temporarily fixable positioning system for secure , play - free fastening to the human body , preferably to the human skull has a base , a clampable ball - and - socket joint , a linear axis mechanism fixed securely to it and a carriage guided along the linear axis mechanism for receiving an actuator or sensor instrument . for intraoperative use , the positioning system is combined with a retractor . a head support is provided for noninvasive diagnostic or therapeutic applications . the combination of the axial degree of freedom of linear axis with the three rotational degrees of freedom of the ball - and - socket joint allows four - axis , in situ positioning of the instrument on or in the skull , especially in the external auditory canal and an opened middle ear . the positioning system in combination with one of the holders enables precise positioning of an active end of a guided instrument relative to sensitive anatomical structures while avoiding risky relative movements .