Knee joint. Coloured X-ray of a healthy human knee joint. Two bones meet at the knee forming a joint that works like a hinge. At top is the large femur (thigh-bone), which articulates with the tibia (shin-bone) at bottom. Next to the tibia (at lower right) is the smaller fibula bone. The patella or kneecap (above centre, faint pink oval) is a protective bone at the front of the knee held in position by muscles and tendons. Two discs of protective cartilage cover the surfaces of the femur and tibia to reduce friction between these bones. This joint, the largest in the body, allows a backward-forward hinge movement with slight rotation.
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