An orthopaedic surgical instrument assembly is disclosed. The assembly includes a tibial base trial adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, a base fastener, and a stem adaptor secured to the base fastener. The stem adaptor includes a first adaptor body engaged with a lower surface of the tibial base trial and a second adaptor body pivotally coupled to the first adaptor body. A stem trial is removably coupled to the second adaptor body of the stem adaptor.
