Causes of knee joint effusion include knee synovitis, knee injuries (e.g. ligament and meniscus tears), arthritis, etc., which can be determined by physical signs and symptoms, and imaging tests. 1. Signs and symptoms: The amount of fluid in the knee joint can be determined by signs and symptoms, such as squeezing the knee joint. When the knee joint effusion is small, squeezing the medial patellar space can make the fluid fill up the lateral side of the knee joint, and a positive beat sign can be observed on the lateral side of the knee joint; when the knee joint effusion is medium, a positive floating patella test can be seen; and when the knee joint effusion is large, the disappearance of the patellar depression can be observed. 2. Symptoms: can be judged by symptoms, if the amount of knee joint effusion is large, there will be swelling, pain, floating feeling. Laboratory tests for arthritis-induced effusion can also identify the type of infection. 3. Imaging: For ligament and meniscus tears, X-ray, magnetic resonance and other imaging tests can not only detect the extent of the effusion, but also determine the damage to the surrounding tissues caused by trauma. The above methods usually need to be used in combination to make an accurate judgment. If symptoms such as joint pain and swelling occur, it is recommended to go to the hospital in time to identify the cause of the disease and then undergo regular treatment.