Atlas (anatomy)

The anterior arch forms about one-fifth of the ring: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the Longus colli muscles and the anterior longitudinal ligament; posteriorly it is concave, and marked by a smooth, oval or circular facet (fovea dentis), for articulation with the odontoid process (dens) of the axis.

The upper and lower borders respectively give attachment to the anterior atlantooccipital membrane and the anterior atlantoaxial ligament; the former connects it with the occipital bone above, and the latter with the axis below.[3]

Median sagittal section through the occipital bone and first three cervical vertebræ, showing ligamentous attachments

The posterior arch forms about two-fifths of the circumference of the ring: it ends behind in the posterior tubercle, which is the rudiment of a spinous process and gives origin to the Recti capitis posteriores minores and the ligamentum nuchae. The diminutive size of this process prevents any interference with the movements between the atlas and the skull.

The posterior part of the arch presents above and behind a rounded edge for the attachment of the posterior atlantooccipital membrane, while immediately behind each superior articular process is the superior vertebral notch (sulcus arteriae vertebralis). This is a groove that is sometimes converted into a foramen by ossification of the posterior atlantooccipital membrane to create a delicate bony spiculum which arches backward from the posterior end of the superior articular process. This anatomical variant is known as an arcuate foramen.

This groove transmits the vertebral artery, which, after ascending through the foramen in the transverse process, winds around the lateral mass in a direction backward and medially to enter the vertebrobasilar circulation through the foramen magnum; it also transmits the suboccipital nerve (first spinal nerve)

On the under surface of the posterior arch, behind the inferior articular facets, are two shallow grooves, the inferior vertebral notches. The lower border gives attachment to the posterior atlantoaxial ligament, which connects it with the axis.

The lateral masses are the most bulky and solid parts of the atlas, in order to support the weight of the head.

Each carries two articular facets, a superior and an inferior.

• The superior facets are of large size, oval, concave, and approach each other in front, but diverge behind: they are directed upward, medially, and a little backward, each forming a cup for the corresponding condyle of the occipital bone, and are admirably adapted to the nodding movements of the head. Not infrequently they are partially subdivided by indentations which encroach upon their margins.
• The inferior articular facets are circular in form, flattened or slightly convex and directed downward and medially, articulating with the axis, and permitting the rotatory movements of the head.

Just below the medial margin of each superior facet is a small tubercle, for the attachment of the transverse atlantal ligament which stretches across the ring of the atlas and divides the vertebral foramen into two unequal parts:

• the anterior or smaller receiving the odontoid process of the axis
• the posterior transmitting the spinal cord (medulla spinalis) and its membranes

This part of the vertebral canal is of considerable size, much greater than is required for the accommodation of the spinal cord.

The transverse processes are large; they project laterally and downward from the lateral masses, and serve for the attachment of muscles which assist in rotating the head. They are long, and their anterior and posterior tubercles are fused into one mass; the foramen transversarium is directed from below, upward and backward.

The atlas ossifies from three centers

The atlas is usually ossified from three centers.[4]

Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with cartilage.

Between the third and fourth years they unite either directly or through the medium of a separate center developed in the cartilage.

At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth year.

The lines of union extend across the anterior portions of the superior articular facets.

Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line.

Foramen arcuale or a bony bridge above the vertebral artery on the posterior arch of the atlas may be present. This foramen has an overall prevalence of 9.1%.[5] Arch defects refer to the condition where a gap or cleft exists at the anterior arch or posterior arch of the atlas. The prevalence of the posterior arch defect and anterior arch defect was 0.95% and 0.087%, respectively.[6] The anterior arch defect may be presented along with posterior arch defect, a condition known as combined arch defect or bipartite atlas.[7]

Posterior arch defect on axial CT

Upper surface:

• rectus capitis anterior – occipital bone (inferior surface of the base)
• rectus capitis lateralis – occipital bone (beneath the jugular process)
• obliquus capitis superior – occipital bone (between the superior and inferior nuchal lines)

Interior and dorsal part:

• obliquus capitis inferior – spinous process of the axis

Lower surface:

• splenius cervicis (part) – spinous processes of T02–T05
• levator scapulae (part) – superior part of medial border of the scapula
• intertransversarius posterior cervicis – transverse process of the axis (posterior tubercle)
• intertransversarius anterior cervicis – transverse process of the axis (anterior tubercle)

Upper surface:

• rectus capitis posterior minor – occipital bone (medial part of the interior nuchal line, and the surface between it and the foramen magnum)

Lower surface:

• interspinalis cervicis – spinous process of the axis

• longus colli (superior oblique) – transverse processes of C03–C05.