For a dynamic x-ray the radiologist takes two x-rays, one with you standing and with you sitting. Comparing these x-rays shows whether your coccyx moves more than normal when you sit down. If your coccyx does move too much, that shows that it is unstable. The method was developed by by Dr Jean-Yves Maigne, who describes it in detail in the paper Management of Common Coccygodynia. He found that when he used this method, he could give a firm diagnosis of the cause of coccyx pain in 70% of cases. Dr Maigne has explained how to take and read a dynamic x-ray (scroll down to 'Technique of dynamic films').
This method deserves to be much more widely used, as doctors have a much better chance of curing you if they know exactly what is wrong. Unfortunately, as the method was developed fairly recently, most orthopedic specialists have never heard of it. Some people who have written to me have told me that they persuaded their doctor to carry out a dynamic x-ray for them, but others say their doctor refused. Sally Cowell persuaded her doctor to give her a dynamic x-ray by showing him a print out of a medical paper about it. Here is Sally's description of what she did. Alternatively, if your doctor refuses, you could try a chiropractor. They often have an x-ray machine of their own, and may be more willing than a doctor to try out this method.
Note: One patient wrote that her doctor could not get a good picture of her coccyx when she was sitting. She then had an x-ray taken lying on her side, with her knees up to her chest. She writes: "At the time we did the x-ray with me curled up in the ball, I was still in pain from trying the sitting x-ray, and I forced myself to be in as much pain as possible before we did that laying on my side x-ray. He said there is a significant difference between the x-ray where I laid on my side and the standing x-ray. He did not know if the act of curling up in a ball moved my tailbone, or if it was still out of place from recently sitting."
For patients in the UK: Dynamic sit/stand x-rays are recommended by the Royal College of Radiologists in their booklet for radiologists, 'Making the best use of Clinical Radiology', eighth edition, 2017, under T18, Trauma to coccyx or coccydynia. Any patient having trouble persuading a radiologist to carry out this procedure can point this out to the radiologist. See tailbonedoctor.com/uk-sitting-standing-coccyx-x-rays-for-tailbone-pain-coccydynia
A positive dynamic x-ray is helpful in directing the treatment:
MRI scanners are now available that you can stand or sit in, called open MRI. These could be used for a dynamic sit/stand MRI.
The pictures below show examples of dynamic x-rays. The coccyx is the last few segments of the spine, and above it the sacrum is visible.
Hypermobility (the coccyx bending upwards far more than normal)
Four examples of posterior luxation (dislocation of a joint, with the lower joint surface slipping backwards)
All of the x-rays on this page are courtesy of Dr Jean-Yves Maigne, Paris, France