The head of the human femur (thigh bone), the long bone between the hip & the knee. The head of the femur (top, well defined knob) articulates with the acetabulum of the hip bone, forming a strong ball & socket joint which allows a wide range of movement. The other two protruberances are the greater & lesser trochanters (upper & lower respectively in image). An iliofemoral ligament binds the greater trochanter to the ilium (the largest of the pelvic bones), reinforcing the strength of the joint. The femur is the largest & heaviest bone in the body, although it may weaken in the neck region. Fractures of the neck of the femur occur most often in the elderly.