The atlanto-occipital joint (also known as the C0-C1 joint) is a paired symmetrical articulation between the cervical spine and the base of the skull. Along with the atlantoaxial joint, it makes up a group called the craniovertebral joints.
The principal movement at the atlanto-occipital joint is flexion-extension. This movement permits nodding of the head, as seen when indicating approval (the “yes” movement). Functionally, these two ellipsoid (condyloid) joints perform their actions simultaneously, and thus can be mechanically considered as a single joint.
The upper cervical spine region is designed to allow for significant mobility, at the sacrifice of stability. This is why joint stability in the craniocervical region is maintained mainly by the fibrous capsules, ligaments, articular surfaces and surrounding muscles.
|Type||Synovial ellipsoid joint; biaxial|
|Articular surfaces||Occipital condyles, superior articular facets of atlas|
|Ligaments||Posterior atlanto-occipital ligament, anterior atlanto-occipital ligament|
|Innervation||C1 spinal nerve|
|Blood supply||Anastomosis between deep cervical, occipital and vertebral arteries|
|Movements||Principal movement; Flexion - extension
Limited lateral flexion
This article will discuss the anatomy and function of the atlanto-occipital joint.
- Articular surfaces
- Ligaments and joint capsule
- Blood supply
- Muscles acting on the atlanto-occipital joint
The atlanto-occipital 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 atlanto-occipital 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 atlanto-occipital joint and participate in its stability. These are the ligamentum nuchae, alar ligament, apical ligament, lateral atlanto-occipital ligament, anterior atlanto-occipital membrane, posterior atlanto-occipital membrane and tectorial membrane. Of these, only two specifically connect the occipital bone with the atlas and thus are considered the principal ligaments of the atlanto-occipital joint. These are the:
- Anterior atlanto-occipital membrane
- Posterior atlanto-occipital membrane
The anterior atlanto-occipital membrane 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, this membrane is strengthened by the anterior longitudinal ligament, while laterally it blends with the joint capsule of the atlanto-occipital joint.
The posterior atlanto-occipital membrane is a thin membrane that covers the posterior aspect of the atlanto-occipital joint. It runs from the posterior margin of the foramen magnum superiorly to the upper border of the posterior arch of atlas, inferiorly. It’s lateral margins blend with the posteromedial joint capsule along its course. The proximity of the posterior atlanto-occipital membrane to the C1 nerve and vertebral artery is an important clinical landmark.
The atlanto-occipital joint is innervated by the ventral rami of C1 spinal nerve.
Being an ellipsoid joint, the atlanto-occipital joint allows movement in two degrees of freedom. These are flexion-extension and lateral flexion. However the principal movement available at the atlanto-occipital 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 atlanto-occipital 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 atlanto-occipital 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 atlanto-occipital 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°.
Muscles acting on the atlanto-occipital joint
The postvertebral and anterior neck muscles act on the atlanto-occipital joint, contributing to the mobility of the cervical spine.
|Flexion from the upright position||Trapezius, splenius capitis, longissimus capitis, semispinalis capitis, rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis superior|
|Flexion from the supine position||Sternocleidomastoid, longus capitis, rectus capitis anterior muscles|
|Extension from the upright position||Sternocleidomastoid, longus capitis, rectus capitis anterior muscles|
|Extension from the prone position||Rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis superior, semispinalis capitis, splenius capitis muscles, cervical part of trapezius|
|Lateral flexion||Rectus capitis lateralis, trapezius, splenius capitis, semispinalis capitis, sternocleidomastoid|
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.
Dislocation of the atlanto-occipital joint (AOD) is a potentially fatal craniocervical injury. It can result from a high speed impact such as a motor vehicle accident. According to the direction of the occipital dislocation, AOD is classified in three types; anterior displacement, posterior displacement or longitudinal distraction. Dislocation is accompanied by injury of the associated ligaments. The severity of the injury depends primarily on the degree of dislocation. Stage I and II describe none or minimal displacement with sufficient preservation of the surrounding ligaments. Stage III is a highly unstable injury with severe dislocation. Stage III may be associated with injury of the cervical part of the spinal cord. This can be fatal. Survivors of this type of injury may suffer from neurological impairments, including lower cranial nerve deficits, unilateral or bilateral muscle weakness or even paralysis of all four limbs (quadriplegia).