The guiding instrument consists of a base with three pins in order to be rigidly placed on the bone surface over the burr hole. A semicircular guide rod is mounted to the base in which an angle scale is engraved. Within the guide rod, a tube can be individually adjusted in one orientation at different angulations with a range from -600 to 600. Different tubes are designed with inner diameters of 2, 2.5, and 3 mm in order to guide catheters at different sizes. At the respective angle adjustment, the tube is fixed to the guide rod by a mounting screw. The base is opened at one side to enable the view or insert any instrument towards the lower opening of the tube. The guiding tool is positioned parallel to the midline along the engraved linear markings on the base of the instrument. Thereby, a rectangular insertion towards the sagittal convexity and an individual insertion angle towards the coronal tangent at the entry point can be established via the tube.