What Is Causing Your Knee Pain? (Illustrated)

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Step 1 of 5 - Normal Joint (Knee)

Normal knee joint
The structure of a normal knee joint. Stocktrek Images/Getty Images

The knee is a hinge joint formed by two bones, the femur and tibia, which are held together by four ligaments— the medial collateral ligament, lateral collateral ligament, anterior cruciate ligament, and posterior cruciate ligament.

The patella (knee cap) is also part of normal knee anatomy. The ends of the femur and tibia, as well as the underside of the patella, are covered by articular cartilage, a slippery surface that allows friction-free, pain-free movement.

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Step 2 of 5 - Synovial Fluid (Normal Knee Joint)

Cut-section view of normal knee joint
Cut-section view of normal knee joint. Photo © A.D.A.M.

A normal knee joint is surrounded by a membrane, the synovium, which produces a small amount of thick fluid, known as synovial fluid. Synovial fluid helps to nourish the cartilage and keep it slippery. The synovium also has a tough outer layer (the joint capsule) which protects and supports the joint.

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Step 3 of 5 - Inflamed Synovium (Rheumatoid Arthritis)

Cut section view of knee joint - rheumatoid arthritis
Cut section view of knee joint - rheumatoid arthritis. Photo © ADAM

In rheumatoid arthritis, an autoimmune disease, the body attacks its own joints. White blood cells, which are agents of the immune system, travel to the synovium and cause an inflammatory process to occur, referred to as active synovitis. The inflamed synovium causes warmth, redness, swelling, and pain in and around the affected joint.

Specifically, during the inflammatory process, the synovium thickens and causes the joint to swell. As rheumatoid arthritis progresses, abnormal synovial cells invade and erode cartilage and bone within the joint. Surrounding muscles, ligaments, and tendons weaken.

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Step 4 of 5 - Joint Damage (Osteoarthritis of Knee)

Erosion of cartilage in osteoarthritis
Erosion of cartilage - Osteoarthritis. Photo © A.D.A.M.

In osteoarthritis, commonly called wear-and-tear arthritis, the surface layer of cartilage breaks down and wears away . With advanced disease, the breakdown of cartilage can become so severe that the bones of the joint rub together (referred to as bone-on-bone).

Pain, swelling, and limited range of motion result. The joint may lose its normal shape over time and become deformed, too. Bone spurs (osteophytes) can develop along the edges of the joint. Bits of bone or cartilage can break off and float inside the joint space, causing even more pain and damage (i.e., loose bodies).

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Step 5 of 5 - Specific Location of Knee Pain

Location of knee pain
Specific location of knee pain. Photo © A.D.A.M.

The location of the knee pain can be useful information when trying to obtain an accurate diagnosis. Pain at the front of the knee can be caused by bursitis, arthritis, or softening of the patella cartilage, as in chrondromalacia patella.

Pain on the side of the knee is usually associated with injury to the collateral ligaments, arthritis, or tears to the menisci. Pain in the back of the knee can be caused by arthritis or a Baker's cyst (an accumulation of synovial fluid behind the knee). Infection may be another possible cause of knee pain.

While location of knee pain offers significant clues as to the cause, imaging studies (x-ray, CT scan, or MRI) provide visible evidence of damage and abnormalities. Appropriate treatment clearly depends on an accurate diagnosis of what's causing your knee pain. 

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