In one embodiment of the present invention a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a concave superior surface on a tray and a convex inferior surface on a tibial insert. In another embodiment of the present invention a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a convex superior surface on a tibial tray and a concave inferior surface on a insert. In another embodiment of the present invention a mobile bearing knee prosthesis may include a bi-concave interface (e.g., having a “wave” like surface geometry). This “wave” like surface geometry may be at the second bearing (i.e., at the interface between a tibial insert and a tray in the mobile bearing knee as opposed to the interface between the tibial insert and a femoral component).