1. A computer-assisted orthopaedic 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 implants; and a computer configured to select from the database a first implant and display on a screen the three dimensional implant model of the first implant, determine a degree of fit of the selected first implant to the first bone by superimposing on the screen an implant model of the first implant on top of the model of the first bone, and visually represent the degree of fit on the screen by a color map, wherein various colors of the color map represent various distances the implant model of the first implant departs from the model of the first bone.