Knee prosthesis tibial componentry has a bearing insert defining a superior condyle articular surface and a planar inferior bearing surface. A tray having a platform defining a superior bearing surface and an inferior stem engages the insert. The tray has a quadrant of retention buttresses having anterior and posterior buttresses, each extending superiorly with respect to the superior bearing surface and extending in from a peripheral edge of the superior bearing surface. The insert has a quadrant of respective spaced apart retention recesses having anterior and posterior recesses, each recessed superiorly with respect to the inferior bearing surface and extending in from a peripheral edge of the inferior bearing surface.