Coccyx (Tailbone)

What is coccyx (tailbone) and where is it located?

Coccyx (tailbone) is the lowest part of the spinal column. It extends down from the sacrum between the buttocks and ends about an inch (2.5 cm) above the anus ^[12-p.329]. At this site, coccyx pain appears.

Picture 1. Coccyx location

Human coccyx consists of 3-5 separate or fused bony parts (vertebrae), most commonly of 4 ^[2,3,14]. The average coccyx length is 1.8 inch (4.5 cm) but can vary a lot and is often greater in men ^[3].

Coccyx has got its name from its resemblance to cuckoo beak (in Greek cuckoo = kokkux) ^[1]. The anatomical name for coccyx is os coccyx.

Coccyx pronunciation: koksiks ^[1].

“Coccygeal” means pertaining to coccyx, for example, coccygeal ligament.

Coccyx Anatomy Diagram

Picture 2. Sacrum and coccyx diagram

The lower end of the coccyx is bent forward, so its anterior surface is concave and its posterior surface is convex. The tip (apex) of the coccyx is rounded.

Coccyx Function

Coccyx assists in weight-bearing when a person is seated, especially when leaning backward ^[2].

The anterior coccygeal surface serves as the attachment for the anterior sacrococcygeal ligament and the muscles of the pelvic floor (levatores ani) that support parts of the rectum and, in women, urethra and vagina.

The borders of the coccyx give attachments on either side to the sacrotuberous and sacrospinous ligaments, to the coccygeus muscle in front of the ligaments, and to the gluteus maximus muscle behind them.

Coccygeal Joints

The joints between the tailbone segments can be true (synovial) joints, they can contain thin cartilaginous discs as in most other parts of the spine or can be fused (ossified) as in the sacrum ^[7,9].

The coccyx makes a joint with the sacrum via the sacrococcygeal junction ^[2].

Coccygeal Ligaments

The anterior, posterior and lateral sacrococcygeal ligaments attach the sacrum to the coccyx ^[2].

The sacrospinous ligament, which connects the sacrum with the spine of the ischium (the lower part of the hip bone), is also attached to the coccyx ^[2].

The sacrotuberous ligament, which connects the sacrum and the tuberosity of the ischium, is also attached to the coccyx ^[2].

The anococcygeal ligament or raphe is in the middle of the part of the levator ani muscle that attaches the anus and the coccyx.

Coccygeal Muscles

Muscles attached to the ANTERIOR coccyx surface:

Levator ani, which consists of iliococcygeus, pubococcygeus and coccygeus muscle ^[11]

• Origin: Pubic bone and the ischium of the hip bone
• Insertion: Anterior surface of the coccyx
• Innervation: 3rd, 4th and 5th sacral nerve, perineal nerve or pudendal nerve ^[5,8]
• Functions:
  • Forms the “pelvic floor,” which supports the pelvic organs from sagging; it also supports the urethra and prostate
  • It constricts the lower end of the rectum and thus helps in maintaining bowel continence; it also elevates the rectum during the bowel movement and thus assists in bowel emptying ^[8]
  • It constricts the lower end of the vagina ^[8]

Muscles attached to the POSTERIOR coccyx surface:

Gluteus maximus ^[4]

• Origin: Posterior surface of the sacrum and coccyx
• Insertion: gluteal tuberosity of the femur
• Function: extends backward and rotates laterally the thigh in hip, abducts (extends laterally) the thigh

Muscles attached to the TIP of the coccyx:

External anal sphincter, which surrounds the outer part of the anus ^[5].

• Function: Maintains the anus closed
• Innervation: Pudendal plexus ^[6]

Nerves in the Coccygeal Area

The 5th sacral nerves (the right and left root) leave the spinal canal between the sacrum and coccyx ^[6].

The coccygeal nerves (the right and left root) arise below the 5th sacral nerve and run around the upper part of the coccyx ^[6].

The 5th sacral and coccygeal nerve supply the posterior part of the levator ani muscle and the external anal sphincter ^[6]. They also form the coccygeal plexus, which gives rise the sensory anococcygeal nerves, which enable sensitivity of the skin in the coccyx area above the anus ^[10].

The pudendal plexus is formed by the 2^nd to 5^th sacral nerve and the coccygeal nerve ^[6]. It gives anococcygeal and pudendal nerve, visceral and muscular branches.

Coccyx Deformities

• Not centered (misaligned)
• Not curved, straight
• Excessively curved, crooked
• Angled (L-shaped, hooked)
• Curved sideways (scoliosis)
• Extended
• Protruding, making a hard bump above the anus
• Bifurcated
• Missing (in caudal regression syndrome) ^[13]

• References

  1. Coccyx definition and pronunciation  Dictionary.com
  2. Coccyx pain  Emedicine
  3. Woon JT et al, 2013, CT morphology and morphometry of the normal adult coccyx  PubMed Central
  4. Gluteus maximus muscle  GetBodySmart
  5. The rectum and anal canal  Darmouth College
  6. Grey H, Anatomy of the Human Body, 1918, The Sacral and Coccygeal Nerves  Bartleby.com
  7. Maigne JY, 2002, Management of common coccygodynia  Coccyx.org
  8. Grey H, Anatomy of the Human Body, 1918, The Muscles and Fasciæ of the Pelvis  Bartleby.com
  9. Postacchini F et al, 1983, Idiopathic coccygodynia. Analysis of fifty-one operative cases and a radiographic study of the normal coccyx  PubMed
  10. Woon JT et al, 2014, Redefining the coccygeal plexus  PubMed
  11. Kearney R et al, 2004, Levator Ani Muscle Anatomy Evaluated by Origin-Insertion Pairs  PubMed Central
  12. Snell RS, 2012, Clinical Anatomy by Regions, 9th Edition
  13. Caudal regression syndrome  Rare Diseases
  14. Grey H, Anatomy of the Human Body, 1918, The sacral and coccygeal vertebrae  Bartleby.com