A modular knee joint prosthesis using variably sized components for joining a variably resected tibia bone and a femur bone, having a tibial platform defining peripheral edges on its bottom surface for engaging beveled edges of the plates or spacers. The platform has an upraised boss on a top surface for engaging a tibial insert which can be locked in place with a lock key. The tibial insert serves as an articulating surface for a femoral component. An implantable stem extends below the platform and may be complementarily joined to a second stem; the stems lock together using a Morse taper and a locking bolt. Full-sized or half-sized extension plates have beveled edges on a top surface to engage complementing edges on the bottom surface of the tibial base plate. The plates act as a spacer or cap to the resected tibia and act to distribute load or make up for bone deficits.