Methods are disclosed for designing a tibial implant to minimize cortical impingement of a keel or other fixation structure when the tibial implant is implanted in the tibia bone. The design of the keel or other fixation structure on the tibial baseplate can be based on determining a common area between defined cancellous regions of at least two tibia bones. Methods are disclosed for designing a femoral component having a stem extension such that the stem can be sufficiently placed in the diaphysis of the femur when the femoral component is implanted. The method includes determining a canal axis in a femur that creates adequate engagement between a reamer and the diaphysis of the femur.