Atlantooccipital joint

Articular surfaces

The atlantooccipital joint is the synovial articulation between the occipital bone and the first cervical vertebra (atlas). The articular surfaces are oval (elliptical) in shape and reciprocally concave-convex, with the concave articular facets of C1 vertebra articulating with the convex surfaces of the occipital bone. There is no intervertebral disc between C1 and the occiput. All the articular surfaces are lined with hyaline cartilage. The first cervical vertebra houses the inferior articular facets. These facets are located on the superior aspect of the vertebra’s lateral mass. They are oval-shaped, tilted slightly medially and are concave. The two long axes of each facet run obliquely in the anteromedial direction and meet at the midline just anterior to the atlas. The superior articular facets are located on the inferior aspect of the occipital bone, in the region of the occipital condyles. These two rounded protuberances are elliptical in shape, elongated and convex on both their long and short axes. The occipital condyles are directed anteromedially, and are situated immediately lateral to the anterior half of the foramen magnum. Learn more about the general features of the synovial joints by exploring articles, diagrams, videos and quizzes.

Ligaments and joint capsule

Each atlantooccipital joint is enveloped by a loose, thin articular capsule. This capsule is composed of fibrous tissue and lined with synovial membrane. It attaches to the margins of the articular facets. Capsular thickenings can be noted on its posterior and lateral aspects. 

Several ligaments cross the atlantooccipital joint and participate in its stability. These are the ligamentum nuchae, alar ligament, apical ligament, lateral atlantooccipital ligament, anterior atlantooccipital membrane and ligament, posterior atlantooccipital membrane and tectorial membrane. Of these, two primarily connect the occipital bone with the atlas and thus are considered the principal ligaments of the atlantooccipital joint. These are the:

• Anterior atlantooccipital ligament (and membrane)
• Posterior atlantooccipital membrane

The anterior atlantooccipital ligament is a dense band of fibrous tissue that spreads from the anterior border of the foramen magnum to the upper border of the anterior arch of the atlas. Medially, it is strengthened by the anterior longitudinal ligament as the anterior atlantooccipital membrane, while laterally it blends with the joint capsule of the atlantooccipital joint. 

The posterior atlantooccipital membrane is a thin membrane that covers the posterior aspect of the atlantooccipital joint. It runs from the posterior margin of the foramen magnum superiorly to the upper border of the posterior arch of atlas, inferiorly. Its lateral margins blend with the posteromedial joint capsule along its course. The proximity of the posterior atlantooccipital membrane to the C1 nerve and vertebral artery is an important clinical landmark.

Innervation

The atlantooccipital joint is innervated by the anterior rami of  spinal nerve C1.

Blood supply

The atlantooccipital joint receives its blood supply from an anastomosis formed by the deep cervical, occipital and vertebral arteries.

Movements

Being an ellipsoid joint, the atlantooccipital joint allows movement in two degrees of freedom. These are flexion-extension and lateral flexion. However the principal movement available at the atlantooccipital joint is that of flexion – extension. This is because of the shape of the atlantal sockets, which are sufficiently deep to prevent excessive translation of the occipital condyles and allow the atlantooccipital joint to provide some stability for the head as it balances on the cervical spine.

Flexion and extension movements occur in the anteroposterior plane, around a transverse axis. During flexion, the convex occipital condyles simultaneously roll forwards and slide posteriorly over the concave facets of the atlas. Thus moving the occipital bone away from the posterior arch of the atlas. This allows a forward tilting, or downwards nod, of the head to occur such as that seen when indicating approval (the “yes” movement). Flexion is limited to approximately 5°-10° by the fibrous tissues surrounding the joint (joint capsules, posterior atlantooccipital membrane, ligamentum nuchae) and the posterior suboccipital muscles.

In extension, the opposite movements occur. The occipital condyles roll backwards and glide anteriorly on the atlantal facets, closing the space between the occipital bone and the posterior arch of the atlas. Approximation of the occipital bone, the atlas and the axis limits the extension range of motion to approximately 10°.

Lateral flexion is not a significant movement at the atlantooccipital joint, in terms of range of motion. In fact, cadaveric studies limit lateral flexion of the upper cervical spine to approximately 5-8° on either side. Furthermore, lateral flexion in the upper cervical spine is both a coupled movement and a double-joint movement. Coupled movement means that the lateral flexion occurs concurrently with a small degree of contralateral rotation. Double-joint movement means that movement occurs in more than one joint at the same time. Thus the combined movements to achieve upper cervical spine lateral flexion are as follows; a small degree of contralateral glide occurs at the occipital condyles (lateral flexion), at the same time one occipital condyle moves somewhat anteriorly while the other moves in a posterior direction (rotation), alongside these movements the second vertebra is rotated (relatively) against the third cervical vertebra, producing an overall range of motion for upper cervical spine lateral flexion of 5-8°.

Test yourself on the craniovertebral joints and the associated ligaments with our quiz!

Muscles acting on the atlantooccipital joint

The postvertebral and anterior neck muscles act on the atlantooccipital joint, contributing to the mobility of the cervical spine. 

The main flexors of the head on the neck are the rectus capitis anterior and longus capitis muscles. The main extensor muscles are rectus capitis posterior major, rectus capitis posterior minor, obliquus superior capitis, semispinalis capitis, splenius capitis and trapezius. However, the exact muscles involved in these movements can differ, depending on the initial position of the head.

When in a supine position strong action is needed to lift the head and flex it forwards on the neck. In this circumstance the anterior neck muscles act as the prime movers. This includes the sternocleidomastoid, longus capitis and rectus capitis anterior muscles.

When the head is in an upright position, the same strength is not required as the weight of the head can move the head into a flexed position. In this circumstance the posterior muscles of the back and neck act in order to control the forward bend movement. These include the trapezius, splenius capitis, longissimus capitis, semispinalis capitis and the short suboccipital muscles.

Similarly, extension of the head from an upright position is controlled by the anterior neck muscles (e.g. sternocleidomastoid, longus capitis), which act against the weight of the head. When in a prone position, prime movers are needed to lift the head into extension. These include rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis superior, semispinalis capitis, splenius capitis muscles and the cervical part of trapezius.

Lateral flexion of the head occurs by simultaneous actions of the anterior and posterior neck muscles. These include rectus capitis lateralis, trapezius, splenius capitis, semispinalis capitis, sternocleidomastoid, obliquus capitis superior and rectus capitis posterior minor muscles. The coupled movement of rotation is supported by obliquus capitis superior, rectus capitis posterior minor, splenius capitis and sternocleidomastoid.