An orthopaedic surgical instrument system for use during a surgical procedure to implant an orthopaedic knee prosthesis, comprising: a tibial base trial component including (i) a superior surface, (ii) an inferior surface 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 being defined by an inner wall extending inwardly from the superior surface to a shelf surface positioned between the superior surface and the inferior surface, and (iv) a number of fixation pinholes (80, 82) extending through the tibial base trial component, an insert component shaped to be received in the opening defined in the tibial base trial component, the insert component comprising a base plate, and a posterior buttress extending upwardly from a superior surface of the base plate, and a tibial bearing trial component having an inner sidewall that defines an opening therein, wherein 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, and characterised in that the insert component further comprises an anterior buttress extending outwardly from an anterior edge of the base plate, wherein the anterior buttress of the insert component is configured to engage the tibial bearing trial component to prevent rotation of the tibial bearing trial component relative to the tibial base trial component, and in that the fixation pinholes (80, 82) extend through a posterior section of the shelf surface and the inferior surface of the tibial base trial component. a tibial base trial component including (i) a superior surface, (ii) an inferior surface 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 being defined by an inner wall extending inwardly from the superior surface to a shelf surface positioned between the superior surface and the inferior surface, and (iv) a number of fixation pinholes (80, 82) extending through the tibial base trial component, an insert component shaped to be received in the opening defined in the tibial base trial component, the insert component comprising a base plate, and a posterior buttress extending upwardly from a superior surface of the base plate, and a tibial bearing trial component having an inner sidewall that defines an opening therein, wherein 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, and characterised in that the insert component further comprises an anterior buttress extending outwardly from an anterior edge of the base plate, wherein the anterior buttress of the insert component is configured to engage the tibial bearing trial component to prevent rotation of the tibial bearing trial component relative to the tibial base trial component, and in that the fixation pinholes (80, 82) extend through a posterior section of the shelf surface and the inferior surface of the tibial base trial component.