Coccydynia is a medical term meaning pain in the coccyx or tailbone area, often brought on by a fall onto the coccyx or by persistent irritation usually from sitting.

Coccydynia is also known as coccygodynia, coccygeal pain, coccyx pain, or coccalgia.

Coccydynia occurs in the lowest part of the spine, the coccyx, which is believed to be a vestigial tail, or in other words the "tail bone". The name coccyx is derived from the Greek word for 'cuckoo' due to its beak-like appearance. The coccyx itself is made up of three to five vertebrae, some of which may be fused together. The ventral side of the coccyx is slightly concave whereas the dorsal aspect is slightly convex. Both of these sides have transverse grooves that show where the vestigial coccygeal units had previously fused. The coccyx attaches to the sacrum from the dorsal grooves, with the attachment being either a symphysis or as a true synovial joint, and also to the gluteus maximus muscle, the coccygeal muscle, and the anococcygeal ligament.

There are four different orientations for the coccyx, as described by Postacchini and Massobrio. In type I the coccyx is curved anteriorly with its apex facing downward and caudally. In type II this forward curvature is more dramatic and the apex extends forward. Type III is where the coccyx angles forward sharply. Lastly, type IV is characterized by the coccyx being subluxated at the sacrococcygeal joint.

There are common pathophysiological ways that a person may develop coccydynia. The two main causes for this condition are sudden impact due to fall, and coccydynia caused by childbirth pressure in women. Other ways that coccydynia develops are partial dislocation of the sacrococcygeal synchondrosis that can possibly result in abnormal movement of the coccyx from excessive sitting, and repetitive trauma of the surrounding ligaments and muscles, resulting in inflammation of tissues and pain.

A number of different conditions can cause pain in the general area of the coccyx, but not all involve the coccyx and the muscles attached to it. The first task of diagnosis is to determine whether the pain is related to the coccyx. Physical rectal examination, high-resolution X-rays and MRI scans can rule out various causes unrelated to the coccyx, such as Tarlov cysts and pain referred from higher up the spine. Note that, contrary to most anatomical textbooks, most coccyxes consist of several segments: 'fractured coccyx' is often diagnosed when the coccyx is in fact normal or just dislocated at an intercoccygeal joint.

A simple test to determine whether the coccyx is involved is injection of local anesthetic into the area. If the pain relates to the coccyx, this should produce immediate relief.

If the anesthetic test proves positive, then a dynamic (sit/stand) X-ray or MRI scan may show whether the coccyx dislocates when the patient sits.