The Anatomy of the Coccyx

The last four vertebrae that make up the tailbone

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The last three to five (usually four) vertebrae of the spinal column are fused together to make the coccyx, colloquially known as the tailbone. Fusion begins during a person's 20s and is usually complete by the age of 30. Before fusion occurs, the coccygeal vertebra articulate just like any other section of the spinal column. There is a debate among spinal specialists and others about whether the coccyx is a relevant and useful part of human anatomy.

The coccyx can be a source of pain in many people. It is commonly damaged from trauma due to falls and can be the location of idiopathic pain, meaning pain that doctors don't know the cause of.

Anatomy

The coccyx is the most distal portion of the spine in primates that don't have tails, including humans.

For the first 20 years of life in humans, the coccyx is made up of separate coccygeal vertebrae, which then fuse together to make a single sphenoid bone that is more commonly known as the tailbone.

The fusion of the coccyx is usually completed by the 30th birthday if it is to be completed at all.

Structure

The coccyx is an inverted triangle with the base (wide part) at the top and the apex (pointy end) at the bottom. Even before the fusion of the coccyx is complete, all but the first coccygeal vertebrae are little more than underdeveloped vertebrae that look a bit like nodules of bone rather than independent structures.

There are usually four coccygeal vertebrae that attach to the apex (small, bottom part) of the sacrum. Most of the time, they are referred to as Co1-Co4. It is common and quite natural to be born with as few as three and as many as five coccygeal vertebrae.

Viewed as a single bone, the coccyx resembles a bull's head. It has two "horns" on top (the base) that are called the coccygeal cornua. The "ears" would be the transverse processes that provide a fulcrum point for articulation with the sacrum.

Location

The coccyx is located at the distal tip of the sacrum and is the most distal portion of the spinal column. The base of the coccyx articulates with the apex of the sacrum. Some articulation is possible between coccygeal vertebrae until they are fused, but they do not move very much.

As the lowest point of the vertebral column and sitting at the bottom of the pelvic girdle, the coccyx acts as one insertion point for the muscles of the pelvic floor, a group of three muscles called levator ani at the apex, the coccygeus muscle across the anterior (front) surface, and the gluteus maximus across the posterior (back) surface. It is connected to the sacrum via the sacrococcygeal ligament.

Anatomical Variations

As mentioned above, the coccyx is usually comprised of four coccygeal vertebrae. One study found four coccygeal vertebrae in 76% of healthy coccyges (the plural of coccyx). The coccyx can contain as few as three (13%) or as many as five (11%).

The shape and curvature of the coccyx can vary between individuals and is noticeably different between sexes. The female coccyx is more narrow, less triangular, and more likely to be straight or curved outwardly instead of inwardly.

In more than half of adults (57%), the sacrococcygeal joint (the joint between the sacrum and the coccyx) is fused. The joint between Co1 and Co2 is only fused in 17% of coccyges.

The further along you go on the coccyx, the more common it is for the segments to be fused together.

Function

If humans had tails, the coccyx would have a much more satisfying job. Unfortunately for it, humans don't, and there are some who say that the coccyx really doesn't have any function at all.

Several pelvic floor muscles are attached to the coccyx, but every muscle has multiple redundant attachment points. Most of those redundant attachment points are considerably stronger and more stable than the coccygeal vertebrae.

Contraction of those muscles can create enough movement of the coccyx to cause pain in some individuals.

One common treatment for traumatic pain or atraumatic pain that originates in the coccyx for no discernable reason (idiopathic coccyx pain) is for doctors to remove some or all of the coccyx. In patients who've had the coccyx surgically removed, there doesn't appear to be any common side effects, which suggests that the coccyx truly doesn't have a function.

Associated Conditions

The most common condition associated with the coccyx is pain, which is called coccydynia or coccygodynia.

Trauma is the most common cause. The location of the coccyx makes it vulnerable to trauma if a person falls to a sitting position. It can become broken or bruised.

In cases of coccydynia, contraction of the pelvic floor muscles can be very painful in the area of the coccyx that is damaged or inflamed. The movement of muscles can lead to movement of the coccyx itself, causing pain.

Because of the number of pelvic floor muscles attached to the coccyx, certain bodily functions, including sex or defecation, can lead to pelvic pain after trauma to the coccyx.

Idiopathic pain of the coccyx is pain caused for no discernible reason. It is more common in females than in males. This is a diagnosis of exclusion, meaning that it can only be diagnosed after all other possible causes have been ruled out.

Not every coccyx gets completely fused. In some people, the coccyx remains mobile and can continue to move as the person sits and moves. There is some evidence that a rigid coccyx is more likely to cause a certain type of pain due to the fact that it is constantly irritating surrounding soft tissues as the person changes positions.

Sacrococcygeal teratomas are the most common type of neonatal tumor and develop on the sacrum or coccyx. The prognosis for a sacrococcygeal teratoma is very good as long as it is diagnosed correctly and early. Teratomas generally appear when the patient is very young.

Treatment

Treatment options depend on which condition is causing pain in the coccyx.

Conservative Treatment

In the case of trauma, the most common treatment is a conservative mix of therapies.

  • Use of special cushions (donut style) is encouraged to reduce pressure on the coccyx.
  • Physical therapy stretches and muscle building are used to provide more strength to the surrounding tissues.
  • Avoid high impact exercises, such as running, jumping, calisthenics, and bike riding while the coccyx is healing.
  • Use over-the-counter pain medications while the coccyx is healing.

It is probably a good idea to give conservative treatment a long leash. It is thought to be successful in 90% of cases of coccydynia.

Surgical Treatment

If a conservative approach is not working, your doctor might suggest surgical removal of the coccyx, known as coccygectomy.

There is not a standard timeline for how long you should wait to consider surgery. Some doctors will consider it in as little as two months if nothing seems to be working. Other doctors might want to continue to try other options for as long as a year.

Even though it is more aggressive than nonsurgical treatment options, complete or partial coccygectomy is considered to be very safe and relatively effective. Patients that have the procedure have good outcomes. About 75% of coccygectomies have a complete reduction of pain.

It is up to you to decide if a 75% success rate is sufficient to undergo surgery. The most common predictor of a poor outcome or failure to relieve pain in all coccydynia patients is whether or not the surgical removal was complete or partial. Evidence suggests that complete coccygectomies lead to better outcomes than partial removal of the coccyx.

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