Kneecaps. Coloured X-ray of the kneecaps of a healthy adult man, as viewed from above when the knees are bent. The kneecap (or patella) is a disc of bone that covers and protects the front of the knee joint. The knee joint, the largest joint in the body, is formed where the thigh bone (femur, seen here below the kneecaps) articulates with the tibia bone (not seen).