Coccydynia

Coccydynia refers to pain in the coccyx, or tailbone, typically triggered by a direct fall onto that area or by ongoing irritation from prolonged sitting. It is also called coccygodynia, coccygeal pain, coccyx pain, or coccalgia. The coccyx sits at the very bottom of the spine and is considered a vestigial tail—essentially a leftover "tail bone." Its name comes from the Greek word for "cuckoo," because the bone resembles a cuckoo's beak. The coccyx consists of three to five vertebrae, some of which may be fused. Its front side is slightly concave, while the back is slightly convex; both sides have transverse grooves marking where the vestigial coccygeal units once fused. The coccyx connects to the sacrum via dorsal grooves, forming either a symphysis or a true synovial joint, and also attaches to the gluteus maximus, the coccygeal muscle, and the anococcygeal ligament.

Postacchini and Massobrio described four coccyx orientations. In type I, the coccyx curves slightly forward with its apex pointing downward and forward. Type II shows a more dramatic forward curve, with the apex extending forward. Type III involves a sharp forward angle. Type IV features a subluxation at the sacrococcygeal joint.

Two main pathophysiological causes are a sudden impact from a fall and pressure during childbirth in women. Other ways coccydynia develops include partial dislocation of the sacrococcygeal synchondrosis, which can lead to abnormal coccyx movement from excessive sitting, and repetitive trauma to surrounding ligaments and muscles, causing tissue inflammation and pain.

Diagnosis first aims to confirm that the pain originates from the coccyx. Physical rectal examination, high-resolution X-rays, and MRI scans help rule out unrelated causes such as Tarlov cysts or pain referred from higher in the spine. Contrary to most anatomy textbooks, most coccyxes consist of several segments; a "fractured coccyx" is often misdiagnosed when the bone is actually normal or simply dislocated at an intercoccygeal joint. A simple test involves injecting a local anesthetic into the area—if the pain is coccyx-related, this should provide immediate relief. If the anesthetic test is positive, a dynamic (sit/stand) X-ray or MRI may reveal whether the coccyx dislocates when sitting.