An orthopaedic surgical instrument assembly comprising: (a) an attachment device comprising: (i) a housing having a longitudinal axis and a passageway defined therein that extends along the longitudinal axis, (ii) a pair of rails (36, 38) extending outwardly from the housing, each rail having a longitudinal axis that extends orthogonal to the longitudinal axis of the housing, and (iii) a mounting frame positioned on the pair of rails, the mounting frame being configured to slide relative to the housing along the pair of rails, (b) an intramedullary orthopaedic surgical instrument (16, 22) including a shaft (242, 422) positioned in the passageway of the housing, and (c) a cutting block removably coupled to the mounting frame, characterised in that the intramedullary orthopaedic surgical instrument comprises: (i) a surgical reamer having a first end attached to its shaft and a second end positioned opposite the first end, and a stem trial secured to the second end of the surgical reamer, or (ii) a broach which is attached to its shaft, and which has a tapered outer surface and a plurality of cutting teeth extending outwardly from the tapered outer surface, and the attachment device includes an attachment device locking mechanism attached to the housing, the attachment device locking mechanism being configured to secure the attachment device to the shaft of the intramedullary orthopaedic surgical instrument. (a) an attachment device comprising: (i) a housing having a longitudinal axis and a passageway defined therein that extends along the longitudinal axis, (ii) a pair of rails (36, 38) extending outwardly from the housing, each rail having a longitudinal axis that extends orthogonal to the longitudinal axis of the housing, and (iii) a mounting frame positioned on the pair of rails, the mounting frame being configured to slide relative to the housing along the pair of rails, (i) a housing having a longitudinal axis and a passageway defined therein that extends along the longitudinal axis, (ii) a pair of rails (36, 38) extending outwardly from the housing, each rail having a longitudinal axis that extends orthogonal to the longitudinal axis of the housing, and (iii) a mounting frame positioned on the pair of rails, the mounting frame being configured to slide relative to the housing along the pair of rails, (b) an intramedullary orthopaedic surgical instrument (16, 22) including a shaft (242, 422) positioned in the passageway of the housing, and (c) a cutting block removably coupled to the mounting frame, characterised in that the intramedullary orthopaedic surgical instrument comprises: (i) a surgical reamer having a first end attached to its shaft and a second end positioned opposite the first end, and a stem trial secured to the second end of the surgical reamer, or (ii) a broach which is attached to its shaft, and which has a tapered outer surface and a plurality of cutting teeth extending outwardly from the tapered outer surface, and the attachment device includes an attachment device locking mechanism attached to the housing, the attachment device locking mechanism being configured to secure the attachment device to the shaft of the intramedullary orthopaedic surgical instrument. (i) a surgical reamer having a first end attached to its shaft and a second end positioned opposite the first end, and a stem trial secured to the second end of the surgical reamer, or (ii) a broach which is attached to its shaft, and which has a tapered outer surface and a plurality of cutting teeth extending outwardly from the tapered outer surface,