A knee prosthesis comprises a tibial tray (1012) having a platform and a tibial insert (1014) which can be fitted to the tibial platform and has a stem (1042) extending downwardly into a cavity (1030) in the tibial tray. The stem has a threaded internal passageway (1048) at its distal end. A screw (1016) can be screwed into the threaded internal passageway of the stem to cause the width of the stem at the distal end to increase.
