Reinnervation of skeletal muscle: a comparison of nerve implantation with neuromuscular pedicle transfer in an animal model

Direct muscle neurotization: Previous advancements in animal models

Peripheral nerve repair is daily activity for several microsurgeons. Numerous nerve repair techniques are applied, including neurorrhaphy, nerve grafting and nerve transfer, depending on the nature and extent of the injury. However, these techniques become unfeasible when the distal nerve end is not preserved during the peripheral nerve injury or a segment of the muscle is transferred without the nerve supplying it. However, direct muscle neurotization (DMN) achieves muscle reinnervation by suturing the nerve directly into the muscle tissue, without requiring a distal nerve end for coaptation. Despite promising results in the literature, DMN is not widely adopted in clinical practice. Animal models may help in advancing novel therapeutic approaches, due to their anatomic and physiologic similarities to humans. This review highlights the current scientific understanding and recent advancements in DMN as well as the animal models and target muscle that have been used in the past to investigate the basic principles behind this surgical technique. The presented information should aid in establishing the clinical importance of DMN in peripheral nerve injury.

Modified first or second cervical nerve transplantation technique for the treatment of recurrent laryngeal neuropathy in horses

BACKGROUND

In horses, the only established method for reinnervation of the larynx is the nerve-muscle pedicle implantation, whereas in human medicine, direct nerve implantation is a standard surgical technique for selective laryngeal reinnervation in human patients suffering from bilateral vocal fold paralysis.

OBJECTIVES

(1) To describe a modified first or second cervical nerve transplantation technique for the treatment of recurrent laryngeal neuropathy (RLN) in horses and (2) evaluate the outcomes of reinnervation using direct nerve needle-stimulation of the first cervical nerve and exercising endoscopy before and after surgery.

STUDY DESIGN

Case series.

METHODS

Nerve transplantation surgery, in which the first or second cervical nerve is tunnelled through the atrophied left cricoarytenoideus dorsalis muscle, was performed in combination with ipsilateral laser ventriculocordectomy. Ultrasound-guided stimulation of the first cervical nerve at the level of the alar foramen was used to confirm successful reinnervation post-operatively. Exercising endoscopy was performed before and after surgery. The exercising RLN grade of the left arytenoid was blindly determined at the highest stride frequency for each examination.

RESULTS

Surgery was performed in 17 client-owned animals with RLN. Reinnervation was confirmed by nerve stimulation and subsequent arytenoid abduction observed in 11 out of 12 cases between 4 and 12 months post-operatively. Fourteen horses had exercising endoscopy before and after surgery. Nine horses had an improved exercising RLN grade, four horses had the same exercising grade and one horse had a worse exercising grade after surgery.

MAIN LIMITATIONS

A sham-operated control group was not included and follow-up beyond 12 months and objective performance data were not obtained.

CONCLUSIONS

The modified first or second cervical nerve transplantation technique, using tunnelling and direct implantation of the donor nerve into the cricoarytenoideus dorsalis muscle, resulted in reinnervation in 11 out of 12 cases and improved exercising grade in 9 out of 14 horses within 12 months after surgery.

Reinnervation of denervated muscle by implantation of nerve-muscle-endplate band graft to the native motor zone of the target muscle

INTRODUCTION

Motor endplate reinnervation is critical for restoring motor function of the denervated muscle. We developed a novel surgical technique called nerve-muscle-endplate band grafting (NMEG) for muscle reinnervation.

METHODS

Experimentally denervated sternomastoid muscle in the rat was reinnervated by transferring a NMEG from the ipsilateral sternohyoid muscle to the native motor zone (NMZ) of the target muscle. A NMEG pedicle contained a block of muscle (~ 6 × 6 × 3 mm), a nerve branch with axon terminals, and a motor endplate band with numerous neuromuscular junctions. At 3 months after surgery, maximal tetanic muscle force measurement, muscle mass and myofiber morphology, motoneurons, regenerated axons, and axon-endplate connections of the muscles were analyzed.

RESULTS

The mean force of the reinnervated muscles was 82% of the contralateral controls. The average weight of the treated muscles was 89% of the controls. The reinnervated muscles exhibited extensive axonal regeneration. Specifically, the mean count of the regenerated axons in the reinnervated muscles reached up to 76.8% of the controls. The majority (80%) of the denervated endplates in the target muscle regained motor innervation.

CONCLUSIONS

The NMZ of the denervated muscle is an ideal site for NMEG implantation and for the development of new microsurgical and therapeutic strategies to achieve sufficient axonal regeneration, rapid endplate reinnervation, and optimal functional recovery. NMEG-NMZ technique may become a useful tool in the treatment of muscle paralysis caused by peripheral nerve injuries in certain clinical situations.

Muscle reinnervation with nerve-muscle-endplate band grafting technique: correlation between force recovery and axonal regeneration

BACKGROUND

This study was designed to determine the correlation between functional recovery and the extent of axonal regeneration after muscle reinnervation with our recently developed nerve-muscle-endplate band grafting (NMEG) technique in a rat model.

MATERIALS AND METHODS

The right experimentally paralyzed sternomastoid (SM) muscle by nerve transection was immediately reinnervated with an NMEG pedicle harvested from a neighboring sternohyoid muscle. The NMEG pedicle contained a muscle block (6 × 6 × 3 mm), a donor nerve branch with nerve terminals, and a motor endplate band. Three months after surgery, the tetanic force of the SM muscle was measured and the regenerated axons in the muscle were detected using neurofilament immunohistochemistry.

RESULTS

The results showed that the maximal tetanic force (a measure of muscle functional recovery) of the NMEG-reinnervated SM muscle reached up to 66.0% of the normal control. The wet weight of the reinnervated SM muscle (a measure of muscle mass recovery) was 87.2% of the control. The area fraction of the regenerating axons visualized with neurofilament staining within the NMEG-reinnervated SM muscle (a measure of muscle reinnervation) was 42.3%. A positive correlation was revealed between the extent of muscle reinnervation and maximal muscle force.

CONCLUSIONS

Our newly developed NMEG technique results in satisfactory functional outcomes and nerve regeneration. Further improvement in the functional recovery after NMEG reinnervation could be achieved by refining the surgical procedure and creating an ideal environment that favors axon-endplate connections and accelerates axonal growth and sprouting.

Comparison of muscle force after immediate and delayed reinnervation using nerve-muscle-endplate band grafting

BACKGROUND

Because of poor functional outcomes of currently used reinnervation methods, we developed novel treatment strategy for the restoration of paralyzed muscles-the nerve-muscle-endplate band grafting (NMEG) technique. The graft was obtained from the sternohyoid muscle (donor) and implanted into the ipsilateral paralyzed sternomastoid (SM) muscle (recipient).

METHODS

Rats were subjected to immediate or delayed (1 or 3 mo) reinnervation of the experimentally paralyzed SM muscles using the NMEG technique or the conventionally used nerve end-to-end anastomosis. The SM muscle at the opposite side served as a normal control.

RESULTS

NMEG produced better recovery of muscle force as compared with end-to-end anastomosis. A larger force produced by NMEG was most evident for small stimulation currents.

CONCLUSIONS

The NMEG technique holds great potential for successful muscle reinnervation. We hypothesize that even better muscle reinnervation and functional recovery could be achieved with further improvement of the environment that favors axon-end plate connections and accelerates axonal growth and sprouting.

Force recovery and axonal regeneration of the sternomastoid muscle reinnervated with the end-to-end nerve anastomosis

BACKGROUND

End-to-end nerve anastomosis (EEA) is a commonly used nerve repair technique. However, this method generally results in poor functional recovery. This study was designed to determine the correlation of functional recovery to the extent of axonal reinnervation after EEA procedure in a rat model.

MATERIALS AND METHODS

Seven adult rats were subjected to the immediate reinnervation of an experimentally paralyzed sternomastoid (SM) muscle. The SM nerve was transected and immediately repaired with EEA. The SM muscle at the opposite side, without nerve transection, served as a control. Three months after EEA nerve repair, the muscle force of the SM muscle was measured and the regenerated axons in the muscle were detected using neurofilament immunohistochemistry.

RESULTS

Three months after surgery, the reinnervated SM muscle produced limited anatomical and functional recovery (calculated as the percentage of the control). Specifically, the wet weight of the operated SM muscle (a measure of muscle mass recovery) was 78.0% of the control. The maximal tetanic force (a measure of muscle functional recovery) was 56.7% of the control. The area fraction of the neurofilament stained intramuscular axons (a measure of axonal regeneration and muscle reinnervation) was measured to be only 13.4% of the control. A positive correlation was revealed between the extent of muscle reinnervation and maximal muscle force.

CONCLUSIONS

The EEA reinnervated SM muscle in the rat yielded unsatisfactory muscle force recovery as a result of mild to moderate nerve regeneration. Further work is needed to improve the surgical procedure, enhance axonal regeneration, and/or develop novel treatment strategies for better functional recovery.

Nerve-muscle-endplate band grafting: a new technique for muscle reinnervation

BACKGROUND

Because currently existing reinnervation methods result in poor functional recovery, there is a great need to develop new treatment strategies.

OBJECTIVE

To investigate the efficacy of our recently developed nerve-muscle-endplate band grafting (NMEG) technique for muscle reinnervation.

METHODS

Twenty-five adult rats were used. Sternohyoid (SH) and sternomastoid (SM) muscles served as donor and recipient muscle, respectively. Neural organization of the SH and SM muscles and surgical feasibility of the NMEG technique were determined. An NMEG contained a muscle block, a nerve branch with nerve terminals, and a motor endplate band with numerous neuromuscular junctions. After a 3-month recovery period, the degree of functional recovery was evaluated with a maximal tetanic force measurement. Retrograde horseradish peroxidase tracing was used to track the origin of the motor innervation of the reinnervated muscles. The reinnervated muscles were examined morphohistologically and immunohistochemically to assess the extent of axonal regeneration.

RESULTS

Nerve supply patterns and locations of the motor endplate bands in the SH and SM muscles were documented. The results demonstrated that the reinnervated SM muscles gained motor control from the SH motoneurons. The NMEG technique yielded extensive axonal regeneration and significant recovery of SM muscle force-generating capacity (67% of control). The mean wet weight of the NMEG-reinnervated muscles (87% of control) was greater than that of the denervated SM muscles (36% of control).

CONCLUSION

The NMEG technique resulted in successful muscle reinnervation and functional recovery. This technique holds promise in the treatment of muscle paralysis.

Locations of the motor endplate band and motoneurons innervating the sternomastoid muscle in the rat

Sternocleidomastoid (SCM) is a long muscle with two bellies, sternomastoid (SM) and cleidomastoid (CM) in the lateral side of the neck. It has been widely used as muscle and myocutaneous flap for reconstruction of oral cavity and facial defects and as a candidate for reinnervation studies. Therefore, exact neuroanatomy of the SCM is critical for guiding reinnervation procedures. In this study, SM in rats were investigated to document banding pattern of motor endplates (MEPs) using whole-mount acetylcholinesterase (AChE) staining and to determine locations of the motoneurons innervating the muscle using retrograde horseradish peroxidase (HRP) tracing technique. The results showed that the MEPs in the SM and CM were organized into a single band which was located in the middle portion of the muscle. After HRP injections into the MEP band of the SM, ipsilaterally labeled motoneurons were identified in the caudal medulla oblongata (MO), C1, and C2. The SM motoneurons were found to form a single column in lower MO and dorsomedial (DM) nucleus in C1. In contrast, the labeled SM motoneurons in C2 formed either one (DM nucleus), two [DM and ventrolateral (VL) nuclei], or three [DM, VL, and ventromedial (VM)] columns. These findings are important not only for understanding the neural control of the muscle but also for evaluating the success rate of a given reinnervation procedure when the SM is chosen as a target muscle.

Characteristics of tetanic force produced by the sternomastoid muscle of the rat

The sternomastoid (SM) muscle plays an important role in supporting breathing. It also has unique anatomical advantages that allow its wide use in head and neck tissue reconstruction and muscle reinnervation. However, little is known about its contractile properties. The experiments were run on rats and designed to determine in vivo the relationship between muscle force (active muscle contraction to electrical stimulation) with passive tension (passive force changing muscle length) and two parameters (intensity and frequency) of electrical stimulation. The threshold current for initiating noticeable muscle contraction was 0.03 mA. Maximal muscle force (0.94 N) was produced by using moderate muscle length/tension (28 mm/0.08 N), 0.2 mA stimulation current, and 150 Hz stimulation frequency. These data are important not only to better understand the contractile properties of the rat SM muscle, but also to provide normative values which are critical to reliably assess the extent of functional recovery following muscle reinnervation.

Laryngeal reinnervation in the horse

Left laryngeal hemiplegia is a frustrating condition for the equine athlete and equine veterinarian. Treatment for the past 30 years has centered on the prosthetic laryngoplasty ("tie-back") with or without ventriculectomy. Laryngeal reinnervation has been used successfully in people and has been shown experimentally to benefit affected horses. This article reviews equine laryngeal reinnervation using the nerve muscle pedicle graft and describes the surgical technique, its complications, and the follow-up in 146 cases treated over the past 10 years. Also discussed is ongoing research into stimulation studies to improve the success of equine laryngeal reinnervation.

Experimental reinnervation of a strap muscle with a few roots of the phrenic nerve in rabbits

In order to compare application of the roots of the phrenic nerve to the ansa hypoglossi for laryngeal muscle neurotization, 1 or more roots from the phrenic nerve were implanted into the right sternothyroid (RST) muscle of rabbits (n = 36). Controls were intact animals (in which RST innervation is provided by the ansa; n = 6) and denervated ones (n = 6). At 66 +/- 2 days (mean +/- SE) after neurotization, during quiet breathing, inspiratory electromyographic activity and isometric contraction force were observed in all reinnervated RST muscles (n = 24). During maximal inspiratory effort, electromyographic activity and force increased. In animals reinnervated by the C4 root alone, forces (46.22 +/- 7.8 g) were significantly higher than in intact animals (10.83 +/- 5.0 g). Retrograde labeling proved the phrenic origin of the neurotization. Electromyography of the diaphragm was recorded. We conclude that in rabbits, neurotization of a strap muscle by 1 or 2 roots of the phrenic nerve allows inspiratory contraction, even during quiet breathing. Such inspiratory activity is not observed in sternothyroid muscles of intact animals innervated by the ansa hypoglossi.

An experimental comparison of different kinds of laryngeal muscle reinnervation

In this study, we attempted to determine which method was the best for reinnervating the laryngeal adductor muscles by comparing nerve suture, nerve implantation, and nerve-muscular pedicle (NMP) transfer, as well as the length of time that could elapse after denervation and still allow for successful reinnervation with the ansa cervicalis. Reinnervation was performed in 36 dogs, at 6-, 8-, 10-, 12- and 18-month intervals after denervation via the three methods of muscle reinnervation described above. We noted some return of adduction in the cases using nerve suture before a 10-month interval after denervation, and with nerve implantation and NMP transfer before the 8-month intervals. The variable adduction was caused by reinnervation of the adductor muscles from the ansa cervicalis, as demonstrated by laryngeal spontaneous and evoked electromyography, the strength of muscle contraction, and histologic findings. Adduction was not observed in the cases after the above-mentioned intervals but partial improvement of the bulk and strength of the reinnervated vocal cord was still achieved. An analysis of the experimental results showed that nerve suture was superior to nerve implantation and the NMP technique. Little difference was noted between nerve implantation and the NMP technique.

Long-term excitability and fine tuning of nerve pedicles reinnervating strap muscles in the dog

Contraction of paralyzed striated muscles has been restored by stimulating reinnervating pedicles with currents of low intensity. In order to allow clinical application, stable, long-term excitability must emulate the parameters necessary for the stimulation of normal motor nerves. In 6 dogs, the ansa hypoglossi nerve was implanted into the contralateral denervated sternohyoid muscle and surrounded with a bipolar cuff electrode. Three of the reinnervating pedicles were chronically paced with a Medtronic Itrel II Multiprogrammable Pulse Generator (0.5 V, 0.2 second on [30 pulses per second, 0.21-millisecond pulse width], 2.9 seconds off). At reexploration after 8 months (6 months for 1 dog), frank contraction confirmed by electromyography tracings occurred in all animals with currents in the range of 0.1 to 0.5 mA. Muscle force was further manipulated by selective release of blocking currents (600 Hz, 1.7 to 0.4 mA) superimposed over regular stimulation (50 Hz, 0.3 to 1.7 mA). Nerve and muscle vitality were histologically confirmed. Long-term, low-intensity conduction capabilities, fine tuning, good tolerance of implanted electrodes, and lack of fatigue suggest that reinnervating pedicles may be successfully used for pacing when clinically indicated.

Restoration of orbicularis oculi function by contralateral orbicularis oculi innervated muscle flap vs neuromuscular pedicle technique

In preliminary experiments with dogs and cats, unilateral paralysis of the orbicularis oculi muscle group was produced by a section of the seventh nerve that included the posterior auricular branch. Either one of two procedures was then employed in attempts to reinnervate the paralyzed eyelid. In one group of animals, a neuromuscular pedicle was employed and in another, a contralateral orbicularis innervated muscle flap was used. Both methods restored synchronous, reflex blinking to the denervated eyelid. Of the two procedures, neurotization appears to offer the greater promise because the use of a neuromuscular pedicle requires an expendable nerve that is functional, and no such suitable substitute is available in humans.

Correlation between histology and nerve excitability after reinnervation of paralyzed strap muscles in the rabbit

We have recently shown that the mean muscle chronaxie for nerve pedicle implanted into denervated rabbit strap muscle is comparable to that of normal nerve. This study correlates excitability with histologic characteristics of muscles reinnervated via nerve-muscle pedicles (NMP) and direct nerve implants (DNI). Strength duration curves were measured in 13 rabbits 3.5 to 5 months after reinnervation by NMP (n = 6) and DNI (n = 7). Following this, control (n = 5) and reinnervated straps were harvested immediately before the animals were killed and frozen in liquid nitrogen. The material was submitted for hematoxylin-eosin stains as well as trichrome stains for general morphology, myofibrillar ATPase and NADH for fiber typing, and cholinesterase for determination of denervated fibers. In all animals with low chronaxie, expected type grouping from reinnervation was noted (n = 10). By contrast, the three animals in which chronaxie was abnormally elevated demonstrated fibrosis, inflammation, and absence of or poor type grouping. This suggests that type grouping is necessary for excitability after reinnervation of paralyzed striated muscles.

Selective reinnervation of the abductor and adductor muscles of the canine larynx after recurrent nerve paralysis

Functional rehabilitation of the larynx after unilateral vocal cord paralysis was attempted in the dog by selective reinnervation of the laryngeal muscles. The intralaryngeal branches of the right recurrent nerve were dissected. The adductor branch was anastomosed with the ansa cervicalis; the abductor branch was anastomosed with the trunk of the phrenic nerve either within the larynx or through the recurrent nerve, the adductor branch of which was sectioned. Results could be analyzed in seven dogs: mobility of the vocal cord was checked, and electromyography, stimulation of the nerves, and histologic studies were performed. Functional reinnervation of both the adductor and abductor muscles was obtained in only one case, with good abduction. Adduction was recorded in five cases. False-positive results emphasize the necessity of collecting several types of data before concluding that functional reinnervation has been accomplished. The reliability of the procedure can and must be improved.

Direct nerve implantation vs. nerve-muscle pedicle: a comparative study of reinnervation in the rabbit

To determine the optimal method for reinnervation of the paralyzed head and neck musculature, we compared direct muscular nerve implants (DNI) with nerve-muscle pedicles (NMP) in rabbits. In 25 anesthetized animals, one ansa hypoglossi nerve was cut. Five animals served as controls and two groups of 10 each received cross-over DNIs or NMP from one sternothyroid to the contralateral sternohyoid muscle. The transplanted nerves of animals that survived long enough for neurotization to occur (8 DNIs, 5 NMPs) were stimulated with 3 to 10 mA. 0.05 msec pulse trains to obtain force curves from corresponding straps. Fiber diameters and areas were calculated on muscles harvested before the animals were killed. There was a nonsignificant trend toward stronger contraction in the NMP group, but NMP fibers were significantly larger than those in DNI and control groups (p less than 0.001).

Facial muscle reinnervation: a comparison of neuromuscular pedicle with direct nerve implant

Two methods of reinnervation, the neuromuscular pedicle (NMP) and the nerve implant (NI), were compared in a model using the rabbit's denervated mentalis muscle. Results from evoked electromyographic (EMG) and muscle tension studies (twitch and tetanic contraction) provided the basis of comparison. In addition, the timing of denervation was studied (ie, at the time of implantation of the NMP or NI, or 2 weeks following implantation). The NMP achieved more rapid reinnervation and produced stronger contractions than the NI. Demonstrable reinnervation with an NMP was accelerated when the implantation occurred prior to the denervation. This was not the case with the NI. There was poor correlation between the evoked EMG potential and the strength of both twitch and tetanic contraction. It was concluded on the basis of this study that muscle tension provided a more accurate means of assessing reinnervated muscle function. Of the two methods, the NMP would seem, therefore, to be the technique of choice when it is available. The NI is certainly effective and should be used when a satisfactory NMP is not available.

Reinnervation of the trapezius muscle

The eleventh cranial nerve shoulder syndrome, which results from denervation of the trapezius muscle, contributes significantly to the postoperative morbidity of radical neck dissections. Multiple techniques exist for the reinnervation of muscles that have injured motor nerves. Reinnervation of denervated trapezius muscles was examined in the New Zealand white rabbit by use of three techniques of reinnervation: (1) neuromuscular pedicle transfer of the accessory nerve from the trapezius muscle, (2) direct accessory nerve implantation, and (3) neuromuscular pedicle transfer of the accessory nerve from the sternocleidomastoid muscle. The reinnervated trapezius muscles were examined grossly by direct nerve stimulation, electrophysiologically by evoked electromyography, and histologically by enzymatic muscle staining and silver-reducing nerve staining. The gross, electrophysiologic, and histologic results confirmed successful reinnervation of the trapezius muscle within 6 weeks of operation. No significant difference was observed between the various techniques of reinnervation.