A hip resurfacing CAS system for guiding an operator in altering a femoral head in computer-assisted surgery for subsequent implanting of a femoral head implant, comprises a trackable reference on the femur. A registration tool is trackable. A bone-altering tool is associated with a resurfacing of the femoral head. A tracking apparatus tracks the tools. A resurfacing processing unit is connected to the tracking apparatus so as to receive tracking data for the tools. The resurfacing processing unit has a position/orientation calculator to calculate from the tracking data a position and orientation of the trackable reference to track the femoral frame of reference, and of the registration tool and the bone-altering tool. A model generator receives position and orientation data of the registration tool to produce a model of the femoral head and neck with respect to the femoral frame of reference. A resurfacing evaluator determines an evaluated bone resurfacing alteration as a function of a position and/or orientation of the bone-altering tool with respect to the bone model of the femoral head and neck, and a tool geometry model, at least prior to resurfacing being performed.
