An assembly for use in orthopaedic surgery which comprises: an orthopaedic prosthesis assembly which is a femoral implant of a joint prosthesis and comprises: a femoral component, and a stem component having an elongate body and a tapered post at one end, and an orthopaedic surgical instrument for disassembling the orthopaedic prosthesis assembly, comprising: a main component including a housing and an elongated body extending from the housing, the housing and the elongated body defining a longitudinal axis, in which a passageway is defined in the elongated body along the longitudinal axis, and the outer surface of the elongated body is threaded at its end, a rod component, having (i) a head configured to be received in the housing of the main component and (ii) an elongated shaft extending from the head, configured to pass through the elongated body of the main component, and having a length greater than a length of the elongated body of the main component, and a spindle component including a threaded body configured to thread into the housing to move the rod component along the longitudinal axis, characterised in that the orthopaedic prosthesis assembly is a femoral implant of a knee joint prosthesis, and the femoral component has a medial condyle surface, a lateral condyle surface, a bone facing surface, and a tapered stem post which extends from its bone facing surface and has a passageway extending through it which (a) ends at an opening which is accessible from a notch between the medial and lateral condyle surfaces, (b) has a tapered bore formed in it at its end remote from the opening between the condyle surfaces into which the tapered post of the stem component may be advanced to taper lock the post and the femoral component to one another, (c) has a distal compartment defined at its distal end, and (d) has a threaded passage connecting the tapered bore and the distal compartment, in which the end of the elongated body can be inserted into the passageway within the tapered stem post through the opening and attached to the femoral component by engaging the threads on the outer surface of the elongated body with the threaded passage. an orthopaedic prosthesis assembly which is a femoral implant of a joint prosthesis and comprises: a femoral component, and a stem component having an elongate body and a tapered post at one end, and a femoral component, and a stem component having an elongate body and a tapered post at one end, and an orthopaedic surgical instrument for disassembling the orthopaedic prosthesis assembly, comprising: a main component including a housing and an elongated body extending from the housing, the housing and the elongated body defining a longitudinal axis, in which a passageway is defined in the elongated body along the longitudinal axis, and the outer surface of the elongated body is threaded at its end, a rod component, having (i) a head configured to be received in the housing of the main component and (ii) an elongated shaft extending from the head, configured to pass through the elongated body of the main component, and having a length greater than a length of the elongated body of the main component, and a spindle component including a threaded body configured to thread into the housing to move the rod component along the longitudinal axis, a main component including a housing and an elongated body extending from the housing, the housing and the elongated body defining a longitudinal axis, in which a passageway is defined in the elongated body along the longitudinal axis, and the outer surface of the elongated body is threaded at its end, a rod component, having (i) a head configured to be received in the housing of the main component and (ii) an elongated shaft extending from the head, configured to pass through the elongated body of the main component, and having a length greater than a length of the elongated body of the main component, and a spindle component including a threaded body configured to thread into the housing to move the rod component along the longitudinal axis,