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.