Water in the knee joint is really swelling inside the knee joint, suggesting that the joint is inflamed. It could be from arthritis, it could be from a torn meniscus, it could be from a torn ACL. What I tell my patients is, “this means that the joint is crying”. Simply aspiration of the water in the knee will not resolve the knee pain. If the pain is from the arthritis, injection in the joint will usually decrease the inflammation and decrease the swelling, as well as the pain.