Radiographic Measurement of Cochlear Duct Length in an Indian Cadaveric Population - Importance of Custom Fit Cochlear Implant Electrodes

THIEME 492 Original Research Radiographic Measurement of Cochlear Duct Length in an Indian Cadaveric Population - Importance of Custom Fit Cochlear Implant Electrodes Anup Singh1 Rajeev Kumar1 David Victor Kumar Irugu1 Smita Manchanda2 1 Department of Otolaryngology and Head and Neck Surgery, All India Institute of Medical Sciences, New Delhi, India 2 Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi, India Ashu Seith Bhalla2 Prem Sagar1 Address for correspondence Rajeev Kumar, MS-ENT, Department of Otolaryngology and Head and Neck Surgery, All India Institute of Medical Sciences, Room No.-4057, 4th Floor, Teaching Block, New Delhi-110029, India (e-mail: ). Int Arch Otorhinolaryngol 2020;24(4):e492–e495. Abstract Keywords ► cochlear implantation ► cochlear duct ► tomography ► adult ► cadaver Introduction Successful cochlear implantation requires an appropriate insertion depth of the electrode, which depends on cochlear duct length CDL). The CDL can vary due to ethnic factors. Objective The objective of the current study was to determine the CDL in an Indian adult cadaveric population. Methods The present was a cadaveric study using the temporal bones obtained after permission of the Institutional Review Board. The temporal bones were subjected to high-resolution computed tomography (HRCT), and the double oblique reformatted CT images were reconstructed through the basal turn of the cochlea. The reformatted images were then viewed in the minimum-intensity projection (minIP) mode, and the ‘A’ value (the diameter of the basal turn of the cochlea) was calculated. The CDL was then measured using the formula CDL ¼ 4.16A - 4 (Alexiades et al). The data analysis was performed using the Microsoft Excel software, version 2016. Results A total of 51 temporal bones were included for imaging analysis. The CDL varied from 27.6 mm to 33.4 mm, with a mean length of 30.7 mm. There was no statistically significant difference between the two sides. Conclusion The CDL can be calculated with preoperative high-resolution CT, and can provide a roadmap for effective cochlear implant electrode insertion. The populationbased anatomical variability needs to be taken into account to offer the most efficient and least traumatic insertion of the electrode. Introduction Cochlear ductal length (CDL) refers to the length of cochlea measured as the spiral distance from round window to the helicotrema. Measurements of the CDL have been made as early as 1865.1 The initial studies to report on this parameter were histological studies.1–3 With time and the evolution of technology, various techniques, including thin reformatted received October 6, 2018 accepted November 20, 2019 DOI https://doi.org/ 10.1055/s-0040-1701272. ISSN 1809-9777. high-resolution computed tomography (HRCT) scan cuts with 3-D reconstruction of the temporal bone are now being used to estimate the CDL. The values of these investigations have been compared with the histological findings of previous studies, and a good correlation between them has been obtained. The CDL estimation holds importance during the insertion of electrodes in cochlear implant surgeries. Especially Copyright © 2020 by Thieme Revinter Publicações Ltda, Rio de Janeiro, Brazil Radiographic Measurement of Cochlear Duct Length in an Indian Cadaveric Population Singh et al. with the advent of residual hearing preservation techniques, the precise insertion of electrodes to a certain depth to avoid damaging residual hearing cannot be overemphasized. The tactile force feedback during implant insertion cannot be solely relied upon to avoid insertional trauma, and a preoperative determination of the length of the cochlear duct will help in the insertion of an accurate and desirable electrode length. The CDL has been shown to harbor gender and ethnic variability. In the current study, we estimated the CDL in Indian cadaveric temporal bones using HRCT. Materials and Methods The present was a radiological study of human cadaveric temporal bones conducted at a tertiary care center in Northern India. Cadaveric temporal bones were obtained from unidentified/unclaimed cadavers from the Department of Forensic Medicine and Toxicology. The approval for the study was obtained from the institutional ethics committee. The fresh cadaveric temporal bones were stored in 10% formalin and were subjected to imaging in the form of HRCT. Fig. 1 Measurement of the variable ‘A’ in double oblique reformatted computed tomography (CT) images. The variable ‘A’ was measured from the center of the round window to the farthest point on the opposite wall of the cochlea passing through the modiolus in the minimum-intensity projection (minIP) mode of the reformatted image. Imaging Protocol Image Acquisition and Reconstruction The CT scans were acquired in the HRCT temporal-bone protocol on a 40 slice CT Scanner (Sensation 40, Siemens, Erlangen, Germany). From this dataset, thin slice images (0.6 mm) were reconstructed in the axial plane in high resolution bone algorithm. Image Transfer and Analysis These images were then transferred to the advanced viewing workstation (Syngovia, Siemens, Erlangen, Germany) for advanced analysis and to the Picture Archiving and Communication Systems (PACS) (Syngoplaza, Siemens, Erlangen, Germany) for permanent storage. Interpretation Each dataset was then opened in the multiplanar reconstruction mode. Double oblique reformatted CT images were reconstructed through the basal turn of cochlea. The reformatted image was then viewed in the minimumintensity projection mode (minIP), with thickness varying from 1.3 mm to 1.5 mm to visualize the cochlear turns. The variable A was measured as the linear measurement from the center point of round window to the farthest point on the opposite wall of the cochlea passing through modiolus (►Fig. 1). All of these values were saved as screenshots and then archived again in the PACS system. The radiologically obtained ‘A’ value was used to calculate the cochlear length using the equation proposed by Alexiades et al4 (CDL ¼ 4.16A - 4). The data was entered and analyzed using the Microsoft Excel (Microsoft Corp. Redmond, WA, US) software, version 2016. Results A total of 51 adult cadaveric temporal bones with normal cochleovestibular morphology (on HRCT temporal bone scans) were included in the study. Information on gender could not be obtained. In total, there were 26 right-sided and 25 left-sided bones. The CDL varied from 27.6 mm to 33.4 mm, with a mean of 30.7 mm and standard deviation (SD) of 1.66. The mean cochlear length on the right side was of 30.5 mm (SD: 1.59), and, on the left side, it was of 30.8 mm (SD: 1.74). There was no statistically significant difference between the CDL on the two sides (unpaired t-test; p ¼ 0.52). ►Fig. 2 shows the graphical distribution of the CDLs on right and left sides. Discussion With the introduction of hybrid technology or electro-acoustic stim (...truncated)