An orthopaedic surgical instrument assembly includes a tibial base trial (182) adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, a base fastener (242), and a stem adaptor (184) secured to the base fastener. The stem adaptor includes a first adaptor body (260) engaged with a lower surface of the tibial base trial and a second adaptor body (262) pivotally coupled to the first adaptor body. A stem trial (14) is removably coupled to the second adaptor body of the stem adaptor.
