Axis

Key Features & Anatomical Relations

The axis (second cervical vertebra, vertebra C2, or epitropheus) is one of the seven cervical vertebrae of the vertebral column. It is considered an atypical cervical vertebra because some of its features differ to those of typical cervical vertebrae, such as the presence of its dens. The axis is classified as an irregular bone, and it includes the following bony features:

- parts: dens, vertebral body, laminae, pedicles, superior and inferior articular processes, and transverse and spinous processes;

- surfaces: inferior intervertebral surface, inferior annular epiphysis, and vertebral arch;

- landmarks: apex of dens, inferior vertebral notches, superior and inferior articular facets, and anterior and posterior articular facets of dens.

More information regarding these and other bony features can be found in the Parts, Surfaces, and Landmarks tabs for this bone.

The axis is located:

- superior to the third cervical vertebra;

- inferior to the atlas (first cervical vertebra).

It articulates with the:

- third cervical vertebra at the intervertebral symphysis and zygapophyseal joints;

- atlas at both the median and lateral atlantoaxial joints.

Ossification

Ossification of the axis occurs at five ossification centers, these are found in the:

- vertebral body, which appears in utero during the fourth to fifth months;

- right and left halves of the vertebral arch, with one center found in each, which appear in utero during the second month;

- right and left halves of the dens, with one center found in each, which appear in utero during the fourth to fifth months.

The ossification centers for both the right and left sides of the dens fuse with each other before birth. The dens fuses with the vertebral body during the third to fourth years. The right and left halves of the vertebral arch fuse with each other during the third to fourth years and these fuse with the vertebral body during the fourth to sixth years (Cunningham, Scheuer and Black, 2016).

Variations

In some individuals:

- the apex of dens does not fuse with the rest of the dens and instead exists as an accessory bone, known as the ossiculum terminale;

- the dens does not fuse with the vertebral body and instead exists as an accessory bone, known the os odontoideum;

- the dens may be present in a bifid form (Tubbs, Shoja and Loukas, 2016).

Surface Anatomy

The spinous process of the axis is difficult to palpate, as it lies deep to the nuchal ligament.

List of Clinical Correlates

- Fracture

- Osteoporosis

- Spinal stenosis

- Scoliosis

- Hypoplasia of axis

- Dislocation of atlantooccipital joints

- Dislocation of atlantoaxial joints

- Spina bifida

- Block vertebra

References

Cunningham, C., Scheuer, L. and Black, S. (2016) Developmental Juvenile Osteology. Elsevier Science.

Tubbs, R. S., Shoja, M. M. and Loukas, M. (2016) Bergman's Comprehensive Encyclopedia of Human Anatomic Variation. Wiley.