Atlanto-axial joint

Original Editor - Rachael Lowe

Lead Editors  

Description

Atlanto-occipital joint (anterior)

Atlanto-occipital joint (posterior)

The atlanto-axial joint is a joint in the upper part of the neck between the first and second cervical vertebrae; the atlas and axis.  It is a complex joint made up of three synovial joints.

Articulating Surfaces

There are two lateral atlanto-axial joints which are concave in an anterior-posterior direction, this allows rotation.

The median atlantoaxial joint is the articulation of the: .  

• posterior surface of the anterior arch of atlas and the front of the odontoid process
• anterior surface of the transverse ligament and the back of the odontoid process
  

Between the articular processes of the two bones there is on either side an arthrodial or gliding joint.

Capsule

The atlantoaxial articular capsules are thick and loose, and connect the margins of the lateral masses of the atlas with those of the posterior articular surfaces of the axis.

Each is strengthened at its posterior and medial part by an accessory ligament, which is attached below to the body of the axis near the base of the odontoid process, and above to the lateral mass of the atlas near the transverse ligament.

Ligaments

The ligaments connecting these bones are:

• Articular capsules
• Anterior atlantoaxial ligament
• Posterior atlantoaxial ligament
• Transverse ligament of the atlas

The atlantoaxial joint in common terminology is actually a composition of three: two lateral and one median atlantoaxial joints. Because of its proximity to the brain stem and importance in stabilization, fracture or injury at this level can be catastrophic. Common trauma and pathologies include (but are not limited to):

• The Dens: significant depression on the skull can push the dens into the brainstem, causing death. The dens itself is vulnerable to fracture due to trauma or ossification.
• Transverse ligament: Should the transverse ligament of the atlas fail due to trauma or disease, the dens is no longer anchored and can travel up the cervical spine, causing paralysis. If it reaches the medulla death can result. Alar ligaments: stress or trauma can stretch the weaker alar ligaments, causing an increase in range of motion of approximately 30%.

Posterior Atlanto-Occipital Membrane: genetic traits can sometimes result in ossification, turning the groove into an foramen.

Motions Available

The movements permitted in this joint are movements of the head on neck, consisting of:

1. flexion and extension around the mediolateral axis, which give rise to the ordinary forward and backward nodding of the head.
2. slight lateral motion, lateroflexion, to one or other side around the anteroposterior axis.

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.

Recent Related Research (from Pubmed)

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References

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