*3.4. Impedances*

The course of impedances over time is depicted in Figure 2. Here, the median values of the overall impedances of all electrode contacts of the IES and SPLIT groups were plotted and compared to those of the REGULAR group (Figure 2). The median values were almost equal between the three groups immediately after the insertion at the intraoperative measurement. Thereafter, an increase in the impedance values up to the first fitting (FF; 4–6 weeks after implantation) was observed in all groups, but at slightly different levels. In the IES group, impedances continued to rise with a slight linear increase up to 12 months after the FF, while in the REGULAR group, a slight decrease was observed and finally constant impedance values were measured at a much lower level than the IES group. The values of the SPLIT group stayed relatively stable on all contacts up to 3 months. Thereafter, a strong increase in impedance values up to 12 months was observed that went far beyond the measured values of the IES group. Statistically significant differences could be observed at the FF between the IES and REGULAR groups (*p* < 0.05) and at M6 between the IES/CONTROL group (*p* < 0.01) and the IES-/SPLIT group (*p* < 0.05). At M12, all three groups showed a significant difference to each other (IES/REGULAR, *p* < 0.001; IES/SPLIT, *p* < 0.05; SPLIT/REGULAR, *p* < 0.001).

**Figure 2.** Change in impedances over time across the IES, SPLIT, and REGULAR groups. Median overall electrode contacts C1–C12. Since the number of patients varied over time, the existing patients were additionally marked as individual dots at each point in time. The red dots represent the IES patients, the blue dots represent the patients from the SPLIT group, and the black dots represent the patients from the REGULAR group. Asterisks mark the significant differences between groups.

## 3.4.1. Impedances at Different Indications of IES Use in the IES Group

In order to examine the diverse IES group in more detail, a retrospective subdivision into four subgroups, based on the indications for which the IES CMD was prescribed in the implanted ears and thus on the aetiology of the patients' deafness, was performed. Figure 3 shows the impedance data over time of the subgroup, where the IES CMD was applied due to an existing ossification in the inner ear (magenta dots, the O group, Figure 3), as well as the subgroup with IES CMD application due to fibrosis in the inner ear (red dots, the F group, Figure 3). The third subgroup indicates that IES CMD use after a resistance occurred during the actual implantation of the stimulation electrode (purple dots, the R group, Figure 3). In the fourth subgroup, the IES was used to determine the depth of the cochlea (light blue dots, the DM group, Figure 3). To facilitate a constant comparison, the REGULAR group was also depicted here with impedance data over time (black dots, Figure 3).

**Figure 3.** Change in impedances over time in subjects with different indications of IES use. Median overall electrode contacts C1–C12. The magenta dots represent patients with cochlear ossification (O), the red dots represent patients with fibrosis (F), the purple dots represent patients with occurring resistance (R), and the light blue dots represent patients for which the IES was used for cochlear depth measurement (DM). The black dots represent patients from the REGULAR group. Asterisks mark significant differences between groups.

Figure 3 shows the median impedance values over time over all electrode contacts. From the FF onwards, there was a slight drop in the impedance values and finally a constant lower level in the REGULAR group, while all other groups continued to show slight impedance increases. The O group showed the highest values over the whole time, followed by the F group. The R and DM groups, on the contrary, were at almost the same median level, but showed slightly higher values than the REGULAR group. From month 3 (M3) onwards, there was a statistically significant difference between the O group and the REGULAR group (*p* < 0.05). After 12 months, the F group also showed significantly higher impedance values than the REGULAR group (*p* < 0.01). All other groups showed no significant differences at any time.

#### 3.4.2. Impedances Re-Implantation vs. First Implantation

In order to evaluate whether the re-implantation procedure and the usage of the IES CMD has an influence on impedance development, we distinguished between the cases of first implantation and the cases of re-implantation in both the IES group and the REGULAR group, as follows:

1. Re-implantation in the REGULAR group (RR): consists of 10 patients who received a re-implantation without using the IES;


The course of median values over all electrode contacts over time is presented in Figure 4. Up to the FF, there was a similar increase in impedance values in all groups. In the further course, the median values of the RR, RI, and FI groups increased, while a decrease in the impedances could be observed in the FR group. Statistically significant differences between the FR and FI groups (*p* < 0.05), FR and RI groups (*p* < 0.01), and FR and RR groups (*p* < 0.05) could also be observed, which remained until month 6 (M6). After 12 months, there was no longer any significant difference between the FR and RR groups.

**Figure 4.** Change in impedances over time re-implantation vs. first implantation in the IES and REGULAR groups. Median over all electrode contacts C1–C12. The filled red dots represent the patients from the IES group who received first implantation (FI) and the hollow red dots represent the patients from the IES group who underwent re-implantation (RI). The filled black dots represent the patients with first implantation from the REGULAR group (FR) and the hollow black dots represent the patients who underwent re-implantation from the REGULAR group (RR). Asterisks mark significant differences between groups.
