A bulging bag on the elbow can be caused by trauma, or epidermal cysts, bursitis and other diseases. 1. Trauma: If the patient’s arm has recently been hit, resulting in soft tissue damage, subcutaneous capillary rupture, it is easy to appear elbow bulging a bag. 2. Epidermal cysts: the occurrence of this disease is related to the destruction of the hair follicle structure, surgical operation or trauma resulting in epithelial cells implanted in the subcutaneous growth and other factors. Epidermoid cysts mostly occur in the face and upper trunk, the cysts are normal skin color, hard texture, so some patients will have a bulging elbow. 3. Bursitis: The causes of this disease include long-term excessive friction, bacterial or tuberculosis infection, usually also secondary to rheumatoid arthritis, osteoarthritis and other diseases. The main symptom is that there will be swelling and stiffness around the joints, often accompanied by painful cystic masses, which can be manifested as a bulging bag on the elbow. A bulging elbow can also be caused by lipoma, boils, sebaceous cysts and other diseases, and it is recommended that the patient go to the hospital to complete the relevant examinations to determine the specific cause.