An orthopaedic surgical instrument system comprising : a tibial base trial including a plate configured to be positioned on a proximal end of a patient's tibia and a post extending outwardly from a superior surface of the plate , a tibial insert trial including (i) an inferior surface configured to confront the superior surface of the plate , (ii) a pair of curved surfaces positioned opposite the inferior surface , (iii) a spine positioned between the pair of curved surfaces , and (iv) an aperture defined in the inferior surface , and an insert adaptor sized to be positioned in the aperture defined in the tibial insert trial , the insert adaptor including (i) a base sized to be positioned over the post of the tibial base trial and (ii) a locking tab positioned in the base ; wherein the post of the tibial base trial includes a superior flange configured to engage the locking tab of the insert adaptor to prevent movement of the insert adaptor in an inferior-superior direction relative to the tibial base trial , the post includes a stem extending outwardly from the plate to the superior flange ; wherein a channel is defined between the superior flange and the superior surface of the plate , and the locking tab is sized to be positioned in the channel , the locking tab includes a pair of arms configured to engage the stem of the post , wherein each arm of the pair of arms is a spring clip characterized in that the insert adaptor includes a curved wall that defines an inferior section of aperture , and each spring clip is positioned in the inferior section of the aperture and is spaced apart from the curved wall . a tibial base trial including a plate configured to be positioned on a proximal end of a patient's tibia and a post extending outwardly from a superior surface of the plate , a tibial insert trial including (i) an inferior surface configured to confront the superior surface of the plate , (ii) a pair of curved surfaces positioned opposite the inferior surface , (iii) a spine positioned between the pair of curved surfaces , and (iv) an aperture defined in the inferior surface , and an insert adaptor sized to be positioned in the aperture defined in the tibial insert trial , the insert adaptor including (i) a base sized to be positioned over the post of the tibial base trial and (ii) a locking tab positioned in the base ; wherein the post of the tibial base trial includes a superior flange configured to engage the locking tab of the insert adaptor to prevent movement of the insert adaptor in an inferior-superior direction relative to the tibial base trial , the post includes a stem extending outwardly from the plate to the superior flange ; wherein a channel is defined between the superior flange and the superior surface of the plate , and the locking tab is sized to be positioned in the channel , the locking tab includes a pair of arms configured to engage the stem of the post , wherein each arm of the pair of arms is a spring clip characterized in that the insert adaptor includes a curved wall that defines an inferior section of aperture , and each spring clip is positioned in the inferior section of the aperture and is spaced apart from the curved wall . <EOS>