Occipital condyles and a superior articular facets of the atlas.
Ligaments & Joint Capsule
The capsule is rather roomy and relaxed and the joint possesses no individual accessory ligaments. Stability is provided by:
Degrees of Freedom
Two degrees of freedom are available at this joint:
- Lateral flexion/conjunct rotation
- Cardinal (pure) swing
- Total ROM: 30° (10° flexion, 20° extension)
- Axis: Transverse axis through external auditory meatus
- Lateral flexion
- Arcuate (impure) swing with coronal conjunct rotation (e.g. right lateral flexion occurs with conjunct left rotation)
- Total ROM: 15° (8-10° in each direction)
- Axis: Oblique sagittal axis through the nose (approximately)
- Associated with AA joint rotation
- Rotation (conjunct)
- Occurs with lateral flexion due to the slanted articular surfaces, anteromedial orientation of the articular facets, and the alar ligaments
- Total ROM: 5-7° in each direction
- Axis: Vertical axis anterior to the foramen magnum
- Forward tilt/nod of the head
- Posterior glide of the convex occipital condyles on the concave atlantal facets
- Atlas moves anterior and cranially
- Dens follows atlas and approaches the clivus
- Occiput and the posterior atlantal arch separate
- Backward tilt of the head
- Anterior glide of the convex occipital condyles on the concave atlantal facets
- Lateral flexion (left lateral flexion example provided below)
- Left lateral tilt of the head causes a contralateral glide of the occiput
- Conjunct right rotation occurs due to the anteriormedial axis of the joint surfaces
- Right glide of the occipital condyles causes relative anterior glide of the left condyle on and a relative posterior glide of the right condyle
- Occipital portion of the right alar ligament tightens, with the left being slackened
- Tension in the right alar ligament pulls on the dens and causes a left rotation of the axis
- Spinous process of the axis moves to the right creating a relative right rotation of the AA joint
Muscle Action at this Joint
Flexion is produced mainly by the action of the rectus capitis anterior. Limited by tectoral membrane.
Extension by the rectus capitis posterior major and minor, assisted by the obliqus capitis superior, the semispinalis capitis, splenius capitis, sternocleidomastoid, and upper fibers of the trapezius.
The rectus capitis lateralis are concerned in the lateral flexion, assisted by the trapezius, splenius capitis, semispinalis capitis, and the sternocleidomastoid of the same side, all acting together.
- Jefferson fracture
- Atlanto-occiptal dislocation
- Orthopaedic Division of the Canadian Physiotherapy Association. Biomechanics of the Atlanto-Occipital Joint. Clinical Technique Manual: Level II Upper Quadrant (2010). Pg 28-30.