, backward sliding has a bony limitation with impaction of the anterior arch of the atlas against the odontoid process. There is no bony restraint to forward sliding. Forward sliding is limited by the transverse ligament of the atlas and the alar ligaments. Dislocation in sliding doesn't occur if one of these is intact.
For lateral gliding, the primary restraint is the contralateral alar ligament, and bony impact of the lateral mass on the side of the odontoid process.
References
- Bogduk et al. Biomechanics of the cervical spine. I: Normal kinematics. Clinical Biomechanics. 2000
