An orthopaedic surgical instrument assembly (10) includes a handle (16), a keel punch (220) configured to be inserted into a proximal end of a surgically-prepared tibia of a patient, and a guide tower (14). The handle includes an elongated body (170) and a lever (308) pivotally coupled to the body at a joint (370). The lever includes a first flange (248) positioned below the joint, the first flange extending in a first direction, and a second flange (328) positioned above the joint such that the joint is positioned between the first flange and the second flange. The second flange extends in a second direction opposite the first direction. The keel punch includes a lip (242) configured to be engaged by the first flange of the lever. The guide tower includes a tower base (80) having an inner surface that defines a passageway (112) sized to receive the keel punch and the body of the handle. The guide tower also includes an inner wall (124) that defines a rectangular aperture (118) in the tower base. The inner wall is configured to be engaged by the second flange of the lever.
