Slow Insertion of Cochlear Implant Electrodes

By Thomas J. Balkany, MD

9.13.16

Introduction

Since preservation of residual hearing during cochlear implantation (CI) was first described in 19891, it has become clear that hearing preservation is possible in most cases2,3 and that it can result in better CI outcomes.4,5 Over the last several years, slow electrode insertion speed has been evaluated as a surgical technique to optimize hearing preservation.

What’s New

Timed observations estimate that surgeons insert electrodes over a period of roughly 10 to 30 seconds.6 Slower insertions (30 seconds or more) have been associated with better hearing preservation as well as better vestibular function.7 Further, two mechanistic explanations for the traumatic effects of fast insertion were investigated with plastic models of the cochlea and support the notion that slower may be better:

  • Higher insertion speed increases insertion force,6 which increases electrode insertion trauma.8
  • Higher insertion speed also causes increased intra-cochlear fluid pressure, which itself appears to be traumatic. 9
  • We should also note that stop-and-go insertion and surgeon tremor may cause intermittent increases in intra-cochlear fluid pressure. 10

However, it is relevant to mention that plastic models, while useful in understanding certain mechanics of insertion, cannot account for many of the variables of human electrode insertion:

  • In plastic cochleae, cochleostomy size is fixed. In surgery, cochleostomy size is variable (e.g.: RWM linear incision vs. flap), resulting in differences in capacity for fluid egress/pressure relief.
  • In human cochleae, instantaneous pressure relief may also occur via the internal auditory canal, cochlear aqueduct, vestibular aqueduct, or mobile stapes footplate.
  • Other than fluid egress, plastic cochleae filled with fluid are incompressible systems. Conversely, human cochleae have compressible elements that may mitigate pressure peaks and trauma.

Take Home

Based on current research, electrode insertion should be slow and continuous, taking 30 seconds or more to complete. Using appropriate cochleostomies, insertion angles and electrode trajectories, along with slow-speed insertion, should minimize trauma and improve hearing preservation. Slow insertions have also been associated with less resistance and a higher rate of complete insertions.

References

    1. Boggess WJ, Baker JE, Balkany TJ. Loss of residual hearing after cochlear implantation. Laryngoscope. 1989;99:1002-5.

    2. Hodges AV, Schloffman J, Balkany T. Conservation of residual hearing with cochlear implantation. Am J Otol. 1997 Mar;18(2):179-83.

    3. Balkany TJ, Connell SS, Hodges AV, Payne SL, Telischi FF, Eshraghi AA, Angeli SI, Germani R, Messiah S, Arheart KL. Conservation of residual acoustic hearing after cochlear implantation. Otol Neurotol. 2006 Dec;27(8):1083-8.

    4. Gifford, R., H., Dorman, M. F., Skarzynski, H., Lorens, A., Polak, M., et al. (2007). Cochlear implantation with hearing preservation yields significant benefit for speech recognition in complex listening environments. Ear and Hearing, 34(4), 413-425.

    5. Sheffield SW, Jahn K, Gifford RH. Preserved acoustic hearing in cochlear implantation improves speech perception. J Am Acad Audiol. 2015 Feb;26(2):145-54

    6. Kontorinis G, Lenarz T, Stover T, Paasche G. Impact of insertion speed of cochlear implant electrodes on the insertion forces. Otol Neurotol. 2011 32:565-570.

    7. Rajan GP1, Kontorinis G, Kuthubutheen J. The effects of insertion speed on inner ear function during cochlear implantation: a comparison study. Audiol Neurootol. 2013;18(1):17-22.

    8. Ishii T, Takayama M, Takahashi Y. Mechanical properties of human round window, basilar and Reissner's membranes. Acta Otolaryngol Suppl 1995;519:78-82.

    9. Todt I, Mittmann P, Ernst A. Intracochlear fluid pressure changes related to the insertional speed of a CI electrode. Biomed Res Int. (Online only: http://dx.doi.org/10.1155/2014/507241.

    10. Todt I, Ernst A, Mittmann P. Effects of Different Insertion Techniques of a Cochlear Implant Electrode on the Intracochlear Pressure. Audiol and Neurotol 2016;21:30-37.

Leave a Comment

Dan Schwartz 1 year ago

Dr Balkany, thank you for pounding home the necessity of residual hearing preservation during electrode insertion, which I have amplified in The Hearing Blog. Question for a future blog entry: Where are we with robotic electrode insertion? As I understand it, this is still in the R&D stage, but without quite enough multidisciplinary engineering "heft" to make it a reality just yet. The reason I ask is that we are now at a half-million people with CI's, and industry forecasts are for one million by 2020 — That's over 100,000 per year divided among the five CI manufacturers — but the number of surgeons who have the "touch" in their fingertips who can "feel" any obstructions without making hash of the delicate structures while threading the inch-long electrode into the pea-sized hearing organ is very limited, hence the need to automate the insertion process to handle the volume. Let's put these numbers in perspective: If we assume about half the CI volume is here in the US, then that's about 50,000 procedures per year, or a thousand per week… And to be blunt, based on feedback from a longtime electrode engineer with two CI mfr's, not that many have the "touch" in their fingertips. What's more, MEMS–based tactile sensors can easily detect vibration at least an order of magnitude better than even the finest surgeon's fingertips can, so that, combined with very precise multiaxis servo motion control, the technology building blocks are already available "off the shelf," so all it takes is an intensive effort to make this a reality. Dan Schwartz, Electrical Engineer. Editor, The Hearing Blog.