Mobile/fixed prosthetic knee systems

An orthopaedic prosthesis comprising:

• a tibial tray configured to be coupled to a surgically-prepared surface of the proximal end of a tibia, the tibial tray including (i) a platform having an upper surface and a bottom surface and (ii) a cavity defined by an inner sidewall and having an opening defined in the upper surface of the platform;
• a tibial insert coupled to the tibial tray, the tibial insert having (i) an upper bearing surface configured to contact a pair of femoral condyles, (ii) a bottom surface in contact with the upper surface of the tibial tray, and (iii) a stem extending downwardly from the bottom surface and received in the cavity of the tibial tray, the stem including a distal end having a threaded internal passageway;
• a screw positionable in the threaded internal passageway of the stem, wherein:
• (i) the distal end of the stem has a first width when the screw is positioned in the threaded internal passageway and (ii) the distal end of the stem has a second width when the screw is removed from the threaded internal passageway, the first width being greater than the second width;
• and
• wherein the tibial insert is rotatable with respect to the tibial tray when the screw is removed from the threaded internal passageway and prevented from rotating with respect to the tibial tray when the screw is positioned in the threaded internal passageway.