Digital mucous cysts (digital myxoid cysts) are common, benign, fluid-filled sacs that develop on the fingers. These small bumps typically form near the nail on the back of the finger. They are localized collections of a thick, gelatinous material under the skin, often prompting medical consultation due to their appearance or location.
Physical Characteristics and Location
These cysts are usually found on the dorsal surface of the finger, specifically over the distal interphalangeal (DIP) joint, the joint closest to the fingertip. They often arise near the nail fold. Cysts are small, generally ranging from 2 to 10 millimeters in diameter.
The appearance is typically smooth and dome-shaped; the overlying skin may look thin, shiny, or slightly pinkish. When felt, the cyst is firm or rubbery, filled with a clear or pale yellow, sticky, mucin-like fluid. They are anchored to the joint beneath and are not freely movable under the skin.
The Underlying Cause of Formation
Mucous cyst formation is strongly linked to degenerative changes in the underlying finger joint. The primary cause is osteoarthritis (OA) in the distal interphalangeal (DIP) joint. This arthritis breaks down cartilage and leads to the development of bony outgrowths called osteophytes, or bone spurs, around the joint margins.
The bone spurs irritate and weaken the joint capsule, the fibrous sac surrounding the joint. As the joint degenerates, pressure causes the synovial lining to push out through a small defect in the joint capsule. This outpouching forms a stalk connecting to the joint, allowing synovial fluid to leak out and collect beneath the skin.
The fluid is a thick, gelatinous substance rich in hyaluronic acid, which is normally found in joint fluid. The cyst is essentially an extension of the joint space, a mechanical consequence of joint wear and tear. This condition is most common in adults over 40, and the likelihood increases with pre-existing joint disease.
Treatment and Management Strategies
Initial management for a small, painless mucous cyst is often watchful waiting, as some may resolve spontaneously. If the cyst causes pain, restricts movement, or leads to a nail deformity, intervention is typically considered. Non-surgical options include aspiration, which involves draining the fluid with a needle, sometimes followed by an injection of a corticosteroid.
These minimally invasive procedures are convenient but carry a high rate of recurrence, often up to 40% or more, because they do not address the underlying joint connection. Repeated aspiration also risks introducing infection into the joint. Surgical excision is generally regarded as the most definitive treatment for a persistent or problematic cyst.
The procedure involves removing the entire cyst sac and its stalk connecting to the joint. To minimize recurrence, the surgeon often removes or smooths out the underlying osteophyte (bone spur) that caused the joint capsule defect. Surgical treatment combined with osteophyte removal has a significantly lower recurrence rate, often below 10%.
Outlook and Signs Requiring Professional Care
Mucous cysts are benign and pose no risk of becoming cancerous, and they may occasionally shrink or disappear without treatment. Even after treatment, recurrence is possible, particularly following less invasive procedures. The long-term outlook is generally favorable, but the underlying osteoarthritis remains.
A cyst requires prompt professional attention if the overlying skin breaks down or ruptures, as this creates an open pathway for bacteria to enter the joint, leading to a serious joint infection. Signs of infection include increasing redness, swelling, warmth, and pain. If the cyst is positioned directly over the nail matrix, the pressure can result in a permanent longitudinal groove or ridging in the fingernail plate, requiring intervention to prevent irreversible nail deformity.