For a controlled placement of the ventricular catheter, the THOMAL GUIDE is placed directly on the skull bone with three feet and with the line markings parallel to the centerline above the drill hole to ensure a right angle of insertion in the sagittal plane. In the coronal plane, an individual angle can be determined using an angle adjustment on the THOMAL GUIDE. The rotation of the catheter guide, which is guided over the rails, is done via a pivot point located very close above the borehole.

In order to measure the insertion angle, it is necessary in principle to determine the desired angle of the ventricular catheter to the tangent in the coronal plane above the borehole.

There are two methods for this:

1. The most precise method for determining the individual insertion angle of the ventricular catheter requires the reading of a volume or thin-film data set into a planning software. In this software the optimal trajectory for the ventricular catheter can be defined. The coordinates of the entry point, e.g. to the nasion and the centerline, can be measured. At the entry point, the tangent would have to be drawn in the projected coronal plane and the angle of the trajectory to the tangent measured. Now the data of the entry point can be transferred to the patient and intraoperatively the angle can be applied on the THOMAL GUIDE for the insertion of the catheter. This method is especially recommended for very narrow ventricle systems. The accuracy of determining the angle depends on the image used.

A coronary sectional image should be used to facilitate individual measurement of the angle of entry. If possible, this should be at the level of the anterior horn in front of the Foramen Monroi. The entry point, the projected target point and the projected trajectory should be marked on the foramen monroi. To mark the projected target point, the distance to the midline and the depth of the ventricle should be taken into account. Now the tangent can be drawn. The easiest way to do this is to set two points at equal distances medially and laterally from the entry point. The connection of these two points corresponds to the tangent. Accordingly, the angle between the trajectory and the tangent can then be determined and set on the THOMAL GUIDE.

For both methods the THOMALE GUIDE App for iPhone and iPad will be used. It allows - e.g. via an image import - a simple determination of the angle of the trajectory to the tangent.