Pinned kneecap fracture. Coloured X-ray of a pin (grey, centre right) in the fractured kneecap (patella) of an adult man. The metal pin is used to hold the broken bone in the correct position, allowing it to heal properly. The kneecap is a disc of bone that covers the front of the knee joint. The knee, the largest joint in the body, is formed where the femur (thigh bone, upper centre) articulates with the tibia (shin bone, lower centre). The smaller fibula (left of tibia) is also seen.