The coccyx is the small, triangular bone at the very bottom of your spine, commonly called the tailbone. It sits just below the sacrum (the broad, flat bone at the base of your back) and is typically made up of three to five small vertebral segments, with four being the most common. Despite its small size, the coccyx serves as an anchor point for muscles and ligaments that support your pelvic floor, and it plays a direct role in how your body bears weight when you sit.
Structure and Anatomy
The coccyx is often described as a single fused bone, but imaging studies show it’s more complex than that. It contains three to five separate bony segments that may or may not be fused together, depending on your age and individual anatomy. These segments develop from about eight ossification centers, small points where cartilage gradually hardens into bone. This process begins before birth and continues in an organized pattern all the way into your late twenties or around age 30.
The degree of fusion between segments varies from person to person. Some people have fully fused coccygeal bones by adulthood, while others retain small joints or disc-like spaces between segments that allow slight movement. The joint between the sacrum and coccyx also fuses in many people over time, with rates differing by age and sex.
What the Coccyx Does
The coccyx is far from useless. It serves as an attachment site for a network of muscles and ligaments critical to pelvic stability. The levator ani, a key muscle group in the pelvic floor that supports the bladder, intestines, and uterus, connects to the front surface of the coccyx. The gluteus maximus, the largest muscle in the body, attaches fibers along its edges. Several important ligaments also anchor here, including the sacrospinous and sacrotuberous ligaments, which help stabilize the pelvis.
When you sit, the coccyx flexes slightly and shares some of your body weight. Normal coccygeal movement during sitting falls between 5 and 25 degrees of flexion. Too much movement (over 25 degrees) or too little (under 5 degrees) can both contribute to pain. This weight-bearing role is why tailbone injuries make sitting so uncomfortable.
The Evolutionary Story
The coccyx is what remains of the tail our distant ancestors once had. During early embryonic development, every human embryo actually grows a small tail containing 10 to 12 vertebral segments. By the end of the fifth week, this tail reaches its maximum length, roughly one-sixth the size of the entire embryo. Over the following weeks, white blood cells gradually break down and absorb the tail’s extra vertebral segments. By the eighth week of development, the tail is fully gone, leaving behind only the three to five segments that become the coccyx.
While the coccyx is sometimes called a vestigial structure, its role as a muscle and ligament anchor point means it still has genuine functional value in the modern human body.
Tailbone Pain: Causes and Risk Factors
Tailbone pain, known medically as coccydynia, is most commonly caused by trauma. A direct fall onto the tailbone accounts for roughly half of all cases. Childbirth is another significant trigger, responsible for about 10% of cases. Obesity and female sex are both associated with higher risk, and low mood has also been linked to the condition.
The shape of the coccyx itself can play a role. Natural variations in how the segments are angled or how mobile the joints between them are can predispose certain people to pain. In about 69% of patients with coccydynia, imaging reveals abnormal movement or positioning of the coccyx when they shift between standing and sitting. People who are overweight tend toward a specific pattern where the coccyx subluxes (partially dislocates) backward when they sit, while thinner individuals are more prone to excessive flexion of the bone under direct pressure.
How Tailbone Problems Are Diagnosed
Standard X-rays and CT scans can identify fractures, dislocations, or structural abnormalities. But when those come back normal, doctors may use dynamic imaging, which involves taking X-rays in both standing and seated positions to see how the coccyx moves under load. A bone that flexes more than 25 degrees or less than 5 degrees when sitting is considered abnormal. This technique, while recommended, isn’t yet standardized, so it’s not universally available.
Treatment for Coccyx Pain
Most tailbone pain is managed without surgery. Common approaches include cushions with a cutout at the back to reduce pressure on the coccyx while sitting, physical therapy focused on the pelvic floor muscles, and injections to reduce inflammation at the site.
When pain persists despite months of conservative treatment and significantly limits daily activities, surgical removal of part or all of the coccyx (a coccygectomy) becomes an option. Success rates for this procedure range from 60% to 100% across studies. Patients whose pain started from a specific injury or childbirth tend to have the best outcomes, with about 75% reporting significant improvement. Those with tailbone pain that developed without a clear cause have a lower but still meaningful success rate of around 58%.