Fluid in knee joint

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

Click here: Second Opinion Arthritis Treatment Kit

The knee is completely enclosed in a joint capsule.

The capsule is lined with synovial tissue, which secretes synovial fluid to lubricate the joint. Cartilage covers the ends of the thighbone (femur) and shinbone (tibia)and provides impact protection.

Additional pads of cartilage (menisci) act as cushions between the two bones and help distribute body weight in the joint. Fluid-filled sacs (bursae) 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.

Synovial fluid is present in all mobile joints (hips, knees, shoulders, etc). The fluid serves two main functions.

One function of the fluid is to help lubricate the cartilage surfaces, so they move smoothly.

The other function of the fluid is to help nourish and protect the cartilage surfaces of the joint.

With osteoarthritis, the cartilage begins to wear away and becomes thinner. The underlying 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.

When the arthritis is severe enough, it causes the synovial lining to produce more fluid than necessary. When fluid builds up to the point where the knee feels tight, the technical term used is a "joint effusion." A knee effusion may be caused by different conditions:

1. Arthritis: Either osteoarthritis -- called degenerative joint disease -- or inflammatory arthritis, such as rheumatoid arthritis

2. Trauma: The fluid in certain kinds of trauma may be blood, rather than synovial fluid

3.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 in the back of the knee 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. Most people will be able to feel the cyst behind the knee joint

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