Illustration of a healthy knee joint, seen from the back. The knee is the largest joint in the body. It is formed by the articulation of the femur (thigh bone, top) with the tibia (shin bone, bottom centre). The smaller fibula (right of tibia) is also seen. The kneecap (patella, centre left) lies over the lower part of the femur. Purple and red areas show the origins and insertions of the muscles that attach to the bones.