The coccyx is a triangular arrangement of bone that makes up the very bottom portion of the spine below the sacrum. It represents a vestigial tail, hence the common term tailbone.
Depending on an individual’s development, the coccyx may consist of three to five different bones connected by fused—or semi-fused—joints and/or disc-like ligaments. While it was originally thought that the coccyx is always fused together, it is now known that the coccyx is not one solid bone, but there is some limited movement between the bones permitted by fibrous joints and ligaments.
The coccyx connects with the sacrum through the sacrococcygeal joint, and there is normally limited movement between the coccyx and the sacrum. The coccyx usually moves slightly forward or backward as the pelvis, hips, and legs move. When a person sits or stands, the bones that make up the pelvis (including the coccyx) rotate outward and inward slightly to better support and balance the body.
Function of the Coccyx
Although the tailbone is considered vestigial (or no longer necessary) in the human body, it does have some function in the pelvis. For instance, the coccyx is one part of a three-part support for a person in the seated position. Weight is distributed between the bottom portions of the two hip bones (or ischium) and the tailbone, providing balance and stability when a person is seated.
The tailbone is the connecting point for many pelvic floor muscles. These muscles help support the anus and aid in defecation, support the vagina in females, and assist in walking, running, and moving the legs.
Why Do More Coccyx Injuries Occur in Women Than Men?
Coccydynia is generally much more common in women; some sources from the medical literature find that women are five times more likely to develop coccydynia than men.1
The majority of coccyx injuries occur in women because:
- A broader pelvic structure, which may decrease the amount of pelvic rotation and leave the coccyx more exposed to injury.
- Women tend to place more weight on the coccyx when sitting, which leaves it more susceptible to injury.
- Childbirth, which may cause acute damage as the baby moves over the tailbone
Pelvic muscle cramps can also play a role in increased coccyx pain in women. In physical evaluations, women have reported significantly increased coccyx pain during the premenstrual period.