Instrument assembly for use in knee joint replacement surgery

An instrument assembly for use in a knee joint replacement surgery involving a knee joint, wherein the knee joint includes a tibia and a femur having medial and lateral compartments located on a medial-lateral axis that is transverse to an anterior-posterior axis, comprising:

• a base instrument that comprises:
• a tibial plate having a generally planar surface and an anterior edge;
• a femoral plate;
• a tibial arm having a plate end and a control end, wherein the tibial plate is fastened to the plate end of the tibial arm at its anterior edge, and wherein, when the tibial plate contacts the tibia, the tibial arm extends from the tibial plate in a direction generally parallel to the medial-lateral axis;
• a femoral arm having a plate end and a control end, wherein the femoral plate is rotatably fastened to the plate end of the femoral arm, and wherein, when the femoral plate contacts the femur, the femoral arm extends from the femoral plate in a direction generally parallel to the tibial arm, and wherein the tibial arm and the femoral arm are pivotably connected to one another at a point between their respective plate and control ends so that pivotal movement of the femoral arm relative to the tibial arm changes a distance between their respective control ends;
• and
• an ancillary instrument, the ancillary instrument connected to the base instrument and positionable relative to the tibial plate in a direction parallel to the medial-lateral axis and in a direction parallel to the anterior-posterior axis when the tibial and femoral plates are in contact with the tibial and femur, respectively, and the tibial and femoral arms generally extend along an axis parallel to the medial-lateral axis.