Our goal was to determine the optimal orientation of insertion of the Slim Modiolar electrode and develop an easy-to-use method to aid implantation surgery. In some instances, the electrode arrays cannot be inserted in their full length. This can lead to buckling, interscalar dislocation or tip fold-over. In our opinion, one of the possible reasons of tip fold-over is unfavourable orientation of the electrode array. Our goal was to determine the optimal orientation of the Slim Modiolar electrode array relative to clear surgical landmarks and present our method in one specified case. For the measurement, we used the preoperative CT scan of one of our cochlear implant patients. These images were processed by an open source and free image visualization software: 3D Slicer. In the first step we marked the tip of the incus short process and then created the cochlear view. On this view we drew two straight lines: the first line represented the insertion guide of the cochlear implant and the second line was the orientation marker (winglet). We determined the angle enclosed by winglet and the line between the tip of the incus short process and the cross-section of previously created two lines. For the calculation we used a self-made python code. The result of our algorithm for the angle was 46.6055°. To validate this result, we segmented, from the CT scan, the auditory ossicles and the membranaceous labyrinth. From this segmentation we generated a 3D reconstruction. On the 3D view, we can see the position of the previous lines relative to the anatomical structures. After this we rotated the 3D model together with the lines so that the insertion guide forms a dot. In this view, the angle was measured with ImageJ and the result was 46.599°. We found that our method is easy, fast, and time-efficient. The surgery can be planned individually for each patient, based on their routine preoperative CT scan of the temporal bone, and the implantation procedure can be made safer. In the future we plan to use this method for all cochlear implantation surgeries, where the Slim Modiolar electrode is used.