Quote from the link: "The load axis of the limb is the line passing from the hip carrier to the ankle carrier. The mechanical axis of the femur is the line from the hip center to the knee center, and for the tibia it is from the knee carrier to the ankle carrier. In an ideally aligned limb, the knee center fails in line with the load axis; i.e., the hip-knee-ankle angle (HKA) is a straight line. (in the example shown the limb has a marked varus alignment.)"