1. A method for controlling a surgical navigation system, the method including: using user gestures as touch-free system control inputs; associating a detection unit for detecting the touch-free system control inputs made by a user with a surgical navigation system; controlling the navigation system in accordance with the control inputs; moving a medical instrument by the user, wherein predetermined movements of the instrument are detected and identified by the detection unit as user gestures; carrying out an initiating user gesture that comprises drawing, with a tip of the medical instrument, a geometrical shape in a free three-dimensional working field of the user above or near a patient; recognizing the geometrical shape by the navigation system as the initiating user gesture and defining a virtual volume surrounding the shape drawn in a sterile working field of the user; and starting an interaction mode on the navigation system in which at least one further predetermined gestures made in the virtual volume is identified and recognized as a control input.