The knee joint is a very cleverly composed large joint with a large skeletal surface and a slightly weaker soft tissue cover, and a large synovial cavity. The knee joint requires both stability and flexibility and plays a role in all joints of the lower extremity. The synovial cavity is lined with synovial membrane, and synovial cells secrete synovial fluid, which lubricates and nourishes the articular cartilage. The normal joint cavity has a small amount of synovial fluid, which is a physiological state. When pathogenic factors, such as trauma, infection, degeneration and strain, change the physiological environment or functional state of the knee joint, the secretion and absorption of the synovial membrane becomes disturbed, which not only fails to play a normal role in lubrication and nutrition, but also causes the synovial membrane to proliferate and thicken, destroying the articular cartilage and subchondral bone, which can cause serious damage to the joint, such as rheumatoid arthritis. The question is how to determine the cause of joint effusion? Given that there are many factors that can cause joint effusion, it is important to determine the cause and adopt a reasonable strategy to eliminate the effusion and cure the joint disease. The following may be helpful: 1. If you notice significant knee swelling, with or without pain and fever, you should see your doctor promptly, especially if you have multi-joint edema, or if you have recurrent swelling in one joint. 2, you can compare the swollen knee joint with another joint, the swelling is often very obvious. 3. Occasional swelling of the surface of the knee joint in adults after exertion, if it is still noticeable after a night’s rest, it is necessary to see a doctor. 4. Do not hide the fact that your doctor will often ask about the circumstances in which the swelling was found, the accompanying symptoms, and whether there is any suspicious reason for the swelling, such as trauma. 5.Non-invasive examination, including general physical and mechanical examination, can initially determine the possible pathological state. 6.Plain X-ray examination is necessary. If necessary, MRI examination should be taken to understand the joint structure in more detail, especially the situation of intra-articular structures, which can determine the cause in many cases. 7.Arthrocentesis. Physical and chemical examination can be performed on the extracted joint fluid, which is helpful in identifying the cause. 8.Peripheral blood test, mainly rheumatology series. Peripheral blood test is necessary when trauma, strain, aging and other factors are excluded, especially when systemic immune diseases are suspected. When the clinical manifestation of multi-joint swelling, and accompanied by fatigue skin abnormal fever and other symptoms, the cause of fluid accumulation is more complex, and stubborn, often not simply orthopedic disease. 9.Short-term symptomatic treatment is based on rest, hot and cold compresses, local physiotherapy, etc. It is not recommended to play closed (use of hormones) at will. 10, three weeks of symptomatic treatment, if there is no improvement or aggravation, it is recommended to see a specialist.