kneecap (patella), saucer-shaped bone at the front of the knee joint; it protects the ends of the femur, or thighbone, and the tibia, the large bone of the foreleg. The kneecap is embedded in the tendon tissue of the quadriceps femoris, a large thigh muscle. As the leg bends and straightens, the kneecap glides up and down in a groove of the femur. Dislocation of the kneecap is a common athletic injury, occurring when stress on the powerful extensor muscles that straighten the leg pulls the kneecap to one side. See skeleton.