THOMALE GUIDE - Application
Measurement of the insertion angle
Basically, for the measurement of the insertion angle, determine the selected angle of the ventricular catheter to the tangent in the coronal plane above the burrhole.
Therefore we propose two possibilities:
1. The most accurate method to determine the individual insertion angle of the ventricular catheter guide to the tangent is done by importing biparietal data of the skull into a planning software (for example, Osirix or a similar navigation software). An optimal trajectory for the ventricular catheter can thereby be
defined. The coordinates of the entry point, e.g. into the nasion and the centre line can be measured. Draw the tangent of the entry point on the projected coronal plane and measure the angle of the trajectory to the tangent. Now the insertion parameters can be applied on to the patient data and intra-operatively the angle can be applied to the THOMALE GUIDE for catheter insertion. This method is especially recommended in case of very narrow ventricles.
2. To allow an easier unique measurement of the trajectory angle, import a coronary crosssection. Preferably, the image should show the level of the anterior horn in front of the foramen Monro. Mark on this image the entry point, the projected target location in the ventricle and thereby the resulting trajectory. The distance to the centre line and the depth of the ventricle need to be considered when marking the projected target. Now, draw the tangent. Proceed by setting two points, both with the same distance medial and laterally to the entry point. The relation between those two points correspond to a tangent. Accordingly, the angle between trajectory and tangent can be determined and set on the THOMALE GUIDE.
For both methods, the Thomale Guide App for iPhone and iPad is available. For example: by importing an image, the angle of the trajectory to the tangent can now easily be determined with the help of the application.
Adjustment of the insertion angle
After loosening the lock nut slightly (a quarter turn in counterclockwise direction) the catheter guide can slide along the guide rails and the required angle can be set.
The scale of the guide rail is marked by thick solid lines in increments of 10° and the thin half-lines at increments of 5°. It is recommended to use a lens or a magnifying glass for the adjustment of the angle. The setting of an angle is done by aligning the arrow on the slider to the desired marking on one of the two rails. Our recommendation is to start the adjustment always at the 0° marking. The 0° mark is characterized by a higher lineweight.
Angles between 0 ° and 45 ° can be set on both sides. The overall THOMALE GUIDE, except for the base ring, is completely symmetrical. This allows the angle to be set on either side of the scale. Once the correct angle has been set, it can be fixed by lightly tightening the locking nut clockwise.
Handling during surgery
Prior to implantation of ventricular catheters using the THOMALE GUIDE, clear the skull from the periosteum in a circular area of about 20 to 25 mm around the burrhole.
Place the THOMALE GUIDE on the skull bone and aligned in such a way, that its rotation axis with the open side of the base ring is aligned parallel to the median plane. Particular attention should be taken during alignment that the coronary insertion angle setting is correct and that the lateral opening of the
base ring is aligned towards the median plane.
The ventricular catheter can now be inserted and pushed through the catheter guide up to the dura. The positioning of the catheter tip can be controlled by the lateral opening in the base ring.
Please note that while cautiously pushing the ventricular catheter, the catheter tip points directly to the previously created dura opening and that the dura boarders will not be affected. Each contact with the dura bears the risk of deflecting the catheter. Control the insertion depth whilst moving the catheter in the direction of the ventricle. Through the lateral openings of the base ring, the markings on the VC can be identified and read. The standard insertion length in adult patients is about 60 to 65 mm. Ideally, the length can be determined individually by preoperative measurement of a coronary image.
When positioned correctly, the catheter is then fixed at the burrhole level with the aid of atraumatic forceps through the lateral opening of the base ring. Remove the stylet and check for correct CSF flow. Now the THOMALE GUIDE can be removed from the site without changing the position of the catheter.