An orthopaedic surgical instrument system includes a tibial base trial (12) and a tibial insert trial (16). An insert adaptor (14) is sized to be positioned in an aperture (330) defined in the tibial insert trial. The insert adaptor includes a base (102) sized to be positioned over a post (58) of the tibial base trial and a locking tab (162) positioned in the base and configured to engage the post of the tibial base trial.