The left knee joint cavity and suprapatellar bursa effusion is considered to be related to synovitis, gouty arthritis, rheumatoid arthritis and so on. 1. Synovitis: traumatic stimulation, viral infection, etc. can cause synovitis, which will lead to more synovial exudation and joint effusion. 2. Gouty arthritis: Due to the deposition of urate crystals, the soft tissues of the joints are irritated, leading to acute inflammation. If it is more serious, there will be effusion in the left knee joint cavity and suprapatellar bursa. 3. Rheumatoid arthritis: a relatively common autoimmune system joint inflammation, may be related to genetics, microbial infections, etc., the left knee has inflammation, synovial fluid will increase, which can be manifested as a small amount of left knee joint cavity and suprapatellar bursa effusion. There may be other causes of left knee joint cavity and suprapatellar bursa effusion, such as meniscus injury, etc. It is recommended to go to the hospital in time to find out the cause.