A computer-assisted surgery system for outputting at least one of a leg length discrepancy and an offset between a preoperative leg condition and a post-implant rejointing leg condition comprises instruments. An inertial sensor unit is connected to one of the instrument, the inertial sensor unit producing readings representative of its orientation. A computer-assisted surgery processor unit operating a surgical assistance procedure and comprises a coordinate system module for setting a pelvic coordinate system from readings of the at least one inertial sensor unit when the at least one instrument is in a given orientation relative to the pelvis, a tracking module for tracking an orientation of the at least one instrument relative to the pelvic coordinate system during movements thereof using the readings from the inertial sensor unit on the instrument, and a geometrical relation data module for recording preoperatively a medio-lateral orientation of the at least one instrument representative of a medio-lateral axis of the legs relative to the pelvic coordinate system and a distance between the legs along the medio-lateral axis, for recording after implant rejointing the medio-lateral orientation and said distance, and for calculating at least one of a leg length discrepancy and an offset, based on said distances and said medio-lateral orientations. An interface outputs at least the leg length discrepancy or the offset between the preoperative leg condition and the post-implant rejointing leg condition.Système de chirurgie assistée par ordinateur, comprenant des instruments, et servant à donner au moins une incohérence de longueur pour une jambe. Le système donne aussi l'écart entre l'état préopératoire et l'état post-implantatoire d'une jambe. Une unité de capteurs d'inertie est connectée à l'un des instruments, l'unité de capteurs d'inertie produisant des données représentatives de son orientation. Une unité de processeurs chirurgicaux assistée par ordinateur met
