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, wherein the post includes a stem extending outwardly from the plate to a superior flange; 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, wherein the tibial insert trial includes a first posterior slot that opens into an aperture; 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 insert adaptor has a second posterior slot positioned superior of the locking tab, wherein the locking tab includes a spring clip (178,186) configured to engage the stem of the post; wherein the post of the tibial base trial includes the 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, wherein when the insert adaptor is positioned in the aperture defined in the tibial insert trial, the first posterior slot and the second posterior slot are aligned to define a posterior passageway sized to receive the superior flange of the post to permit the insert adaptor and the tibial insert trial to move in an anterior-posterior direction relative to the tibial base trial, characterized in that the insert adaptor includes a curved wall that defines an inferior section of the aperture, and the 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, wherein the post includes a stem extending outwardly from the plate to a superior flange; 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, wherein the tibial insert trial includes a first posterior slot that opens into an aperture; 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 insert adaptor has a second posterior slot positioned superior of the locking tab, wherein the locking tab includes a spring clip (178,186) configured to engage the stem of the post; wherein the post of the tibial base trial includes the 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, wherein when the insert adaptor is positioned in the aperture defined in the tibial insert trial, the first posterior slot and the second posterior slot are aligned to define a posterior passageway sized to receive the superior flange of the post to permit the insert adaptor and the tibial insert trial to move in an anterior-posterior direction relative to the tibial base trial, characterized in that the insert adaptor includes a curved wall that defines an inferior section of the aperture, and the spring clip is positioned in the inferior section of the aperture and is spaced apart from the curved wall.