An orthopaedic surgical instrument assembly comprises a cutting block having a base plate, and a pair of curved arms extending posteriorly from the base plate. Each of the curved arms has a posterior surface and a cutting guide defined in the posterior surface. An intramedullary orthopaedic surgical instrument is configured to be inserted into a medullary canal of a patient's femur, and an adaptor positioned in a slot defined in the base plate. The adaptor includes a mounting bracket engaged with the base plate, a first adaptor body coupled to the mounting bracket, and a second adaptor body pivotally coupled to the first adaptor body, the second adaptor body including a fastener coupled to the intramedullary orthopaedic surgical instrument. The first adaptor body defines a first axis, and the intramedullary orthopaedic surgical instrument includes an elongated body that defines a second axis extending parallel to the first axis. The elongated body is pivoted about the first axis when the second adaptor body is pivoted relative to the first adaptor body.
