The present invention relates to a system and method for adjusting a thickness of a prosthesis (e.g., a knee implant). In one embodiment, a knee prosthesis for implantation in a patient is 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, wherein the tibial insert comprises an upper surface and a lower surface; a tibial spacer, wherein the tibial spacer comprises an upper surface and a lower surface, and wherein the tibial 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 tibial tray to form a locked spacer/tray assembly such that the locking mechanism substantially prohibits at least relative up-down movement between the tibial spacer and the tibial tray when the locked spacer/tray assembly is implanted in the patient. In one example, the locking mechanism may further lock the tibial spacer to the tibial insert to form a locked insert/spacer/tray assembly such that the locking mechanism substantially prohibits at least relative up-down movement between the tibial spacer and the tibial insert when the locked insert/spacer/tray assembly is implanted in the patient. In another embodiment, a method for implanting a knee prosthesis in a patient is provided, comprising: providing 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; providing a tibial insert which has a shelf life and which is not readily re-sterilizable after the shelf life has expired, wherein the tibial insert comprises an upper surface and a lower surface; and providing a tibial spacer which has a shelf life and which is re-sterilizable after the shelf life has expired, wherein the tibial spacer comprises an upper surface and a lower surface, and wherein the tibial 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.