A surgical instrument and method determine the orientation of the mechanical axis of a femur. The instrument may include a rod with a first end of the rod rotatably secured at a first point on the surface of distal femur that corresponds to center of the knee joint so that the first end is coincident with the mechanical axis. The connection between the first end of the rod and the distal end of the femur defines a first point of rotation. The second end of the rod is slideably received within a sleeve that is rotatably connected to a support positioned distally away from the distal end of the femur. The connection between the sleeve and the support defines a second point of rotation. When the distance between the first and second points of rotation is minimized, a line drawn therebetween is coincident with the mechanical axis of the femur.