An assembly 10 for use in knee replacement surgery comprising a tibial base plate 14 which, in use, engages a tibia 200, and a tensioning element 33, 34 operable to engage between the tibial base plate 14 and a femur 300 to set the gap between the tibial base plate 14 and the femur 300 when the femur 300 and the tibia 200 are in extension. A ratchet mechanism is operable to change the gap. The tensioning element 33, 34 has a central tensioning element. A measuring arrangement 40, 42 contacts the lateral and medial femoral condyles when spring loaded elements 48 are released, which may be a seesaw element. The springs may be located in a main body portion, connected to the elements 48 by rods, and may be slideable in barrel elements. The femoral block 12 may be height adjustable and may rotate in the varus and valgus direction with respect to a support portion which attaches to a femur. The femoral block 12 may also be connectable to the tibial base plate 13 by a connecting element 18.
