A computer-assisted surgery system includes a display, an input device configured to receive data input by a user, and a processor, coupled to the input device and the display. The processor is configured to establish a first position of a pre-operative center of rotation of a joint in a first coordinate space of a first bone and a second coordinate space of a second bone and establish a second position of the pre-operative center of rotation of the joint in the first coordinate space, wherein the second position is a projection into the first coordinate space of the position of the pre-operative center of rotation maintained in a constant position in the second coordinate space. The processor is further configured to determine a change in a parameter associated with the joint based on the first and second positions and output a result indicating the determined change to the display.
