An orthopaedic surgical instrument assembly includes a tibial base trial (436) adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, an intramedullary orthopaedic surgical instrument (440) engaged with a lower surface of the tibial base trial, and a fastener (512) configured to pivot relative to the tibial base trial. The fastener is configured to engage with the intramedullary orthopaedic surgical instrument to removably couple the intramedullary orthopaedic surgical instrument to the tibial base trial.
