A mobile tibial assembly includes a tibial tray and a unicompartmental tibial insert configured to move relative to the tibial tray. The tibial insert includes a platform having an upper bearing surface and a stem extending downwardly from a bottom surface of the unicompartmental insert. The tibial tray includes a corresponding recess, such as a cavity and/or a channel, configured to receive the stem therein such that the stem and the recess cooperate to constrain the movement of the unicompartmental tibial insert relative to the tibial tray. The stem of the unicompartmental tibial insert is configured to extend below the bottom surface of the tibial tray when received in the cavity.
