The present invention relates to a system and method for adjusting a thickness of a prosthesis. A knee prosthesis for implantation may be provided, comprising: a tibial tray, wherein the tibial tray comprises an upper surface and a lower surface, and the lower surface of the tibial tray is disposed adjacent a tibia of the patient； a tibial insert, with an upper surface and a lower surface； a tibial spacer, with an upper surface and a lower surface. The spacer is disposed between the tibial tray and the tibial insert such that the lower surface of the tibial spacer is adjacent the upper surface of the tibial tray and the upper surface of the tibial spacer is adjacent the lower surface of the tibial insert； and at least one locking mechanism, wherein the locking mechanism locks the tibial spacer to the tibiat tray to form a locked spacer/tray assembly.
