Hip and Knee Questions and Answers.

Edited By: Seth S. Leopold, M.D.
Last updated Wednesday, February 09, 2005

Fluid on the knee

What is fluid on the knee? What is joint effusion?

Everyone has fluid in all mobile joints (hips, knees, shoulders, etc), but usually the amount of fluid is very small (really just enough to coat the surfaces of the joints themselves) and under normal conditions you don't know it's there. The fluid serves two main functions. One function of the fluid is to help lubricate the cartilage surfaces, so they move smoothly. In fact, normal cartilage that is lubricated by normal joint fluid (called "synovial fluid") is many times more slippery than a hockey puck gliding across a smooth ice rink. The other function of the fluid is to help nourish and protect the cartilage surfaces of the joint.

But when you have enough fluid in a joint to where you notice it (either by being able to see it, or by being able to feel it as a tense swelling) that's what people call "water on the joint." The technical term is a "joint effusion." Such a visible or detectable swelling of a joint is never normal, and it may be caused by any of a number of conditions, including:

• Arthritis: Either osteoarthritis -- called degenerative joint disease -- or inflammatory arthritis, such as rheumatoid arthritis
• Trauma: The fluid in certain kinds of trauma may be blood, rather than synovial fluid
• Infection: The fluid in this case may be pus, rather than synovial fluid

Depending on the cause of the fluid accumulation, the treatments may be very different.

It is reasonable to speak to an orthopaedic surgeon should you detect water on a joint if that joint is also painful.

Surgery for Hip and Knee at the University of Washington

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