The coccyx, commonly known as the tailbone, is the small, triangular bone at the bottom of the spinal column. It functions as an anchor point for numerous ligaments, tendons, and muscles of the pelvic floor. When a person experiences persistent pain (coccydynia) or suffers a traumatic injury, a doctor may request a coccyx X-ray. The goal of this imaging is to determine if the bone is fractured, dislocated, or has structural changes that might explain the discomfort.
Understanding the Coccyx and Imaging Procedure
The coccyx is composed of three to five small, fused or semi-fused vertebral segments; the first segment is the largest and most distinct. This bone provides one of the three points of support, along with the two ischial tuberosities, when a person is seated. The sacrococcygeal joint maintains the relationship between the coccyx and the sacrum, the large bone directly above it.
A standard coccyx X-ray typically involves two main projections: the anteroposterior (AP) view and the lateral view. The lateral view is usually the most informative, providing a clear profile of the bone’s alignment and curvature, which the AP view often obscures. To assess the bone under stress, a radiologist may also perform “dynamic” X-rays, capturing images while the patient is both standing and sitting. Comparing these images helps determine if the coccyx moves excessively or abnormally when weight is applied, which can cause chronic pain.
Key Features of a Normal Coccyx X-ray
A radiologist interpreting an X-ray looks for specific features that define a healthy, normal appearance. The bone is typically formed from three to five distinct coccygeal segments, though the exact number varies. If the segments have not yet fused, the spaces between them should appear uniform, without signs of narrowing or widening that might suggest joint issues.
The alignment and curvature are scrutinized on the lateral view, where a normal coccyx displays a smooth, gentle forward curve, known as an anterior angulation. This natural curvature should be continuous with the curve of the sacrum above it, creating a smooth line. Significant deviations, such as a sharp break or angulation, may indicate an abnormal fracture or dislocation.
The bone density across the segments should appear uniform and consistent throughout the structure. Abnormal variations in density, such as areas of unusual hardening (sclerosis) or areas where bone appears thinned out (lysis), can point toward underlying conditions like infection, arthritis, or a tumor. The overall contour of the bone should be smooth, without any breaks or discontinuities that would suggest a fracture.
Natural Variations That Are Not Injuries
The coccyx exhibits one of the highest degrees of natural variation in the human skeleton. What may appear unusual is often a benign variant, not a sign of injury. A common variation is segment fusion, where two or more coccygeal vertebrae are naturally joined together by bone. This fusion can occur between any of the segments or between the first segment and the sacrum, and it is a common finding not linked to pain.
Radiologists often classify the coccyx into various morphological types based on its curvature and angle, with four to six types commonly recognized. Type I, the most frequent variant, shows a gentle forward curve that continues naturally from the sacrum. Other types show more prominent forward curves, sharp angulations, or a retroverted (backward-pointing) tip. All these are considered normal variations unless they are the source of documented pain. For instance, a sharp anterior angulation (Type III) or an anterior subluxation (Type IV) are recognized morphological types classified as normal variants unless the patient is experiencing symptoms.
Limitations of Coccyx X-rays
A common clinical scenario involves a patient with chronic tailbone pain (coccydynia) whose X-ray is reported as entirely normal. This highlights the limitations of standard plain film X-rays in evaluating coccyx pain. X-rays are excellent for visualizing dense bone structure but cannot show soft tissues, which are often the source of persistent pain.
Inflammation, ligament damage, muscle sprains, or issues with the intervertebral discs are not visible on a static X-ray. Therefore, a “normal” X-ray only confirms sound bone integrity, ruling out a fracture or dislocation, but it does not exclude soft tissue injuries. If pain continues despite a normal static X-ray, dynamic X-rays may be necessary to check for instability, or a Magnetic Resonance Imaging (MRI) scan may be ordered to visualize soft tissues and rule out other conditions.