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Skeletal System


Os coccygis

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Key Features & Anatomical Relations

The coccyx (or tail bone) is one of the five regional groups of the vertebral column. It is triangular, classified as an irregular bone, and includes the following bony features:

- parts: first, second, third, and fourth coccygeal vertebrae;

- surfaces: apex, base, pelvic, and dorsal surfaces, and lateral borders;

- landmarks: coccygeal cornu and transverse processes.

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

The coccyx is located inferior to the sacrum and articulates with it at the sacrococcygeal joint. It contributes to the formation of the pelvic girdle.


Ossification of the coccyx occurs at four ossification centers, with one ossification center found in each of its four coccygeal vertebrae that unite to form the coccyx. The ossification center for the first coccygeal vertebra appears during the first year after birth, with the other centers appearing sequentially up until the twentieth year or later. The ossification centers for the coccygeal vertebrae fuse with each other during adulthood to form one unified coccyx (Standring, 2016).


In some individuals:

- the coccyx may consist of three or five coccygeal vertebrae instead of four;

- the coccyx may appear shorter or longer than usual;

- the first coccygeal vertebra may be fused with the sacrum, which is known as sacralization of the first coccygeal vertebra (Tubbs, Shoja and Loukas, 2016).

Surface Anatomy

The dorsal surface and apex of the coccyx can both be palpated at the inferior end of the vertebral column.

List of Clinical Correlates

- Fracture of coccyx

- Coccydynia

- Hypoplasia of coccyx

- Aplasia of coccyx


Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edn.: Elsevier Limited.

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

Learn more about this topic from other Elsevier products


ScienceDirect image

The coccyx is a small triangular bone at the bottom of the vertebral column consisting of three to five (usually four) fused rudimentary vertebrae.

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