A knee prosthesis system for total knee replacement procedures comprising: a. a plurality of distinctly-sized femoral components (10, 20), b. a plurality of distinctly-sized fixed tibial components, each of which has a fixed platform and a fixed stem extending distally from the bottom of the fixed platform, and each of the fixed platforms having an anterior buttress and a posterior buttress, c. a plurality of distinctly-sized mobile tibial components, each of which has a mobile platform and a mobile hollow stem extending distally from the bottom of the mobile platform, d. a plurality of mobile inserts (40, 60), each of which is sized and shaped such that each may be optimally matched to one of the femoral components and may be used with any one of the mobile tibial components, and each having a post (48, 68) extending inferiorly from its inferior surface which can rotate within the hollow stem of the mobile tibial component when the inferior surface of the mobile insert articulates against the platform of the mobile tibial component, e. a plurality of fixed inserts (30, 50), each of which is sized and shaped such that each may be optimally matched to one of the femoral components and may be used with any one of the fixed tibial components, in which the anterior buttress and the posterior buttress on a fixed tibial component can fixedly retain a fixed insert in place with its inferior surface on the platform, and in which the fixed and mobile stems for a particular anatomical size have approximately the same external size and configuration, such that approximately the same surgical preparation of the proximal tibia is required for each of the fixed and the mobile tibial components for the said particular anatomical size. a. a plurality of distinctly-sized femoral components (10, 20), b. a plurality of distinctly-sized fixed tibial components, each of which has a fixed platform and a fixed stem extending distally from the bottom of the fixed platform, and each of the fixed platforms having an anterior buttress and a posterior buttress, c. a plurality of distinctly-sized mobile tibial components, each of which has a mobile platform and a mobile hollow stem extending distally from the bottom of the mobile platform, d. a plurality of mobile inserts (40, 60), each of which is sized and shaped such that each may be optimally matched to one of the femoral components and may be used with any one of the mobile tibial components, and each having a post (48, 68) extending inferiorly from its inferior surface which can rotate within the hollow stem of the mobile tibial component when the inferior surface of the mobile insert articulates against the platform of the mobile tibial component, e. a plurality of fixed inserts (30, 50), each of which is sized and shaped such that each may be optimally matched to one of the femoral components and may be used with any one of the fixed tibial components, in which the anterior buttress and the posterior buttress on a fixed tibial component can fixedly retain a fixed insert in place with its inferior surface on the platform, and in which the fixed and mobile stems for a particular anatomical size have approximately the same external size and configuration, such that approximately the same surgical preparation of the proximal tibia is required for each of the fixed and the mobile tibial components for the said particular anatomical size.