There are several possible causes of knee swelling: First, osteoarthritis of the knee joint. Osteoarthritis of the knee joint can involve the synovial membrane and lead to secondary inflammation and effusion of the synovial membrane, which can lead to swelling of the knee joint, and X-ray and CT examinations of the knee joint should be performed for diagnosis. Second, synovitis, synovitis is often due to strain, cold, or gout, rheumatoid and secondary synovitis, can appear swelling of the knee joint, the knee joint cavity effusion, to conduct blood tests, and MRI of the knee joint to diagnose and treatment. Third, tumors, when there is a tumor in the synovial membrane of the knee or a tumor in the bone around the knee, swelling of the knee joint can also occur, and MRI of the knee joint should be performed to make a diagnosis. Fourth, infection, when there is bacterial infection or tuberculosis infection in the knee joint, swelling of the knee joint can also occur, and blood pathogenic tests, as well as X-rays and MRI examinations should be performed for early diagnosis and treatment.