An instrument assembly for use in knee joint replacement surgery is provided including a base instrument having a tibial plate, a femoral plate, a tibial arm having a plate end and a control end, 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. The assembly also includes a distractor mechanism configured to act between the tibial arm and the femoral arm at a point between their respective plate ends and control ends by which the distance between the plate ends of the arms can be adjusted and an ancillary instrument mountable on the base instrument and configured such that the position of the ancillary instrument relative to the tibial plate is fixed during subsequent steps in the knee replacement procedure, while the knee joint is distracted as a result of forces applied across the joint between the tibial plate and the femoral plate.