Atlanto-Axial Articulation

In the last issue, we reviewed the atlanto-occipital articulation. The articulations of the entire upper cervical spine are highly complex as it is the meeting place of the skull, the spine and spinal cord. and the brainstem. In the Gonstead technique, the atlas misalignment is analyzed in relation to the axis, unlike in upper cervical techniques where misalignment is determined by the relationship of the atlas to the occiput (the latter bears some thinking due to the nature of the altanto-occipital articulation).

There is no intervertebral foramen between the atlas and axis. The second spinal nerve passes in a space between the atlas and axis. It is unlike other regions of the spine where the spinal nerve does not pass subadjacent to its numerical vertebra—e.g., C8 spinal nerve passes through the C7/T1 IVF. As you know, the axis is—the atlas as well—one of the atypical vertebrae. The axis has the dens or odontoid process, what should have been the vertebral body for the atlas. The atlantoaxial joints are dissimilar to its articulation with C3. These differences allow greater rotation at the atlanto-axial juncture than at any other motion unit in the cervical spine or at any other level in the spine.

Articulations

There are seven articular surfaces on the axis. The intervertebral disc and facets join the axis to C3. The articulation between the inferior articulating surface of the atlas lateral mass to the superior articulating surface on the axis is a synovial joint with a loose capsule that is able to allow large movements. Both surfaces are, of course, lined with hyaline cartilage. The superior facet (inferior articulating surface of the atlas) is concave with a matching orientation of the convex inferior facet—superior articulating surface—atop the axis. The surfaces are not perfectly matching, and hyaline cartilage helps to even the surfaces. Unlike the typical cervical vertebrae, the superior articulating surface of the axis is a flattish, oval shaped area on the pedicle rather than at that pediculolaminar junction. Also, the orientation is more horizontal than the more vertically aligned zygapophyeal joints in the rest of the cervical spine.

This along with a thin, lax capsule allows considerable axial rotation of the atlas (45° or more to each side) on the axis which is up to 50% of the axial rotation of the entire cervical spine (1,3). During rotation, there is also a “screwing down” coupled motion, wherein the atlas descends 2 to 3 mm with a minute amount of lateral flexion (3). These coupled motions are due to the shape of the atlantoaxial articulations. Flexion and extension are less than 20° and lateral flexion is 5°, minimal in comparison to axial rotation.

The antero-superior region of the odontoid process fits in a facet that is located behind the anterior tubercle of the anterior arch of the atlas and is a synovial joint. The opposing surfaces are oval in shape but the surface on the dens is longer in the longitudinal axis while that on the atlas is longer horizontally (1). Holding the two in proximity is the transverse ligament that is anchored to the medial surfaces of the lateral masses.

The posterior aspect of the medial atlanto-axial joint is a joint between the transverse ligament and the posterior surface of the superior aspect of the odontoid which is a synovial joint, often with a bursa. The joint surface on the axis is larger than the anterior joint surfaces (1). The trapping of the odontoid between the anterior arch of the atlas and the transverse ligament prevents anterior and posterior translation of the atlas, especially in flexion and extension.

Ligaments

Ligaments are the straps that are required to maintain stability of the craniocervical region. They, along with the suboccipital musculature and upper cervical articulations are the transition between the movements required by the head and the movements of the rest of the cervical spine.

Along the anterior aspect of the vertebral body is the anterior longitudinal ligament and to the posterior is the posterior longitudinal ligament (PLL). These bridge together the adjacent vertebrae. The tectorial membrane is a continuation of the PLL that goes from the posterior body of the axis, along the dens to the clivus on the anterior rim of the foramen magnum on the basiocciput (1,4,5). The more superficial fibers attach to the dura mater (1).

Of importance is the transverse ligament that bridges one atlas lateral mass to the other. As noted above, there is an unique synovial joint between this fibbrous band to the posterior odontoid (4). Fibrocartilage often forms in the area of its approximation with the odonoid, much as occurs with the Achilles tendon, rotator cuff tendons, among others (1). It forms the horizontal element of the cruciate ligament (more below). It is bow-shaped in order to pass behind the odontoid.

The paired alar ligaments are short, thick ligaments that connect the apex of the odontoid to the medial surface of the ipsilateral medial occipital condyle with some fibers that attach to the lateral mass of the atlas, and occasionally, to the anterior arch of the atlas (1,2,6). It is known that these ligaments limit contralateral axial rotation, i.e., the left alar ligament limits right axial rotation (2). It also limits lateral motion of the atlas in lateral flexion (2). Stretching or tearing, such as occurs following a motor vehicle accident, can cause hypermobility and instability in the upper cervical spine (1).

The apical ligament extends from the superior and posterior aspect of the apex of the odontoid to the clivus on the anterior margin of the foramen magnum (1,6). The fibers diverge superiorly as it approaches its termination on the basiocciput which gives it a v-shape (1). It has traces of the notochord and may have a small ossicle that represents the proatlas centrum (1,6).

The cruciate ligament is posterior to the alar ligament. It is formed by the transverse ligament and the superior and inferior longitudinal bands, thereby presenting a cross-like appearance. The superior band passes from the transverse ligament to the anterior margin of the foramen magnum, and the inferior band attaches the transverse ligament to the axis body. The latter helps hold the transverse ligament in place and limits upper cervical flexion (1).

The accessory atlantoaxial ligament joins the atlas and axis, and also links them to the occiput. This ligament is anchored on the body near the juncture with the dens and extends to the lateral mass of the atlas near the attachment for the transverse ligament. A major function of the accessory atlantoaxial ligament, maybe more appropriately called the atlantoaxialoccipital ligament, appears to assist the alar ligament in attenuating craniocervical rotation and also bolster the strength of the atlantoaxial joint capsule. (1,5)

Posteriorly, the ligamentum nuchae begins at the external occipital crest and attaches to the posterior tubercle of the atlas and the spinous processes of the entire cervical spine.

There are numerous cervical muscles that create gross movement of the cervical spine. The obliquus inferior muscle has its origin on the tip of the axis spinous process and passes superiorly and laterally to the posterior tip of the atlas lateral mass. It receives motor fibers from the first cervical nerve and sensory fibers are from the second cervical spinal nerve (6).

Odontoid Alterations

Between 20 and 25 percent of those who suffer from rheumatoid arthritis have involvement of the medial atlanto-axial joint. There may be horizontal displacement of the atlas on axis due to erosive synovitis of the capsules. In many cases, there are signs of myelopathy. In a few cases, the odontoid shifts cephalically and is closer to the foramen magnum. Those without rheumatoid arthritis may also have degeneration of the medial atlanto-axial joints that can lead to loss of those joint spaces (1). Rupture of the transverse ligament is suspected if the space between the anterior tubercle of the atlas and the anterior of the dens is greater than 3 mm on the lateral film.

The odontoid and C2 vertebral body generally fuse by ages 3 to 6 years, although a radio-opaque line may persist until age 11 years (4). Some people have a remnant mini-disc between the base of the odontoid and the main body of the axis even if the odontoid has fused to the vertebral body (4). This seems to be similar to the persisting discs in the fused, mature sacrum. Os odontoideum is incomplete fusion of the dens to the axis body. In some cases, there is cartilage in the area of fusion. Non-union is thought to be due to fracture during early childhood (4).

Fracture of the odontoid has been classified into three types. Type I is fracture superior to the odontoid-body junction. Fracture at the odontoid-body juncture is Type II. Type III is fracture that of the odontoid that includes part of the axis body (4).

Learn more about the upper cervical spine at the 9th Meeting of the Minds at Logan College of Chiropractic near St. Louis, Missouri, on October 13-14, 2012.

References
1 Cramer GA. The Cervical Region. In: Cramer GA, Darby SA (ed). Basic and Clinical Anatomy of the Spine, Spinal Cord, and ANS (2nd Ed). St. Louis MO: Elsevier Mosby. 2005:164-177.

2 Dvorák J, Dvorák V, Gilliar W, et al. Musculoskeletal Manual Medicine: Diagnosis and Treatment. New York: Thieme. 2008:44-54.

3 Enebo BA, Gatterman MI. Kinesiology: An Essential Approach Towards Understanding the Chiropractic Subluxation. In: Gatterman (ed). Foundations of Chiropractic Subluxation. St. Louis MO: Elsevier Mosby. 2005:254-258.

4 Mercer S. Kinematics of the Spine. In: Boyling JD, Jull GA. Grieve’s Modern Manual Therapy: The Vertebral Column (3rd Ed). New York: Churchill Livingstone. 2004:31-32.

5 Tubbs RS, Salter EG, Oakes WJ. The Accessory Atlantoaxial Ligament. Neurosurgery August 2004; 55(2):400-404.

6 Vernon H. Upper Cervical Syndrome.: Chiropractic Diagnosis and Treatment. Baltimore: Williams & Wilkins, 1988:7-9.