Atlas

Where is the atlas?

It is the topmost vertebra, just below the occiput. It links the cranium to the cervical spine through its joints with the occipital bone’s condyles above and the axis below. (StatPearls, C1 Fractures)

The atlas and the axis form the craniocervical junction together with the occiput. (StatPearls, Cervical Vertebrae (Head and Neck))

What does the atlas look like?

It is a ring of an anterior and a posterior arch joined by two lateral masses. The lateral masses bear load in place of a body. (StatPearls, Atlantoaxial Instability and Cervical Vertebrae (Back))

Each lateral mass has a concave superior facet that faces inward and receives an occipital condyle, and an inferior facet that meets the axis. A posterior tubercle sits where other vertebrae have a spinous process. (StatPearls, Cervical Vertebrae (Head and Neck) and Cervical Vertebrae (Back))

The vertebral arteries rise through the transverse foramina of C2 and C1, curve inward over the arch of C1 and enter the skull base. (StatPearls, Cervical Vertebrae (Head and Neck))

What does the atlas do?

The atlanto-occipital joint passes the weight of the head to the spine, gives much of the neck’s flexion and extension, and limits sideways shift of the occiput. (StatPearls, Cervical Vertebrae (Head and Neck))

With no body, the ring of C1 has room for the spinal cord as the cord leaves the foramen magnum. (StatPearls, Cervical Vertebrae (Head and Neck))

C1 rotates around the dens of C2 when the head turns. The atlantoaxial joints provide 50% of the rotation in the cervical spine, and the atlanto-occipital joints 50% of its flexion. (StatPearls, Cervical Vertebrae (Back) and C1 Fractures)

What attaches to the atlas?

The transverse ligament runs between the lateral masses of C1, behind the dens. StatPearls calls it the strongest ligament attachment in the cervical spine. (StatPearls, Atlantoaxial Instability)

Two suboccipital muscles arise from it: the rectus capitis posterior minor from the posterior tubercle and the obliquus capitis superior from the transverse process. The obliquus capitis inferior inserts on that transverse process. (StatPearls, Suboccipital Muscles)

The posterior longitudinal ligament runs from C2 to the sacrum and does not attach to C1, matching the lack of an intervertebral disc at the atlas. (StatPearls, Cervical Vertebrae (Back))

How does the atlas vary?

A bony bridge, the ponticulus posticus, can arch over the groove for the vertebral artery on the atlas. A meta-analysis found it in 16.7% of people, and complete in 9.3%. (StatPearls, Cervical Vertebrae (Back))