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. Author manuscript; available in PMC: 2018 Oct 1.
Published in final edited form as: Otol Neurotol. 2017 Oct;38(9):e369–e377. doi: 10.1097/MAO.0000000000001545

Selective stimulation of facial muscles following chronic intraneural electrode array implantation and facial nerve injury in the feline model

Ronald Sahyouni 1, Yarah M Haidar 1, Omid Moshtaghi 1, Beverly Y Wang 1, Hamid R Djalilian 1, John C Middlebrooks 1, Harrison W Lin 1
PMCID: PMC5597489  NIHMSID: NIHMS893745  PMID: 28834941

Abstract

Background

Our group has previously shown that activation of specific facial nerve (FN) fiber populations and selective contraction of facial musculature can be achieved through acute intraneural multi-channel microelectrode array (MEA) implantation in the feline model.

Hypothesis

Selective stimulation of facial muscles will be maintained in the setting of (1) chronic and (2) acute MEA implantation following FN injury and subsequent recovery.

Methods

This study included seven cats. In three cats with normal facial function, 4-channel penetrating MEAs were implanted chronically and tested biweekly for six-months. Electrical current pulses were delivered to each channel individually, and elicited electromyographic (EMG) voltage outputs were recorded for each of several facial muscles. For FN injury experiments, two cats received a standardized hemostat-crush injury, and two cats received a transection-reapproximation injury to the FN main trunk. These four underwent acute implantation of MEA and EMG recording in terminal experiments four months post-injury.

Results

Stimulation through individual channels selectively activated restricted nerve populations, resulting in contraction of individual muscles in cats with chronic MEA implantation and following nerve injury. Increasing stimulation current levels resulted in increasing EMG voltage responses in all cases. Nerve histology showed only minor neural tissue reaction to the implant.

Conclusion

We have established in the animal model the ability of a chronically implanted MEA to selectively stimulate restricted FN fiber populations and elicit contractions in specific facial muscles. Likewise, following FN injury, selective stimulation of restricted FN fiber populations and subsequent contraction of discrete facial muscles can be achieved following acute MEA implantation.

Keywords: facial nerve, implant, stimulation, multi-channel electrode array

Introduction

The facial nerve (FN) is a critical motor nerve that animates the face, closes the eyelid, facilitates communication and enunciation, and maintains oral competency, among other functions. Irreversible FN damage can lead to permanent facial paralysis, a debilitating disorder with substantial functional and psychological sequelae. FN damage can lead to corneal irritation, visual impairment, drooling, intraoral food retention, dryness in the ipsilateral eye and nose, and demoralizing cosmetic deformities. Complete facial paralysis results in atonic facial musculature that can atrophy and irreversibly convert to scar tissue over time.1 Facial paralysis is not uncommon, with an annual incidence of 11–40 cases per 100,000.2,3 Over 127,000 new cases of facial paralysis are diagnosed annually in the United States alone.4 Facial paralysis can be caused by tumors, vascular lesions (e.g. hemangioma), mass effects (e.g. cholesteatoma independent of active infection), stroke, congenital defects, trauma, infectious processes, or can be idiopathic in nature.5 Bell’s palsy is a common type of facial paralysis that carries a 65–71% chance of full recovery, yet 29–35% of patients develop persistent dysfunction, including reduced facial function, spasms, and/or synkinesis.6,7,8

Current static surgical strategies for patients with chronic facial paralysis have important but limited functional and cosmetic utility. While dynamic surgical interventions for patients with permanent unilateral facial paralysis, such as myoneurovascular gracilis transfer, can provide meaningful movement, these interventions focus on specific facial regions, can require multiple procedures and surgical sites, and carry a 10–15% failure rate.9,10 Furthermore, while functional goals may be achieved and quality-of-life improved,11 aesthetic outcomes are uncommonly fully satisfactory and infrequently restore normal facial form and function.

Our group has recently demonstrated in short-term animal studies the ability of a penetrating multi-channel microelectrode array (MEA) to selectively stimulate neural fibers within the FN, building on prior work done in the cochlear nerve.12,13,14,15 In the present study, we sought to test the safety and efficacy of long-term MEA implantation in the FN over the course of six months. Furthermore, we aimed to simulate the human condition of permanent unilateral facial paralysis in cats using standardized and validated injuries to the nerve, which would represent a clinical injury such as a FN sacrifice during parotid cancer surgery, FN damage during vestibular schwannoma surgery, or complete idiopathic facial paralysis. The extent of spontaneous recovery in such patients would be determined over many months, and those with incomplete or unsatisfactory function would become potential candidates for rehabilitative reanimation surgery. Accordingly, in our animal study, we conducted intraneural stimulation testing following a post-injury recovery period. Based on previous experiments with implantation of similar MEAs in nerve tissue, we hypothesized that the FN implant will be well-tolerated by the nerve and stimulation following either transection-reapproximation or crush nerve injury will result in minor threshold increases.16 This work serves to elucidate the functional and histological changes that occur following chronic MEA implantation and stimulation post-FN injury in reanimating facial musculature following facial paralysis.

Materials and Methods

Electrode array and stimuli

The custom-designed, multi-channel intraneural stimulating MEAs (Microprobes, Gaithersburg, MD, USA) have four platinum/iridium-plated electrode sites (channels), at 250µm intervals spanning a distance of five mm along a single, 241µm diameter polyimide tube [Figure 1]. System 3 Tucker-Davis Technologies (TDT; Alachua, FL, USA) equipment and custom MATLAB (The MathWorks, Natick, MA, USA) software were used for stimulus presentation. Charge-balanced biphasic electrical stimulus pulses or pulse trains were produced by a 4-channel current source controlled by a 4-channel digital-to-analog converter (TDT RX8). The pulses were initially cathodic, 41 or 82µs per phase, with illustrated responses obtained with stimulus charge levels of 26 to 41nC per phase.

Figure 1.

Figure 1

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(A) Photograph of a MicroProbes 4-channel stimulating electrode array. (B) Microscopic picture of the shank and distal flexible output table with a metric ruler size reference. (C) High-magnification microscopic photograph of the distal end of the penetrating shank; the 4 electrode sites can be seen. Superficial or proximal electrodes are those furthest from the tip of the array (to the left in this picture), while deep or distal electrodes are those closest to the array tip (to the right). (D) Microscopic picture of the male Omnetics connector and ground reference wire.

Surgery (Chronic Electrode Implantation)

All procedures were performed with the approval of the University of California at Irvine Institutional Animal Care and Use Committee (IACUC) according to the National Institutes of Health guidelines. We conducted chronic, survival experiments in three cats. For nerve implantation surgeries, a pre-auricular incision was made and dissection through the parotid gland proceeded until the FN trunk was identified and skeletonized. The intraneural MEA was then inserted into the main trunk of the FN, and secured to the adjacent connective tissue with nylon suture [Figure 2]. The wire connections to the MEA were brought to a skull-mounted Omnetics (Minneapolis, MN) connector that was protected by a stainless-steel cylinder. The floor of the cylinder and the base of the connector were sealed with acrylic to ensure that the cylinder interior was isolated from tissue spaces. A screw-on cap was subsequently placed over the cylinder, and the muscle and skin were sutured closed in layers.

Figure 2.

Figure 2

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Image demonstrating a multichannel microelectrode array (MEA) inserted within the feline facial nerve (FN) as it exits the stylomastoid foramen during a chronic implantation surgery. The cable is tunneled underneath adjacent tissue to an Onmetics connector secured to the cranial vertex.

Every two weeks following implantation, for a period of six-months, measurements of stimulation thresholds were conducted. Each of the MEA channels were individually stimulated and EMG voltage responses from four selected facial muscles (orbicularis oris, orbicularis oculi, nasalis, and levator auris longus) were recorded using a nerve integrity monitoring system (NIM Response 2.0, Medtronic Inc., Minneapolis, MN). At the end of the six-month period, the animal received a lethal dose of barbiturate and was transcardially perfused with a formaldehyde solution. The nerves were then harvested for histologic processing and evaluation.

Surgery (Facial Nerve Injury)

For survival surgeries, to produce a standardized FN injury, a pre-auricular incision was made and the FN trunk was identified and skeletonized. The nerve was then intentionally damaged by either a nerve crush injury17 consisting of a 30 second one-click crush with a serrated hemostat (n=2) to produce an axonotmesis injury, or complete transection with scissors and reapproximation (n=2), to produce a Sunderland 5th degree18 neurotmesis injury. For the transection injury model, the nerve endings were aligned but not sutured together. The incision was closed in layers.

Immediate and complete unilateral facial paralysis was observed in all cats that received either a crush or transection-reapproximation injury. Eye ointment was routinely applied to protect the cornea, and animals were carefully monitored per IACUC protocol. In terminal surgeries four months post-injury, the FN trunk was again identified, and a MEA was introduced via a micropositioner into the FN proximal to its bifurcation and either directly adjacent or proximal to the injury site, such that all four channels were in neural tissue. Insertion site and angle was dictated by surgical anatomy and micropositioner-mounted electrode access to the nerve. Each of the MEA channels was individually stimulated, and EMG voltage responses from the four selected facial muscles were recorded by the nerve integrity monitoring system. To vary which neural populations were stimulated, the MEA was withdrawn and reinserted into the nerve in variable trajectories and angles along the course of the exposed FN trunk, and each channel was again stimulated. The experimental stimulus parameters of interest included current level, pulse rate, time interval between pulses on pairs of channels, and electrode configuration (i.e., location of active and return electrodes). Following a lethal dose of barbiturate and transcardial fixation, the FN was harvested for histological examination.

Statistical analysis was performed to determine the correlation strength between current levels and EMG response (Pearson’s correlation coefficient). A paired samples t-test was conducted to determine whether individual muscles significantly differed in activation based on the channel activated. An independent samples t-test was used to determine if there was a difference in the mean activation between various muscles within the same stimulation channel, as well as between EMG response following acute vs. chronic stimulation experiments. Statistical analysis was performed using PASW Statistics 18.0 software (SPSS Inc., Chicago, IL). A p value of < 0.05 was considered statistically significant.

Results

Chronic Electrode Implantation in Normal FN

In chronically-implanted MEAs, selective activation of two or more distinct muscles was achieved by stimulation of individual channels during each post-operative testing over the course of 6-months. Figure 3A [cat 3] shows representative data of EMG voltages from a single muscle following increasing levels of current injected through an individual channel over a succession of measurements at varying stimulation levels. When using cat 3 as a representative example, EMG response of the orbicularis oculi two weeks following implantation was significantly greater than that of the nasalis when channel 1 was activated (p < 0.01); however, activation of channel 2 resulted in similar EMG responses from both muscles, with a trend towards selectivity of nasalis activation. Although levator auris longus and orbicularis oris were also being recorded, they failed to elicit detectable EMG responses.

Figure 3.

Figure 3

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(A) Graphic representation of increasing current levels (from left to right on the x-axis) and the corresponding EMG voltage response of the orbicularis oculi on the y-axis (logarithmic scale). EMG voltage responses respond in a dose-responsive manner to increasing current [Cat 3; phase duration: 82µs; pulse duration: 200µs]. (B) and (C) Graphic representation of successive stimulation of channels (2 weeks and 4 months post-chronic implant) from superficial (channel #1) to deep (channel #4) electrodes, from left to right on the x-axis, and the corresponding EMG voltage response of four distinct muscles on the y-axis. In panel B, only 2 channels (channels 1 and 2) elicited EMG voltage responses, albeit in a selective manner, while all four channels in panel C elicited strong responses from the nasalis [Cats 2 and 3, phase duration: 82µs; pulse duration: 200µs]. (D) Graphic representation of successive stimulation of channels (6 months post-chronic implant) from superficial (channel #1) to deep (channel #4) electrodes, from left to right on the x-axis, and the corresponding EMG voltage response of four distinct muscles on the y-axis. All four channels elicited strong responses from the levator [Cat 3, phase duration: 82µs; pulse duration: 200µs].

Figure 3B [cat 3] and Figure 3C [cat 2] show representative data of EMG voltages from four different muscles based on stimulation channel at two different time points. In cat 3, two weeks after implantation, channel 1 of the MEA robustly stimulated the orbicularis oculi, while channel 2 activated the levator auris longus to a stronger degree [Figure 3B]. Figure 3C [cat 2] demonstrates activation of the nasalis across all stimulation channels four-months following implantation, while Figure 3D [cat 2] shows activation of the levator auris longus across all stimulation channels six-months following implantation. Figure 4 [cat 2] demonstrates changes in EMG voltage responses of three different facial muscles over a six-month period. Supplemental video content 1–3 shows representative graded movement of the nasalis [cat 2] 4-weeks post implantation with increasing current levels.

Figure 4.

Figure 4

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Graphical depiction of EMG voltage response changes over time in 4 distinct muscles at current: 0 [Cat 2; current level: 1mA; phase duration: 82µs; pulse duration: 200µs].

There were notable threshold changes and marked decreases in the degree of visible facial muscle contraction when comparing these chronic implantation experiments to our previously published acute experimental data in normal FN.11 EMG responses following chronic MEA implantation in normal, uninjured FN were significantly lower, with maximal EMG responses reaching only 800µV in chronic implants compared to responses of 4,000µV at maximum stimulation levels in acute experiments (p < 0.05).

On histological examination, facial nerve fibers demonstrated show a desmoplastic foreign body reaction (i.e. granuloma formation) adjacent to the MEA and suture, as well as pale axonal fibers, vacuolization and irregular atrophic changes indicative of a foreign body reaction manifested in the axonal neural tissue [Figure 5].

Figure 5.

Figure 5

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Light microscopic histological images of implanted nerves at 10 and 20×. (A) Section of the facial nerve adjacent to a chronically implanted microelectrode array (hematoxylin and eosin [H&E)] stain, 10× magnification) showing a piece of nonabsorbable suture (star), and a foreign body reaction (granuloma) adjacent to the suture (arrow). (B) Section of the facial nerve adjacent to a chronically implanted microelectrode array (trichrome stain, 10× magnification) showing pale axonal fibers (arrowhead), vacuolization (arrow); note the pale appearance of the neural fibers at the top of the image compared to the bottom darker fibers. (C) Section of the facial nerve adjacent to a chronically implanted microelectrode array (H&E stain, 20× magnification) showing pale axonal fibers (arrowhead) and vacuolization (arrow). (D) Section of the facial nerve adjacent to a chronically implanted microelectrode array (H&E stain, 20× magnification) showing pale axonal fibers (arrowhead) and vacuolization (arrow).

Responses to electrical stimulation following facial nerve injury

Four months following FN injury, all four cats had good recovery of facial function, with only subtle to minor mimetic deficits, roughly corresponding to House-Brackmann grade II or III function. In acute, terminal experiments four months after experimental FN injury, EMG activity in individual muscles was elicited by selective FN stimulation through individual channels. Selective contraction of two or more distinct muscles was accomplished by selection of appropriate stimulation channels in both transection-reapproximation [Figure 6A] and hemostat crush [Figure 6C] injuries. Subsequent insertion of the MEA into the FN at varying locations and angles resulted in similarly diverse patterns of stimulation responses. Figure 6B (cat 4, position 2) demonstrates the escalation of stimulation current levels resulted in mounting EMG voltage responses recorded in two different muscles (p < 0.01). Furthermore, we found that the nasalis had a statistically increased EMG response compared to the orbicularis oculi when only channel 4 is activated (p < 0.05). Increases in EMG voltage in response to escalating stimulation current were also demonstrated following crush injury [Figure 6D].

Figure 6.

Figure 6

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(A) Graphic depiction of channel-specific activation of four different muscles demonstrating no selectivity but strong activation of levator auris longus and orbicularis oculi [Cat 1; position 2; current level: 1mA; phase duration: 82µs; pulse duration: 200µs]. (B) Graphic depiction of escalating stimulation current levels in four individual muscles 4 months after a standardized and validated facial nerve hemostat crush injury resulting in complete unilateral facial paralysis [Cat 1; position 2; channel: 2; phase duration: 82µs; pulse duration: 200µs]. (C) Graphic depiction of channel-specific activation of four different muscles demonstrating strong activation of the orbicularis oculi in all 4 channels, with selective activation of the levator auris longus in channel 1 compared to the other channels [Cat 2; position 2; current level: 1mA; phase duration: 82µs; pulse duration: 200µs]. (D) Graphic depiction of escalating stimulation current levels in four individual muscles 4 months after a standardized and validated facial nerve transection-reapproximation injury resulting in complete unilateral facial paralysis [Cat 2; position 2; channel: 2; phase duration: 82µs; pulse duration: 200µs].

When comparing EMG response of an individual muscle, such as the levator auris longus between individual channels in cat 2 [Figure 6C], significantly elevated EMG output was identified when channel 1 was stimulated compared to channel 4 (p < 0.05). Similarly, the nasalis in cat 2 had a significantly increased response when channel 1 was activated compared to channels 2, 3, or 4 (p < 0.05) [Figure 6C].

Similar to data obtained from chronically-implanted electrodes, stimulation following FN injury resulted in significantly diminished EMG output and marked reduction in the degree of visible facial muscle contraction compared to the previously published acute FN experiments,11 with maximal EMG responses reaching 400µV in acute stimulation of injured FN compared to a 4,000µV maximal response in uninjured FN experiments (p < 0.01).

On histological examination, crushed and transected facial nerve fibers show desmoplastic and irregular atrophic changes and perineural thickening indicative of a post-traumatic neuroma under hematoxylin and eosin (H&E) staining, and thickened perineurium with fibrosis within the neural bundles on trichrome staining [Supplementary Figure 1].

Discussion

In this study, we demonstrated the feasibility of long-term implantation of an intraneural MEA in eliciting facial muscle contraction, as well as a proof-of-concept validation of acute stimulation after FN injury. Furthermore, we confirmed previous reports suggesting that efficacy of stimulation changes after chronic implantation19,20 and following nerve injury due to desmoplasia, and is dependent on the degree of FN recovery following MEA implantation or injury.23 Despite decreases in maximal EMG responses and less robust visible muscle contraction, these results suggest that we can continue to selectively elicit contractions in discrete muscles and regions of the face in a graded, current-dependent fashion following chronic nerve implantation and after complete nerve injury. The selectivity of muscle contraction and current-dependent EMG voltage responses reported here are similar to findings reported in our previous acute studies in the uninjured FN.11 The primary distinction in the results reported presently and our previously reported results lies in the markedly reduced EMG voltage responses and reduction of visible muscle contraction following chronic MEA implantation.

In our chronic implantation experiments, muscular contraction was present in a current-dependent graded fashion during the majority of the 6-month testing period. While the strength of elicited contractions were not as strong following chronic implantation when compared to acute stimulation of an uninjured facial nerve, Supplementary Video Content 1–3 demonstrates graded contraction of facial muscles with escalating levels of current delivered to the chronic MEA implant. Importantly, however, beginning at approximately 8–12 weeks following chronic implantation, visible contractility and movement of facial musculature steadily diminished until eventually no movement could be appreciated with intraneural stimulation, despite continued robust EMG voltage responses throughout the six-month implantation. Intriguingly, visible spontaneous muscle movement of the facial muscles was observed when the cat was awake, and visible contraction was elicited following stimulation of the FN by a stimulating probe during the terminal surgery, at which intraneural positioning of the MEA was confirmed in all animals. It is possible that the desmoplastic tissue response at the nerve-electrode interface reduced the current delivery to the nerve, and thus diminished the extent of facial musculature contraction. Furthermore, isometric contraction may have contributed to reduced visibility of facial muscle contraction (interestingly, contractions were accompanied by robust EMG voltage outputs). Isometric contraction is a fatigue-induced impairment in excitation-contraction coupling of muscle fibers. Although the exact cause of isometric contraction remains unknown, it can result from decreased intracellular calcium release, decreased sensitivity of myofilaments to calcium, and/or reduced force produced by each active cross-bridge.15 Other described theories into why visible implant-mediated muscle contraction diminished over time include muscle21,22 and/or neural23 fatigue, and ionic dysregulation24,25. Further studies will need to be conducted to provide additional insight into the origins of this unusual observation.

Based on the chronic implantation literature reported in other animal nerves (e.g. sciatic nerve), histologic or physiologic consequences of intraneural array placement are most likely to occur within the first several weeks to months26,27,28,27 Moreover, any inflammatory or desmoplastic response to chronic foreign body implantation and electric stimulation has been demonstrated to increase stimulation thresholds, but maintain the ability to elicit muscle contractions.29 We found relatively stable stimulation thresholds, but attenuated levels of maximal muscular contraction as measured by EMG response in both chronically implanted FNs and following acute stimulation of injured FNs when compared to acute experiments in normal FN. Figure 4 demonstrates changes in EMG voltage responses of four different facial muscles over a six-month period. Despite a lack of a global increase or decrease in EMG voltage responses across all facial muscles over time, there was considerable variability in EMG voltage responses throughout the chronic implant testing period. Sources of this variability could include minuscule movement of the implanted array residing in the extratemporal portion of the facial nerve, variability in intramuscular EMG needle placement between testing sessions, and variations in the electrochemical microenvironment at the neural-electrode interface. Although maximal EMG voltage responses peaked at 1200µV during the six-month implantation period, the responses were typically below 400µV.

The observed deterioration of functional changes are likely a result of fibrous tissue formation around the electrode site, as described by Bowman et al.27 It has been shown in chronically implanted intrafascicular electrodes of peripheral nerves, the immune system elicits a macrophage-mediated response resulting in an increase in endoneural connective tissue.30 However, intraneural electrode studies in both rabbits and feline models have illustrated no significant difference in nerve conduction velocities between chronically or acutely implanted electrodes.31 In our experiments, on histological examination of injured FN four months post-injury and following acute MEA implantation, we observed post-traumatic scarring within neural bundles, manifested by perineural fibrosis and neuroma-like irregular and atrophic neural proliferation [Supplementary Figure 1]. Although these chronic changes likely contributed to the reduction in voltage response levels when compared to acute stimulation of a normal nerve11, the MEA was able to selectively stimulate distinct neural fibers and elicit activation in specific muscles and regions of the face following complete FN injury similar to the acute and chronic experimental results.

With regards to our FN injury experiments, the results demonstrate that the direct electrode-to-nerve interface is sufficient to generate movement in a cat following a severe neural injury and partial recovery of facial function. Similarly, Frigerio et al. successfully and routinely elicited a full blink in patients with House-Brackmann grade VI paralysis with transcutaneous stimulation.32 This suggests that even in cases of severe neural injury, there are enough axonal fibers to stimulate muscle contraction. Overall, despite the fact that some degree of selectivity in muscle activation was present following FN injury in our experiments, it was not as robust as that previously demonstrated in acute experiments in uninjured nerve. However, despite the decreased EMG response and selectivity, there continued to be a current-dependent graded EMG response, similar to the chronic implantation experiments.

Selectivity of muscular activation following chronic MEA implantation was demonstrated in only a subset of our animals. This variation likely results from natural variation in the somatotopic distribution of the facial nerve fibers innervating specific muscles, the limitations of the MEA design, and the variability in the surgical placement of the MEA. Notably, the neurostimulator will not be effective if implanted into a distal nerve trunk that lacks a proximal neuronal cell body/ganglion, as the distal neural fibers would permanently degenerate. However, the distal FN can potentially be anastomosed with or without a nerve graft to any regional viable motor nerve prior to or at the time of implantation. Axonal regeneration from a brainstem motor nucleus (e.g. trigeminal, facial, and hypoglossal) could provide the excitable neural circuitry susceptible to neuroprosthesic stimulation. The clinical utility of a FN implant system can potentially be negatively impacted by the extent of randomly-targeted neural regeneration, which results in synkinesis. The ability to satisfactorily address this issue with individual electrode stimulation parameter adjustment and optimization remains to be determined. Nonetheless, the stability of implanted MEAs to elicit EMG voltage potentials in selective facial musculature has been suggested in this study. Further refinements of the electrode composition, current characteristics, and intraoperative testing of intraneural stimulation may help enhance the selectivity of stimulation. Improving the selectivity of specific muscular contraction by stimulating restricted nerve fiber populations within the FN requires further modulation of MEA materials, design, and array dimensions prior to contemplating clinical applicability. The deterioration of the MEAs ability to selectively elicit muscle contraction following chronic implantation highlights the necessity of enhancing the biocompatibility of the MEA to avoid the chronic foreign body response that is characteristic of implanted electrode arrays.33,34 One such example of enhancing the biocompatibility characteristics of the MEA, would be by applying a CD200 coating on the array which would mitigate the macrophage-mediated foreign body response.35 Additional work will focus on improving the biocompatibility of the MEA and ensuring that the current delivered to the nerve and surrounding tissue will not result in long-term damage. Furthermore, recapitulating the clinical condition in which the efficacy of chronic MEA implantation following complete FN injury and recovery will be assessed.

This proof-of-concept study demonstrated the feasibility of chronic MEA implantation in initiating muscular activation in normal nerve, as well as stimulation of previously injured FN. This study served to further elucidate the possibility and limitations of FN intraneural implantation in facial nerve reanimation. Future studies could be directed as investigating the function of chronically implanted intraneural electrodes in these nerve injury models.

Conclusion

We have established in the animal model the ability of a chronically implanted MEA to selectively stimulate restricted fiber populations within the FN and to elicit contractions in specific facial muscles. These results hold true in acute experiments four months following complete FN injury and recovery. Despite the need to further refine the selectivity of muscle stimulation in both cases, the feasibility of chronically-implanted intraneural MEAs to activate specific muscles with functional muscular activation warrants further investigation of a FN implant system in the setting of facial paralysis.

Supplementary Material

Supplementary Figure 1

Supplementary Figure 1. Light microscopic histological images of injured nerves at 4×. Hematoxylin and eosin (H&E) staining of (A) crushed and (B) transected facial nerve. Normal facial nerve fibers (arrowheads) have thin perineurium with parallel fibers, while crushed or transected facial nerve fibers show irregular atrophic changes and perineural thickening indicative of a post-traumatic neuroma (arrows). Trichrome staining of (C) crushed and (D) transected facial nerve. Normal facial nerve fibers (arrowheads) have thin perineurium with parallel fibers, while crushed or transected facial nerve fibers show thickened perineurium with fibrosis within the neural bundles (arrows).

Supplementary Video

Supplementary Video Content Legend. Intraneural electrode array activation of the nasalis at increasing current levels from 52, 54, and 58 dB re: 1µA (Video Content 1, 2, and 3 respectively). Video Content 4 demonstrates fine contractions of several facial muscles (orbicularis oculi and nasalis) in response to short bursts of high levels of current when Channel 1 is activated. Video Content 5 demonstrates fine contractions of the orbicularis oculi muscle when Channel 4 is activated.

Download video file (14.8MB, mp4)

Acknowledgments

This project was supported by grants from the UC Irvine Interdisciplinary Innovation Initiative, American Neurotology Society, and the National Center for Research Resources and the National Center for Advancing Translational Sciences, National Institutes of Health, through Grant UL1 TR001414, and will be presented as the American Neurotology Society Trainee Award recipient at the American Neurotology Society 52nd Annual Spring Meeting at COSM, April 28–29, 2017 in San Diego, California.

Footnotes

The authors report no conflicts of interest.

References

  • 1.Mann CJ, Perdiguero E, Kharraz Y, et al. Aberrant repair and fibrosis development in skeletal muscle. Skelet Muscle. 2011;1(1):21. doi: 10.1186/2044-5040-1-21. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.May M, Schaitkin B, Shapiro A. The facial nerve. New York: Thieme; 2001. [Google Scholar]
  • 3.McCaul JA, Cascarini L, Godden D, Coombes D, Brennan PA, Kerawala CJ. Evidence based management of Bell's palsy. British Journal of Oral and Maxillofacial Surgery. 2014;52(5):387–91. doi: 10.1016/j.bjoms.2014.03.001. [DOI] [PubMed] [Google Scholar]
  • 4.Bleicher JN, Hamiel S, Gengler JS, Antimarino J. A survey of facial paralysis: etiology and incidence. Ear Nose Throat. 1996;75:355–8. [PubMed] [Google Scholar]
  • 5.Bleicher JN, Hamiel S, Gengler JS, Antimarino J. A survey of facial paralysis: etiology and incidence. Ear, nose, & throat journal. 1996 Jun;75(6):355–8. [PubMed] [Google Scholar]
  • 6.May M, Wette R, Hardin WB, Sullivan J. The use of steroids in Bell's palsy: a prospective controlled study. Laryngoscope. 1976;86(8):1111–22. doi: 10.1288/00005537-197608000-00003. [DOI] [PubMed] [Google Scholar]
  • 7.May M, Klein SR, Taylor FH. Idiopathic (Bell's) facial palsy: natural history defies steroid or surgical treatment. Laryngoscope. 1985;95(4):406–9. doi: 10.1288/00005537-198504000-00007. [DOI] [PubMed] [Google Scholar]
  • 8.Peitersen E. Bell's palsy: the spontaneous course of 2,500 peripheral facial nerve palsies of different etiologies. Acta Oto-Laryngologica. 2002;122(7):4–30. [PubMed] [Google Scholar]
  • 9.Bhama PK, Weinberg JS, Lindsay RW, Hohman MH, Cheney ML, Hadlock TA. Objective outcomes analysis following microvascular gracilis transfer for facial reanimation: a review of 10 years' experience. JAMA Facial Plastic Surgery. 2014;16:85–92. doi: 10.1001/jamafacial.2013.2463. [DOI] [PubMed] [Google Scholar]
  • 10.Adams JE, Kircher MF, Spinner RJ, Torchia ME, Bishop AT, Shin AY. Complications and outcomes of functional free gracilis transfer in brachial plexus palsy. Acta Orthopedic Belgica. 2009;75:8–13. [PubMed] [Google Scholar]
  • 11.Lindsay RW, Bhama P, Hadlock TA. Quality-of-life improvement after free gracilis muscle transfer for smile restoration in patients with facial paralysis. JAMA Facial Plastic Surgery. 2014;16:419–24. doi: 10.1001/jamafacial.2014.679. [DOI] [PubMed] [Google Scholar]
  • 12.Middlebrooks JC, Snyder RL. Auditory prosthesis with a penetrating nerve array. Journal of the Association for Research in Otolaryngology. 2007;8:258–79. doi: 10.1007/s10162-007-0070-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Middlebrooks JC, Snyder RL. Intraneural stimulation for auditory prosthesis: modiolar trunk and intracranial stimulation sites. Hearing Research. 2008;242:52–63. doi: 10.1016/j.heares.2008.04.001. [DOI] [PubMed] [Google Scholar]
  • 14.Middlebrooks JC, Snyder RL. Selective electrical stimulation of the auditory nerve activates a pathway specialized for high temporal acuity. Neuroscience. 2010;30:1937–46. doi: 10.1523/JNEUROSCI.4949-09.2010. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Sahyouni R, Bhatt J, Djalilian HR, Tang WC, Middlebrooks JC, Lin HW. Selective stimulation of facial muscles with a penetrating electrode array in the feline model. Laryngoscope. 2016;127:460–465. doi: 10.1002/lary.26078. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Branner A, Stein RB, Fernandez E, Aoyagi Y, Normann RA. Long-term stimulation and recording with a penetrating microelectrode array in cat sciatic nerve. IEEE transactions on biomedical engineering. 2004;51(1):146–57. doi: 10.1109/TBME.2003.820321. [DOI] [PubMed] [Google Scholar]
  • 17.Bridge PM, Ball DJ, Mackinnon SE, et al. Nerve crush injuries--a model for axonotmesis. Exp Neurol. 1994;127(2):284–90. doi: 10.1006/exnr.1994.1104. [DOI] [PubMed] [Google Scholar]
  • 18.Sunderland S. A classification of peripheral nerve injuries producing loss of function. Brain. 1951;74(4):491–516. doi: 10.1093/brain/74.4.491. [DOI] [PubMed] [Google Scholar]
  • 19.Crenshaw AG, Karlsson S, Gerdle B, Fridén J. Differential responses in intramuscular pressure and EMG fatigue indicators during low- vs. high-level isometric contractions to fatigue. Acta Physiol Scand. 1997;160(4):353–361. doi: 10.1046/j.1365-201X.1997.00168.x. [DOI] [PubMed] [Google Scholar]
  • 20.Van boxtel A, Goudswaard P, Van der molen GM, Van den bosch WE. Changes in electromyogram power spectra of facial and jawelevator muscles during fatigue. J Appl Physiol Respir Environ Exerc Physiol. 1983;54:51–8. doi: 10.1152/jappl.1983.54.1.51. [DOI] [PubMed] [Google Scholar]
  • 21.Van boxtel A, Goudswaard P, Van der molen GM, Van den bosch WE. Changes in electromyogram power spectra of facial and jaw-elevator muscles during fatigue. J Appl Physiol Respir Environ Exerc Physiol. 1983;54(1):51–8. doi: 10.1152/jappl.1983.54.1.51. [DOI] [PubMed] [Google Scholar]
  • 22.Brach JS, VanSweringen JP. Measuring fatigue related to facial muscle function. Archives of Physical Medicine and Rehabilitation. 1995;76(10):905. doi: 10.1016/s0003-9993(95)80064-6. [DOI] [PubMed] [Google Scholar]
  • 23.Jones DA, Bigland-ritchie B, Edwards RH. Excitation frequency and muscle fatigue: mechanical responses during voluntary and stimulated contractions. Exp Neurol. 1979;64(2):401–13. doi: 10.1016/0014-4886(79)90279-6. [DOI] [PubMed] [Google Scholar]
  • 24.Fitts RH. Cellular mechanisms of muscle fatigue. Physiol Rev. 1994;74(1):49–94. doi: 10.1152/physrev.1994.74.1.49. [DOI] [PubMed] [Google Scholar]
  • 25.Allen DG, Lamb GD, Westerblad H. Skeletal muscle fatigue: cellular mechanisms. Physiol Rev. 2008;88(1):287–332. doi: 10.1152/physrev.00015.2007. [DOI] [PubMed] [Google Scholar]
  • 26.Fang X, Sakaguchi H, Fujikado T, et al. Electrophysiological and histological studies of chronically implanted intrapapillary microelectrodes in rabbit eyes. Graefes Arch Clin Exp Ophthalmol. 2006;244(3):364–75. doi: 10.1007/s00417-005-0073-9. [DOI] [PubMed] [Google Scholar]
  • 27.Agnew WF, McCreery DB, Yuen TGH, et al. Histologic and physiologic evaluation of electrically stimulated peripheral nerve: Considerations for the selection of parameters. Ann Biomed Eng. 1989;17:39. doi: 10.1007/BF02364272. [DOI] [PubMed] [Google Scholar]
  • 28.Grill WM, Mortimer JT. Neural and connective tissue response to long-term implantation of multiple contact nerve cuff electrodes. J Biomed Mater Res. 2000;50(2):215–26. doi: 10.1002/(sici)1097-4636(200005)50:2<215::aid-jbm17>3.0.co;2-a. [DOI] [PubMed] [Google Scholar]
  • 29.Polasek KH, Hoyen HA, Keith MW, Kirsch RF, Tyler DJ. Stimulation stability and selectivity of chronically implanted multicontact nerve cuff electrodes in the human upper extremity. IEEE Trans Neural Syst Rehabil Eng. 2009;17(5):428–37. doi: 10.1109/TNSRE.2009.2032603. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Lefurge T, Goodall E, Horch K, Stensaas L, Schoenberg A. Chronically implanted intrafascicular recording electrodes. Ann Biomed Eng. 1991;19(2):197–207. doi: 10.1007/BF02368469. [DOI] [PubMed] [Google Scholar]
  • 31.Bowman BR, Erickson RC. Acute and chronic implantation of coiled wire intraneural electrodes during cyclical electrical stimulation. Ann Biomed Eng. 1985;13(1):75–93. doi: 10.1007/BF02371251. [DOI] [PubMed] [Google Scholar]
  • 32.Frigerio A, Heaton JT, Cavallari P, Knox C, Hohman MH, Hadlock TA. Electrical stimulation of eye blink in individuals with acute facial palsy: progress toward a bionic blink. Plast Reconstr Surg. 2015;136(4):515–23. doi: 10.1097/PRS.0000000000001639. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Sahyouni R, Chang DT, Moshtaghi O, Mahmoodi A, Djalilian HR, Lin HW. Functional and Histological Effects of Chronic Neural Electrode Implantation. Laryngoscope Investigative Otolaryngology. 2017;2(2):80–93. doi: 10.1002/lio2.66. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Agnew WF, McCreery DB. Considerations for safety with chronically implanted nerve electrodes. Epilepsia. 1990;31(s2):S27–32. doi: 10.1111/j.1528-1157.1990.tb05845.x. [DOI] [PubMed] [Google Scholar]
  • 35.Kim YK, Que R, Wang SW, Liu WF. Modification of biomaterials with a self-protein inhibits the macrophage response. Adv Healthc Mater. 2014;3(7):989–994. doi: 10.1002/adhm.201300532. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

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Supplementary Materials

Supplementary Figure 1

Supplementary Figure 1. Light microscopic histological images of injured nerves at 4×. Hematoxylin and eosin (H&E) staining of (A) crushed and (B) transected facial nerve. Normal facial nerve fibers (arrowheads) have thin perineurium with parallel fibers, while crushed or transected facial nerve fibers show irregular atrophic changes and perineural thickening indicative of a post-traumatic neuroma (arrows). Trichrome staining of (C) crushed and (D) transected facial nerve. Normal facial nerve fibers (arrowheads) have thin perineurium with parallel fibers, while crushed or transected facial nerve fibers show thickened perineurium with fibrosis within the neural bundles (arrows).

NIHMS893745-supplement-Supplementary_Figure_1.tiff (2.7MB, tiff)
Supplementary Video

Supplementary Video Content Legend. Intraneural electrode array activation of the nasalis at increasing current levels from 52, 54, and 58 dB re: 1µA (Video Content 1, 2, and 3 respectively). Video Content 4 demonstrates fine contractions of several facial muscles (orbicularis oculi and nasalis) in response to short bursts of high levels of current when Channel 1 is activated. Video Content 5 demonstrates fine contractions of the orbicularis oculi muscle when Channel 4 is activated.

Download video file (14.8MB, mp4)

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