A computer-assisted surgery system comprising: at least one calibrating instrument (13, 14, 15) adapted to be applied to a pelvis in a known manner; at least one surgical instrument (16, 17); a computer-assisted processor unit operating a surgical assistance procedure and comprising: at least one portable inertial sensor unit configured to be connected to the at least one calibrating instrument (13, 14, 15) and the at least one surgical instrument (16, 17), the portable inertial sensor unit outputting readings representative of its orientation; a geometrical relation data module providing a geometrical relation data between the orientation of the portable inertial sensor unit, of the at least one calibrating instrument (13, 14, 15) and of the at least one surgical instrument (16, 17); a coordinate system module for setting a coordinate system of the pelvis in which a first axis is representative of an anterior-posterior axis of the pelvis and is generally aligned with of gravity, and in which a second axis is aligned with a medio-lateral axis of the pelvis, wherein the medio-lateral axis of the pelvis is obtained from readings of the at least one portable inertial sensor unit on the at least one calibrating instrument (13, 14, 15) using the geometrical relation data therebetween, wherein the at least one calibrating instrument is applied to the anterior-superior iliac spines of the pelvis; a tracking module for tracking movements of the at least one surgical instrument (16, 17) relative to the coordinate system using readings from the inertial sensor unit on the surgical instrument (16, 17) using the geometrical relation data therebetween, and calculating navigation data for the movements, the navigation data relating the orientation of the surgical instrument (16, 17) to the orientation of the pelvis; and an interface for outputting the navigation data. at least one calibrating instrument (13, 14, 15) adapted to be applied to a pelvis in a known manner; at least one surgical instrument (16, 17); a computer-assisted processor unit operating a surgical assistance procedure and comprising: at least one portable inertial sensor unit configured to be connected to the at least one calibrating instrument (13, 14, 15) and the at least one surgical instrument (16, 17), the portable inertial sensor unit outputting readings representative of its orientation; a geometrical relation data module providing a geometrical relation data between the orientation of the portable inertial sensor unit, of the at least one calibrating instrument (13, 14, 15) and of the at least one surgical instrument (16, 17); a coordinate system module for setting a coordinate system of the pelvis in which a first axis is representative of an anterior-posterior axis of the pelvis and is generally aligned with of gravity, and in which a second axis is aligned with a medio-lateral axis of the pelvis, wherein the medio-lateral axis of the pelvis is obtained from readings of the at least one portable inertial sensor unit on the at least one calibrating instrument (13, 14, 15) using the geometrical relation data therebetween, wherein the at least one calibrating instrument is applied to the anterior-superior iliac spines of the pelvis; a tracking module for tracking movements of the at least one surgical instrument (16, 17) relative to the coordinate system using readings from the inertial sensor unit on the surgical instrument (16, 17) using the geometrical relation data therebetween, and calculating navigation data for the movements, the navigation data relating the orientation of the surgical instrument (16, 17) to the orientation of the pelvis; and an interface for outputting the navigation data. at least one portable inertial sensor unit configured to be connected to the at least one calibrating instrument (13, 14, 15) and the at least one surgical instrument (16, 17), the portable inertial sensor unit outputting readings representative of its orientation; a geometrical relation data module providing a geometrical relation data between the orientation of the portable inertial sensor unit, of the at least one calibrating instrument (13, 14, 15) and of the at least one surgical instrument (16, 17); a coordinate system module for setting a coordinate system of the pelvis in which a first axis is representative of an anterior-posterior axis of the pelvis and is generally aligned with of gravity, and in which a second axis is aligned with a medio-lateral axis of the pelvis, wherein the medio-lateral axis of the pelvis is obtained from readings of the at least one portable inertial sensor unit on the at least one calibrating instrument (13, 14, 15) using the geometrical relation data therebetween, wherein the at least one calibrating instrument is applied to the anterior-superior iliac spines of the pelvis; a tracking module for tracking movements of the at least one surgical instrument (16, 17) relative to the coordinate system using readings from the inertial sensor unit on the surgical instrument (16, 17) using the geometrical relation data therebetween, and calculating navigation data for the movements, the navigation data relating the orientation of the surgical instrument (16, 17) to the orientation of the pelvis; and an interface for outputting the navigation data.