Slow Insertion of Cochlear Implant Electrodes
Since preservation of residual hearing during cochlear implantation (CI) was first described in 1989(1), it has become clear that hearing preservation is possible in most cases (2,3) 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.
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.
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.
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.