Evaluation of Intracochlear Position of a Slim Modiolar Electrode Array, by Using Different Radiological Analyses

Abstract

INTRODUCTION: The radiological analysis following a cochlear implantation offers insight into the audiological outcomes of cochlear implant recipients. The wrapping factor (WF) is the most common radiological analysis measuring the modiolar position and depth of insertion of an electrode array. New measurements like the intracochlear position index (ICPI) or the homogeneity factor (HF) can offer more accurate information regarding the electrode's intracochlear position. We have also studied a new method to calculate the WF, by normalizing it with a new methodology (WFn). OBJECTIVES: To analyze and compare the results of the WF, ICPI, HF, and WFn obtained using a cone beam computer tomography (CBCT) with the histological analysis on temporal bone. MATERIAL: A perimodiolar electrode array (Nucleus Slim CI532) was inserted in three temporal bones. A perfect insertion was performed in the first temporal bone, according to the correct specifications. In the second specimen, a slightly over-inserted electrode was analyzed and in the third specimen a completely over-inserted electrode array was studied. METHOD: A CBCT was performed following the implantations and then, a histological analysis with slices perpendicular to the cochlea axis (modiolus). Each measurement was made 10 times by 10 experts (radiologist and otologist) with a total amount of 600 measurements (100 for each data, 3 CBCT and 3 histology). A t test statistical analysis was performed to compare the measurements between CBCT and histology. RESULTS: It was observed that the ICPI and the HF correctly identify the three different insertions. Regarding the WF no significant difference in the two over-inserted specimens was found. The ICPI was the only measurement that shows no statistical difference between the CBCT and the histology, so it was considered the most accurate method. Finally, the WF shows a statistical difference between the CBCT and the histology in all cases, indicating the poor value of the radiological method. The WFn analysis includes the modiolar wall length in the measurement. This improves the final result as it reduces the error induced by the size of the cochlea. CONCLUSION: The ICPI and the HF provide better radiological information than the WF, regarding the intracochlear position of the electrode array. The most relevant difference is that the ICPI, HF, and WFn include modiolar and lateral wall dimensions, thereby using the diameter of the cochlear duct for the analysis.