Anatomy of the Coccyx (Tailbone)

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Learn about the anatomy of the coccyx (tailbone)

At the end of this article, you should be able to recognize the anatomy of the coccyx (or tailbone). The coccyx is the very bottom portion of the spine. It represents a vestigial tail (hence the common term “tailbone“) and consists of three or more very small bones fused together. The coccyx is made up of between three and five separate or fused vertebrae. While it was originally thought that the coccyx is always fused together (with no movement between the coccygeal vertebrae), it is now known that the entire coccyx is not one solid bone but often there is some limited movement between the bones permitted by the fibrous joints and ligaments.

The coccyx articulates with the sacrum through a vestigial disc, and is also connected to the sacrum with ligaments. There is very limited movement between the coccyx and the sacrum.

Structure of the coccyx

The coccyx is usually formed of four rudimentary vertebrae (sometimes five or three). It articulates superiorly with the sacrum. In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. The transverse processes are most prominent and noticeable on the first coccygeal segment. All the segments are destitute of pedicles, laminae and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the last three diminish in size from above downward.

Most anatomy books wrongly state that the coccyx is normally fused in adults. In fact it has been shown that the coccyx may consist of up to five separate bony segments, the most common configuration being two or three segments.

Surfaces of the coccyx

The anterior surface is slightly concave and marked with three transverse grooves that indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the Levatores ani and supports part of the rectum.The posterior surface is convex marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles, the rudimentary articular processes of the coccygeal vertebrae. Of these, the superior pair are large, and are called the coccygeal cornua; they project upward, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve.

Borders of the coccyx

The lateral borders are thin and exhibit a series of small eminences, which represent the transverse processes of the coccygeal vertebrae. Of these, the first is the largest; it is flattened from before backward, and often ascends to join the lower part of the thin lateral edge of the sacrum, thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size from above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the coccygeus in front of the ligaments, and to the gluteus maximus behind them

Pathology

Injuring the coccyx can give rise to a condition called coccydynia. A number of tumors are known to involve the coccyx; of these, the most common is sacrococcygeal teratoma. Both coccydynia and coccygeal tumors may require surgical removal of the coccyx (coccygectomy). One complication of cocygectomy is a coccygeal hernia.[14] Fortunately, most cases of coccyx pain respond well to nonsurgical treatment, such as medications given by local injection (often done under fluoroscopic guidance).

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