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 comprising: at least one instrument ; at least one inertial sensor unit connected to the at least one instrument, the inertial sensor unit producing readings representative of its orientation; a computer-assisted surgery processor unit operating a surgical assistance procedure and comprising: a coordinate system module adapted to 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 adapted to track 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 adapted to record 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, to record after implant rejointing the medio-lateral orientation and said distance, and to calculate at least one of a leg length discrepancy and an offset, based on said distances and said medio-lateral orientations; an interface adapted to output at least the leg length discrepancy or the offset between the preoperative leg condition and the post-implant rejointing leg condition. at least one instrument ; at least one inertial sensor unit connected to the at least one instrument, the inertial sensor unit producing readings representative of its orientation; a computer-assisted surgery processor unit operating a surgical assistance procedure and comprising: a coordinate system module adapted to 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 adapted to track 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 adapted to record 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, to record after implant rejointing the medio-lateral orientation and said distance, and to calculate at least one of a leg length discrepancy and an offset, based on said distances and said medio-lateral orientations; a coordinate system module adapted to 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 adapted to track 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 adapted to record 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, to record after implant rejointing the medio-lateral orientation and said distance, and to calculate at least one of a leg length discrepancy and an offset, based on said distances and said medio-lateral orientations; an interface adapted to output at least the leg length discrepancy or the offset between the preoperative leg condition and the post-implant rejointing leg condition. <EOS>