A computer-assisted surgery, CAS, system for tracking an orientation of a pelvis comprising: 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; 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 configured to set 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 configured to set 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 configured to track 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, and an interface configured to output orientation data as a function of the pelvic coordinate system. 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; 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 configured to set 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 configured to set 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 configured to track 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, and an interface configured to output orientation data as a function of the pelvic coordinate system. a coordinate system module configured to set 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 configured to set 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 configured to track 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, and an interface configured to output orientation data as a function of the pelvic coordinate system.