Influence of basal-most electrode positioning on the accuracy of angular insertion depth predictions in cochlear implantation

Objective Accurate prediction of the angular insertion depth (AID) is essential for optimising cochlear implant (CI) outcomes. However, discrepancies between preoperative predictions and postoperative results persist. In this study, we evaluated systematically the accuracy of equation-based AID prediction to identify the main contributors to prediction errors. Methods In this retrospective study, preoperative AID predictions for various MED-EL electrode arrays were generated by using an equation-based method, assuming full insertion with the electrode stopper at the round window. These predictions were compared with postoperatively measured AIDs obtained by digital volume tomography. The primary variable investigated was the linear insertion depth of the electrode closest to the basal position (LID E12 ), defined as its distance from the round window. Results The LID E12 ranged from − 13.8 mm to 11.0 mm; negative values indicate incomplete insertions, which were observed in 9% of cases. Between predicted and measured AID a median difference of − 11° (interquartile range 74°) was observed. After correcting the prediction by incorporating the measured LID E12 instead of assuming insertion to the stopper, the median difference shifted to + 35° with a markedly reduced interquartile range (37°). Conclusion While current AID prediction models generally show clinically acceptable accuracy, considerable discrepancies can still occur. One major contributing factor is the actual surgical placement of the electrode array. This variability, captured by the LID E12 measurement, demonstrates that the real-life result of the electrode placement often differs from the assumed placing, and that this factor should be considered when planning electrode positioning.