A computer-assisted surgery (CAS) system for tracking an orientation of a pelvis comprises at least one instrument, the instrument having an acetabulum abutment end adapted to be received in an acetabulum, a rim abutment adapted to be abutted against a rim of the acetabulum, and an indicator representative of a physical orientation of the instrument. An inertial sensor unit is connected to the at least one instrument, the inertial sensor unit producing readings representative of its orientation. A computer-assisted surgery processor unit 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 has the acetabulum abutment end received in the acetabulum, the coordinate system module setting the pelvic coordinate system by obtaining a plurality of orientation values from the at least one inertial sensor unit when the rim abutment is abutted against locations of the rim, one of said orientation values having the indicator aligned with a reference landmark, the coordinate system module defining an acetabular plane representative of the pelvic coordinate system from the plurality of orientation values; and a tracking module for tracking an orientation of the at least one inertial sensor unit relative to the pelvic coordinate system during movements thereof using the readings from the inertial sensor unit. An interface outputs orientation data as a function of the pelvic coordinate system.