A modular keel or stem for use as a tibial implant has a central stem portion including a tongue-like coupling portion for insertion into a grooved portion in a bone contacting portion of 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 and posteriorly on the lateral side. Proximal portions of the ribs or fins may engage 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 into the groove to prevent the disassembly the keel from the baseplate. In one embodiment, the baseplate is capable of being inserted in a direction offset from the anterior-posterior direction, such as a medial-lateral direction, after the keel has been implanted. A polymeric bearing insert is provided for placement onto the baseplate in a standard manner.