The commonest cause of coccydynia is an unstable coccyx - one which dislocates or moves more than normal when you sit down. It may either bend upwards more than normal, or the joint may partly dislocate, slipping backwards. An unstable coccyx may be caused by trauma, such as from falls, from childbirth, or from car accidents, or it may be caused by weakening of the joint, which can occur in pregnancy. In many cases the cause of an unstable coccyx is unknown. You can see a video about tailbone pain and pregnancy from Dr Patrick Foye, www. tailbonedoctor.com. The medical paper by Bayne et al includes the cases of four women whose coccyx pain was apparently caused by anal intercourse. Beating a child on the bottom can also cause this.
The pictures below show unstable coccyxes.
This picture is based on x-rays of someone facing to the left, standing and then sitting. It shows a coccyx which bends up more than normal on sitting (a hypermobile coccyx).
In this picture, the coccyx is made up of three segments (coccyxes vary a lot). The x-rays show that the whole coccyx slips backwards when this patient sits down.
It is not surpising that slippage of a joint like this, pulling or tearing the surrounding tissues, can cause inflammation and pain. Because normal life is impossible without sitting, the joint is repeatedly forced out of its normal position, and may not be able to heal properly. Sitting down with an unstable coccyx is like spraining your ankle, then forcing the ankle to bend more than normal.
The most common trauma is a fall on to the bottom. Accidents like slipping when walking down stairs, or falls when skating or just on a slippery floor are often the cause. Childbirth can also strain the coccyx. Apparently the joints around the pelvis become more flexible towards the end of pregnancy, to allow delivery of the baby. This allows the coccyx to be pushed out of position more easily. Some women start getting the pain during the pregnancy, and some get it during and after delivery of the baby.
Repetitive strain from cycling or rowing has also been blamed for this condition. One of the medical papers says that anal intercourse can also be a cause. And in some cases there is no known cause, but the joint has become dislocated.
'Fractured coccyx' Doctors quite commonly diagnose 'fractured coccyx', and sometimes 'incomplete fusion of the coccyx'. So far as I can make out, both of these diagnoses are usually wrong, and are based on the idea, given in medical textbooks, that the coccyx is normally fused into one piece in adults. This is not true - doctors who have examined uninjured coccyxes have found that they normally consist of two or more pieces. Doctors who have no experience in this area look at an x-ray of a coccyx of someone in pain, and find that the coccyx is in several pieces, and so assume that it must either have been broken, or it never fused together properly.
A survey of the causes of coccyx pain published in the medical journal Spine by Dr Maigne and colleagues found that 55% of cases of coccyx pain were apparently caused by instability of the coccyx on sitting down.
The best method of diagnosing an unstable coccyx is by comparing x-rays of the coccyx taken when the patient was sitting and standing. A history of the pain starting immediately after a fall, or starting in pregnancy or childbirth, is also good evidence (though not everyone with an unstable coccyx can identify the cause). Some people can feel the coccyx dislocate by putting some hand cream on the area of the coccyx/sacrum joint, and feeling the joint with a finger as they sit down.
Dr Maigne found that all of the patients he tested who had acute pain while moving from sitting to standing had an unstable coccyx .
Sometimes the joint will heal by itself over a period of weeks or months. If it does not, injections of corticosteriods usually help. Some people get permanent relief with this treatment, while most get relief for a few months. The treatment can be repeated. Oddly, when the treatment is successful, the joint is still unstable, but no longer hurting. If corticosteroid treatment is unsuccessful, removal of the coccyx is successful in most cases.