Sometimes there is a spur of bone on the coccyx. Dr Maigne, in a paper in Spine, found that this was the cause of the coccyx pain in 14% of patients.The spur is usually invisible on x-rays, unless they are very high quality, but may be seen on MRI images. The spur points backwards at the tip of the coccyx, and can always be felt through the skin.
Three different coccyx spurs
There is usually a corresponding pit in the skin over the spur, which suggests that the spur has been present since before birth. Dr Maigne considers that some cases in the medical literature described as a pilonidal sinus (a pit associated with a hair follicle) in this position may in fact be the pit corresponding to a spicule. This link suggests to me that these two conditions are part of a spectrum of conditions which have their origin in the way that the spine is formed in the embryo and the skin closes over it. But that is just my guess.
Spicules only caused pain in thin patients, who do not have any padding of fat over the spur.
The spur can be felt through the skin. The diagnosis can be confirmed with high-quality x-rays or MRI scans.
According to Dr Maigne, "The usual treatment for spicules is a local injection (around the spicule), with a good response rate (around 70%). We have operated on some cases, with a good result."