The Femoral Head would be the other bone.
Under most circumstances, hip replacement is actually more of a surgery involving the Neck and Head of the femur. The Neck and Head of the femur is relatively fragile in contract to the overall femur, and so is the part that will normally break in falls. Most of the time this is the parts that are replaced. Normally the pelvic bone, where it is connected to the femur is merely smoothed, and debrided.