Normal joints have joint fluid in the middle, which acts as a lubricant when the joint moves. Joint fluid is produced by the secretion of the synovial membrane that covers the surface of the joint capsule. When the synovial membrane is hyperplastic for some reasons, or the secretion of the synovial membrane is increased, it leads to joint effusion. The most common causes are as follows: First, osteoarthritis in the elderly, due to bone proliferation, cartilage wear and tear, bone redundancy, and synovial membrane hyperplasia, when stimulated by over-exertion or catching a cold, the synovial membrane secretion increases, resulting in joint effusion, which tends to be more acute in onset. The second type, rheumatoid arthritis in the middle-aged and elderly, is often symmetrical and bilateral, often accumulating in the wrist and finger joints. Laboratory tests are positive for rheumatoid-related factors. The disease usually starts slowly and lasts for a long time. In the third type, septic infection, due to bacterial infection, there may be localized redness, high skin temperature, pain is very obvious, and the inflammatory factor is elevated when blood tests are drawn. A puncture of the joint cavity may produce pus. The fourth type, tuberculosis of joints, usually redness, swelling, heat and pain are not particularly obvious, but the patient may have symptoms such as emaciation, anorexia, fever in the afternoon, or sweating at night, and so on, and the test for tuberculosis may be positive. There may also be some other causes of joint effusion, such as gout, and there are also some unknown causes of joint effusion.