A tool for digitizing a mechanical axis of a tibia using a computer-assisted surgery system is described. The tool includes upper and lower mounting ends interconnected by an alignment rod extending therebetween. The upper mounting end is releasably fastenable to an upper reference point on a tibial plateau and the lower mounting end includes a self-centering malleoli engaging mechanism having opposed caliper arms displaceable in a common plane relative to each other for clamping engagement with the medial and lateral malleoli of the ankle. At least one trackable member is mounted to the alignment rod of the tool and is in communication with the computer assisted surgery system for providing at least orientation information of the alignment rod. The mechanical axis of the tibia is parallel to the alignment rod and extends between the upper and lower reference points when the tool is mounted on the tibia.
