A method and system for determining the orientation of a surgical tool with respect to a bone of a patient discloses initializing a first active sensor and a second active sensor relative to each other with the first active sensor and second active sensor having a predetermined (relative) orientation. The first active sensor is configured to sense a change in its orientation, provide first information relating to the change and be attached to a bone in a relationship to a reference plane therefor, for use during an operation. The second active sensor is configured to sense a change in its orientation, provide second information relating to the change and be attached to a surgical tool. The relative orientation of the active sensors is useful to determine the relative orientation of the tool and the reference frame of the bone.