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.

Analysis of Neural Interface When Using Modiolar Electrode Stimulation. Radiological Evaluation, Trans-Impedance Matrix Analysis and Effect on Listening Effort in Cochlear Implantation

“…From this image analysis, valuable information of the electrode position inside the cochlea can be obtained. We considered three measurements: Wrapping Factor, the Intracochlear Position Index, and the Homogeneity Factor, as previously described [25].…”

Simpler and effective radiological evaluations for modiolar proximity of a slim modiolar cochlear implant electrode

“…By definition, ICPI values indicate the distance of each contact relative to the modiolus. This measurement is normalized by the electrode, being zero “0” the closest position to the modiolus and one “1” the closest position to the lateral wall 25 . Amongst four ICPI values, the ICPI at point1, which presented the lowest intracochlear position index (ICPI) value, displayed the lowest ECAP threshold (Fig.…”

“…With reference to a recent study by Miguel et al 25 , ICPI(i) refers to the ratio of the Euclidean distance between the modiolus (M) and the electrode (Ei) relative to the distance between the modiolus and the lateral wall (LWi). Specifically, the modiolus (M) value was determined as a crossing point in relation to the two lines associated with the modiolus and the round window.…”

“…The wrapping factor has been suggested to be inversely correlated with the degree of modiolar proximity. However, the wrapping factor has been reported to yield an inconsistency to determine the relative relationship of the electrode array against the lateral wall between the cone-beam CT and the histologic finding, requiring a more accurate and refined radiological value for modiolar proximity 25 . Recently, intracochlear positioning of the electrode array in close proximity to the modiolus—with good modiolar hugging, as evidenced by the wrapping factor normalized or intracochlear positioning index (ICPI) 25 —would lead to better hearing outcomes for cochlear implantees with the slim modiolar electrodes.…”

“…Although a previous ex vivo study showed that the average electrode-to-modiolus distance value measured by two imaging modalities (conventional CT vs. cone-beam CT) was not different 28 , measurement of electrode-to-modiolus distance using conventional CT is not feasible due to metal artifacts blurring the region of interest. A recent study proposed the most reliable and finely tuned indicator reflecting the degree of modiolar proximity, called ICPI 25 , which was obtained exclusively through ‘metal artifact-less’ cone-beam CT. However, lack of such cone-beam CT in many institutions preclude widespread use of this index.…”