Axis

Where is the axis?

It lies below the atlas and passes weight down through its body to the third cervical vertebra. With the atlas and the occiput it forms the craniocervical junction. (StatPearls, Cervical Vertebrae (Back) and Cervical Vertebrae (Head and Neck))

What are the parts of the axis?

It has two lateral masses that meet the atlas, a vertebral body, and the dens rising from the top of the body. The dens meets the back of the anterior arch of C1. (StatPearls, Cervical Vertebrae (Back))

Unlike the atlas, it has a spinous process. Two suboccipital muscles arise from it, the rectus capitis posterior major and the obliquus capitis inferior. (StatPearls, Suboccipital Muscles and Cervical Vertebrae (Back))

What does the axis do?

StatPearls describes the axis as the primary weight-bearing bone of the upper cervical spine. (StatPearls, Cervical Vertebrae (Head and Neck))

The atlantoaxial junction has three joints. In the midline the anterior arch of the atlas pivots on the dens, and two shallow facet joints lie to the sides. (StatPearls, Cervical Vertebrae (Head and Neck))

This junction gives more of the head’s rotation than flexion or extension. The atlantoaxial joints provide 50% of the rotation in the cervical spine. (StatPearls, Cervical Vertebrae (Head and Neck) and C1 Fractures)

What holds the dens in place?

The transverse ligament of the atlas runs behind the dens. Paired alar ligaments run from the tip of the dens to the margins of the foramen magnum, and the apical ligament joins the tip to the basion. (StatPearls, Atlantoaxial Instability and Cervical Vertebrae (Back))

The tectorial membrane, the upward continuation of the posterior longitudinal ligament, attaches firmly to the axis and to the clivus of the occipital bone. (StatPearls, Cervical Vertebrae (Back) and Occipital Bone, Artery, Vein, and Nerve)

How does the axis develop?

The dens joins the body of C2 at around 6 years of age. Its secondary ossification centers appear around age 3 but do not fuse to its tip until age 12. The ossification centers of the dens are often mistaken for fractures in children. (StatPearls, Cervical Vertebrae (Head and Neck))

StatPearls states that the dens evolved from the body of C1. An os odontoideum, a separate ossicle cut off from the base of C2, appeared in 0.11% of patients in a large series. (StatPearls, Cervical Vertebrae (Head and Neck) and Cervical Vertebrae (Back))

Where the sources differ

The axis has a bifid spinous process in both StatPearls reviews, but they disagree on the vertebrae below it. The head and neck review gives bifid spinous processes to C2 to C6; the back review finds consistent bifidity only from C2 to C4, with C6 single in nearly half of people. (StatPearls, Cervical Vertebrae (Head and Neck) and Cervical Vertebrae (Back))