The atlanto-occipital joint is formed by the condyles of the occipital bone and the concave articular fossae of the atlas. Both pairs of articulating surfaces are enclosed in separate articular capsules, but they move simultaneously, forming a single combined joint.
Stability of the atlanto-occipital joint is ensured by:
- two joint capsules;
- posterior atlanto-occipital membrane;
- anterior atlanto-occipital membrane.
Atlanto-occipital joint: what is it, where is it located?
AZ node
The Atlanto-occipital node (AZ) is a cartilaginous node that connects two important bones of the body: the skull and the spine. It is located at the top of the neck. It can be easily felt with your fingers near the head.
The movements of the articular microcapsules occur at the same time. This indicates the combined operation of the node. The connection between the surface of the cartilaginous unit and the protrusions of the occipital fossae forms the condyles. Therefore, such a joint is also called a condylar joint.
Atlanto-occipital joint: structure, anatomy
Structure of the AZ node
Between the atlas junction and the axial vertebral circle there are 3 cartilaginous nodes:
- Two lateral nodules, which are formed by the cartilage pits in the atlas region and the spinal pits.
- In the middle there is a tooth that connects to the arched atlas and transverse tissue. Around this tooth there is a ring, which consists of fibrous and bone tissue. All this together forms a cylindrical assembly that rotates.
See what the atlas node looks like from the back and to the right - in the picture above. In the picture below, you will see what the knot looks like when you look at it from above and below.
Top and bottom view of the AZ unit
2 bundles consisting of fibrous cells extend from the edge of the transverse tissue. One is directed upward, and the other is directed downward. Together they create two lines crossed in the middle. This ligament is important for the functional features of the atlanto-occipital ganglion: the first directs the movement of the odontoid bone, and the second keeps it from injury. Thanks to these elements, the spinal and medulla oblongata fluid is protected.
AZ node
The auxiliary function is performed by the ligamentous node, which extends from the top of the tooth. On the side of the vertebral circles, the node is covered with a special membrane membrane, which extends from the slope of the bone at the back of the head.
Muscles affecting the atlanto-occipital joint
Suboccipital muscles
Flexion is carried out mainly by contraction of the anterior rectus capitis muscle, which is limited by the integumentary membrane.
Extension is achieved by contracting the posterior rectus capitis major and minor muscles, the superior oblique capitis muscle, the semispinalis capitis muscle, the splenius capitis muscle, the sternocleidomastoid muscle and the upper portion of the trapezius muscle.
The rectus lateral capitis muscle performs lateroflexion with the help of the trapezius muscle, the splenius capitis muscle and the sternocleidomastoid muscle on one side, which contract simultaneously.
Atlanto-occipital joint: ligaments, muscles
Ligaments and muscles of the AZ node.
A stable and constant position of the head, as well as its mobility, is ensured by the following ligaments:
- The membrane, which is located in the front, stretches from the front of the occipital bone to the 1st link of the spinal column.
- The membrane that is located at the back is structured in the same way as the front one, but it is an articulation of bones behind the node.
Muscles and ligaments are responsible for the following movements:
- Mobility of the head relative to the neck.
- Fixation of the back of the head and cranium in the desired position.
- The special structure allows the circulatory system and nerve roots to be located unhindered.
- Providing space for the functioning of the central nervous system.
- Nerve endings in this area allow us to stand upright and walk.
The functional features of this node may be impaired after injury. Such injuries can even be fatal. If the bones are displaced by even a few tenths of one degree, this can lead to deformation of the spinal cord. Even if a person remains alive after injury, there is a high probability that he will be paralyzed.
Anatomy of the human Atlanto-occipital joint - information:
Atlanto-occipital joint, art. atlantooccipitdlis , belongs to the condylar; it is formed by two condyles of the occipital bone, condyli occipitales, and the concave superior articular fossae of the atlas, foveae articulares superiors atlantis. Both pairs of articular surfaces are enclosed in separate articular capsules, but move simultaneously, forming a single combined joint.
Auxiliary ligaments:
- anterior, membrana atlantooccipitalis anterior, stretched between the anterior arch of the atlas and the occipital bone;
- the posterior one, membrana atlantooccipitalis posterior, is located between the posterior arch of the atlas and the posterior circumference of the foramen magnum.
In the atlanto-occipital joint, movement occurs around two axes: frontal and sagittal. Around the first of them, nodding movements are made, i.e., bending and extending the head forward and backward (expressing agreement), and around the second axis, tilting the head to the right and left. The sagittal axis is slightly higher at its anterior end than at its posterior end. Due to this oblique position of the axis, simultaneously with the lateral tilt of the head, a slight rotation in the opposite direction usually occurs.
Joints between the atlas and axial vertebra
There are three joints here. Two lateral joints, artt. atlantoaxiales laterales , formed by the lower articular fossae of the atlas and the upper articular fossae of the axial vertebra in contact with them, forming a combined articulation. The tooth located in the middle, dens axis, is connected to the anterior arch of the atlas and the transverse ligament, lig. transversum atlantis, stretched between the inner surfaces of the lateral masses of the atlas. The tooth is covered by an osteofibrous ring formed by the anterior arch of the atlas and the transverse ligament, resulting in a cylindrical rotatory joint, art. atlantoaxidlis medna.
Two fibrous bundles extend from the edges of the transverse ligament: one upward, to the anterior circumference of the large foramen of the occipital bone, and the other downward, to the posterior surface of the body of the axial vertebra. These two bundles, together with the transverse ligament, form the cruciate ligament, lig. cruciforme atlantis. This ligament is of great functional importance: as already noted, on the one hand, it is the articular surface for the tooth and directs its movements, and on the other hand, it keeps it from dislocation, which could damage the spinal cord and the medulla oblongata nearby near the large foramen of the occipital bone, which leads to death.
The auxiliary ligaments are lig. apicis dentis, coming from the apex of the tooth, and ligg. alaria - from its lateral surfaces to the occipital bone. The entire described ligamentous apparatus is covered from behind, from the side of the spinal canal, by a membrane, membrana tectoria (continuation of the lig. longitudinale posterius, spinal column), coming from the slope of the occipital bone.
In artt. atlantoaxiales, the only kind of movement occurs - rotation of the head around a vertical axis (turn to the right and left, an expression of disagreement) passing through the tooth of the axial vertebra, and the head moves around the process along with the atlas (cylindrical joint). At the same time, movements occur in the joints between the atlas and the axial vertebra. The apex of the tooth during the rotational movement is held in its position by the above-mentioned ligg. alaria, which regulate movement and thus protect the adjacent spinal cord from shocks.
Movements in the connections of the skull with the two cervical vertebrae are small. More extensive movements of the head usually occur with the participation of the entire cervical part of the spinal column. The cranial-vertebral joints are most developed in humans due to upright posture and head elevation.
Movements, functions of the atlanto-occipital joint
Functional feature of the AZ node
In the AZ node, movement is performed around two axes. With the help of one axis we can nod our head forward and bend it back. The second axis allows you to tilt to the right and left. The front of one of these axles is located slightly higher than the rear. This oblique position allows for simultaneous tilting of the skull to the side with a slight rotation in the other direction.
Motor capacity of the AZ unit
The top of the tooth-like bone is held in a constant position during head rotations by ligamentous nodes that regulate movement and protect the spinal cord from concussions. Motor ability in the area of the connections of the skull and vertebrae in the neck is performed with a small amplitude. Deeper movements occur when the entire neck is involved. The cranial-vertebral ligaments are well developed, as we walk straight and also hold our heads straight.
Osteokinematics
Flexion/extension
- Swing-type movement.
- Total amplitude: 15-30° (10° flexion, 20° extension).
- Axis: transverse axis through the external auditory canal.
Lateroflexion (side bending)
- “Arc-shaped” swing + rotation (for example, lateroflexion to the right occurs simultaneously with rotation to the left).
- Total amplitude: 15° (8-10° in each direction).
- Axis: oblique sagittal axis through the nose (approximately).
- The movement is associated with rotation at the atlantoaxial joint.
Rotation
- It occurs during lateroflexion due to inclined articular surfaces (their anteromedial orientation) and alar ligaments.
- Overall amplitude: 5-7° in each direction.
- Axis: vertical axis in front of the foramen magnum.
Blood circulation of the atlanto-occipital joint
Arteries of the AZ node
The first vertebra in the neck area has a special structure. It is thin and wide at the same time. This allows the upper part of the spinal cord to fit freely inside. Behind the atlanto-occipital ganglion is the vertebral artery, as well as many nerve roots that transmit information from the brain.
Blood circulation in this area should be free. If it is violated, then the following negative effects can occur:
- Headaches, various types of migraines, attacks of hypertension.
- Lack of nutrition in the brain area.
- Constant nausea, vomiting, dizziness.
- Temporary loss of consciousness, fainting.
- Confused consciousness, tinnitus, spots before the eyes.
If blood circulation in this area is impaired for some reason, then the brain suffers. It does not receive the required amount of nutrients. In this case, medications are prescribed in the form of vitamins with the addition of microelements.
What bones are involved in the formation of the atlanto-occipital joint?
The bones of the AZ node
of the Atlanto-occipital node surround the condyles of the occipital bones and the articular apparatus. The following bones of the skull are involved in the formation of this node:
- Frontal
- Parietal
- Occipital
- Temporal
- Wedge-shaped
The spine originates in this area. The two upper cervical vertebral circles, the atlas and the axis, have a specific anatomical structure that differs from the structure of all other vertebrae. The first cervical vertebral bone consists of arches that are connected not by bones, but by bone thickenings. The second cervical vertebra, as mentioned above, has a tooth-like process, which is fixed with the help of ligamentous tissues.
The atlanto-occipital joint has a special structure that is both strong and vulnerable. In addition to excellent mechanical strength, this section must have excellent flexibility and motor ability. All this is achieved by special joints, articular tissues and a special arrangement of ligaments.
Arthrokinematics
Flexion
- Lean forward/nod head.
- Posterior sliding of the convex condyles of the occipital bone along the concave articular surfaces of the lateral masses of the atlas.
- The atlas moves anteriorly and cranially.
- The axis tooth follows the atlas and approaches the clivus.
- The occipital bone and posterior arch of the atlas are removed.
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Extension
- Tilt your head back.
- Anterior sliding of the convex condyles of the occipital bone along the concave articular surfaces of the lateral masses of the atlas.
Lateroflexion
- Lateral tilt of the head to the left causes contralateral sliding of the occipital bone.
- Conjugate rotation to the right occurs along the anteromedial axis of the articular surfaces.
- The sliding of the occipital condyles to the right causes relative anterior sliding of the left condyle and relative posterior sliding of the right condyle.
- The occipital part of the right alar ligament tenses, and the left one relaxes.
- The tension of the right alar ligament due to the impact of the axis on the tooth leads to its rotation to the left.
- The spinous process of the axis shifts to the right, creating a relative right rotation in the atlantoaxial joint.