*3.1. Intraoperative Course*

The surgery was uneventful for both patients. The approach to the cochlea was typically completed-by antro-mastoidectomy and posterior tympanotomy. The bone bed for the transducer was drilled. The cochlea was opened by the round window in the first patient and by performing cochleostomy (due to poor visibility of the round window) in the second. The internal coil of CI was inserted into the subperiosteal pocket and the transducer was positioned in the bone bed and fixed with surgical thread. Then the insertion tool was connected to a dedicated connector and coupled for a moment to the robot arm by Boglock (Collin, Bagneux, France; AB Mid-Scala: RBT-0406) to confirm the optimal position of the robot arm (Figure 5), and then decoupled. In the next stage, the Mid-Scala array was positioned on the insertion tool and connected to the robot (Figure 6). The electrode was moved directly to the cochlear opening and slowly inserted to the first blue marker using a robot (Figure 7). Further insertion was carried out by hand with a slider on the insertion tool. However, the stable position of the tool allowed for a very slow and gentle insertion. Moreover, it was possible to stop the electrode insertion and keep it in one position for a few or even several seconds if the ECochG potential decreased. What is more, the insertion axis of the electrode array was slightly modified when iECochG potentials decreased in case one, which improved the iECochG results. The full insertion (till the second blue marker) was carried out in both cases. Then the intraoperative measurements were completed. The connecting cable to the electrode was positioned in the antro-mastoidectomy and the wound was typically closed.

**Figure 5.** The RobOtol® system is ready to use. The insertion tool is attached to a dedicated connector and coupled to the robot arm by Boglock to confirm the optimal position of the robot arm.

**Figure 6.** The Mid-Scala array is positioned on the insertion tool and connected to the robot. The system is ready for electrode array insertion.

**Figure 7.** The electrode array insertion via typical (antromastoidectomy and facial recess) approach with a RobOtol® (case 1).
