The coccyx or tailbone is the final section of the spine in humans and many other mammals. It is small, with a triangular shape, and resembles a very short tail. Typically, the coccyx consists of between three and five vertebrae. In some individuals, the vertebrae fuse together to form a single solid bone. In others, up to the third vertebrae may remain separate from the remaining vertebrae. Research shows the coccyx performs some potentially important functions and acts as a connection for many muscles and ligaments.
The anterior or frontal surface of the coccyx is slightly concave, crossed with several grooves. This area connects to important ligaments and muscles and provides support for a portion of the rectum. The posterior area is convex and possesses similar grooves. A row of paired round projections lies on either side of the posterior surface. The largest pair, the coccygeal cornua, project towards the spine and connect the coccyx to the sacrum.
The main structure to which the coccyx connects is the sacrum. This large triangular bone sits between the two wings of the pelvis. The sacrum and the first coccygeal vertebra share a similar appearance. The coccyx connects with the sacrum at an amphiarthrodial joint, the sacrococcygeal symphysis. Unlike other joints, amphiarthrodial joints allow for little movement. In addition to connecting to the sacrum, the coccyx also serves as the attachment point for some muscles and ligaments.
Many muscles connect to the tailbone. The levator ani is a broad but thin muscle that sits on either side of the pelvis. It connects to the anterior area of the coccyx. It consists of three smaller muscles, the iliococcygeus, the pubococcygeus, and the puborectalis. Both the iliococcygeus and the pubococcygeus connect to the coccyx. The coccygeus also attaches to the coccyx. This muscle, along with the levator ani, forms the pelvic diaphragm. On the posterior side of the coccyx, there is an attachment site for the gluteus maximus, the large muscle that helps extend the thigh.
In addition to acting as an attachment site for muscles, the tailbone also serves as an attachment site for many ligaments. Both the anterior and posterior sacrococcygeal ligaments connect to the coccyx. These are continuations of the ligaments that stretch along the spine. The lateral sacrococcygeal ligaments stretch from the lower area of the sacrum and attach to the coccyx, helping stabilize the joint where the sacrum and the coccyx meet. Some of the fibers of the sacrotuberous and sacrospinous ligaments also connect to the coccyx.
Some physicians argue that tailbone does not play any important roles or perform any essential functions in the body. This is because all of the muscles and ligaments that attach to the coccyx also attach to much stronger structures. The coccyx does act as one leg of the tripod of support, along with the ischial tuberosities, bones that provide weight-bearing support for the body when it is in a sitting position. Leaning back increases the amount of pressure on the coccyx.
The coccyx gained its nickname of the tailbone because of its probable evolutionary origin point. Every mammal possesses a tail at some point in their development, even humans. Human tails appear during fetal development for around four weeks and are most noticeable when the fetus is between 30 and 35 days old. Typically, the body absorbs this structure, with the coccyx remaining as the only evidence of its existence. Full absorption of the vestigial tail usually completes around the eighth week of fetal development.
There are cases of a person damaging their tailbone, resulting in severe pain. This is coccydynia or, historically, coccygodynia. Many physicians minimize or dismiss the severity of the pain coccydynia causes, though this has begun to change in recent years. The pain is typically severe and persistent, often rendering the person incapable of daily activities or sitting and leaning back. Individuals with coccydynia often lead forward while sitting to alleviate pressure on their coccyx.
An individual can damage their coccyx in many different ways. Additionally, some cases of coccydynia develop from abnormal curvature of one of the coccygeal vertebrae. Acute and abrupt trauma can lead to coccydynia. This includes injuries such as falling onto the tailbone. During childbirth, the coccyx receives a substantial amount of pressure and many women experience coccyx injuries as a result. Prolonged sitting and a sedentary lifestyle may also damage the bone. Medical procedures such as a colonoscopy can cause tailbone pain, particularly in older individuals.
Initial diagnosis of coccydynia usually requires radiographs. Technicians may observe plain radiographs to find fractures, dislocations, abnormal curvature, or bone spurs. Magnetic resonance imaging or MRIs can also help identify whether the pain originates at the coccyx, or if it begins in the lumbar region. In some instances, a physician may observe the patient’s response after an injection of local anesthetics. This can allow a physician to determine from where the pain originates without using expensive scans.
Coccydynia can be acute or chronic. Acute cases are typically easy for physicians to treat. Physical therapy focusing on the levator ani or the coccygeus can help alleviate pressure on the region and provide greater support. Chronic cases of coccydynia may require more advanced treatments. Surgery to remove the tailbone, a coccygectomy, can alleviate pain and has very few side effects. Radiofrequency ablation or chemical ablation can destroy or inhibit the nerve fibers around the coccyx, preventing the nerves from sending pain signals to the brain.
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