The atlantoaxial joint is a complex joint between the atlas (C1) and the axis (C2). It is composed of three synovial joints; one median atlantoaxial joint and two lateral atlantoaxial joints.
The median atlantoaxial joint is formed between the dens of axis and an osteoligamentous ring of the atlas anteriorly and transverse ligament posteriorly. It is classified as a pivot joint. The lateral atlantoaxial joints are bilateral joints formed between the lateral masses of the atlas and axis. These joints are classified as gliding, or plane joints.
The primary movement of the atlantoaxial joint complex is rotation. In rotation the atlas together with the head, rotate around the dens of axis. This movement allows us to turn our head to look towards the left or towards the right. This is also the movement that allows us to shake our head in the familiar ‘no’ pattern.
|Type||Atlantoaxial joint complex: Synovial joint; biaxial|
Median atlantoaxial joint: dens of axis (C2), osteoligamentous ring (anterior arch of atlas [C1], transverse ligament of atlas)
Lateral atlantoaxial joints: inferior articular surface of lateral mass for axis, superior articular facet of atlas
|Ligaments||Cruciform ligament (transverse ligament of atlas, superior and inferior longitudinal bands), tectorial membrane, alar ligaments, apical ligament of dens|
|Innervation||Ventral primary ramus of the second cervical spinal nerve|
|Blood supply||Deep cervical, occipital, vertebral arteries|
|Movements||Principal movement; axial rotation,
Limited flexion, extension, lateral flexion
This article will discuss the anatomy and function of the atlantoaxial joint.
- Lateral atlantoaxial joints
- Median atlantoaxial joint
- Blood supply
- Muscles acting on the atlantoaxial joint
Lateral atlantoaxial joints
The left and right lateral atlantoaxial joints are the articulations between the inferior articular surface of the lateral mass of atlas (C1) and the superior articular surface of the lateral mass of axis (C2). Since these are synovial joints, their articular surfaces are covered with hyaline cartilage.
Although classified as planar-type joints, the articular surfaces of the lateral atlantoaxial joints are somewhat more complex. The articular surfaces of both the axis and atlas are oval in shape, with their long axis running on an oblique angle, from anteromedial to posterolateral. The surfaces are also inclined somewhat inferiorly, with the anterior aspect being more superior than the posterior. Lastly, the articular surfaces are convex in the sagittal plane, with the incongruent anterior and posterior joint margins being filled by flexible meniscoids.
The lateral atlantoaxial joints are wrapped with loose fibrous capsules, lined by synovial membrane. The joint capsule attaches on the lateral articular facets of the joints, enclosing the lateral masses of both the atlas and axis. Each side of the joint capsule is reinforced by an accessory atlantoaxial ligament.
The accessory atlantoaxial ligament supports the posterior aspect of the lateral atlantoaxial joints. It is formed as a lateral extension from the deep laminae of the tectorial membrane. The ligament attaches superiorly on the posterior aspect of the lateral mass of the atlas near its transverse ligament, blending with the fibers of the posterior capsules. From there it courses obliquely downwards and medially to attach to the back of the body of the axis, near the base of the dens.
The accessory atlantoaxial ligament is thought to play a role in rotational stability at the craniocervical junction, but its exact significance is uncertain. Note that some authors do not mention this as an independent ligament, and instead consider these fibers to be part of the tectorial membrane.
Median atlantoaxial joint
The median atlantoaxial joint is a pivot type of synovial joint. Broadly, the joint is formed by the dens of axis (odontoid process) surrounded by an osteoligamentous ring composed of the anterior arch of atlas anteriorly and the transverse ligament of atlas posteriorly.
Within this osteoligamentous ring there are actually two sets of articulations, each with their own synovial cavity. The first is the articulation between the anterior articular facet of the dens of axis (cervical vertebra 2) and the inner surface of the anterior arch of the atlas (cervical vertebra 1). The second articulation is formed between the posterior articular facet of the dens of axis and the anterior surface of the transverse ligament of atlas.
The anterior facet of the dens is rectangular in shape and convex in both the vertical and transverse plane. It fits against the correspondingly concave surface of the anterior arch of the atlas. The posterior articular facet of the dens is concave vertically and convex transversely. It sits against the fibrocartilaginous surface of the transverse ligament.
All the articulating surfaces of the median atlantoaxial joint are covered with hyaline cartilage.
The median atlantoaxial joint contains two synovial cavities, one on each side of the dens of axis. The posterior synovial cavity, between the dens and the transverse ligament, is the larger of the two. Each synovial cavity is enclosed by a thin joint capsule lined by synovial membrane. This joint capsule is relatively loose, especially in the superior portion, thus allowing a substantial range of motion movement to occur within the joint.
There are several ligaments securing the median atlantoaxial joint. The main ligaments of the joint connect the atlas to the axis, these ligaments are collectively known as the cruciform ligament complex.
The cruciform ligament is a complex of three ligaments, one horizontal and two longitudinal, that together resemble a cross, hence the name. The three bands that form the cruciform ligament are as follows:
- Transverse ligament of atlas: the transverse ligament of atlas is a strong, broad ligament that runs transversely between the lateral masses of the atlas, attaching to tubercles on their medial aspects. Also called the transverse atlantal ligament, it arches behind the dens, broadening in its central part where it is covered anteriorly by a layer of articular cartilage. This ligament takes part in the formation of the osteoligamentous ring around the dens of axis, articulating with it posteriorly. The transverse ligament is the main stabilizator of the dens of axis, acting to resist forward translation of the atlas relative to the axis.
- Superior longitudinal band of cruciform ligament: arises from the superior margin of the median part of the transverse ligament of atlas and ascends to insert at the basilar part of the occipital bone. The attachment lies in between the apical ligament of the dens and the tectorial membrane.
- Inferior longitudinal band of cruciform ligament: arises from the inferior margin of the median part of the transverse ligament of atlas and descends to attach on the posterior aspect of the body of axis.
In addition to the main ligaments, there are several accessory ligaments of the median atlantoaxial joint that connect the axis (C2) with the occipital bone.
- Tectorial membrane of cervical vertebral column: represents the superior continuation of the posterior longitudinal ligament. This strong, broad band originates on the posterior aspect of the body of the axis, from which it ascends to insert on the anterior edge of the foramen magnum. It attaches near the hypoglossal canals, blending with the spinal dura mater. The tectorial membrane covers the posterior surface of the dens. It is situated posterior to the cruciform and alar ligaments.
- Alar ligaments: these paired ligaments course obliquely in a superolateral direction from the posterolateral margins of the apex of dens of axis (C2), to attach on the medial parts of the occipital condyles. The short but strong alar ligaments act to limit excessive motion within the atlantoaxial joint.
- Apical ligament of dens: originates from the apex of dens fanning out superiorly to attach to the anterior margin of the foramen magnum. The apical ligament lies in front of the superior longitudinal band of the cruciform ligament, and inserts in between the two alar ligaments. Also termed the apical dental ligament, this represents the vestigial cranial continuation of the notochord. It’s biomechanical relevance is disputed.
- Anterior atlantoaxial membrane: a continuation of the anterior longitudinal ligament, this membrane spans the length from the inferior border of the atlas to the inferior part of the axis. Continues to the occiput as the anterior atlanto-occipital membrane
- Posterior atlantoaxial membrane: a superior continuation of the ligamentum flavum, it traverses the gap between the atlas and axis, from the inferior border of the posterior arch of the atlas to the upper borders of the laminae of the axis. Continues to the occiput as the posterior atlanto-occipital membrane
The atlantoaxial joint is innervated by branches of the ventral primary ramus of the second cervical spinal nerve.
The atlantoaxial joint is one of the most mobile joints in the spine. It allows us to turn our head to look left and to look right, or to shake our head in the familiar ‘no’ pattern. This movement is achieved by simultaneous action at all three atlantoaxial joints, the one medial and two lateral, which together produce axial rotation.
For left rotation, the right lateral mass of axis translates anteriorly on the articular facet of the axis. At the same time the left lateral mass of axis displays reciprocal posterior displacement. These movements occur at the plane type lateral atlantoaxial joint. Simultaneously, the atlas pivots around the dens, which is held within its osteoligamentous ring. The head and atlas (C1) move together as one unit over the axis (C2) resulting in turning of the head towards the left.
For right rotation, the opposite occurs. The range of motion for rotation at the atlantoaxial joint is 40° (range 39-49°). Due to the shape of the articular surfaces (convex with oblique orientation) rotation is accompanied by a slight “screwing down” of the atlas on the axis, in which the atlas descends vertically by around 1 millimetre. Rotation movement is limited mainly by the alar ligaments.
The atlantoaxial joint also allows limited flexion, extension and lateral flexion. Flexion and extension are produced by a small degree of roll and slide of the anterior arch of atlas on the dens of axis. In flexion, an anteroinferior glide of the atlas on the axis is coupled with a slight anterior translation. In extension, the atlas slides superoposteriorly allowing a slit backwards tilting to occur. These movements are made possible due to the joint capsule being weak and loose, as well as the relatively flexible transverse ligament, which bends downwards during flexion, and upwards during extension. The range of flexion – extension motion has been reported as between 11 – 21°. Lateral flexion is produced by the inferior articular facet gliding down the convex oval facet on one side, with a reciprocal displacement occurring on the opposite side. Due to the inferior inclination of the joint surfaces this produces a small amount of lateral tilt, and thus lateral flexion, of the atlas on the axis. Contralateral lateral flexion has been shown to be coupled with rotation. The reported range of motion of lateral flexion within the atlantoaxial joint is minimal, at 5 – 10°.
As a whole, the atlantoaxial joint takes a closed-packed position when the head is in full extension, while it is in an open-pack position when the head and neck are semiflexed. The capsular pattern of the atlantoaxial joint describes equally limited lateral flexion, extension and rotation. Although we can consider the segmental motion at the C1-C2 joint, craniovertebral movements do not occur in isolation. Rather the entire upper cervical spine functions as a united complex of joints.
Muscles acting on the atlantoaxial joint
The primary muscles that produce rotation in the atlantoaxial joint are the suboccipital muscles, mainly the ipsilateral obliquus capitis inferior, rectus capitis posterior major. The ipsilateral splenius capitis muscle and contralateral sternocleidomastoid muscle are also involved.
Besides rotation, the suboccipital muscles also have a proprioceptive function and play a role in maintaining posture. Additionally, obliquus capitis inferior stabilizes the atlantoaxial joint through its attachment on the transverse process of the atlas.