Aspectos Legales

to stabilize and that its final position can differ from the position as inserted by the surgeon. This hypothesis might be tested by future prospective research studying electrode stability using multiple postoperative imaging moments.

In addition to the first objective, postoperative imaging was assessed using the extensive database to study various electrode position related variables and their possible relation with speech perception scores. This study, revealing the answers to the third objective, confirmed the influence of several patient-specific variables in both prelingual and postlingual patients and controlled for these influences when studying the relation between electrode position and performance. However, the study found no correlations between electrode-position related variables and performance. Even when the analysis was performed without controlling for patient specific variables, none of the electrode-position related variables showed any correlation to performance. This might be explained by the relative similar electrode position within the studied population. Possibly, only extreme electrode position may be found to correlate with performance outcome.

In line with this, a study performed among patients with device failure showed that the electrode position can be restored very accurately during a reimplantation. Prior to reimplantation the postoperative CT scan was extensively studied to acquire a similar position with the new implant. After reimplantation, new imaging was obtained which confirmed almost similar or small displacements in every case. More importantly, the performance was restored within weeks to at least the level obtained with the original implant. This study covered the other part to the first objective of examining the value of CT during the postoperative period.

These outcomes encouraged us to study the feasibility of developing a surgical guidance tool predicting final electrode position for an individual patient based on preoperative available variables. Such a model could allow surgeons greater control over the ultimate position. Since cochlear size was found to have a significant influence on modiolus proximity as well as insertion depth, several variables describing this size were evaluated as predictors in the insertion model. This study established an extension to the second objective next to the study on cochlear morphology. A surgical guidance tool predicting surgical insertion distance necessary to reach a predefined insertion depth was developed. For this model a combination of 4 cochlear diameters and the preferred insertion depth was able to predict up to 78.1% of the variation in surgical insertion distance.

The research described in this thesis was performed among patients who received a HiRes90K implant with HiFocus electrode using one surgical approach, the extended round window insertion. As described by previous studies, design of the implant and surgical approach are important factors influencing electrode position. Performing the studies under these controlled circumstances allowed clear interpretations of the outcomes. Though, it also implies that conclusions about the outcomes can only be applied to the specific electrode design inserted under the same circumstances. It will be left to future research to analyze these topics with other designs and surgical techniques.

While performing the study on the relation between electrode position and performance, we also found some leads on the most preferable position with regard to minimal frequency mismatch. It is important to note that the goal of this research was not to determine the optimal electrode position. However, for the studied population the analysis of the relation between surgical insertion distance and insertion depth respectively, to frequency mismatch illustrated a range of positions, either measured as distance from round window or insertion depth, wherein minimal frequency mismatch may occur. Positioning of the electrode within this range might thereby provide optimal chances of obtaining good performance with the implant. Future studies focusing on the relation between frequency mismatch and performance, instead of a direct relation between electrode position and performance might reveal new insights.

This research found no relation between any of the electrode position related variables and performance. This outcome may be explained by the fact that for many of the studied patients the electrodes where positioned within this observed range of insertion depth and surgical insertion distance where frequency mismatch is minimal.

Nonetheless, the calculated frequency mismatch might in fact be different considering the findings of the study on electrode stability. This study raised doubts about the actual electrode position as migrations had often occurred when comparing the CT scan obtained 1 day after surgery with a later performed CT scan. The insight that a migration could occur after postoperative imaging was obtained, made us realize that there was a significant chance that the actual electrode position differed from the position detected with imaging. This weakens all previous studies which used direct postoperative imaging to define electrode position and investigate any relations between position and performance.

Indeed, this important finding encouraged us to evaluate our own imaging protocol. Given the chance of migration within the first weeks and the fact that implant is activated around

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