perception in quiet showed no significant correlation, it is hypothesized on the basis of available research with vocoder simulations that the patients with a small frequency mismatch may show a faster initial increase in phoneme scores than the patients with a larger frequency mismatch and/or better speech perception in noise (Baskent & Shannon, 2007; Li & Fu, 2010). However, the current dataset does not provide the information required to perform such an analysis. Further studies are currently conducted on this topic.
This study applied two different methods (spiral fitting and multiple linear regression) to develop insertion models, which could assist the surgeon during surgery in reaching the preferred position of the electrode array within the cochlea. In a previous study principal components (PCA) and spiral fitting coefficients of the outer cochlear wall were considered as input parameters in regression models (Van der Marel et al. 2014). However, those variables required a more time-consuming analysis, and did not lead to substantially higher correlation coefficients compared to the direct cochlear size measures. Therefore they were discarded as input parameters in the present study.
To summarize, the present study has led to the formulation of insertion models of varying complexity (Eqs. 11-13) which are expected to predict over 75% to 78% of the variation in insertion depth by providing an estimated surgical insertion distance (L’E16) in future data. Further prospective research is needed to analyze the validity of the model and to evaluate whether this guidance tool will lead to better performance of implantees.
At the risk of over-simplifying, the following general guidelines for everyday clinical practice can be formulated on the basis of this study, which will help surgeons to more consistently reach the preferred position of HiFocus 1 and 1J electrode arrays in future surgeries:
• The HiFocus 1 or 1J array of Advanced Bionics should be positioned at a surgical insertion distance of 5.5 to 8 mm to achieve minimal frequency mismatch. A deeper insertion is likely to result in a large frequency mismatch, which might influence performance outcomes adversely.
• If a specific insertion depth is aimed at, the optimal surgical insertion distance differs approximately 2.5 mm between a ‘small’ and a ‘large’ cochlea (e.g., between 5.5 mm and 8 mm for 480˚) (Figure 3).
• Therefore, it is beneficial to determine cochlear size with preoperative imaging (based on diameter measurements), as it can tell the surgeon whether he/she should try to achieve a surgical insertion distance of around 6, 7 or 8 mm for a ‘small’, ‘medium’ or ‘large’ cochlea, respectively, in order to reach an optimal tonotopic position (Figure 2).
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