*2.2. Study Design*

Based on the retrospective design of the study, ethical board approval was not required. The data for the present analysis were extracted from our cochlear implantation database. This database was established to routinely collect all clinical, audiological, radiological, and surgical data of the patients. We retrospectively identified patients, in whom either a IES CMD or a Aplit-Array CMD was used for cochlear implantation. In addition to the data obtained from the database, we also collected data retrospectively from the patients' surgical reports to determine the indication criteria used to determine the electrode type in individual cases. After identifying these patients, measurement protocols for impedances

and hearing tests were evaluated, as well as clinical data collected from patient records to ensure a comprehensive evaluation. The REGULAR group served as a control group and included routinely treated patients without any cochlear abnormalities, as radiologically and intraoperatively determined.

## *2.3. The Fibrotic Obliteration Probe*

The IE stiff CMD (IES, MED-EL, Innsbruck, Austria) (Figure 1) is a custom-made device which can be used individually when fibrotic tissue is present in the inner ear. It is used for the dilation of the cochlear lumen prior to electrode insertion. Thus, the IES consisted of an electrode dummy, which was used prior to electrode insertion as a surgical tool with no electrical function. The outer geometry was the same as the distal 50 mm of MED-EL's STANDARD electrode, with a diameter of 1.3 mm at the proximal end and 0.5 mm at the distal end of the array (Figure 1a,b). It was made of medical-grade silicone with multiple stiff metal wires incorporated into its matrix. The markings on the IES CMD array at 20, 24, and 28 mm indicate the possible insertion depths (Figure 1b).

**Figure 1.** Dimensions of the IE stiff CMD. (**a**) Intraoperative; (**b**) markings on the IES CMD array. The IES CMD has a maximal insertion length of 31.5 mm.

#### *2.4. The Surgical Procedure*

Electrode insertion via the round window is the preferred approach in cochlear implantation. After opening the round window for insertion, the IES CMD was carefully inserted into the inner ear for the dilatation of obstructed scala tympani or for depth measurement prior to the actual insertion of the electrode array in the IES group. In any case, the electrode array was inserted slowly into the scala tympani up to the previously defined insertion depth. The insertion site was sealed with muscle fascia from the temporal muscle after electrode placement. The array was usually fixed in a 1 mm bony notch drilled in the chorda facial angle. At the end of the surgical intervention, cone beam computed tomography (CB-CT) was performed to assess the intracochlear position of the electrode array.

In cases of ossification of the round window region or of ossification commencing at the basal turn, cochleostomy was performed in order to insert the electrode array. Therefore, the opening of the membrane was extended in an antero-inferior direction with a bur.

In order to supply the severe ossification patients with a split electrode array, superior cochleostomy at the level of the second turn was performed for the full insertion of the apical array (the SPLIT group).

#### *2.5. Impedance Measurement*

The impedance values were measured with the MED-EL telemetry system (MAX interface box, clinical software Maestro), enabling an impedance field telemetry (IFT) on all 12 electrode contacts. The used stimuli were biphasic pulses (24.2 μs) with a nominal amplitude of 300 current units (cu), where 1 cu approximates 1 μA [12]. The measurements were carried out at defined points after surgery as follows. The first measurement was taken during surgery (intraoperatively, after electrode insertion). The second measurement was taken at the end of the first fitting (FF), usually 4–10 weeks after surgery, or sometimes longer in rare exceptional cases due to previous complications. The following measurements were performed 3, 6, and 12 months (±4 weeks) after the FF. Until the FF, the implant was not activated since there was no audio processor worn by the patient, meaning that there was no electrical stimulation. After the first fitting, the implant was activated and the cochlea was electrically stimulated on a daily basis.
