An orthopaedic surgical instrument assembly (10) includes a base trial (12) adapted to be positioned on a proximal end of a patient's resected tibia, and a guide tower (14). The base trial includes a plate (30) having an opening (38) defined therein, and a pair of fixation pin holes (92) defined in an anterior aspect. The guide tower includes a tower base (80) adapted to be positioned on the base trial and a pair of fixation pins (82) extending downwardly from an anterior aspect of the tower base. The tower base has a passageway (112) defined therein that is configured to be substantially aligned with the opening of the plate when the tower base is positioned on the base trial. Each of the fixation pins is sized to be received in, and extend outwardly from, each of the fixation pin holes of the base trial when the tower base is positioned on the base trial.