A mobile tibial assembly includes a tibial tray and a tibial insert. The tibial tray includes a track that is defined in an upper surface, and the track has a bottom wall that is flat in the medial-lateral direction and convex in the anterior-posterior direction. The tibial insert includes a bottom bearing surface configured to contact the upper surface of the tibial tray and a stem including a circular neck connected to the bottom bearing surface. The bottom bearing surface has a surface area that is less than the cross-sectional area of the circular neck. The convex upper surface of the tibial tray contacts the concave bottom bearing surface of the tibial insert and the tibial insert is freely rotatable about a central axis defined by the stem as the tibial insert is moved along the track relative to the tibial tray in the generally anterior-posterior direction.
