Prediction of the Cochlear Implant Electrode Insertion Depth: Clinical Applicability of two Analytical Cochlear Models

Abstract

Although the spiral anatomy of the human cochlea seems evident, measuring the highly inter-variable true dimensions is still challenging. Today, only a few three-dimensional reconstruction models of the inner ear are available. Previously, spiral equations were applied to two-dimensional computed tomography (CT) images to predict the electrode insertion depth prior to cochlear implantation. The study aimed primarily to compare the clinical applicability of two analytical cochlear models using a recently introduced planning software to predict the insertion depth of the electrode array of 46 cochlear implant recipients. One was based upon the Escudé formula, which relies only on the basal turn diameter, and another based upon the Elliptic-Circular Approximation (ECA), using the diameter and width. Each case was measured twice by two ENT surgeons. Secondly, in order to measure the benefit of the new planning software over the use of the existing clinical routine method, the results were compared to the prediction based upon a two-dimensional CT image. The intra -and inter-observer agreement using the planning software was significantly better when the ECA was applied, compared to the Escudé formula (p < 0.01). As a reference, the predicted insertion depth was compared to the actual insertion depth measured on post-operative images. The mean absolute error was |2.36| (|1.11|) mm in case of the Escudé approach and |1.19| (|0.92|) mm in case of the ECA. The use of a new planning software that allows three-dimensional handling, integrating the diameter and width of the basal turn (ECA formula), resulted in the most accurate predictions of the electrode insertion depths.

Cochlear Duct Length Measurements in Computed Tomography and Magnetic Resonance Imaging Using Newly Developed Techniques

“…Several studies have investigated the ECA approach, demonstrating a high precision of CDL computation 23 and low intra-and interobserver variability. 24 Only 1 study has described a CDL value (32.91 mm) with CT, 25 but in this investigation the CDL was corrected to the organ of Corti, which results in about 10% to 15% reduction. 21 Thus, the mean CDL of 37.24 mm measured in CT data sets via ECA in the present study is similar and can be compared with MRI.…”

“…The measurements were done only by 1 experienced observer. However, it has been shown that the ECA formula has low intra- and interobserver variability, 24 so this was not further investigated. When different imaging modalities are compared and not the quality and reliability of a formula or an approach checked, measurements by 1 observer might be better to not have additional variables, such as different settings of contrast and brightness.…”

Implementation of secondary reconstructions of flat-panel volume computed tomography (fpVCT) and otological planning software for anatomically based cochlear implantation

“…One potential limitation of this study is the sample size concerning pre-and postoperative measurements. In addition, the study has the limitation to observe only intraobserver variability, but low interobserver variability in the use of this otologic planning software has been shown before [27][28][29]. We also cannot rule out a possible influence of the investigator's training curve.…”

“…The clinical applicability of the software, in particular an angular insertion depth prediction for preoperative electrode selection has been demonstrated [27]. In various recently published studies, the CDL was also measured with this software, using MSCT and MRI [28][29][30]. They revealed low inter-as well as intraobserver variability and reliability [28,29].…”

“…In various recently published studies, the CDL was also measured with this software, using MSCT and MRI [28][29][30]. They revealed low inter-as well as intraobserver variability and reliability [28,29]. So far, evaluation of pre and postoperative clinical cochlear data created with fpVCT using the otological software has not been performed.…”