The most common causes of a hard lump in the middle finger joint are tendon sheath cysts, tenosynovitis, and joint degeneration. 1. Tendon sheath cyst: due to trauma, wear and tear and other factors caused by the joint capsule, ligaments and other tissues undergo degenerative changes, manifested in the joint capsule thick mucus, and the joint capsule wall is a dense, hard and tough fibrous connective tissue, mucus is not easy to be absorbed, which is manifested in the middle finger joint has a hard lump. 2. Tendovaginitis: If the patient repeatedly carries out the activities of the middle finger joint for a long time, it is easy to cause muscle and tendon injuries and inflammation, with the progress of the disease, local adhesion may occur, which may cause thickening of the tendon sheaths, which is manifested as a hard lump. 3. Osteoarthritis: is a chronic joint disease, due to a variety of factors leading to cartilage cell damage, which in turn causes degenerative changes in the articular cartilage, as well as secondary osteophytes, is also prone to hard lumps in the middle finger joint. Caused by a hard lump in the middle finger joints of more reasons, may also be related to fractures, rheumatoid arthritis, gout and other reasons, if the symptoms continue to have no relief, it is recommended to promptly go to the hospital to find out the cause of the disease, by the doctor targeted treatment.