as illustrated in fig6 the distal surface of block 14 is substantially flat a protrusion 44 extends transversely from proximal surface 36 and includes a diverging outer wall 46 and a flat wall 46 a cross section of protrusion 44 is illustrated in fig8 from where it can be seen that the protrusion is generally dovetail in shape to substantially match the dovetail shaped slot 26 in the tibial tray 16 from the elevational view of fig7 it is evident that the protrusion of 44 is generally round a second protrusion 50 extends transversely from proximal surface 36 adjacent the anterior edge of the augmentation block 14 protrusion 50 includes a pair of side walls 52 which diverge with distance from the proximal surface 36 of the augmentation block as illustrated best in fig9 the cross section of protrusion 50 is generally dovetail in shape and is configured for sliding accommodation within a slot 26 the posterior wall 54 of protrusion 50 generally follows the contour of the anterior edge of the block 14 finally augmentation block 14 includes an anterior extension 56 and a posterior extension 58 which extend laterally from block 14 as illustrated and define a gap therebetween each extension 56 and 58 terminates in a generally flat end wall at a midpoint of the tibial tray 16 fig1 through 6 illustrate in progression the.