*3.4. Pure Tone Audiometry and the Estimated Audiogram*

The results of pure tone audiometry and the estimated audiograms are presented in Figure 10. Preoperative BC thresholds compared to intraoperative estimated ECochG thresholds and 2-day postoperative BC thresholds had similar values at frequencies where all thresholds were measurable. However, for patient 1 the estimated threshold for 250 Hz seems an outlier (80 dB HL vs. 55 and 50 dB HL). The results of the ECochG performed one month after the surgery showed that in both patients the hearing residues were preserved for the selected frequencies. Moreover, in patient 2, the pure-tone audiometry results confirmed the maintenance of postoperative auditory thresholds for most frequencies.

**Figure 9.** Postoperative CT scans of operated patients.

**Figure 10.** The patients' preoperative BC thresholds (triangles), intraoperative estimated ECochG thresholds (asterisks), 1-month postoperative estimated ECochG threshold (solid asterisks), postoperative BC thresholds after 2 days (solid triangles) and 1 month (solid diamonds).

#### **4. Discussion**

Cochlear implantation can benefit from robotic assistance in several steps of the surgical procedure: (i) the approach to the middle ear by automated mastoidectomy and posterior tympanotomy or through a tunnel from the postauricular skin to the middle ear (i.e., direct cochlear access); (ii) a minimally invasive cochleostomy by a robot-assisted drilling tool; (iii) alignment of the correct insertion axis on the basal cochlear turn; (iv) insertion of the electrode array with a motorized insertion tool [10]. Currently, there are four systems for clinical robotic cochlear implantation available. Three of them, Microtable® (Vanderbilt), HEARO® (Bern) and ROSA® (Amiens), are used for direct cochlear access but the number of cases implanted with these devices is still very limited [10,23–26]. The fourth system (RobOtol®) is not intended for drilling, but for robotic alignment of the electrode array and its insertion into the scala tympani. This system is clinically used in many European countries, mainly France and in China. More than 250 cochlear implantations with this system have been performed, both in adults and children [10,27–29].

The primary assumption of introducing RobOtol® was to optimize the electrode array insertion into the scala tympani and preserve the anatomical structures of the cochlea, which can have many benefits, mainly in patients with residual hearing. It was supposed to have an effect in better hearing if damage to the basilar membrane is avoided and residual hearing preservation is possible [30]. Currently, the RobOtol® can be used with many straight electrodes: SlimJ (Advanced Bionics), 522 and 622 (Cochlear), Flex (Medel), Evo (Oticon) and with one perimodiolar electrode–MidScala (Advanced Bionics) [5,6,9,13,14,28]. The system shows its advantage in eliminating human involuntary tremors and augmenting accuracy during micromanipulation. It can safely assist cochlear implantation to realize minimally invasive and full tympanic scala insertion of the electrode array and to ensure the preservation of the fine intracochlear structure [12]. Despite the promising results in laboratory tests in terms of minimal invasiveness, reduced trauma and better hearing preservation, so far, no clinical benefits on residual hearing preservation or better speech performance have been demonstrated [10]. It is emphasized that new robotic insertion tools should be provided with loop feedback systems capable of modifying the insertion parameters based on both insertion forces and ECochG responses. A preliminary study in vivo sheep model tested the feasibility of an ECochG-guided robotics-assisted CI insertion system [31].

The main goal of our study was to show the application of robotic electrode insertion with simultaneous iECochG measurements, which constituted intraoperative control. We wanted to clarify how the iECochG can improve the robotic cochlear electrode array insertion. To the best of our knowledge, the association between an innovative method of supporting CI surgery and an equally current method of tracking the intraoperative effect has not been published so far. During the electrode insertion with a RobOtol® in our patients with residual hearing, the continuous measurements of ECochG responses were recorded and the insertion speed and axis were constantly modified. For example, electrode insertion was slowed down or interrupted and the insertion axis of the electrode array was modified as the ECochG potential decreased. Perhaps thanks to this, we have managed to partially preserve residual hearing, confirmed by estimated audiograms at the end of the surgeries and during the system's activation and by measuring bone conduction thresholds by pure tone audiometry. However, our patients require longer follow-ups and subsequent measurements to explain the differences between obtained results and long-term effects.
