An improved apparatus provided to position the femur and tibia of a leg relative to each other during surgery includes a femoral section which is fixedly mounted on the femur and a tibial section which is fixedly mounted on the tibia. The femoral and tibial sections are movable relative to each other during bending of the leg. An index assembly is provided to interconnect the femoral and tibial sections when the femur and tibia are in any one of a plurality of predetermined orientations relative to each other. A pair of cams and cam followers are provided on opposite sides of the leg to support and guide movement of the femur and tibia relative to each other during bending of the leg. The cams have surfaces with configurations which are a function of the relative movement between the femur and tibia during bending of the leg. The cams and cam followers can be used to effect distraction of the knee joint and to maintain the extent of distraction of the knee joint constant during bending of the leg. The medial and lateral portions of the knee joint may have different extents of distraction.