Patients who are treated with a mastoidectomy usually suffer from an inflammation of the petrosal bone. The intervention is a time consuming landmark based surgery and usually performed with a powered drill. Delicate risk structures must be respected. Navigated Control (NC) describes the control for a power driven instrument which is controlled by a surgeon and additionally controlled according to the position of the instrument relatively to a deliberated position known from a preoperatively segmented work space which excludes risk structures. The force of a drill can be regulated by the principle of NC. Following results were received: 1. Risk structure segmentation is feasible 2. The drill and a phantom can be registered. 3. With NC the resection is faster, more accurate and with no risk structures damage. 4. The phantom is suitable.