An orthopaedic surgical instrument system (10) includes an orthopaedic surgical instrument for positioning on a surgically-prepared proximal end of a patients tibia. The instrument includes a central post (94) that defines a longitudinal axis. The system also includes a tibial bearing trial assembly (20) coupled to the orthopaedic surgical instrument. The tibial bearing trial assembly includes one of a plurality of tibial bearing surface trial components (192), each tibial bearing surface trial component having an articular surface, and a shim (190). The shim is configured to be coupled to a first tibial bearing surface trial component of the plurality of tibial bearing surface trial components in a first orientation in which the tibial bearing trial assembly is permitted to pivot about the axis. The shim is configured to be coupled to a second tibial bearing surface trial of the plurality of tibial bearing surface trial components in a second orientation in which the tibial bearing trial assembly is substantially prevented from rotating about the longitudinal axis.
