Artificial knee. Coloured X-rays of an artificial (prosthetic) knee joint. At left is a frontal X- ray of a straight knee and at right is a side view of a bent knee. The prostheses are implanted into the thigh bone (femur) and shinbone (tibia) with pegs and surgical cement. The artificial joint hinges in an identical way to a normal knee joint. Knee joints need replacing when the protective cartilage that surrounds them is lost. Cartilage can be lost due to injury or osteoarthritis.