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Atlas

Synonym: C1
German: Atlas

1 Definition

The atlas is the first cervical vertebra (C1) and thus forms the part of the spine nearest to the neck. Together with the second cervical vertebra, the axis, it forms one functional unit and is also given names in other languages based on the fact that it permits the inflection of the head toward the anterior.

2 Anatomy

2.1 Basic structure

The structure of the atlas differs greatly from that of the other vertebrae, as does the axis. When observed at a most macroscopic level, a ring-like shape can initially be seen, since the atlas loses its corpus vertebrae over the course of its development. Laterally and ventrally one sees instead bony enlargements called massae laterales (lateral masses). From these, two approximately semicircular bone arches emerge - the Atlas arches:

  • anteriorly: arcus anterior (atlantis)
  • posteriorly: arcus posterior (atlantis).

A spinous process (processus spinosus) is not pronounced on the atlas, it is replaced by an increase on the poserior side of the posterior arch (arcus posterior), the tuberculum posterius. Similarly, a tuberculum anterius can be found on the anterior side of the anterior arch.

Lateral to the lateral masses in the form of additional extensions one finds the processus transversi which represent the evolutionary rudiments of the processus costalis and contain the typical cervical vertebrae foramina transversaria.

2.2 Joint surfaces

On the upper side of the lateral masses one finds the joint surfaces, which together with the joint surfaces of the occipital bone (os occipitale) form the atlantal occipital joint. They are also known as facies articulares superiores.

The joint surfaces of the axis form the atlantoaxial joint and are located on the lower side and the forward inside front of the atlas. The downward-facing surfaces is called facies articulares inferiores. The dens axis articulates with an oval cartilage surface in a recess on the back of the arcus anterior of the atlas. It is called fovea dentis.

2.3 Foramina

The foramen transversarium of the atlas is crossed by the arteria verterbralis, which enters the skull through the foramen magnum.

The foramen vertebrale of the atlas is divided by the ligamentum transversum atlantis into two sections. Ventral to the ligament is the dens axis, dorsally one finds the spinal cord.

3 Pathology

3.1 Developmental disorders

During the course of embryonic development, disorders can occur in the development of the atlas. This includes the partial merging of sclerotomes of the top four somites. As a consequence, a coalescence of occipital bone with the atlas results, which can either be complete or incomplete. This developmental disorder is referred to as atlas assimilation.

3.2 Misalignments and injuries

With the atlas, just as with the other vertebrae, misalignments can occur. Since the spinal cord runs right through the atlas, depending on the severity of the deformity more or less serious disorders of the central nervous system and the statics of the spine can occur. In addition, the circulation of blood and the cerebrospinal fluid can be disrupted and/or be hindered, which may lead to further malfunctions.

Another serious problem is broken neck, which is a fracture of the dens axis, which is located together with the spinal cord in the ring of the atlas. Only in very rare cases does this injury go unnoticed, as such representing a permanent threat to the person concerned. In most cases, however, tearing of the ligamentum transversum atlantis and ligamentum apicis dentis takes place, so that the dens axis acquires considerable freedom of movement and may damage or even completely destroy the medulla oblongata. Since portions of the respiratory centre are located here, death occurs within a few seconds. This injury is often found in hanged persons.

In addition, the so-called Jefferson fracture is a special form of atlas fracture. Here, a total destruction of the ring of the atlas follows as a result of strong longitudinally acting forces.

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