Quantification of human upper extremity nerves and fascicular anatomy

Biomimetic Peripheral Nerve Stimulation Promotes the Rat Hindlimb Motion Modulation in Stepping: An Experimental Analysis

Peripheral nerve stimulation is an effective neuromodulation method in patients with lower extremity movement disorders caused by stroke, spinal cord injury, or other diseases. However, most current studies on rehabilitation using sciatic nerve stimulation focus solely on ankle motor regulation through stimulation of common peroneal and tibial nerves. Using the electrical nerve stimulation method, we here achieved muscle control via different sciatic nerve branches to facilitate the regulation of lower limb movements during stepping and standing. A map of relationships between muscles and nerve segments was established to artificially activate specific nerve fibers with the biomimetic stimulation waveform. Then, characteristic curves depicting the relationship between neural electrical stimulation intensity and joint control were established. Finally, by testing the selected stimulation parameters in anesthetized rats, we confirmed that single-cathode extraneural electrical stimulation could activate combined movements to promote lower limb movements. Thus, this method is effective and reliable for use in treatment for improving and rehabilitating lower limb motor dysfunction.

Typical CIDP, distal variant CIDP, and anti-MAG antibody neuropathy: An ultra-high frequency ultrasound comparison of nerve structure

To date, little is known about the usefulness of ultra-high frequency ultrasound (UHF-US, 50-70 MHz) in clinical practice for the diagnosis of dysimmune neuropathies. We present a prospective study aimed at comparing UHF-US alterations of nerves and fascicles in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), distal CIDP (d-CIDP) and anti-MAG neuropathy and their relationships with clinical and electrodiagnostic (EDX) features. 28 patients were included (twelve CIDP, 6 d-CIDP and 10 anti-MAG) and ten healthy controls. Each patient underwent neurological examination, EDX and UHF-US study of median and ulnar nerves bilaterally. UHF-US was reliable in differentiating immune neuropathies from controls when using mean and/or segmental nerve and/or fascicle cross-sectional area (CSA); furthermore, fascicle ratio (fascicle/nerve CSA) was a reliable factor for differentiating d-CIDP from other types of polyneuropathies. The fascicle CSA appears to be more increased in CIDP and its variant than in anti-MAG neuropathy. UHF-US offers information beyond simple nerve CSA and allows for a better characterization of the different forms of dysimmune neuropathies.

Morphology and morphometry of the ulnar nerve in the forelimb of pigs

The knowledge of the morphology and morphometry of peripheral nerves is essential for developing neural interfaces and understanding nerve regeneration in basic and applied research. Currently, the most adopted animal model is the rat, even though recent studies have suggested that the neuroanatomy of large animal models is more comparable to humans. The present knowledge of the morphological structure of large animal models is limited; therefore, the present study aims to describe the morphological characteristics of the Ulnar Nerve (UN) in pigs. UN cross-sections were taken from seven Danish landrace pigs at three distinct locations: distal UN, proximal UN and at the dorsal cutaneous branch of the UN (DCBUN). The nerve diameter, fascicle diameter and number, number of fibres and fibre size were quantified. The UN diameter was larger in the proximal section compared to the distal segment and the DCBUN. The proximal branch also had a more significant number of fascicles (median: 15) than the distal (median: 10) and the DCBUN (median: 11) segments. Additionally, the mean fascicle diameter was smaller at the DCBUN (mean: 165 μm) than at the distal (mean: 197 μm) and proximal (mean: 199 μm) segments of the UN. Detailed knowledge of the microscopical structure of the UN in pigs is critical for further studies investigating neural interface designs and computational models of the peripheral nervous system.

Selective peripheral nerve recording using simulated human median nerve activity and convolutional neural networks

BACKGROUND

It is difficult to create intuitive methods of controlling prosthetic limbs, often resulting in abandonment. Peripheral nerve interfaces can be used to convert motor intent into commands to a prosthesis. The Extraneural Spatiotemporal Compound Action Potentials Extraction Network (ESCAPE-NET) is a convolutional neural network (CNN) that has previously been demonstrated to be effective at discriminating neural sources in rat sciatic nerves. ESCAPE-NET was designed to operate using data from multi-channel nerve cuff arrays, and use the resulting spatiotemporal signatures to classify individual naturally evoked compound action potentials (nCAPs) based on differing source fascicles. The applicability of this approach to larger and more complex nerves is not well understood. To support future translation to humans, the objective of this study was to characterize the performance of this approach in a computational model of the human median nerve.

METHODS

Using a cross-sectional immunohistochemistry image of a human median nerve, a finite-element model was generated and used to simulate extraneural recordings. ESCAPE-NET was used to classify nCAPs based on source location, for varying numbers of sources and noise levels. The performance of ESCAPE-NET was also compared to ResNet-50 and MobileNet-V2 in the context of classifying human nerve cuff data.

RESULTS

Classification accuracy was found to be inversely related to the number of nCAP sources in ESCAPE-NET (3-class: 97.8% ± 0.1%; 10-class: 89.3% ± 5.4% in low-noise conditions, 3-class: 70.3% ± 0.1%; 10-class: 52.5% ± 0.3% in high-noise conditions). ESCAPE-NET overall outperformed both MobileNet-V2 (3-class: 96.5% ± 1.1%; 10-class: 84.9% ± 1.7% in low-noise conditions, 3-class: 86.0% ± 0.6%; 10-class: 41.4% ± 0.9% in high-noise conditions) and ResNet-50 (3-class: 71.2% ± 18.6%; 10-class: 40.1% ± 22.5% in low-noise conditions, 3-class: 81.3% ± 4.4%; 10-class: 31.9% ± 4.4% in high-noise conditions).

CONCLUSION

All three networks were found to learn to differentiate nCAPs from different sources, as evidenced by performance levels well above chance in all cases. ESCAPE-NET was found to have the most robust performance, despite decreasing performance as the number of classes increased, and as noise was varied. These results provide valuable translational guidelines for designing neural interfaces for human use.

Modeling the excitation of nerve axons under transcutaneous stimulation

Computational models enable a safe and convenient way to study the excitation of nerve fibers under external stimulation. Contemporary models calculate the electric field distribution from transcutaneous stimulation and the resulting neuronal response separately. This study uses finite element methods to develop a multi-scale model that couples electric fields within macroscopic tissue layers and microscopic nerve fibers in a single-stage computational framework. The model included a triaxial myelinated nerve fiber bundle embedded within a volume conductor of tissue layers to represent the median nerve innervating the forearm muscles. The model captured the excitability of nerve fibers under transcutaneous stimulation and their nerve-tissue interactions to a transient external stimulus. The determinants of the strength-duration curve, rheobase, and chronaxie for the proposed model had close correlations with in-vivo experimentation on human participants. Additionally, the excitability indices for the triaxial myelinated nerve fiber implemented using the finite element method agreed well with experimental data from the literature. The validity of the proposed model encourages its use for applications involving transcutaneous stimulation. Capable of capturing field distribution across realistic morphologies, the model can serve as a testbed to improve stimulation protocols and electrode designs with subject-level specificity.

A pilot prospective cohort study using experimental quantification of early peripheral nerve regeneration with high-frequency three-dimensional tomographic ultrasound (HFtUS)

Quantification of peripheral nerve regeneration after injury relies upon subjective outcome measures or electrophysiology assessments requiring fully regenerated neurons. Nerve surgeons and researchers lack objective, quantifiable information on the site of surgical repair and regenerative front. To address this need, we developed a quantifiable, visual, clinically available measure of early peripheral nerve regeneration using high-frequency, three-dimensional, tomographic ultrasound (HFtUS). We conducted a prospective, longitudinal study of adult patients with ulnar and/or median nerve injury of the arm undergoing direct epineurial repair within 5 days of injury. Assessment of morphology, volumetric and 3D grey-scale quantification of cross-sectional views were made at baseline up to 15 months post-surgery. Sensory and motor clinical outcome measures and patient reported outcome measures (PROMs) were recorded. Five participants were recruited to the study. Our data demonstrated grey-scale values (an indication of axonal density) increased in distal stumps within 2-4 months after repair, returning to normal as regeneration completed (4-6 months) with concomitant reduction in intraneural volume as surgical oedema resolved. Two patients with abnormal regeneration were characterized by increased intraneural volume and minimal grey-scale change. HFtUS may quantify early peripheral nerve regeneration offering a window of opportunity for surgical intervention where early abnormal regeneration is detected.

Interfascicular Anatomy of the Motor Branch of the Ulnar Nerve: A Cadaveric Study

PURPOSE

The motor branch of the ulnar nerve contains fascicles that innervate the intrinsic musculature of the hand. This cadaveric study aimed to describe the organization and consistency of the internal topography of the motor branch of the ulnar nerve.

METHODS

Five fresh-frozen cadaveric specimens with an average age of 74 years (range, 65-88 years) were dissected. The ulnar nerve was exposed and transfixed to the underlying tissues to maintain its orientation throughout the dissection. The dorsal cutaneous branch (DCB) and the volar sensory branch were identified and reflected to expose the motor branch. The fascicles to the first dorsal interosseus (FDI), flexor pollicis brevis, and abductor digiti minimi (ADM) were identified. Internal neurolysis was performed distal to proximal to identify the interfascicular arrangement of these fascicles within the motor branch. The organization of these fascicles was noted, and the branch points of the DCB, FDI, and ADM were measured relative to the pisiform using a handheld electronic caliper.

RESULTS

The internal topography of the motor branch was consistent among all specimens. Proximal to the pisiform, the arrangement from radial to ulnar was as follows: volar sensory branch, flexor pollicis brevis, FDI/intrinsic muscles, ADM, and DCB. The position of these branches remained consistent as the deep motor branch curved radially within the palm and traveled to the terminal musculature. The locations of the average branch points of the FDI, ADM, and DCB with respect to the pisiform were as follows: FDI, 4.6 cm distal (range, 4.1-4.9 cm), 4.5 cm radial (range, 4.1-4.9 cm); ADM, 0.65 cm distal (range, 0.3-1.1 cm), 0.7 cm radial (range, 0.3-1.1 cm), DCB, 7.7 cm proximal (range, 4.2-10.1 cm), and 0.4 cm ulnar (range, 0.3-0.8 cm).

CONCLUSIONS

The internal topography of the ulnar nerve motor branch was consistent among the specimens studied. The topography of the motor branches was maintained as the motor branch turns radially within the palm.

CLINICAL RELEVANCE

This study provides further understanding of the internal topography of the ulnar nerve motor branch at the wrist level.

Thermosensitive hydrogel carrying extracellular vesicles from adipose-derived stem cells promotes peripheral nerve regeneration after microsurgical repair

Peripheral nerve injuries are commonly occurring traumas of the extremities; functional recovery is hindered by slow nerve regeneration (<1 mm/day) following microsurgical repair and subsequent muscle atrophy. Functional recovery after peripheral nerve repair is highly dependent on local Schwann cell activity and axon regeneration speed. Herein, to promote nerve regeneration, paracrine signals of adipose-derived stem cells were applied in the form of extracellular vesicles (EVs) loaded in a thermosensitive hydrogel (PALDE) that could solidify rapidly and sustain high EV concentration around a repaired nerve during surgery. Cell experiments revealed that PALDE hydrogel markedly promotes Schwann-cell migration and proliferation and axon outgrowth. In a rat sciatic nerve repair model, the PALDE hydrogel increased repaired-nerve conduction efficacy; contraction force of leg muscles innervated by the repaired nerve also recovered. Electromicroscopic examination of downstream nerves indicated that fascicle diameter and myeline thickness in the PALDE group (1.91 ± 0.61 and 1.06 ± 0.40 μm, respectively) were significantly higher than those in PALD and control groups. Thus, this EV-loaded thermosensitive hydrogel is a potential cell-free therapeutic modality to improve peripheral-nerve regeneration, offering sustained and focused EV release around the nerve-injury site to overcome rapid clearance and maintain EV bioactivity in vivo.

A systematic review: normative reference values of the median nerve cross-sectional area using ultrasonography in healthy individuals

Median nerve cross-sectional area (CSA) was used for screening and diagnosis of neuropathy, but few studies have suggested reference range. Hence, this systematic review was performed to evaluate a normative values of median nerve CSA at various landmarks of upper limb based on ultrasonography. PubMed and Web of science were used to search relevant articles from 2000 to 2020. Forty-one eligible articles (2504 nerves) were included to access median nerve CSA at different landmarks (mid-arm, elbow, mid-forearm, carpal tunnel (CT) inlet and CT outlet). Data was also stratified based on age, sex, ethnicity, geographical location, and method of measurement. Random effects model was used to calculate pooled weighted mean (95% confidence interval (CI), [upper bound, lower bound]) at mid-arm, elbow, mid-forearm, CT inlet and outlet which found to be 8.81 mm², CI [8.10, 9.52]; 8.57 mm² [8.00, 9.14]; 7.07 mm² [6.41, 7.73]; 8.74 mm² [8.45, 9.03] and 9.02 mm² [8.08, 9.95] respectively. Median nerve CSA varies with age, geographical location, and sex at all landmarks. A low (I² < 25%) to considerable heterogeneity (I² > 75%) was observed, indicating the variation among the included studies. These findings show that median nerve CSA is varying not only along its course but also in other sub-variables.

Clinical Basis for Creating an Osseointegrated Neural Interface

As technology continues to improve within the neuroprosthetic landscape, there has been a paradigm shift in the approach to amputation and surgical implementation of haptic neural prosthesis for limb restoration. The Osseointegrated Neural Interface (ONI) is a proposed solution involving the transposition of terminal nerves into the medullary canal of long bones. This design combines concepts of neuroma formation and prevention with osseointegration to provide a stable environment for conduction of neural signals for sophisticated prosthetic control. While this concept has previously been explored in animal models, it has yet to be explored in humans. This anatomic study used three upper limb and three lower limb cadavers to assess the clinical feasibility of creating an ONI in humans. Anatomical measurement of the major peripheral nerves- circumference, length, and depth- were performed as they are critical for electrode design and rerouting of the nerves into the long bones. CT imaging was used for morphologic bone evaluation and virtual implantation of two osseointegrated implants were performed to assess the amount of residual medullary space available for housing the neural interfacing hardware. Use of a small stem osseointegrated implant was found to reduce bone removal and provide more intramedullary space than a traditional implant; however, the higher the amputation site, the less medullary space was available regardless of implant type. Thus the stability of the endoprosthesis must be maximized while still maintaining enough residual space for the interface components. The results from this study provide an anatomic basis required for establishing a clinically applicable ONI in humans. They may serve as a guide for surgical implementation of an osseointegrated endoprosthesis with intramedullary electrodes for prosthetic control.

Median and ulnar nerve fascicle imaging using MR microscopy and high-resolution ultrasound

BACKGROUND AND PURPOSE

Understanding nerve microanatomy is important as different neuropathies and some nerve neoplasms present with fascicle enlargement. The aim of our study was to gain clinically oriented knowledge on nerve fascicular anatomy using imaging modalities.

METHODS

On a cadaveric upper extremity, high-resolution ultrasound (HRUS) scan with 22 MHz probe was performed. Sections of the median and ulnar nerves were excised at the level of the distal arm and after magnetic resonance microscopy (MRM), histological cross-sections (HCS) were prepared. Cross-referencing of the MRM and HRUS images with HCS was performed. Fascicle and nerve contouring was performed with morphometric software in order to assess nerve and fascicular cross-sectional area (CSA), fascicle count, and interfascicular distances. Based on fascicle differentiation, factual fascicle (FF) group and fascicular cluster (FC) group were defined.

RESULTS

On the cross-referenced imaging material, fascicles were differentiated in 92.7% on MRM and in 57.3% on HRUS. High to very high positive correlation among imaging material was observed for the fascicle CSA. FF depiction was 30.1% on HRUS. In comparison to the FF group, the FC group had significantly larger fascicle CSA and shorter interfascicular distances.

DISCUSSION

The findings of our study contribute to understanding of fascicle depiction on imaging modalities. HRUS offers good visualization of fascicles. The capability of differentiating fascicles is modality specific and depends on the fascicle CSA and the amount of interfascicular epineurium.

Directed stimulation with interfascicular interfaces for peripheral nerve stimulation

Objective.Computational models have shown that directional electrical contacts placed within the epineurium, between the fascicles, and not penetrating the perineurium, can achieve selectivity levels similar to point source contacts placed within the fascicle. The objective of this study is to test, in a murine model, the hypothesis that directed interfascicular contacts are selective.Approach.Multiple interfascicular electrodes with directional contacts, exposed on a single face, were implanted in the sciatic nerves of 32 rabbits. Fine-wire intramuscular wire electrodes were implanted to measure electromyographic (EMG) activity from medial and lateral gastrocnemius, soleus, and tibialis anterior muscles.Main results.The recruitment data demonstrated that directed interfascicular interfaces, which do not penetrate the perineurium, selectively activate different axon populations.Significance.Interfascicular interfaces that are inside the nerve, but do not penetrate the perineurium are an alternative to intrafascicular interfaces and may offer additional selectivity compared to extraneural approaches.

Review Article "Spotlight on Ultrasonography in the Diagnosis of Peripheral Nerve Disease: The Evidence to Date"

Neuromuscular ultrasound is rapidly becoming incorporated into clinical practice as a standard tool in the assessment of peripheral nerve diseases. Ultrasound complements clinical phenotyping and electrodiagnostic evaluation, providing critical structural anatomical information to enhance diagnosis and identify structural pathology. This review article examines the evidence supporting neuromuscular ultrasound in the diagnosis of compressive mononeuropathies, traumatic nerve injury, generalised peripheral neuropathy and motor neuron disease. Extending the sonographic evaluation of nerves beyond simple morphological measurements has the potential to improve diagnostics in peripheral neuropathy, as well as advancing the understanding of pathological mechanisms, which in turn will promote precise therapies and improve therapeutic outcomes.

Adjustable conduits for guided peripheral nerve regeneration prepared from bi-zonal unidirectional and multidirectional laminar scaffold of type I collagen

Shortness of donor nerves has led to the development of nerve conduits that connect sectioned peripheral nerve stumps and help to prevent the formation of neuromas. Often, the standard diameters of these devices cannot be adapted at the time of surgery to the diameter of the nerve injured. In this work, scaffolds were developed to form filled nerve conduits with an inner matrix with unidirectional channels covered by a multidirectional pore zone. Collagen type I dispersions (5 mg/g and 8 mg/g) were sequentially frozen using different methods to obtain six laminar scaffolds (P1 to P5) formed by a unidirectional (U) pore/channel zone adjacent to a multidirectional (M) pore zone. The physicochemical and microstructural properties of the scaffolds were determined and compared, as well as their biodegradability, residual glutaraldehyde and cytocompatibility. Also, the Young's modulus of the conduits made by rolling up the bizonal scaffolds from the unidirectional to the multidirectional zone was determined. Based on these comparisons, the proliferation and differentiation of hASC were assessed only in the P3 scaffolds. The cells adhered, aligned in the same direction as the unidirectional porous fibers, proliferated, and differentiated into Schwann-like cells. Adjustable conduits made with the P3 scaffold were implanted in rats 10 mm sciatic nerve lesions to compare their performance with that of autologous sciatic nerve grafted lesions. The in vivo results demonstrated that the tested conduit can be adapted to the diameter of the nerve stumps to guide their growth and promote their regeneration.

Textural markers of ultrasonographic nerve alterations in amyotrophic lateral sclerosis

Ultrasound has revealed cross-sectional nerve area (CSA) reduction in amyotrophic lateral sclerosis (ALS), but little is known about the sonographic nerve texture beyond CSA alterations. In a large cohort of 177 ALS patients and 57 control subjects, we investigated the covariance and disease-specific signature of several sonographic texture features of the median and ulnar nerves and their relationship to the patients' clinical characteristics. ALS patients showed atrophic nerves, a loss of the intranerve structures' echoic contrast, elevated coarseness, and a trend toward lower cluster shading compared with controls. A reduction in intranerve echoic contrast was related to longer disease duration and poorer functional status in ALS. Sonographic texture markers point toward a significant reorganization of the deep nerve microstructure in ALS. Future studies will be needed to further substantiate the markers' potential to assess peripheral nerve alterations in ALS.

Modelling the effects of ephaptic coupling on selectivity and response patterns during artificial stimulation of peripheral nerves

Artificial electrical stimulation of peripheral nerves for sensory feedback restoration can greatly benefit from computational models for simulation-based neural implant design in order to reduce the trial-and-error approach usually taken, thus potentially significantly reducing research and development costs and time. To this end, we built a computational model of a peripheral nerve trunk in which the interstitial space between the fibers and the tissues was modelled using a resistor network, thus enabling distance-dependent ephaptic coupling between myelinated axons and between fascicles as well. We used the model to simulate a) the stimulation of a nerve trunk model with a cuff electrode, and b) the propagation of action potentials along the axons. Results were used to investigate the effect of ephaptic interactions on recruitment and selectivity stemming from artificial (i.e., neural implant) stimulation and on the relative timing between action potentials during propagation. Ephaptic coupling was found to increase the number of fibers that are activated by artificial stimulation, thus reducing the artificial currents required for axonal recruitment, and it was found to reduce and shift the range of optimal stimulation amplitudes for maximum inter-fascicular selectivity. During propagation, while fibers of similar diameters tended to lock their action potentials and reduce their conduction velocities, as expected from previous knowledge on bundles of identical axons, the presence of many other fibers of different diameters was found to make their interactions weaker and unstable.

Influence of limb position on assessment of nerve mechanical properties by using shear wave ultrasound elastography

INTRODUCTION

Evaluation of nerve mechanical properties has the potential to improve assessment of nerve impairment. Shear wave velocity, as measured by using shear wave (SW) ultrasound elastography, is a promising indicator of nerve mechanical properties such as stiffness. However, elucidation of external factors that influence SW velocity, particularly nerve tension, is required for accurate interpretations.

METHODS

Median and ulnar nerve SW velocities were measured at proximal and distal locations with limb positions that indirectly altered nerve tension.

RESULTS

Shear wave velocity was greater at proximal and distal locations for limb positions that induced greater tension in the median (mean increase proximal 89.3%, distal 64%) and ulnar (mean increase proximal 91.1%, distal 37.4%) nerves.

DISCUSSION

Due to the influence of nerve tension when SW ultrasound elastography is used, careful consideration must be given to limb positioning.

Intraoperative Responses May Predict Chronic Performance of Composite Flat Interface Nerve Electrodes on Human Femoral Nerves

Peripheral nerve cuff electrodes (NCEs) in motor system neuroprostheses can generate strong muscle contractions and enhance surgical efficiency by accessing multiple muscles from a single proximal location. Predicting chronic performance of high contact density NCEs based on intraoperative observations would facilitate implantation at locations that maximize selective recruitment, immediate connection of optimal contacts to implanted pulse generators (IPGs) with limited output channels, and initiation of postoperative rehabilitation as soon as possible after surgery. However, the stability of NCE intraoperative recruitment to predict chronic performance has not been documented. Here we report the first-in-human application of a specific NCE, the composite flat interface nerve electrode (C-FINE), at a new and anatomically challenging location on the femoral nerve close to the inguinal ligaments. EMG and moment recruitment curves were recorded for each of the 8 contacts in 2 C-FINE intraoperatively, perioperatively, and chronically for 6 months. Intraoperative measurements predicted chronic outcomes for 87.5% of contacts with 14/16 recruiting the same muscles at 6 months as intraoperatively. In both 8-contact C-FINEs, 3 contacts elicited hip flexion and 5 selectively generated knee extension, 3 of which activated independent motor unit populations each sufficient to support standing. Recruitment order stabilized in less than 3 weeks and did not change thereafter. While confirmation of these results will be required with future studies and implant locations, this suggests that remobilization and stimulated exercise may be initiated 3 weeks after surgery with little risk of altering performance.

Comparison between optical coherence tomography imaging and histological sections of peripheral nerves

OBJECTIVE

Optical coherence tomography (OCT) is an imaging technique that uses the light-backscattering properties of different tissue types to generate an image. In an earlier feasibility study the authors showed that it can be applied to visualize human peripheral nerves. As a follow-up, this paper focuses on the interpretation of the images obtained.

METHODS

Ten different short peripheral nerve specimens were retained following surgery. In a first step they were examined by OCT during, or directly after, surgery. In a second step the nerve specimens were subjected to histological examination. Various steps of image processing were applied to the OCT raw data acquired. The improved OCT images were compared with the sections stained by H & E. The authors assigned the structures in the images to the various nerve components including perineurium, fascicles, and intrafascicular microstructures.

RESULTS

The results show that OCT is able to resolve the myelinated axons. A weighted averaging filter helps in identifying the borders of structural features and reduces artifacts at the same time. Tissue-remodeling processes due to injury (perineural fibrosis or neuroma) led to more homogeneous light backscattering. Anterograde axonal degeneration due to sharp injury led to a loss of visible axons and to an increase of light-backscattering tissue as well. However, the depth of light penetration is too small to allow generation of a complete picture of the nerve.

CONCLUSIONS

OCT is the first in vivo imaging technique that is able to resolve a nerve's structures down to the level of myelinated axons. It can yield information about focal and segmental pathologies.

A translational framework for peripheral nerve stimulating electrodes: Reviewing the journey from concept to clinic

The purpose of this review article is to describe the underlying methodology for successfully translating novel interfaces for electrical modulation of the peripheral nervous system (PNS) from basic design concepts to clinical applications and chronic human use. Despite advances in technologies to communicate directly with the nervous system, the pathway to clinical translation for most neural interfaces is not clear. FDA guidelines provide information on necessary evidence which should be generated and submitted to allow the agency evaluate safety and efficacy of a new medical device. However, a knowledge gap exists on translating neural interfaces from pre-clinical studies into the clinical domain. Our article is intended to inform the field on some of the key considerations for such a transition process specific to neural interfaces that may not be already covered by FDA guidances. This framework focuses on non-penetrating peripheral nerve stimulating electrodes that have been proven effective for motor and sensory neural prostheses and successfully transitioned from pre-clinical through first-in-human and chronic clinical deployment. We discuss the challenges of moving these neural interfaces along the translational continuum and ultimately through FDA approval for human feasibility studies. Specifically, we describe a translational process involving: quantitative human anatomy, neural modeling and simulation, acute intraoperative testing and verification, clinical demonstration with temporary percutaneous access, and finally chronic clinical deployment and functional performance. To clarify and demonstrate the importance of each step of this translational framework, we present case studies from electrodes developed at Case Western Reserve University (CWRU), specifically the spiral cuff, the Flat Interface Nerve Electrode (FINE), and the Composite FINE (C-FINE). In addition, we demonstrate that success along this translational pathway can be further expedited by: appropriate selection of well-characterized materials, validation of fabrication and sterilization protocols, well-implemented quality control measures, and quantification of impact on neural structure, health, and function. The issues and approaches identified in this review for the peripheral nervous system may also serve to accelerate the dissemination of any new neural interface into clinical practice, and consequently advance the performance, utility, and clinical value of new neural prostheses or neuromodulation systems.

Comparison of high-frequency and ultrahigh-frequency probes in chronic inflammatory demyelinating polyneuropathy

OBJECTIVES

High-frequency ultrasound (HFUS 18-20 MHz) performed on patients with chronic inflammatory demyelinating polyneuropathy (CIDP) shows a focal enlargement, particularly in the proximal segments of upper-arm motor nerves. Ultrahigh frequency ultrasound (UHFUS 30-70 MHz), having a higher spatial resolution, enables a better characterization of nerve structures. The aim of this study was to compare the two ultrasound probes in the evaluation of motor nerve characteristics in CIDP patients.

METHODS

Eleven patients with definite or probable CIDP underwent an ultrasound evaluation of median and ulnar nerves, bilaterally. Nerve and fascicle cross-sectional area (CSA), vascularization, and echogenicity were assessed.

RESULTS

Nerve and fascicle CSA were increased in the proximal segments, especially in the median nerve, in 9/11 patients and in 10/11 patients at the HFUS and UHFUS evaluations, respectively. A statistically significant difference between CSA values obtained with the two probes was found only for fascicle values. UHFUS allowed for a more precise estimation of fascicle size and number than the HFUS. We were able to identify nerve vascularization in 4/11 patients at UHFUS only.

CONCLUSION

UHFUS gives more detailed information on the changes in the internal nerve structure in CIDP patients. In particular, it permits to better characterize fascicle size and morphology, and to have a precise estimation of their number. Its frequency range also allows to evaluate nerve vascularization.

SIGNIFICANCE

Ultrasound evaluation could become an adjunctive diagnostic tool for CIDP. Further studies are needed to validate the examined parameters as biomarkers for the evaluation and follow-up of CIDP patients.

Nerve Diameter in the Hand: A Cadaveric Study

Background

Nerve injuries in the hand are common and often pose a challenge for the upper extremity surgeon. A range of reconstructive options exist for nerve repair, but proper matching of nerve diameter is important for many of these techniques. The purposes of this study were to (1) describe the nerve diameters of the hand and their relative differences and (2) investigate whether there is a relationship between nerve diameter and external hand dimensions or body mass index.

Methods

We utilized 18 freshly frozen cadaveric hands from adult donors aged 20-86 of both sexes for this study. Two independent observers measured each nerve diameter to the nearest 0.1 mm using a digital caliper.

Results

Using the flexor zones as boundaries, a total of 33 nerve measurements were performed for each cadaveric hand. Nerve diameter increased from the distal to the proximal flexor zones. The internal common digital nerves in flexor zone 3 were larger than the external digital nerves. The median nerve was found to be nearly 2 times larger than the ulnar nerve at 2 locations within the wrist. There was a positive correlation between body mass index, hand span, hand width, and nerve diameter at several measured locations.

Conclusions

This study provides reference values for nerve diameters of the hand and wrist and describes their relative differences. It is important for surgeons to be aware of these differences and to consider this information as we advance our efforts to reconstruct the hand and develop technologies for nerve repair.

Biomimetic encoding model for restoring touch in bionic hands through a nerve interface

OBJECTIVE

Hand function can be restored in upper-limb amputees by equipping them with anthropomorphic prostheses controlled with signals from residual muscles. The dexterity of these bionic hands is severely limited in large part by the absence of tactile feedback about interactions with objects. We propose that, to the extent that artificial touch mimics its natural counterpart, these sensory signals will be more easily integrated into the motor plan for object manipulation.

APPROACH

We describe an approach to convey tactile feedback through electrical stimulation of the residual somatosensory nerves that mimics the aggregate activity of tactile fibers that would be produced in the nerve of a native hand during object interactions. Specifically, we build a parsimonious model that maps the stimulus-described as time-varying indentation depth, indentation rate, and acceleration-into continuous estimates of the time-varying population firing rate and of the size of the recruited afferent population.

MAIN RESULTS

The simple model can reconstruct aggregate afferent responses to a wide range of stimuli, including those experienced during activities of daily living.

SIGNIFICANCE

We discuss how the proposed model can be implemented with a peripheral nerve interface and anticipate it will lead to improved dexterity for prosthetic hands.

Evaluation of high-density, multi-contact nerve cuffs for activation of grasp muscles in monkeys

OBJECTIVE

The objective of this work was to evaluate whether nerve cuffs can selectively activate hand muscles for functional electrical stimulation (FES). FES typically involves identifying and implanting electrodes in many individual muscles, but nerve cuffs only require implantation at a single site around the nerve. This method is surgically more attractive. Nerve cuffs may also more effectively stimulate intrinsic hand muscles, which are difficult to implant and stimulate without spillover to adjacent muscles.

APPROACH

To evaluate its ability to selectively activate muscles, we implanted and tested the flat interface nerve electrode (FINE), which is designed to selectively stimulate peripheral nerves that innervate multiple muscles (Tyler and Durand 2002 IEEE Trans. Neural Syst. Rehabil. Eng. 10 294-303). We implanted FINEs on the nerves and bipolar intramuscular wires for recording compound muscle action potentials (CMAPs) from up to 20 muscles in each arm of six monkeys. We then collected recruitment curves while the animals were anesthetized.

MAIN RESULT

A single FINE implanted on an upper extremity nerve in the monkey can selectively activate muscles or small groups of muscles to produce multiple, independent hand functions.

SIGNIFICANCE

FINE cuffs can serve as a viable supplement to intramuscular electrodes in FES systems, where they can better activate intrinsic and extrinsic muscles with lower currents and less extensive surgery.

[Visualization research of three-dimensional microstructure of rabbit sciatic nerve bundles by micro-CT]

Objective

To realize the visualization of three-dimensional microstructure of rabbit sciatic nerve bundles by micro-CT and three-dimensional visualization software Mimics17.0.

Methods

The sciatic nerve tissues from 6 New Zealand rabbits were divided into 2 groups ( n=3), and the sciatic nerve tissues were stained by 1% (group A) and 5% (group B) Lugol solution respectively. After staining for 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 hours, the imaging changes of specimens were observed by light microscope and micro-CT. The clear micro-CT images were exported to the Mimics software to complete the visualization of three-dimensional microstructure of rabbit sciatic nerve according to three-dimensional reconstruction tool.

Results

The clear three-dimensional microstructure images could be observed in group A at 2.5 hours after staining and in group B at 1.5 hours after staining by light microscope and micro-CT. The sciatic nerve of New Zealand rabbits were divides into 3 bundles and each of them was relatively fixed. There was no obvious crossing or mergers between each bundle. The cross-sectional area of each bundle was (0.425±0.013), (0.038±0.007), and (0.242±0.026) mm ² respectively. The digital model could clearly reflect the microstructure of the sciatic nerve at all cross sections.

Conclusion

The internal structure of New Zealand rabbits sciatic nerve can be clearly reflected by micro-CT scanning. It provides a reliable method for establishing a nerve microstructure database with large amount specimens.

A look inside the nerve - Morphology of nerve fascicles in healthy controls and patients with polyneuropathy

OBJECTIVE

Polyneuropathies are increasingly analyzed by ultrasound. Summarizing, diffuse enlargement is typical in Charcot-Marie Tooth type 1 (CMT1a), regional enlargement occurs in inflammatory neuropathies. However, a distinction of subtypes is still challenging. Therefore, this study focused on fascicle size and pattern in controls and distinct neuropathies.

METHODS

Cross-sectional area (CSA) of the median, ulnar and peroneal nerve (MN, UN, PN) was measured at predefined landmarks in 50 healthy controls, 15 CMT1a and 13 MMN patients. Additionally, largest fascicle size and number of visible fascicles was obtained at the mid-upper arm cross-section of the MN and UN and in the popliteal fossa cross-section of the PN.

RESULTS

Cut-off normal values for fascicle size in the MN, UN and PN were defined (<4.8mm², <2.8mm² and <3.5mm²). In CMT1a CSA and fascicle values are significantly enlarged in all nerves, while in MMN CSA and fascicles are regionally enlarged with predominance in the upper arm nerves. The ratio of enlarged fascicles and all fascicles was significantly increased in CMT1a (>50%) in all nerves (p<0.0001), representing diffuse fascicle enlargement, and moderately increased in MMN (>20%), representing differential fascicle enlargement (enlarged and normal fascicles at the same location) sparing the peroneal nerve (regional fascicle enlargement). Based on these findings distinct fascicle patterns were defined.

CONCLUSION

Normal values for fascicle size could be evaluated; while CMT1a features diffuse fascicle enlargement, MMN shows regional and differential predominance with enlarged fascicles as single pathology.

SIGNIFICANCE

Pattern analysis of fascicles might facilitate distinction of several otherwise similar neuropathies.

Stability and selectivity of a chronic, multi-contact cuff electrode for sensory stimulation in human amputees

OBJECTIVE

Stability and selectivity are important when restoring long-term, functional sensory feedback in individuals with limb-loss. Our objective is to demonstrate a chronic, clinical neural stimulation system for providing selective sensory response in two upper-limb amputees.

APPROACH

Multi-contact cuff electrodes were implanted in the median, ulnar, and radial nerves of the upper-limb.

MAIN RESULTS

Nerve stimulation produced a selective sensory response on 19 of 20 contacts and 16 of 16 contacts in subjects 1 and 2, respectively. Stimulation elicited multiple, distinct percept areas on the phantom and residual limb. Consistent threshold, impedance, and percept areas have demonstrated that the neural interface is stable for the duration of this on-going, chronic study.

SIGNIFICANCE

We have achieved selective nerve response from multi-contact cuff electrodes by demonstrating characteristic percept areas and thresholds for each contact. Selective sensory response remains consistent in two upper-limb amputees for 1 and 2 years, the longest multi-contact sensory feedback system to date. Our approach demonstrates selectivity and stability can be achieved through an extraneural interface, which can provide sensory feedback to amputees.