Evolution of Cochlear Implant Electrodes: Straight vs. Pre-Curved

 

Introduction

Early intra-cochlear electrodes were simply straight, short wires. The House single-channel electrode was a somewhat variable length (around 4 mm) of copper wire with a flame-balled tip (1). Preserving hearing was not a priority for the anacusic or profoundly deaf patients implanted in the 1960s and 1970s and short electrodes seemed appropriate to the expectations of single channel implantation.

Extra-cochlear electrodes were also in common use at that time. Douek et al (2) implemented a steel, flame-tipped electrode that was initially placed on the round window membrane in 1976. It was later placed on the promontory after surgical collapse of the tympanic membrane (tympano-cochleopexy) where it was held in place by spring-loading it to a hearing aid mold. Other unilateral extra-cochlear systems were used in a number of centers including Portmann (3) in Bordeaux and by Burian and Hochmaier (4) in Vienna.

Banfai et al (5) in Cologne-Duren used a 16-channel extra-cochlear electrode nicknamed the Hedgehog. Anatomic studies allowed promontory surface projections of the scalae. Bone was thinned in the areas to be stimulated and a plate was wedged against the promontory with 16 metal projections in corresponding locations.

As it became clear that extra-cochlear and single channel intra-cochlear devices provided limited benefit, the push was on to optimize multi-channel devices with intra-cochlear electrodes. Two outstanding electrode engineers, among others, who played a critical role in the evolution of CI electrodes deserve recognition for their work: Janusz Kuzma and (Melbourne, Valencia), Claude Jolly (Vienna).

Multichannel electrodes were first used in the 1960s by House and Simmons (later abandoned). More successful prototypes were developed in the 1970s by Michelson and Schindler (San Francisco), Eddington (Salt Lake City), Chouard (Paris), the Hochmairs (Vienna), and Clark (Melbourne), et al.

The most commonly used commercially available multi-channel electrodes of the 1980’s were straight, bulky, stiff, and traumatic (6). In comparison, early peri-modiolar electrodes of the 1980s were less traumatic and had the putative advantage of being close to ganglion cells, limiting current spread during bipolar stimulation. However, the industry has leaned to monopolar stimulation, largely to increase battery life, thereby increasing current spread and reducing some potential benefits of peri-modiolar electrodes. Straight, flexible, low-trauma electrodes came into common use in the 1990s.

The current emphasis in electrode development is on reducing electrode insertion trauma. Doing so helps preserve residual hearing and improve CI outcomes (with and without electro-acoustic stimulation). The very short, very delicate hybrid electrodes developed by Gantz are the best example of low-trauma electrodes (7).

Over the last decade, advanced imaging techniques have been used to estimate scalar location (S. tympani vs. S. vestibuli) in living subjects. It is generally thought that electrode location in S. vestibuli may be a surrogate for cochlear trauma and appears to correlate with poorer hearing outcomes and reduced hearing preservation (8,9).

What’s New

O’Connell, Hunter, Gifford, Rivas, Haynes, Noble and Wanna8 at Vanderbilt University recently compared outcomes using contemporaneous straight and peri-modiolar electrodes with identical processors from the same manufacturer.

Scalar Place Outcomes

In a retrospective sample of 56 implanted ears (20 straight, 36 peri-modiolar electrodes), imaging evidence of electrode transgression from ST into SV was noted in 10% of straight electrodes and in 53% of perimodiolar electrodes (p=0.002).

Hearing Outcomes

Hearing outcomes at one year were better for straight electrodes. CNC word scores were 55.4% for straight electrodes compared with 36.5% for perimodiolar electrodes (p = 0.005). AzBio sentence scores were 71.2% vs. 46.7% (p = 0.004).

However, even when both types of electrodes were entirely within ST, sentence scores were somewhat higher for straight electrodes. This indicates that trauma or other factors unrelated to scalar transgression have not yet been explained and deserve further investigation.

 

Take Home

Electrode design has continuously evolved since the first CIs were implanted. The data suggests that the straight, thin, flexible electrodes used in this study may provide better hearing preservation and hearing outcomes than the perimodiolar electrodes used. Of course, this is not generalizable to all peri-modiolar and all thin-straight electrodes.

Electrodes range from < 1 cm to 3 cm in length. The shortest may have the greatest chance for hearing preservation but may be less efficient without simultaneous acoustical stimulation. The longest may better access low frequency place, but may be somewhat more traumatic. Look for a variety of length options to become available, which may be most suitable for the amount of residual hearing and other factors. Parallel efforts to retain residual hearing include improved surgical techniques and the use of pharmaceuticals.

 

References

1. House WF, Urban J. Long term results of electrode implantation and electronic stimulation of the cochlea in man.Ann Otol Rhinol Laryngol.1973 82(4):504–517.

2. Douek E, Fourcin AJ, Moore BCJ, Rosen S, et al. Clinical Aspects of Extra-cochlear Electrical Stimulation. Annals NY Acad Sci (2006) 405: 332-336.

3. Portmann M, Cazals Y, Negrevergne M. Extra-cochlear Implants. Otolaryngol Clin N America. (Balkany TJ, ed.) (1986) 19: 307-312.

4. Burian K, Hochmaier-Desoyer I, Eisenwort B. The Vienna Cochlear Implant Program. Otolaryngol Clin N America. (Balkany TJ, ed) (1986) 19: 313-328.

5. Banfai P, Karczaq A, Kublik S, Luers P, Surth W. Extra-cochlear sixteen-channel electrode system. Otolaryngol Clin N America. (Balkany TJ, ed) (1986) 19: 371-408.

6. Kennedy DW. Multichannel intracochlear electrodes: mechanism of insertion trauma. Laryngoscope 1987; 97:42-49.

7. Gantz BJ, Turner C, Gfeller KE, Lowder MW. Preservation of hearing in cochlear implant surgery: advantages of combined electrical and acoustical speech processing. Laryngoscope 2005;115:796–802.

8. O'Connell BP, Hunter JB, Gifford RH, Rivas AR, Haynes DS, Noble JH, Wanna GB. Electrode location and audiologic performance after cochlear implantation: a comparative study between Nucleus CI422 and CI512 electrode arrays. Otol and Neurotol 2016;37:1032-35.

9. Boyer E, Karkas A, Attye A, et al. Scalar localization by cone-beam computed tomography of cochlear implant carriers: a comparative study between straight and periomodiolar precurved electrode arrays. Otol Neurotol 2015;36:422–9.