Mucous cysts of the fingers are very common. They are most commonly seen on the backs of the fingers on one side or other of the finger over the last joint of the finger. They often cause irregular growth of the nail (ridging or "dishing" of the nail).
I like to describe mucous cysts as a symptom of a underlying disease rather than a disease in themselves.
The underlying cause of mucous cysts of the fingers is osteoarthritis of the underlying joint.
Osteoarthritis of the smallest joints in your fingers is very common. It can be a condition inherited from your mother or father or it can be the result of wear and tear over the years or a specific traumatic incident such as a broken bone or injury to the joint of that finger.
When you have arthritis in one of these small joints, over time the cartilage covering of the bone is worn away. This leaves the underlying bone exposed so that you have bone on bone contact at either side of the joint as your finger joint moves. This bone on bone contact causing irriatation of the bone forming cells which can make a small bone spur (we call this an osteophyte). Osteophytes are irregular spiky lumps of bone .You may be able to feel them on the backs of the joints involved.
This osteophyte can wear away the covering, stabilising tissues on the back of the joint (the joint capsule) so that a tiny hole develops. When this occurs some of the lining tissue of the joint (the synovium) can poke out through this hole forming a "mucous cyst".
The normal job of this lining tissue of the joint (the synovium) is to make the joint fluid (synovial fluid) so even though it is now outside the joint capsule it continues to make this fluid and thus the cyst can grow bigger.
Often the cysts increase and decrease in size. What is happening when this occurs is that some of the cyst fluid empties back into the joint (and the cyst gets smaller or disappears for a while) but it slowly increases in size again and more fluid is produced.
The ridging of the nail occurs because all this happens in a very confined space and if the cyst is pressing down on the nail bed (base of the nail) over months as the nail grows a "pressure related" ridge or irregularity develops. Is you remove the cyst and the pressure is no longer there the ridge goes away and the new nail grows out over months.