An orthopaedic surgical instrument system for use during a surgical procedure to implant an orthopaedic knee prosthesis, comprising a tibial base trial component (14) including (i) a superior surface (24), (ii) an inferior surface (26) positioned opposite the superior surface that is adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, (iii) an opening (30) being defined by an inner wall (44) extending inwardly from the superior surface to a shelf surface (54) positioned between the superior surface and the inferior surface, and (iv) a number of fixation pinholes (80, 82) extending through a posterior section of the shelf surface and the inferior surface of the tibial base trial component. The system also includes an insert component (16) shaped to be received in the opening defined in the tibial base trial component, the insert component comprising a base plate (120), and a posterior buttress (170) extending upwardly from a superior surface (182) of the base plate. The system further includes a tibial bearing trial component (18) having an inner sidewall that defines an opening (416) therein. The posterior buttress of the insert component is configured to be received in the opening of the tibial bearing trial component to prevent rotation of the tibial bearing trial component relative to the tibial base trial component. The insert component also has an anterior buttress (180) extending outwardly from an anterior edge of the base plate.
