The coccyx is the terminal end of the spine, just inferior to the sacrum. The human coccyx is often considered a vestigial remnant or corollary of a tail; thus, the coccyx is colloquially referred to as the tailbone. The word coccyx comes from the Greek word for cuckoo, the name apparently having been derived from the tailbone’s shape, which resembles that of a cuckoo’s beak. [2]
The human coccyx is composed of 3-5 individual segments (coccygeal vertebrae), with variations occurring with regard to the number of segments, the overall angulation (curve) of the coccyx, and the degree of articulation versus fusion between the individual segments. In 80% of patients, the coccyx is made up of 4 coccygeal vertebrae. Typically, the coccyx is concave anteriorly and convex posteriorly.
The base of the coccyx articulates with the sacral apex via the sacrococcygeal junction. The sacrococcygeal articulation and intracoccygeal articulations contain fibrocartilaginous discs, somewhat comparable to the intervertebral discs present at other spinal levels. The apex (distal tip) of the coccyx is typically rounded, but may be bifid. (See the image below.)
Lateral view of the pelvis and coccyx. The bracket shows the area of focus for radiographs that would provide a coned-down view of mainly the coccyx and distal sacrum. A more common lateral view would often also include larger bony structures, such as the lumbar spine and femur, all of which would make it difficult to optimize visualization of the small bones of the coccyx. In patients with coccyx pain, these coned-down, lateral views of the coccyx can provide important diagnostic information. Coned-down images obtained in the weight-bearing (seated) position can be compared with those obtained in a non–weight-bearing position (eg, side lying), thus allowing assessment for dynamic instability (eg, dislocations that occur only while seated).
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Lateral view of the pelvis and coccyx. The bracket shows the area of focus for radiographs that would provide a coned-down view of mainly the coccyx and distal sacrum. A more common lateral view would often also include larger bony structures, such as the lumbar spine and femur, all of which would make it difficult to optimize visualization of the small bones of the coccyx. In patients with coccyx pain, these coned-down, lateral views of the coccyx can provide important diagnostic information. Coned-down images obtained in the weight-bearing (seated) position can be compared with those obtained in a non–weight-bearing position (eg, side lying), thus allowing assessment for dynamic instability (eg, dislocations that occur only while seated).
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Coccyx pain (coccydynia, or tailbone pain) is typically worse when the patient is sitting. Often, the pain is even worse when sitting leaning slightly backward, since this increases the weight bearing on the coccyx, as shown in this image. Dynamic radiographs of the coccyx involve obtaining coned-down (focused) views of the coccyx while the patient is seated (eg, in his or her most painful position). Often, this involves having the patient lean backward 0-40°, depending on the symptoms. Radiographs obtained in this position are compared with those obtained in a non–weight-bearing position (eg, side lying) to assess for instability or dislocations in the seated position.
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Patients with a painful coccyx often find it more comfortable to sit leaning slightly forward, as shown in this image. This forward-leaning position minimizes any weight bearing on the coccyx itself and thus minimizes the exacerbation of coccyx pain. As shown, when a patient sits leaning forward, most of the weight bearing occurs bilaterally through the inferior ischial regions of the pelvis and the posterior thigh (femur) regions.
