Mucous Cyst

A mucous cyst is a small ganglion cyst that pops up on top of the last joint of your finger — almost always because that joint already has arthritis. It looks like a smooth, tense, slightly see-through bump just behind the nail. If the cyst presses on the cells that grow your fingernail, it can leave a long groove or ridge running down the nail.

They're most common in women over 50, and they most often appear on the middle or index finger.

• Firm bump at the fingertip joint — a smooth, dome-shaped lump on top of the last (DIP) joint, usually just behind the nail
• Nail groove or ridge — a longitudinal line running down the fingernail where the cyst presses on the nail-growing cells
• Translucent appearance — the cyst often looks slightly see-through or bluish because it is filled with clear, jelly-like fluid
• Joint stiffness — mild aching or reduced motion at the DIP joint, reflecting the underlying arthritis

The diagnosis is straightforward by exam — the location is unmistakable, the cyst lights up when shined through with a small flashlight (because the fluid inside transmits light), the underlying joint is usually arthritic, and there's often a nail groove. An X-ray of the finger confirms the arthritis and the small bone spurs at the joint that are the underlying cause.

Surgical removal is recommended for cysts that are painful, that keep coming back, or that are leaking fluid. A draining cyst is particularly important to address — it provides a direct path for bacteria to get into the joint and cause infection.

External patient-education references and related OSI pages for additional background:

• Britannica — Hand (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language