A CAS system for measuring surgical parameters during hip replacement surgery comprises a first tracking reference in fixed relation with the pelvis to form a frame of reference. A registration tool is trackable. A sensor apparatus tracks the first tracking reference and the registration tool. A controller unit receives tracking data for the first trackable reference and the registration tool. The controller unit has a position and orientation calculator for calculating from the tracking data a position and orientation of the pelvic tracking reference to track the frame of reference, and of the registration tool to produce femoral models at a first and a second sequential operative steps. An alignment adjustor aligns the femoral models with the frame of reference. A surgical parameter calculator calculates surgical parameters as a function of the femoral models as aligned by the alignment adjustor.
