The coccyx, commonly known as the tailbone, is a small, triangular bone located at the very end of the vertebral column, just below the sacrum. When persistent pain in this area, known as coccydynia, follows a traumatic injury, an X-ray is often the initial imaging procedure to examine the underlying bony structure. The X-ray provides a quick, non-invasive method to visualize the coccyx and the surrounding region, helping to determine if a fracture, dislocation, or abnormal alignment is the cause of the discomfort. While X-rays are excellent for viewing bone, they represent only one part of the diagnostic process for tailbone pain.
Understanding Coccyx Anatomy
The coccyx is typically composed of three to five small segments, known as coccygeal vertebrae, which are often fused or semi-fused in adults. This final portion of the spine maintains a gentle, forward-angling curve, pointing toward the pelvis. The most superior segment of the coccyx articulates with the sacrum, the large triangular bone above it, at the sacrococcygeal joint. The coccyx serves as an anchor point for numerous pelvic ligaments, tendons, and muscles, including the gluteus maximus and the muscles of the pelvic floor. This structure is also one leg of the “tripod” that helps distribute body weight and stabilize the pelvis when a person is sitting.
The X-Ray Procedure and Positioning
To obtain a comprehensive view of the coccyx, two standard projections are generally taken: the Anteroposterior (AP) view and the Lateral view. The AP view is taken with the patient lying on their back and is useful for assessing the general shape and checking for any lateral deviations or fractures. This view requires the X-ray beam to be angled slightly toward the feet to minimize the obscuring effect of the pelvis. The Lateral view, taken with the patient typically lying on their side, is considered the most informative for evaluating the coccyx’s curvature and its relationship with the sacrum. Both views require the patient to remain still to prevent motion blur, which can make subtle features like hairline fractures difficult to detect. Sometimes a dynamic X-ray may also be performed, which involves taking a lateral image while the patient is standing and another while they are sitting, to assess for abnormal movement or hypermobility of the coccyx under stress.
Identifying Features of a Normal Image
A normal coccyx X-ray image reveals a structure with natural variability, meaning that what looks “normal” can differ significantly between individuals. The most common normal appearance, seen in over half of the population, is a gentle, forward curvature that is a smooth continuation of the sacrum’s curve, with the tip pointing toward the tail. This is often classified as a Type I coccyx.
The number of segments visible can range from three to five, with four segments being the most frequently observed configuration. Fusion between these segments is a normal, age-related finding, and a sacrococcygeal joint that is partially or fully fused is present in a majority of adults. A healthy X-ray will show smooth, uniform spaces between any unfused segments, indicating an absence of significant joint irregularity.
The overall alignment should be smooth, with no clear breaks in the cortical bone, which would suggest a fracture. There should be no significant displacement or abrupt subluxation at the sacrococcygeal joint or between the coccygeal segments. While some angulation is normal, a sharp bend or a tip that points excessively forward or backward is considered an anatomical variant that may predispose an individual to pain. A normal image excludes serious conditions that can affect the bone, such as tumors or bone infections, which would appear as areas of abnormal bone density or destruction.
What Coccyx X-Rays Cannot Reveal
While X-rays are effective for visualizing bone structure, they have significant limitations, especially when a patient experiences chronic pain. Standard X-rays only capture a static image and cannot provide information about the soft tissues that surround the bone. Therefore, causes of coccydynia such as ligament sprains, muscle inflammation, or bursitis will not be visible on a plain X-ray. Furthermore, an X-ray cannot show subtle damage to cartilage or nerve impingement, which can be significant sources of pain. Many cases of coccydynia show a completely normal X-ray, as the pain is often related to abnormal mobility of the joint that is only apparent when the body is in motion. If a static X-ray is normal but pain persists, a doctor may recommend dynamic X-rays or more advanced imaging like Magnetic Resonance Imaging (MRI). The MRI provides superior visualization of soft tissues and can detect subtle bone marrow changes that may not be apparent on a static X-ray.