The common cause of knee swelling is knee effusion, and there are many reasons for knee effusion, including knee trauma and chronic osteoarthritis of the knee, as follows: 1. Knee trauma: sprains or joint fractures or dislocations of the knee can cause traumatic synovitis of the knee, and the synovial vascular permeability increases to produce a large amount of knee effusion, which will be gradually absorbed after cold compresses, braking, and rest. 2. Chronic osteoarthritis of the knee joint: Synovial inflammation caused by chronic osteoarthritis of the knee joint can also cause joint effusion, resulting in joint swelling and pain. Withdrawing the effusion through arthrocentesis and injecting drugs such as prednisolone can promote the inflammation to subside, thus relieving the symptoms. In severe cases, arthroscopic minimally invasive surgical solutions need to be considered. The symptoms of increased joint effusion can be improved by surgically removing the inflammatory hyperplastic synovium, and intra-articular lesions, such as articular cartilage damage and meniscal damage, can also be addressed. After aggressive treatment, the symptoms of knee swelling and pain will be significantly relieved.