A computer-assisted surgery system comprises a calibrating instrument adapted to be applied to a pelvis in a known manner, and a surgical instrument. A computer-assisted processor unit operating a surgical assistance procedure and comprises at least one portable inertial sensor unit configured to be connected to the at least one calibrating instrument and the at least one surgical instrument, the portable inertial sensor unit outputting readings representative of its orientation. A geometrical relation data module provides a geometrical relation data between the orientation of the portable inertial sensor unit, of the calibrating instrument and of the surgical instrument. A coordinate system module sets a coordinate system of the pelvis in which an anterior-posterior axis of the pelvis is generally in a direction of gravity, and in which a medio-lateral axis of the pelvis is obtained from readings of the at least one portable inertial sensor unit on the calibrating instrument using the geometrical relation data therebetween. A tracking module tracks movements of the at least one surgical instrument relative to the coordinate system using readings from the inertial sensor unit on the surgical instrument using the geometrical relation data therebetween, and calculates navigation data for the movements, the navigation data relating the orientation of the surgical instrument to the orientation of the pelvis. An interface outputs the navigation data.