A mucous cyst is a common type of fluid-filled cyst in the finger. These are called “mucous cysts” due to the thick, clear, mucous-like fluid within the cyst. This fluid has leaked out of the joint closest to the fingernail, the distal interphalangeal (DIP) joint. They are common in both men and women aged 50-70 years. Most patients with mucous cysts also have hand arthritis. Bone spurs (also called “osteophytes”) are a sign of arthritis. Bone spurs are seen on x-rays below. They come in different shapes and sizes as seen in the photos below. Occasionally a ridge in the fingernail develops. The nail deformity is caused by pressure on the nailbed by the cyst. These cysts can go up and down in size some days. They may go away on their own. Most of the time they are painless, but they can become painful if they are inflamed. Sometimes the skin over the cyst is very thin. If the skin breaks, an infection can develop. Surgery is often recommended to remove the cyst, clean out the joint, and remove the bone spur to reduce the chance of the cyst coming back. Simply popping the cyst is not often successful since the fluid comes back from the joint. Patients are discouraged from draining these cysts at home, since an infection can develop involving the bone or joint.