Abstract: An orthopaedic navigation system (1) has (a) a reference point sensor (10) for connection to a body to transmit a reference position signal; (b) a trial sensor (20) for connection to a trial implant and to transmit a trial position signal indicating its position relative to the reference point sensor; (c) an implant sensor (41) for connection to a first implant and to transmit a first implant position signal indicating its position relative to the reference position sensor; (d) a computer (30) having a receiver to (i) receive said reference point signal and trial position signal to create a predetermined position value, (ii) receive said reference point signal and first implant position signal to create a comparison position value, and having a transmitter to transmit output signals of the predetermined position value; and (e) a screen (51) to receive said visual signals and create a display to compare the predetermined position value relative to the comparison value, which display can be seen by a user while