A knee prosthesis comprises a tibial tray (712) having a platform (720) with curved first (732,792) and second (732,792) slots formed in its upper surface (734). A tibial insert (714) can be positioned on the platform and has first (750,790) and second (750,790) curved tabs extending downwardly from its bottom surface to be received in the first and second curved slots, so that the insert is able to rotate with respect to the tibial tray.
