What exactly is a ‘coccyx’ and what is its function?
The word coccyx comes from the Greek word for cuckoo, the name apparently having been derived from the tailbone’s shape, which resembles that of a cuckoo’s beak. The coccyx is the terminal end of the spine, just inferior to the sacrum. The human coccyx is composed of 3-5 individual segments (coccygeal vertebrae), with variations occurring with regard to the number of segments, the overall angulation (curve) of the coccyx, and the degree of articulation versus fusion between the individual segments. In 80% of patients, the coccyx is made up of 4 coccygeal vertebrae.
Typically, the coccyx is concave anteriorly and convex posteriorly. The human coccyx is often considered a vestigial remnant or corollary of a tail; thus, the coccyx is colloquially referred to as the tailbone.
Anatomy and function of the coccyx
In the human body, the coccyx serves a variety of important functions, including as an attachment site for various muscles, tendons, and ligaments. Physicians and patients should remember the importance of these attachments when considering surgical removal of the coccyx (coccygectomy).
Muscles inserting on the anterior coccyx include the levator ani, which is sometimes considered as several separate muscle parts, including the coccygeus, iliococcygeus, and pubococcygeus muscles. This important muscle group supports the pelvic floor (preventing inferior sagging of the intrapelvic contents) and plays a role in maintaining fecal continence. A midline component is the anococcygeal raphe, whereby the coccyx supports the position of the anus.1 Muscles originating on the posterior coccyx include the gluteus maximus, which is the largest of the gluteal (buttock) muscles and which functions to extend the thigh during ambulation.
Multiple important ligaments attach to the coccyx. The anterior and posterior sacrococcygeal ligaments attach the sacrum to the coccyx (similar to the functions of the anterior and posterior longitudinal ligaments spanning cervical, thoracic, and lumbosacral spinal segments). Laterally, the transverse process of the coccyx serves as an attachment site for the lateral sacrococcygeal ligaments (arising from the inferolateral sacrum), as well as for fibers from the sacrospinous ligament (arising laterally from the spine of the ischium) and the sacrotuberous ligament (connecting the sacrum with the ischial tuberosity but with fibers attaching to the coccyx as well).
The coccyx serves as somewhat of a weight-bearing structure when a person is sitting down, thus completing the tripod of weight bearing composed of the coccyx and the bilateral ischium. The ischial weight-bearing surfaces are, more specifically, at the ischial tuberosities and inferior rami of the ischium. The coccyx bears more weight when the seated person is leaning backward; therefore, many patients with coccydynia sit leaning forward (flexing at the lumbosacral and hip regions), which shifts more of the weight to the bilateral ischium rather than the coccyx (see images below and Images 1-2). Alternatively, patients with coccydynia may sit leaning toward one side so that the body weight is exerted mainly on one ischial tuberosity or the other, with less pressure on the coccyx. Such side leaning may lead to concomitant ischial bursitis in addition to the antecedent coccydynia.
The base of the coccyx articulates with the sacral apex via the sacrococcygeal junction. The sacrococcygeal articulation and intracoccygeal articulations contain fibrocartilaginous discs, comparable to the intervertebral discs present at other spinal levels. The apex (distal tip) of the coccyx is typically rounded but may be bifid.