Arthritis expert Carol Eustice from About.com explains that moderate joint effusion, or swollen joints, occurs when an abnormal accumulation of fluid in or around a joint causes the joint to swell. This affliction typically affects the knees and has other common names like "water on the knee" or "fluid on the knee."
Eustice explains that the joints in the knees naturally contain a small amount of fluid, but this amount sometimes increases due to several conditions, including a variety of arthritis afflictions. When the fluid builds up in the tissues that line the joints, the joints appear swollen. Eustice says that a doctor usually drains this fluid from the joints to relieve pressure while reducing pain and other symptoms.
Joint effusion does not only occur in the knees. WebMD also describes many other kinds of joint effusion, particularly various forms of arthritis, including ankylosing spondylitis, gout, infectious arthritis, osteoarthritis, psoriatic arthritis and rheumatoid arthritis. These conditions affect various joints in the body, such as the ankles, hands, hips, feet, spine and wrists. WebMD explains that a condition such as osteoarthritis does not cause illness beyond the pain associated with osteoarthritis, while an affliction such as a joint injury often results in stiffness as well as pain.