1. A computer-assisted orthopedic surgery system comprising: a device for generating a three dimensional geometrical surface model of a first bone; a database containing three dimensional implant models of a plurality of available implants; and a computer that is configured to permit a user to select from the database a first implant and display on a screen the three dimensional implant model of the first implant, wherein the computer superimposes on the screen the implant model on top of the model of the first bone such that the two models are visually identifiable from one another and wherein differences between the two models are highlighted on the display in a visually identifiable manner.