of tray 13 a in fig2 post 59 has a hexagonal shape providing a hexagonally shaped flat top 60 hexagonal post 59 also has a plurality of vertical side walls 61 and a rectangular base 62 the base 62 has inclined surfaces 63 that form a taper lock connection with inclined surfaces 48 of tray socket 47 of tray 13 a in fig2 insert 28 a provides a square opening 64 that exactly fits peg 49 in fig2 there is no relative motion between insert 28 a and tray 13 a in fig2 rotational motion only is indicated by arrow 65 between insert 28 a and tray 13 a when peg 42 is used in fig2 the rectangular peg 53 enables only translational movement between the insert 28 a and tray 13 a as indicated by arrow 66 in fig2 the hexagonal peg 59 enables both rotational motion as indicated by arrow 65 and translational motion as indicated by arrow 66 between insert 28 a and tray 13 a an alternate embodiment of mobile bearing knee apparatus 110 is shown generally in fig3 in fig3 the prosthesis 110 is shown positioned upon a patient 39 s proximal tibia 111 specifically upon a flat surgically cut proximal surface 112 as shown in fig2 28 tibial tray 113 is shown which can be of metallic construction such as titanium alloy for example tray 113 has a flat proximal surface 114.