fluid in knee joint

The knee is designed for its own protection. It is completely surrounded by a joint capsule that is flexible enough to allow movement but strong enough to hold the joint together.

The capsule is lined with synovial tissue, which secretes synovial fluid to lubricate the joint. Wear-resistant cartilage covering the ends of the thighbone (femur) and shinbone (tibia) helps reduce friction during movement.

Pads of cartilage (menisci) act as cushions between the two bones and help distribute body weight in the joint. Fluid-filled sacs (bursas) provide cushioning as skin or tendons move across bone. Ligaments along the sides and the back of the knee reinforce the joint capsule, adding stability. The kneecap (patella) protects the front of the joint.

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. When a joint develops osteoarthritis, the cartilage gradually roughens and becomes thin. This happens over the main surface of the knee joint or at the cartilage underneath the patella.

The surrounding bone reacts by growing thicker. The bone at the edge of the joint grows outwards (this forms osteophytes or bony spurs). This bone growth can affect both the femur and the tibia, as well as the patella.

The synovium swells slightly and may produce extra fluid, which then makes the joint swell.

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.

A popliteal cyst is a small, bag-like structure that forms when the joint lining produces too much fluid in the knee. The extra fluid builds up and pushes through the back part of the joint capsule, forming a cyst.

The cyst squeezes out toward the back part of the knee in the area called the popliteal fossa, the indentation felt in the back part of the knee between the two hamstring tendons and the top part of the calf muscle. Most people will be able to feel the cyst in the hollow area right behind the knee joint

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Fluid in knee joint