Modiolar Proximity of Slim Modiolar Electrodes and Cochlear Duct Length: Correlation for Potential Basis of Customized Cochlear Implantation With Perimodiolar Electrodes

Abstract

Objectives: Recent studies have shown that cochlear duct length (CDL) varies among individuals and could significantly influence the final position of the electrode and its trajectory in the cochlea. Given this, we hypothesized that the degree of modiolar proximity of novel slim modiolar electrodes, such as CI532 and CI632, can also be affected by CDL. To test this hypothesis, we retrospectively evaluated individual CDL to determine if there is any significant correlation of CDL with degree of modiolar proximity. Methods: Fifty-one ears from 38 subjects implanted with slim modiolar electrodes by a single surgeon through the round window approach using the pull-back technique were included. Our cohort was classified according to the deafness onset (congenital versus postlingual) and the degree of modiolar proximity (less versus tight) with reference to the spiral diameter made by the slim modiolar electrodes in situ on transorbital x ray. We then analyzed the CDL and its metrics using a readily available surgical preplanning tool (OTOPLAN) to obtain comparable data. Results: Among 30 ears associated with congenital deafness, 9 ears (30%) showed less modiolar proximity, while none of the 21 ears from 19 subjects with postlingual deafness exhibited “less modiolar proximity” based on our criteria. In this study, CDL showed significant variation among subjects. Importantly, a significant inverse correlation between spiral diameter and CDL (ρ = −0.581, p < 0.001) was found, showing that shorter CDLs have longer spiral diameter and less modiolar proximity. Moreover, further pull-back technique characterized by pulling out the electrode a little bit more in cases with shorter CDL, if not always, exhibited tighter modiolar proximity. Conclusion: A preponderance of less modiolar proximity of the electrode was observed exclusively among congenital deafness cases, demonstrated by a less tight spiral configuration even under the pull-back technique. Our data suggest that shorter CDL is associated with a less tight spiral configuration of slim modiolar electrodes postoperatively. Depending on the insertion technique, the differential degree of modiolar proximity of slim modiolar electrodes can be alleviated in cases with short CDL, which justifies cochlear duct length-based customized insertion of slim modiolar electrodes.

Effects of in vivo repositioning of slim modiolar electrodes on electrical thresholds and speech perception

“…Although MR imaing indicates cochlear nerve agenesis, such cases may have extremely low, but still exist residual spiral ganglion neurons (SGNs) near the modiolus 42 . Based on the evidence that the electrode-modiolus distance is closely correlated with the ECAP thresholds of the electrodes including slim modiolar electrodes 43 , a better modiolar proximity with the pull-back maneuver would potentially lead to more favorable outcomes in those with cochlear nerve deficiency than those requiring conventional insertion. In other words, slim modiolar electrodes with pull-back maneuvers could be better in cases with cochlear nerve deficiency, if not completely defective modiolus; however, future studies are required to fully elucidate the effects of modiolar proximity on outcomes among cochlear implant recipients with cochlear nerve deficiency.…”

“…As described in our previous study 43 , a readily available surgical pre-planning tool (OTOPLAN Version 2.0 by MED-EL Innsbruck, Austria, www.otoplan.ch ) used the Elliptic-Circular Approximation (ECA) method 44 was employed to determine individual cochlear duct length from all our subjects.…”

Potential Implications of Slim Modiolar Electrodes for Severely Malformed Cochleae: A Comparison With the Straight Array With Circumferential Electrodes

“…conducted all procedures; a straight array with a full-banded electrode (CI24RE[ST]; Cochlear, Australia) and slim modiolar electrodes (CI632; Cochlear, Australia) were alternately inserted and tested. ECAP thresholds were measured in every channel at a 250 Hz stimulation rate and 35 sweeps; those that did not elicit any response even at the maximum stimulus were noted as 255 current levels (11).…”

“…If such cases show that the nerve and modiolus are extremely small, but still exist, at least some portion of residual SGNs is likely to be present near the modiolar side. In this case, slim modiolar electrodes with the best modiolar proximity would potentially lead to favorable outcomes (11). Theoretically, modiolar hugging electrodes such as slim modiolar electrodes might be eligible and further outperform the lateral straight electrodes or full-banded electrodes in the inner ear malformation cases with a smaller number of SGNs being still present in the modiolus.…”

Variations in Microanatomy of the Human Modiolus: Implications for Cochlear Implants

“…Beyond this point (apical to it) it will be too large and thus come to lie further abmodiolarly, at an intermediate position between the modiolar and the lateral wall (comp. 52 ).…”