ATLANTO-AXIAL JOINT

Three separate synovial joints; median and two laterals, connect the atlas and axis. These joints move as one unit and permit rotation of the atlas along with the entire skull. The atlanto-axial joints are called the joints of ‘no’ or ‘negative expression.

Median Atlanto-Axial Joint

It is pivot joint and is formed by the articulation between the dens or odontoid process of the axis and anterior arch of the atlas.

The anterior surface of the dens presents an oval facet to articulate with the corresponding facet on the posterior surface of the anterior arch of atlas. The dens is held in place with the transverse ligament of atlas which extends between the two tubercles on the medial side of the lateral masses of atlas and lodges in a groove on the posterior surface of the root of the dens; usually a bursa intervenes between the dens and the transverse ligament.

The pivot is formed by the dens and the ring by the anterior arch and the transverse ligament of atlas. During rotation, the pivot is fixed and the ring rotates.

The tip of the dens is connected by the apical ligament to the dorsal surface of basilar part of occipital bone close to the anterior margin of foramen magnum. The apical ligament is said to represent the remnant of notochord.

A pair of alar ligaments lie one on each side of the apical ligament and connect the dens to medial surface of occipital condyles. The alar ligaments are stretched during flexion and relaxed on extension of the head. These ligaments also check the excessive rotation at the atlanto-axial joints.

Lateral Atlanto Axial Joints

Each joint is plane synovial, and formed by the inferior articular facet of the lateral mass of atlas joining with superior articular facet of the axis. The atlantal facet is slightly concave and that of the axis is convex and reciprocally curved. The lateral atlanto-axial joints participate in the rotation of the head and also transmit the weight of the skull through the atlanto- occipital joints. Both atlanto-occipital and lateral atlanto-axial joints are morphologically equivalent to the unco-vertebral joints of lower cervical vertebrae.

The capsular ligaments of the lateral joints are loose and attached to the peripheral margins of the articular surfaces. The laxity of the fibrous capsule permits forward or backward gliding during rotation of the atlas. Simultaneously curvatures of the articular surfaces allow the descent of atlas during translatory movements and thereby prevent stretching of the fibrous capsule.

Nerve Supply

Atlanto-axial nerves are supplied by the C2 nerves.

Movements

The atlas carrying the globe of the head rotates around the dens of the axis, and at the same time the atlas descends somewhat on the axis while translating at the lateral atlanto-axial joints. Eventually, the alar ligaments are relaxed at the initial stage and permit rotation. When full range is reached, the rotation is checked by the tension of anterior fibers of alar ligaments on the rotating side and posterior fibers of alar ligament on the contra-lateral side.

Muscles Producing Rotation

Obliquus capitis inferior, rectus capitis posterior major and splenius capitis of one side, acting with the sterno-cleidomastoid of the opposite side.

Applied Anatomy

Death by judicial hanging may be due to the rupture of the transverse ligament of atlas or fracture of the dens of axis. As a result, the atlas is dislocated from the axis and compresses the spinal cord with fatal outcome.

Fracture of the Dens

Fractures of the dens make up about 40% of axis fractures.

1. The most common dens fracture occurs at its base- i.e. at its junction with the body of the axis. Often these fractures are unstable (do not reunite) because the transverse ligament of the atlas becomes interposed between fragments and      because the separated fragment (the dens) no longer has a blood supply resulting    in avascular necrosis.

2. Almost as common are fractures of the vertebral body inferior to the base of the dens. This type of fracture heals more readily because the fragments retain their blood supply.

3. Other dens fractures result from abnormal ossification patterns.

Rupture of the Transverse Ligament of the Atlas

When the transverse ligaments of the atlas ruptures the dens is set free resulting in atlanto-axial subluxation or incomplete dislocation of the medial-atlanto axial joint.

Pathological softening of the transverse and adjacent ligaments usually resulting from disorders of connective tissue may also cause atlanto-axial subluxation.

Down syndrome exhibits laxity or agenesis of the ligament.

Dislocation owing to transverse ligament rupture or agenesis is more likely to cause spinal cord compression than that resulting from fracture of the dens. In the absence of a competent ligament, the upper cervical region of the spinal cord may be compressed between the approximated posterior arch of the atlas and the dens causing paralysis of all four limbs (quadriplegia) or into the medulla of the brainstem, resulting in death. Approximately 1/3rd of the atlas ring is occupied by the dens, 1/3rd by the spinal cord and the remaining 3^rd by the free fluid space and tissues surrounding the cord: Steele Rule of Thirds. This explains why some patients with anterior displacement of the atlas may be relatively asymptomatic until a large degree of movement (greater than 1/3rd of the diameter of the atlas ring) occurs.

Sometimes inflammation in the craniovertebral area may produce softening of the ligaments of the craniovertebral joints and cause dislocation of the atlanto-axial joints. Sudden movements of a patient from bed to a chair, for example, may produce posterior displacement of the dens and injury to the spinal cord.

Rupture of the Alar Ligaments

Alar ligaments are weaker than the transverse ligament of the atlas. Consequently combined flexion and rotation of the head may tear one or both alar ligaments.

Compression of C2 Spinal Ganglion

Although uncommon, atlanto-axial rotation may compress the C2 spinal nerve. When the neck is severely hyper extended while the head is turned to the side, the spinal ganglion of the C2 nerve on the opposite side may be compressed between C1 and C2 vertebrae. This may be followed by prolonged severe headaches and excruciating cervico-occipital pain.

References:

1. Essentials of Human Anatomy- Head and Neck by A K Datta

2.  B D Chaurasia’s Human Anatomy- Head, Neck and Brain

3. Clinically oriented anatomy by Keith. L.Moore

4. Gray’s Anatomy

5. Clinical Anatomy by Richard S Snell

6. Grant’s method of anatomy by Basmajian and Slonecker