Fluid buildup in the knee is generally referred to as knee joint effusion, which can be determined by whether there is swelling and pain in the knee joint, and whether there is a positive floating patella test. 1. Swelling and pain: when there is a small amount of fluid in the knee joint, there may be no discomfort, but when there is a significant accumulation of fluid in the knee joint, the knee joint will be swollen and painful because of the symptoms of compression of the joint cavity. 2. Positive floating patella test: the patient lies on the bed with his legs flat, one hand presses the anterior side of the knee above the patella, and the other hand presses the patella, when there is effusion in the joint, the patella will float up and down, and then it is a positive floating patella test, which indicates that there may be an effusion in the knee joint. Under normal circumstances, the synovial membrane of the knee joint will secrete a small amount of fluid to protect the articular cartilage from damage. If the synovial membrane is inflamed or there is a problem with the joint, the knee joint will secrete too much fluid, and there are many causes, such as trauma, infection, inflammation, chronic strain and so on, all of which may lead to knee joint effusion. It is recommended that patients with related manifestations should consult a doctor in time for a professional examination to clarify the cause of the disease and timely treatment, so as to avoid delaying the time of treatment.