An orthopaedic surgical instrument assembly is disclosed. The orthopaedic surgical assembly includes a guide tower, an offset tool attached to the guide tower, and a stem trial. A guide tower includes a base surface adapted to be positioned on a proximal end of a patient's tibia and defining a first axis extending through the base surface. The offset tool is configured to rotate about the first axis. The offset tool includes a tool body that is positioned in the guide tower and is integrally formed with a shaft extending through an opening defined in the base surface of the guide tower. The stem trial is removably coupled to a lower end of the shaft.