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, in use, 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 tibia 200 are in extension. A ratchet mechanism is operable to change the gap. The tensioning element 33, 34 contacts the intercondylar notch of the femur. A measuring arrangement 40, 42, contacts the lateral and medial femoral condyles when spring loaded elements 48 are released. 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 the femur. The femoral block 12 may also be connectable to the tibial base plate 14 by a connecting element 18.
