A modular keel or stem for use as a tibial implant has a central stem portion including a coupling portion for insertion into and through an opening in a tibial baseplate. The modular keel or stem may also include a pair of anti-rotation fins or ribs which extend medially and posteriorly on the medial side and laterally-posteriorly on the lateral side for engaging receptacles in the bone contacting of the tibial baseplate. A locking element is provided for engaging the keel or stem coupling portion and the baseplate after the coupling portion of the keel is inserted through the opening to prevent the disassembly the keel from the baseplate. The coupling element is capable of being inserted in a medial-lateral direction after the keel and baseplate have been implanted. A polymeric bearing insert is provided for placement on the baseplate which insert includes a recess or opening for receiving the coupling element.