An orthopaedic surgical instrument assembly for preparing a patient's femur in a knee replacement procedure, comprising: a cutting block comprising a base plate, and a pair of curved arms extending posteriorly from the base plate, each curved arm including a posterior surface and a cutting guide defined in the posterior surface, an intramedullary orthopaedic surgical instrument configured to be inserted into a medullary canal of the patient's femur, and an adaptor positioned in a slot defined in the base plate, the adaptor including (i) a mounting bracket engaged with the base plate, (ii) a first adaptor body coupled to the mounting bracket, (iii) a second adaptor body pivotally coupled to the first adaptor body, and (iv) a first locking mechanism configured to prevent relative movement between the second adaptor body and the first adaptor body, the first adaptor body having a passageway extending along the first axis of the first adaptor body to a proximal end, in which (i) the first adaptor body defines a first axis, (ii) the intramedullary orthopaedic surgical instrument includes an elongated body that defines a second axis extending parallel to the first axis and offset from said first axis, and (iii) when the second adaptor body is rotated relative to the first adaptor body, the elongated body is rotated about the first axis, characterised in that the second adaptor body includes a fastener coupled to the intramedullary orthopaedic surgical instrument, the passageway in the first adaptor body has an aperture defined at the proximal end, and the first locking mechanism includes a threaded pin coupled to the first adaptor body, the threaded pin being moveable between (i) a first position in which a tip of the threaded pin extends through the aperture at the end of the passageway in the first adaptor body to engage an end face of the second adaptor body, preventing relative movement between the second adaptor body and the first adaptor body, and (ii) a second position in which the tip of the threaded pin is disengaged from the second adaptor body to permit relative movement between the second adaptor body and the first adaptor body. a cutting block comprising a base plate, and a pair of curved arms extending posteriorly from the base plate, each curved arm including a posterior surface and a cutting guide defined in the posterior surface, an intramedullary orthopaedic surgical instrument configured to be inserted into a medullary canal of the patient's femur, and an adaptor positioned in a slot defined in the base plate, the adaptor including (i) a mounting bracket engaged with the base plate, (ii) a first adaptor body coupled to the mounting bracket, (iii) a second adaptor body pivotally coupled to the first adaptor body, and (iv) a first locking mechanism configured to prevent relative movement between the second adaptor body and the first adaptor body, the first adaptor body having a passageway extending along the first axis of the first adaptor body to a proximal end, in which (i) the first adaptor body defines a first axis, (ii) the intramedullary orthopaedic surgical instrument includes an elongated body that defines a second axis extending parallel to the first axis and offset from said first axis, and (iii) when the second adaptor body is rotated relative to the first adaptor body, the elongated body is rotated about the first axis, characterised in that the second adaptor body includes a fastener coupled to the intramedullary orthopaedic surgical instrument, the passageway in the first adaptor body has an aperture defined at the proximal end, and the first locking mechanism includes a threaded pin coupled to the first adaptor body, the threaded pin being moveable between (i) a first position in which a tip of the threaded pin extends through the aperture at the end of the passageway in the first adaptor body to engage an end face of the second adaptor body, preventing relative movement between the second adaptor body and the first adaptor body, and (ii) a second position in which the tip of the threaded pin is disengaged from the second adaptor body to permit relative movement between the second adaptor body and the first adaptor body.