A small bag bulging at the joint of the little finger, the possible causes are osteoarthritis, tendon sheath cysts, rheumatoid arthritis, bone tumors and so on. 1. Osteoarthritis: it may be due to heredity, age, inflammation, trauma and other factors, resulting in damage to the articular cartilage of the little finger, causing degeneration and fibrosis of the articular cartilage, resulting in joint pain, swelling and other symptoms. 2. Tendon sheath cyst: Tendon sheath cyst is a benign swelling near the joints, ligaments or tendons of the human body, which is mainly caused by degeneration of the joint capsule and ligaments, and can occur in the area of the little finger, resulting in the little finger joints bulging with a small bag. 3. Rheumatoid arthritis: it may be related to genetic, immune and other factors, resulting in rheumatoid arthritis, and its common symptom is the thumb joint bulge. 4. Bone tumor: it may also be a tumor occurring in the bone or collateral tissues, such as the growth of local bone tumors in the metaphysis of the phalanges, easy to form a hard mass in the local area, appearing in the small thumb joint bulging a small bag, usually benign bone tumors. There may be other reasons for the bulging of a small bag at the joint of the little thumb, it is recommended to go to the regular hospital in time, complete the relevant examinations, clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.