A mucous cyst (mucocele) is a benign, fluid-filled sac that forms under the skin. These soft, dome-shaped lesions can be clear, bluish, or pinkish. They commonly develop on parts of the body with mucous membranes, such as the fingers and inside the mouth.
How Mucous Cysts Form
Mucous cysts form through two main mechanisms: fluid leakage from joints or the obstruction and rupture of ducts within glands. The specific mechanism depends on the cyst’s location.
One common way mucous cysts develop is through the leakage of synovial fluid, a lubricating substance found in joints. This often occurs in digital mucous cysts on fingers or toes. When the joint capsule experiences degeneration or damage, synovial fluid can escape. This fluid then accumulates in the surrounding soft tissues, forming a lump under the skin.
Another mechanism involves the obstruction or rupture of small ducts that transport mucus or saliva. This is relevant for oral mucoceles inside the mouth. If a minor salivary gland duct becomes blocked or damaged, the mucus or saliva it produces cannot drain properly. Instead, the fluid builds up, creating a swollen sac.
Specific Triggers and Common Locations
The triggers for mucous cyst formation vary by location. Distinct factors contribute to digital mucous cysts on fingers and oral mucoceles in the mouth.
Digital mucous cysts, found near the distal interphalangeal (DIP) joints of fingers, are associated with osteoarthritis. This degenerative joint disease causes cartilage to break down, leading to inflammation and joint damage. As the joint deteriorates, bone spurs can develop, and small tears may form in the joint capsule. These changes allow synovial fluid to leak and collect under the skin, forming the cyst. While osteoarthritis is the primary trigger, chronic minor trauma or repetitive stress to the finger joint can also contribute.
Oral mucoceles commonly arise from minor trauma to mouth tissues. Activities such as accidentally biting the lip or cheek, lip-sucking habits, or dental procedures can damage the delicate ducts of minor salivary glands. When a salivary gland duct is ruptured or obstructed, saliva and mucus leak into the surrounding connective tissue, accumulating to form the cyst. These cysts frequently appear on the inner surface of the lower lip, the buccal mucosa, and less commonly, the floor of the mouth, where they are known as ranulas.