Knee joint effusion can lead to pain for two main reasons: 1. When knee joint effusion is present, the secretion of synovial fluid within the joint increases significantly, causing the pressure in the knee joint to increase significantly, and the soft tissues around the knee joint to swell significantly, which, due to the increased pressure in the knee joint, will promote the expression of pressure sensory cells and nociceptive sensory cells, thus 2. Knee pain is also caused by the primary disease of knee joint effusion. The combination of these two causes can lead to significant pain after a knee effusion.