An orthopaedic surgical instrument assembly, comprising: a tibial bearing trial including an articulation surface and a bottom surface opposite the articulation surface, a tibial base trial adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, the tibial base trial includes an upper surface engaged with the bottom surface of the tibial bearing trial, a base fastener attached to the tibial base trial, the base fastener including a shaft configured to rotate relative to the tibial base trial, and a stem adaptor secured to the shaft of the base fastener, the stem adaptor including: (i) a first adaptor body engaged with a lower surface of the tibial base trial, the first adaptor body defining a first axis, and (ii) a second adaptor body defining a second axis extending parallel to the first axis, characterised in that the first adaptor body has an upper surface with an opening in it, a lower wall with an opening in it, and a passageway extending from the upper surface to the lower wall, and the stem adaptor includes an adaptor fastener which secures the first adaptor body to the second adaptor body, the fastener having a lug with a lower end extending through the opening in the lower wall of the first adaptor body and an annular flange at its upper end which is positioned in the passageway in the first adaptor body and sized to prevent the upper end of the fastener from passing through the lower wall opening, the adaptor fastener including a pin which secures the lug to the second adaptor body, in which the lug can move along the first axis between (i) a first position in which the first adaptor body is able to rotate relative to the second adaptor body so that one of the first and second axes moves in a circular path relative to the other of the axes, and (ii) a second position in which an upper edge of the second adaptor body engages an annular flange of the first adaptor body to prevent the first adaptor body from rotating relative to the second adaptor body. a tibial bearing trial including an articulation surface and a bottom surface opposite the articulation surface, a tibial base trial adapted to be positioned on a surgically-prepared proximal end of a patient's tibia, the tibial base trial includes an upper surface engaged with the bottom surface of the tibial bearing trial, a base fastener attached to the tibial base trial, the base fastener including a shaft configured to rotate relative to the tibial base trial, and a stem adaptor secured to the shaft of the base fastener, the stem adaptor including: (i) a first adaptor body engaged with a lower surface of the tibial base trial, the first adaptor body defining a first axis, and (ii) a second adaptor body defining a second axis extending parallel to the first axis, (i) a first adaptor body engaged with a lower surface of the tibial base trial, the first adaptor body defining a first axis, and (ii) a second adaptor body defining a second axis extending parallel to the first axis, characterised in that the first adaptor body has an upper surface with an opening in it, a lower wall with an opening in it, and a passageway extending from the upper surface to the lower wall, and the stem adaptor includes an adaptor fastener which secures the first adaptor body to the second adaptor body, the fastener having a lug with a lower end extending through the opening in the lower wall of the first adaptor body and an annular flange at its upper end which is positioned in the passageway in the first adaptor body and sized to prevent the upper end of the fastener from passing through the lower wall opening, the adaptor fastener including a pin which secures the lug to the second adaptor body, in which the lug can move along the first axis between (i) a first position in which the first adaptor body is able to rotate relative to the second adaptor body so that one of the first and second axes moves in a circular path relative to the other of the axes, and (ii) a second position in which an upper edge of the second adaptor body engages an annular flange of the first adaptor body to prevent the first adaptor body from rotating relative to the second adaptor body.