Current findings indicated that the degree of preoperative low-frequency hearing had no effect on the post-operative hearing findings. Similarly, Cosetti et al. (2013) found that there was no significant relationship between the low-frequency pure-tone average and the post-operative hearing findings.

Current findings are inconsistent with those by Balkany et al. (2006) who found that there was a relationship between the degree of preoperative hearing loss and the hearing findings, in that those patients who had a greater degree of preoperative hearing loss tended to experience a complete loss of residual hearing post-operatively. In the current study, there was no significant correlation between the preoperative hearing threshold levels and the preservation of post-operative residual hearing (p = 0.174).

Despite the fact that there was a correlation between the degree of preoperative hearing threshold levels and post-operative hearing in Balkany et al.’s (2006) study, this was limited to patients who experienced a complete loss of residual hearing post-operatively. In general, there was no significant relationship between the degree of preoperative hearing loss and the unaided post-operative hearing findings.

The mean duration of hearing loss prior to surgery was established to determine the relationship between the duration of the hearing loss before surgery and the post-operative hearing findings.

The study findings indicated that the mean duration of hearing loss prior to surgery was 22 years and 9 months (s.d. = ± 3.1) overall. The effect of duration of hearing loss was not significant for any of the frequencies. Thus, there was no significant difference in terms of post-operative hearing threshold levels between those implantees who had a longer duration of preoperative hearing loss and those implantees who experienced hearing loss for a shorter period of time. This is contrary to what Lenarz et al. (2009) found, that patients with a shorter duration of preoperative hearing loss had better hearing preservation post-operatively than those with a longer duration of loss preoperatively. Current findings are consistent with those of Cosetti et al. (2013) who found that there was no correlation between the duration of hearing loss and the unaided post-operative hearing threshold findings.

The researcher examined the mean time between surgery and first post-operative hearing test to ascertain whether this had had any influence on the post-operative hearing threshold levels. This was used as a covariate in the analysis of the results in order to prevent the findings being affected by any extraneous variables. The mean time between surgery and first post-operative hearing test was 24.7 months (s.d. = ±9.0) (range 0.5–159 months).

Findings on the relationship between the hearing findings and the duration between surgery and unaided post-operative hearing testing indicated that this time was not significant. This means that the duration between surgery and unaided post-operative hearing testing did not affect the preservation of residual hearing post-operatively. This is a notable finding from this study where the upper extreme was 13 years and 4 months. These findings seem contrary to previously published research where a number of patients lost their residual hearing at different times post-surgery, with 30% of the sample experiencing a low-frequency threshold drop of more than 30 dB over time (Gantz et al., 2009).

As shown in Table 2, the category ‘Other’ (17%) included inter alia the electrode type –Nucleus CI24R with Contour Advance electrode (n = 5). In this study, the electrode type that was used most after the two categories mentioned above (namely, the CI512 and the CI24RE) was the CI24R. However, there were only five observations for the CI24R. There were even fewer observations for other electrode types that included CI24M, Nucleus CI24(ST) and Nucleus CI422. Statistical analysis could not be performed on these electrode types as there was not sufficient data because the number of observations was too small.

The effect of electrode type on change in hearing thresholds was not significant for any of the frequencies, with the exception of 2000 Hz: the mean change in hearing thresholds was 5.9 dB higher for other versus Nucleus Freedom (NF). In other words, the electrode type did not have a negative effect on the post-operative hearing threshold levels (or the change in residual hearing), with the exception of that at 2000 Hz. Given that there was no trend with respect to the other frequencies, this could have been a spurious result.

Although the discrepancy of 5.9 dB between the NF and other was only at one frequency, namely 2000 Hz, and the difference was minimal, this was clinically relevant for speech understanding.

The current findings are consistent with those of Cosetti et al. (2013) who found that electrode type did not have any effect on the hearing findings. In a review paper, Brant and Ruckenstein (2016) conclude that although there is some evidence that indicates that shorter electrodes may improve hearing preservation, studies that directly examine the effect of implant length on hearing preservation in similar patient populations are required.

The large majority (see Table 3) of the participants (83% overall) had the Contour Advance electrode array implanted. Other classifications included ‘other’ electrode types (8% overall).

The Contour Advance electrode is the only perimodiolar array in the industry and is pre-curved for complete insertion, with Softip that enables atraumatic insertion (Roland, Shelva, Gibson, & Treaba, 2005).

As only five non-Contour Advance electrodes were implanted, it would not have been possible to compare the results; thus, the researcher found it to be impractical to perform this analysis which looked at the relationship between the hearing findings and the electrode array type. Therefore, the researcher did not perform this analysis as the data obtained would have no relevance to the findings.

In Skarzynski et al.’s (2002) study, it was found that the electrode type did not have any effect on the hearing findings post-operatively. Furthermore, Dhanasingh and Jolly (2017) did not consider electrode type as one of the key factors that influence hearing performance post-implantation in their overview of cochlear implant electrode array designs.

The findings with regard to the percentages for electrode array insertion techniques at the two centres and overall are presented in Table 3. From surgical reports, as this was a retrospective record review, it was evident that the majority of participants had their electrode arrays inserted in the scala tympani during surgery (82% overall). The fact that the current study did not include a review of radiology results for this determination is acknowledged as a limitation and should be taken into consideration when interpreting current findings. The category ‘Other’ included inter alia one case involving electrode array insertion via the scala vestibule, possibly because of obstructed scala tympani (Kiefer, Weber, Pfennigdorff, & Von Ilberg, 2000) and another via the round window. However, given the fact that there was only one patient in each case, a meaningful statistical analysis could not be performed on the results; also no significant effects were anticipated because the electrode is placed via the round window into the scala tympani.

The technique used in this study is supported by Adunka, Pillsbury and Kiefer (2006), as 90% of their patients retained their residual hearing post-operatively. In the current study, there was one case where the electrode was inserted in the scala vestibuli. However, review of the surgical notes of this case did not indicate any surgical complications. This was surprising as in previous studies it has been discovered and reported that insertion via the scala vestibuli can result in intracochlear damage (Adunka et al., 2006), as well as variable speech perception outcomes (Lin, Marrinan, Waltzman, & Roland, 2006).

A total of 91% of patients were implanted in the scala tympani and there was a 92% success rate in the preservation of residual hearing (partial and complete) at both centres combined; therefore, it can be inferred that this technique did not negatively influence the hearing findings. O’Connell, Hunter and Wanna (2016) assert about the superior audiologic outcomes for electrode arrays inserted entirely within the scala tympani.

In the current study (Table 4), the researcher found that the majority of the participants (93%) underwent complete insertion of the electrode array during surgery.

There was only one case of electrode array insertion depth with partial insertion, possibly because of obstructions within the cochlea, such as cochlear otosclerosis, cochlear anomalies and so on (Lee, Nadol, & Eddington, 2011). The remainder of the data consisted of the electrode array depth being complete insertion. As a result of this, the results were not comparable. Thus, the researcher did not perform this analysis as it would not have added any meaningful contribution to the findings.

Nonetheless, current findings where preservation of residual hearing was achieved with full insertion depth for the majority of the cases are supported by Bruce, Bates, Melling, Mawman and Green (2011) who emphasised that the insertion depth was critical to the successful conservation of residual hearing. Numerous other studies reviewed by Miranda, Sampaio, Lopes, Venosa and de Oliveira (2014) revealed that limiting the depth of electrode insertion preserved low-frequency hearing, although this did not include maintenance of ability to discriminate for all patients.

Current findings are inconsistent with what Turner, Gantz, Vidal, Behrens and Henry (2004) have also reported, where they stated that less damage is caused to intracochlear structures by using short electrode arrays rather than the more invasive long electrode arrays. James et al. (2005) studied 12 patients who had received a long electrode and reported that there was preservation of post-operative residual hearing. Contrary to the findings of earlier studies, the post-operative unaided hearing threshold levels in the current study were not affected by the insertion depth.

The findings for the surgical techniques are summarised in Table 4. Two main surgical techniques were used at the two centres: Contour Advance and the advance off-stylet (AOS) techniques. The majority of surgeons (67% overall; 33% at unit A and 94% at unit B) used the AOS technique.

The majority of surgeons utilised a cochleostomy approach, with the exception of four cases where the round window approach was used. Mangus, Rivas Tsai, Haynes and Roland (2012) assert that the round window approach has advantages for the conservation of residual hearing over the cochleostomy approach that may result in intracochlear trauma. However, the positive findings in the current study seem to dispute this because the cochleostomy approach achieved success in preserving residual hearing post-operatively. Briggs et al. (2006) observed in their study that both approaches successfully avoided trauma to the cochlea during surgery, thus conserving post-operative residual hearing.

The examination of the surgical techniques was considered to be relevant in terms of establishing the possible effect or lack of effect on the preservation of residual hearing. By investigating their effects on the hearing findings, the researcher was interested in determining whether certain surgical techniques were more favourable than others in preserving residual hearing. Thus, an analysis was carried out to establish whether a relationship existed between the hearing findings and the surgical techniques employed.

The effect of surgical technique on change in hearing thresholds was not significant for any of the frequencies, except for 2000 Hz, where the mean change in hearing thresholds was 10.1 (10.3) dB lower for Contour on stylet (Contour) versus AOS, and the mean change in hearing thresholds was 7.9 (6.8) dB higher for other versus AOS. As this discrepancy was limited to one frequency (2000 Hz), and as the change in hearing threshold levels was minimal, this was not statistically significant.

From the surgical notes of the surgeons, the researcher investigated surgical complications to determine the possible influence on residual hearing and whether they affected the conservation of functional hearing post-surgery. These are reported in the Gautschi-Mills et al. (2018) study, which included intraoperative complications such as adhesions (3%), drill out of basal turn (3%), Gusher (5%) and trauma (10%), with no complications reported in 68% of the participants.

According to Clark, Tong and Martin (1981) and Balkany et al. (cited in Di Nardo et al., 2007), surgical complications affect anatomical structures following implantation. A relationship between the surgical complications and hearing threshold levels became apparent in the current study where the researcher found a direct correlation between the surgical complications and the hearing findings.

When summarising the relationship between the hearing findings and the various factors listed above, it became evident that the variables investigated had little or no negative effect in general on the preservation of residual hearing, with the exception of surgical complications (Gautschi-Mills et al., 2018). It was found that the aforementioned complications during cochlear implant surgery had a direct influence on the change in residual hearing in that the surgical complications resulted in loss of residual hearing.