Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Facial nerve regeneration via body-brain crosstalk: The role of stem cells and biomaterials

The human body is a complex, integral whole, and disruptions in one organ can lead to dysfunctions in other parts of the organ network. The facial nerve, as the seventh cranial nerve, arises from the brainstem, controls facial expression muscles and plays a crucial role in brain-body communication. This vulnerable nerve can be damaged by trauma, inflammation, tumors, and congenital diseases, often impairing facial expression. Stem cells have gained significant attention for repairing peripheral nerve injuries due to their multidirectional differentiation potential. Additionally, various biomaterials have been used in tissue engineering for regeneration and repair. However, the therapeutic potential of stem cells and biomaterials in treating facial nerve injuries requires further exploration. In this review, we summarize the roles of stem cells and biomaterials in the regeneration and repair of damaged facial nerves, providing a theoretical basis for the recovery and reconstruction of body-brain crosstalk between the brain and facial expression muscles.

Biohacking Nerve Repair: Novel Biomaterials, Local Drug Delivery, Electrical Stimulation, and Allografts to Aid Surgical Repair

The regenerative capacity of the peripheral nervous system is limited, and peripheral nerve injuries often result in incomplete healing and poor outcomes even after repair. Transection injuries that induce a nerve gap necessitate microsurgical intervention; however, even the current gold standard of repair, autologous nerve graft, frequently results in poor functional recovery. Several interventions have been developed to augment the surgical repair of peripheral nerves, and the application of functional biomaterials, local delivery of bioactive substances, electrical stimulation, and allografts are among the most promising approaches to enhance innate healing across a nerve gap. Biocompatible polymers with optimized degradation rates, topographic features, and other functions provided by their composition have been incorporated into novel nerve conduits (NCs). Many of these allow for the delivery of drugs, neurotrophic factors, and whole cells locally to nerve repair sites, mitigating adverse effects that limit their systemic use. The electrical stimulation of repaired nerves in the perioperative period has shown benefits to healing and recovery in human trials, and novel biomaterials to enhance these effects show promise in preclinical models. The use of acellular nerve allografts (ANAs) circumvents the morbidity of donor nerve harvest necessitated by the use of autografts, and improvements in tissue-processing techniques may allow for more readily available and cost-effective options. Each of these interventions aid in neural regeneration after repair when applied independently, and their differing forms, benefits, and methods of application present ample opportunity for synergistic effects when applied in combination.

Facial Aberrant Reinnervation Syndrome Following Facial Nerve Injury and Recovery

A common sequela of peripheral nerve injury is aberrant regeneration and recovery. Aberrant regeneration of injured motor nerves can affect all aspects of the nerve circuit from the motor cortex to the target muscle. A more comprehensive term for the symptoms that develop after aberrant motor neuromuscular reinnervation is aberrant reinnervation syndrome (ARS). Injury to the facial nerve followed by aberrant reinnervation results in a spectrum of symptoms that has been called many things in the literature. The authors support that this commonly encountered sequela of facial nerve injury be called facial aberrant reinnervation syndrome (FARS), a term that is more descriptive of the underlying pathophysiology and more inclusive of the clinical symptoms: facial synkinesis, facial muscle hypertonicity, and facial muscle spasm/twitching, which occur following facial nerve injury and recovery. In the following article, we present the clinical manifestations and sequelae of facial nerve injury and recovery and briefly discuss our evolving understanding of the pathophysiology and treatment of FARS.

Design and Development of a Polymeric-Based Curcumin Nanoparticle for Drug Delivery Enhancement and Potential Incorporation into Nerve Conduits

Peripheral nerve injuries (PNI) impact millions of individuals in the United States, prompting thousands of nerve repair procedures annually. Nerve conduits (NC) are commonly utilized to treat nerve injuries under 3 cm but larger gaps still pose a challenge for successful peripheral nerve regeneration (PNR) and functional recovery. This is partly attributed to the absence of bioactive agents such as stem cells or growth factors in FDA-approved conduits due to safety, harvesting, and reproducibility concerns. Therefore, curcumin, a bioactive phytochemical, has emerged as a promising alternative bioactive agent due to its ability to enhance PNR and overcome said challenges. However, its hydrophobicity and rapid degradation in aqueous solutions are considerable limitations. In this work, a nanoscale delivery platform with tannic acid (TA) and polyvinylpyrrolidone (PVP) was developed to encapsulate curcumin for increased colloidal and chemical stability. The curcumin nanoparticles (CurNPs) demonstrate significantly improved stability in water, reduced degradation rates, and controlled release kinetics when compared to free curcumin. Further, cell studies show that the CurNP is biocompatible when introduced to neuronal cells (SH-SY5Y), rat Schwann cells (RSC-S16), and murine macrophages (J774 A.1) at 5 μM, 5 μM, and 10 μM of curcumin, respectively. As a result of these improved physicochemical properties, confocal fluorescence microscopy revealed superior delivery of curcumin into these cells when in the form of CurNPs compared to its free form. A hydrogen peroxide-based oxidative stress study also demonstrated the CurNP's potential to protect J774 A.1 cells against excessive oxidative stress. Overall, this study provides evidence for the suitability of CurNPs to be used as a bioactive agent in NC applications.

Biodegradable Electrospun Conduit with Aligned Fibers Based on Poly(lactic-co-glycolic Acid) (PLGA)/Carbon Nanotubes and Choline Bitartrate Ionic Liquid

Functionally active aligned fibers are a promising approach to enhance neuro adhesion and guide the extension of neurons for peripheral nerve regeneration. Therefore, the present study developed poly(lactic-co-glycolic acid) (PLGA)-aligned electrospun mats and investigated the synergic effect with carbon nanotubes (CNTs) and Choline Bitartrate ionic liquid (Bio-IL) on PLGA fibers. Morphology, thermal, and mechanical performances were determined as well as the hydrolytic degradation and the cytotoxicity. Results revealed that electrospun mats are composed of highly aligned fibers, and CNTs were aligned and homogeneously distributed into the fibers. Bio-IL changed thermal transition behavior, reduced glass transition temperature (Tg), and favored crystal phase formation. The mechanical properties increased in the presence of CNTs and slightly decreased in the presence of the Bio-IL. The results demonstrated a decrease in the degradation rate in the presence of CNTs, whereas the use of Bio-IL led to an increase in the degradation rate. Cytotoxicity results showed that all the electrospun mats display metabolic activity above 70%, which demonstrates that they are biocompatible. Moreover, superior biocompatibility was observed for the electrospun containing Bio-IL combined with higher amounts of CNTs, showing a high potential to be used in nerve tissue engineering.

Beyond the limiting gap length: peripheral nerve regeneration through implantable nerve guidance conduits

Peripheral nerve damage results in the loss of sensorimotor and autonomic functions, which is a significant burden to patients. Furthermore, nerve injuries greater than the limiting gap length require surgical repair. Although autografts are the preferred clinical choice, their usage is impeded by their limited availability, dimensional mismatch, and the sacrifice of another functional donor nerve. Accordingly, nerve guidance conduits, which are tubular scaffolds engineered to provide a biomimetic environment for nerve regeneration, have emerged as alternatives to autografts. Consequently, a few nerve guidance conduits have received clinical approval for the repair of short-mid nerve gaps but failed to regenerate limiting gap damage, which represents the bottleneck of this technology. Thus, it is still necessary to optimize the morphology and constituent materials of conduits. This review summarizes the recent advances in nerve conduit technology. Several manufacturing techniques and conduit designs are discussed, with emphasis on the structural improvement of simple hollow tubes, additive manufacturing techniques, and decellularized grafts. The main objective of this review is to provide a critical overview of nerve guidance conduit technology to support regeneration in long nerve defects, promote future developments, and speed up its clinical translation as a reliable alternative to autografts.

Comparative Analysis of Various Spider Silks in Regard to Nerve Regeneration: Material Properties and Schwann Cell Response

Peripheral nerve reconstruction through the employment of nerve guidance conduits with Trichonephila dragline silk as a luminal filling has emerged as an outstanding preclinical alternative to avoid nerve autografts. Yet, it remains unknown whether the outcome is similar for silk fibers harvested from other spider species. This study compares the regenerative potential of dragline silk from two orb-weaving spiders, Trichonephila inaurata and Nuctenea umbratica, as well as the silk of the jumping spider Phidippus regius. Proliferation, migration, and transcriptomic state of Schwann cells seeded on these silks are investigated. In addition, fiber morphology, primary protein structure, and mechanical properties are studied. The results demonstrate that the increased velocity of Schwann cells on Phidippus regius fibers can be primarily attributed to the interplay between the silk's primary protein structure and its mechanical properties. Furthermore, the capacity of silk fibers to trigger cells toward a gene expression profile of a myelinating Schwann cell phenotype is shown. The findings for the first time allow an in-depth comparison of the specific cellular response to various native spider silks and a correlation with the fibers' material properties. This knowledge is essential to open up possibilities for targeted manufacturing of synthetic nervous tissue replacement.

Bredigite-Based Bioactive Nerve Guidance Conduit for Pro-Healing Macrophage Polarization and Peripheral Nerve Regeneration

Structural and functional healing of peripheral nerves damaged by trauma or chronic disease remain major clinical challenges, requiring the development of an effective nerve guidance conduit (NGC). The present study investigates a NGC fabrication strategy based on bredigite (BRT, Ca7MgSi4O16) bioceramic for the treatment of peripheral nerve injury. Here, BRT bioceramic shows good biocompatibility and sustainable release of Ca2+, Mg2+, and Si4+ ions. Both BRT extracts and BRT-incorporating electrospun membranes promote the proliferation and myelination potential of RSC96 cells, as well as accelerate vascular formation by human umbilical vein endothelial cells. Notably, BRT facilitates RAW 264.7 cell polarization to the pro-healing phenotype under LPS-induced inflammatory stimulation. More importantly, the macrophages activated by BRT in turn promote RSC96 cell migration and remyelination. In a rat sciatic nerve defect model, improved electrophysiological performance and alleviated gastrocnemius muscle atrophy are observed at 12 weeks post-implantation. Further experiments verify that BRT-loaded NGC facilitates axonal regrowth and revascularization with high M2-like macrophage infiltration. These findings support the beneficial effects of BRT for creating a pro-healing immune microenvironment and orchestrating multicellular processes associated with functional nerve regeneration, indicating the potential of rationally engineered bioceramics as safe, effective, and economical materials for peripheral nerve repair.

Enhancing regeneration and repair of long-distance peripheral nerve defect injuries with continuous microcurrent electrical nerve stimulation

Introduction

Peripheral nerve injuries, especially those involving long-distance deficits, pose significant challenges in clinical repair. This study explores the potential of continuous microcurrent electrical nerve stimulation (cMENS) as an adjunctive strategy to promote regeneration and repair in such cases.

Methods

The study initially optimized cMENS parameters and assessed its impact on Schwann cell activity, neurotrophic factor secretion, and the nerve regeneration microenvironment. Subsequently, a rat sciatic nerve defect-bridge repair model was employed to evaluate the reparative effects of cMENS as an adjuvant treatment. Functional recovery was assessed through gait analysis, motor function tests, and nerve conduction assessments. Additionally, nerve regeneration and denervated muscle atrophy were observed through histological examination.

Results

The study identified a 10-day regimen of 100uA microcurrent stimulation as optimal. Evaluation focused on Schwann cell activity and the microenvironment, revealing the positive impact of cMENS on maintaining denervated Schwann cell proliferation and enhancing neurotrophic factor secretion. In the rat model of sciatic nerve defect-bridge repair, cMENS demonstrated superior effects compared to control groups, promoting motor function recovery, nerve conduction, and sensory and motor neuron regeneration. Histological examinations revealed enhanced maturation of regenerated nerve fibers and reduced denervated muscle atrophy.

Discussion

While cMENS shows promise as an adjuvant treatment for long-distance nerve defects, future research should explore extended stimulation durations and potential synergies with tissue engineering grafts to improve outcomes. This study contributes comprehensive evidence supporting the efficacy of cMENS in enhancing peripheral nerve regeneration.

Bone marrow stromal cell-conditioned medium regenerates injured sciatic nerve by increasing expression of MPZ and NGF and decreasing apoptosis

Objectives

Despite the many benefits of mesenchymal stem cell (MSC) transplantation for tissue regeneration, there are some limitations to using them, including the high costs, applying invasive procedures, the possibility of transplant rejection, and cell malignancy. This study aimed to investigate the effect of secretions of bone marrow stromal cells (BMSCs) with the cell-free strategy on damaged sciatic nerve with an emphasis on the role of apoptosis and the expression of myelin protein zero (MPZ) and nerve growth factor (NGF) proteins.

Materials and Methods

BMSCs were cultured and a 25-fold concentrated conditioned medium (CM) from the cells was provided. After creating a crush injury in the left sciatic nerve of male rats, BMSCs or CM were injected into the injured site of the nerve. Four weeks later, the expression of MPZ, NGF, Bax, and Bcl-2 proteins in the sciatic nerve and histological parameters of the sciatic nerve and gastrocnemius muscle were assessed.

Results

The results demonstrated that injection of CM decreased apoptosis and increased expression of MPZ and NGF proteins, improving remyelination and regeneration of the sciatic nerve almost as much as the transplantation of the BMSCs themselves compared to the control group.

Conclusion

The results suggest that BMSC secretions may improve remyelination and regeneration of damaged sciatic nerve by increasing the expression of MPZ and NGF and decreasing apoptosis.

Biodegradable Nerve Guide with Glial Cell Line-Derived Neurotrophic Factor Improves Recovery After Facial Nerve Injury in Rats

Background: Bioengineered nerve guides with glial cell line-derived neurotrophic factor (GDNF) support recovery after facial nerve injury by acting as regenerative scaffolds. Objective: To compare functional, electrophysiological, and histological outcomes after repair of rat facial nerve transection in control, empty nerve guide, and nerve guide with GDNF conditions. Methods: Rats underwent transection and primary repair of the buccal branch of the facial nerve and were divided into (1) transection and repair only, (2) transection and repair augmented with empty guide, (3) transection and repair augmented with GDNF-guide groups. Weekly measurements of the whisking movements were recorded. At 12 weeks, compound muscle action potentials (CMAPs) at the whisker pad were assessed, and samples were collected for histomorphometric analysis. Results: Rats in GDNF-guide group displayed the earliest peak in normalized whisking amplitude. CMAPs were significantly higher after GDNF-guide placement. Mean fiber surface area of the target muscle, axonal count of the injured branch, and the number of Schwann cells were highest with GDNF guides. Conclusion: The biodegradable nerve guide containing double-walled GDNF microspheres enhanced recovery after facial nerve transection and primary repair.

Enhanced peripheral nerve regeneration by mechano-electrical stimulation

To address limitations in current approaches for treating large peripheral nerve defects, the presented study evaluated the feasibility of functional material-mediated physical stimuli on peripheral nerve regeneration. Electrospun piezoelectric poly(vinylidene fluoride-trifluoroethylene) nanofibers were utilized to deliver mechanical actuation-activated electrical stimulation to nerve cells/tissues in a non-invasive manner. Using morphologically and piezoelectrically optimized nanofibers for neurite extension and Schwann cell maturation based on in vitro experiments, piezoelectric nerve conduits were synthesized and implanted in a rat sciatic nerve transection model to bridge a critical-sized sciatic nerve defect (15 mm). A therapeutic shockwave system was utilized to periodically activate the piezoelectric effect of the implanted nerve conduit on demand. The piezoelectric nerve conduit-mediated mechano-electrical stimulation (MES) induced enhanced peripheral nerve regeneration, resulting in full axon reconnection with myelin regeneration from the proximal to the distal ends over the critical-sized nerve gap. In comparison, a control group, in which the implanted piezoelectric conduits were not activated in vivo, failed to exhibit such nerve regeneration. In addition, at both proximal and distal ends of the implanted conduits, a decreased number of damaged myelination (ovoids), an increased number of myelinated nerves, and a larger axonal diameter were observed under the MES condition as compared to the control condition. Furthermore, unlike the control group, the MES condition exhibited a superior functional nerve recovery, assessed by walking track analysis and polarization-sensitive optical coherence tomography, demonstrating the significant potential of the piezoelectric conduit-based physical stimulation approach for the treatment of peripheral nerve injury.

Development and preclinical evaluation of bioactive nerve conduits for peripheral nerve regeneration: A comparative study

In severe peripheral nerve injuries, nerve conduits (NCs) are good alternatives to autografts/allografts; however, the results the available devices guarantee for are still not fully satisfactory. Herein, differently bioactivated NCs based on the new polymer oxidized polyvinyl alcohol (OxPVA) are compared in a rat model of sciatic nerve neurotmesis (gap: 5 mm; end point: 6 weeks). Thirty Sprague Dawley rats are randomized to 6 groups: Reverse Autograft (RA); Reaxon®; OxPVA; OxPVA + EAK (self-assembling peptide, mechanical incorporation); OxPVA + EAK-YIGSR (mechanical incorporation); OxPVA + Nerve Growth Factor (NGF) (adsorption). Preliminarily, all OxPVA-based devices are comparable with Reaxon® in Sciatic Functional Index score and gait analysis; moreover, all conduits sustain nerve regeneration (S100, β-tubulin) without showing substantial inflammation (CD3, F4/80) evidences. Following morphometric analyses, OxPVA confirms its potential in PNI repair (comparable with Reaxon®) whereas OxPVA + EAK-YIGSR stands out for its myelinated axons total number and density, revealing promising in injury recovery and for future application in clinical practice.

In vivo evaluation of Adipose-Derived Stem Cells (ADSCs) using Nanofat technique and chitosan conduit for peripheral nerve defect repair

INTRODUCTION

Nerve autograft is the gold standard for reconstruction of peripheral nerve loss. The alternative is the interposition of a synthetic regeneration conduit. The purpose of the study was to evaluate the in vivo feasibility and interest of Adipose-Derived Stem Cells (ADSCs) using the Nanofat technique and chitosan conduit for peripheral nerve defect repair.

MATERIAL AND METHODS

In vivo, after the creation of a 10 mm defect of the sciatic nerve, 2 groups were defined according to the nerve repair in rats: "chitosan" group (n = 10) and "chitosan and ADSCs" group (n = 10) with a clinical and paraclinical evaluation at 7 weeks.

RESULTS

The in vivo results seem to show that the adjunction of ADSCs was favorable clinically, histologically and functionally compare to a chitosan reconstruction alone.

DISCUSSION

Peripheral nerve repair with defect using a chitosan conduit associated with ADSCs would constitute a surgical alternative in a single surgical step.

Advances in Biomimetic Nerve Guidance Conduits for Peripheral Nerve Regeneration

Injuries to the peripheral nervous system are a common clinical issue, causing dysfunctions of the motor and sensory systems. Surgical interventions such as nerve autografting are necessary to repair damaged nerves. Even with autografting, i.e., the gold standard, malfunctioning and mismatches between the injured and donor nerves often lead to unwanted failure. Thus, there is an urgent need for a new intervention in clinical practice to achieve full functional recovery. Nerve guidance conduits (NGCs), providing physicochemical cues to guide neural regeneration, have great potential for the clinical regeneration of peripheral nerves. Typically, NGCs are tubular structures with various configurations to create a microenvironment that induces the oriented and accelerated growth of axons and promotes neuron cell migration and tissue maturation within the injured tissue. Once the native neural environment is better understood, ideal NGCs should maximally recapitulate those key physiological attributes for better neural regeneration. Indeed, NGC design has evolved from solely physical guidance to biochemical stimulation. NGC fabrication requires fundamental considerations of distinct nerve structures, the associated extracellular compositions (extracellular matrices, growth factors, and cytokines), cellular components, and advanced fabrication technologies that can mimic the structure and morphology of native extracellular matrices. Thus, this review mainly summarizes the recent advances in the state-of-the-art NGCs in terms of biomaterial innovations, structural design, and advanced fabrication technologies and provides an in-depth discussion of cellular responses (adhesion, spreading, and alignment) to such biomimetic cues for neural regeneration and repair.

Compression Neuropathies: Revisions and Managing Expectations

Most compression neuropathies can be reliably treated with surgical decompression; however, in approximately 25% of the cases, this release fails, requiring revision surgery. Defining the correct diagnosis after a failed nerve decompression (ie, persistent, recurrent, or new symptoms) is of the utmost importance and guides toward the optimal treatment. This article describes the clinical categorization of secondary carpal tunnel syndrome and cubital tunnel syndrome, intraoperative principles of revision surgery and treatment options that are currently available.

Electroceuticals for Regeneration of Long Nerve Gap Using Biodegradable Conductive Conduits and Implantable Wireless Stimulator

Regeneration of over 10 mm long peripheral nerve defects remains a challenge due to the failure of regeneration by prolonged axotomy and denervation occurring in long-term recovery. Recent studies reveal that conductive conduits and electrical stimulation accelerate the regeneration of long nerve defects. In this study, an electroceutical platform combining a fully biodegradable conductive nerve conduit and a wireless electrical stimulator is proposed to maximize the therapeutic effect on nerve regeneration. Fully biodegradable nerve conduit fabricated using molybdenum (Mo) microparticles and polycaprolactone (PCL) can eliminate the unwanted effects of non-degradable implants, which occupy nerve paths and need to be removed through surgery increasing the risk of complications. The electrical and mechanical properties of Mo/PCL conduits are optimized by controlling the amounts of Mo and tetraglycol lubricant. The dissolution behavior and electrical conductivity of biodegradable nerve conduits in the biomimetic solutions are also evaluated. In in vivo experiments, the integrated strategy of a conductive Mo/PCL conduit with controlled therapeutic electrical stimulation shows accelerated axon regeneration for long sciatic nerve defects in rats compared to the use of the Mo/PCL conduit without stimulation and has a significant therapeutic effect based on the results obtained from the functional recovery test.

Medium chain length polyhydroxyalkanoates as potential matrix materials for peripheral nerve regeneration

Polyhydroxyalkanoates are natural, biodegradable, thermoplastic and sustainable polymers with a huge potential in fabrication of bioresorbable implantable devices for tissue engineering. We describe a comparative evaluation of three medium chain length polyhydroxyalkanoates (mcl-PHAs), namely poly(3-hydroxyoctanoate), poly(3-hydroxyoctanoate-co-3-hydoxydecanoate) and poly(3-hydroxyoctanoate-co-3-hydroxydecanoate-co-3-hydroxydodecanoate), one short chain length polyhydroxyalkanoate, poly(3-hydroxybutyrate), P(3HB) and synthetic aliphatic polyesters (polycaprolactone and polylactide) with a specific focus on nerve regeneration, due to mechanical properties of mcl-PHAs closely matching nerve tissues. In vitro biological studies with NG108-15 neuronal cell and primary Schwann cells did not show a cytotoxic effect of the materials on both cell types. All mcl-PHAs supported cell adhesion and viability. Among the three mcl-PHAs, P(3HO-co-3HD) exhibited superior properties with regards to numbers of cells adhered and viable cells for both cell types, number of neurite extensions from NG108-15 cells, average length of neurite extensions and Schwann cells. Although, similar characteristics were observed for flat P(3HB) surfaces, high rigidity of this biomaterial, and FDA-approved polymers such as PLLA, limits their applications in peripheral nerve regeneration. Therefore, we have designed, synthesized and evaluated these materials for nerve tissue engineering and regenerative medicine, the interaction of mcl-PHAs with neuronal and Schwann cells, identifying mcl-PHAs as excellent materials to enhance nerve regeneration and potentially their clinical application in peripheral nerve repair.

Donor nerve selection in free gracilis muscle transfer for facial reanimation. A systematic review and meta-analysis of clinical outcomes

BACKGROUND

One of the critical factors in facial reanimation is selecting the donor nerve. The most favored neurotizers are the contralateral facial nerve with a cross-face nerve graft (CFNG) and motor nerve to the masseter (MNM). A relatively new dual innervation (DI) method has shown successful results. This study aimed to compare the clinical outcomes of different neurotization strategies for free gracilis muscle transfer (FGMT).

METHODS

The Scopus and WoS databases were queried with 21 keywords. Three-stage article selection was performed for the systematic review. Articles presenting quantitative data for commissure excursion and facial symmetry were included in meta-analysis, using random-effects model. ROBINS-I tool and Newcastle-Ottawa scale were used to assess bias and study quality.

RESULTS

One hundred forty-seven articles containing FGMT were systematically reviewed. Most studies indicated CFNG as the first choice. MNM was primarily indicated in bilateral palsy and in elderly. Clinical outcomes of DI studies were promising. 13 studies including 435 observations (179 CFNG, 182 MNM, 74 DI) were eligible for meta-analysis. The mean change in commissure excursion was 7.15 mm (95% CI: 4.57-9.72) for CFNG, 8.46 mm (95% CI: 6.86-10.06) for MNM, and 5.18 mm (95% CI: 4.01-6.34) for DI. In pairwise comparisons, a significant difference was found between MNM and DI (p = 0.0011), despite the superior outcomes described in DI studies. No statistically significant difference was found in resting and smile symmetry (p = 0.625, p = 0.780).

CONCLUSIONS

CFNG is the most preferred neurotizer, and MNM is a reliable second option. Outcomes of DI studies are promising, but more comparison studies are needed to draw conclusions. Our meta-analysis was limited by incompatibility of the assessment scales. Consensus on a standardized assessment system would add value to future studies.

Thermoelectric Freeze-Casting of Biopolymer Blends: Fabrication and Characterization of Large-Size Scaffolds for Nerve Tissue Engineering Applications

Peripheral nerve injuries (PNIs) are detrimental to the quality of life of affected individuals. Patients are often left with life-long ailments that affect them physically and psychologically. Autologous nerve transplant is still the gold standard treatment for PNIs despite limited donor site and partial recovery of nerve functions. Nerve guidance conduits are used as a nerve graft substitute and are efficient for the repair of small nerve gaps but require further improvement for repairs exceeding 30 mm. Freeze-casting is an interesting fabrication method for the conception of scaffolds meant for nerve tissue engineering since the microstructure obtained comprises highly aligned micro-channels. The present work focuses on the fabrication and characterization of large scaffolds (35 mm length, 5 mm diameter) made of collagen/chitosan blends by freeze-casting via thermoelectric effect instead of traditional freezing solvents. As a freeze-casting microstructure reference, scaffolds made from pure collagen were used for comparison. Scaffolds were covalently crosslinked for better performance under load and laminins were further added to enhance cell interactions. Microstructural features of lamellar pores display an average aspect ratio of 0.67 ± 0.2 for all compositions. Longitudinally aligned micro-channels are reported as well as enhanced mechanical properties in traction under physiological-like conditions (37 °C, pH = 7.4) resulting from crosslinking treatment. Cell viability assays using a rat Schwann cell line derived from sciatic nerve (S16) indicate that scaffold cytocompatibility is similar between scaffolds made from collagen only and scaffolds made from collagen/chitosan blend with high collagen content. These results confirm that freeze-casting via thermoelectric effect is a reliable manufacturing strategy for the fabrication of biopolymer scaffolds for future peripheral nerve repair applications.

Bridging Gaps in Peripheral Nerves: From Current Strategies to Future Perspectives in Conduit Design

In peripheral nerve injuries (PNI) with substance loss, where tensionless end-to-end suture is not achievable, the positioning of a graft is required. Available options include autografts (e.g., sural nerve, medial and lateral antebrachial cutaneous nerves, superficial branch of the radial nerve), allografts (Avance^®; human origin), and hollow nerve conduits. There are eleven commercial hollow conduits approved for clinical, and they consist of devices made of a non-biodegradable synthetic polymer (polyvinyl alcohol), biodegradable synthetic polymers (poly(DL-lactide-ε-caprolactone); polyglycolic acid), and biodegradable natural polymers (collagen type I with/without glycosaminoglycan; chitosan; porcine small intestinal submucosa); different resorption times are available for resorbable guides, ranging from three months to four years. Unfortunately, anatomical/functional nerve regeneration requirements are not satisfied by any of the possible alternatives; to date, focusing on wall and/or inner lumen organization/functionalization seems to be the most promising strategy for next-generation device fabrication. Porous or grooved walls as well as multichannel lumens and luminal fillers are the most intriguing options, eventually also including the addition of cells (Schwann cells, bone marrow-derived, and adipose tissue derived stem cells) to support nerve regeneration. This review aims to describe common alternatives for severe PNI recovery with a highlight of future conduits.

Sourcing and development of tissue for transplantation in reconstructive surgery: A narrative review

The wealth of allogeneic and xenogeneic tissue products available to plastic and reconstructive surgeons has allowed for the development of novel surgical solutions to challenging clinical problems, often obviating the need to inflict donor site morbidity. Allogeneic tissue used for reconstructive surgery enters the tissue industry through whole body donation or reproductive tissue donation and has been regulated by the FDA as human cells, tissues, and cellular and tissue-based products (HCT/Ps) since 1997. Tissue banks offering allogeneic tissue can also undergo voluntary regulation by the American Association of Tissue Banks (AATB). Tissue prepared for transplantation is sterilized and can be processed into soft tissue or bone allografts for use in surgical reconstruction, whereas non-transplant tissue is prepared for clinical training and drug, medical device, and translational research. Xenogeneic tissue, which is most often derived from porcine or bovine sources, is also commercially available and is subject to strict regulations for animal breeding and screening for infectious diseases. Although xenogeneic products have historically been decellularized for use as non-immunogenic tissue products, recent advances in gene editing have opened the door to xenograft organ transplants into human patients. Herein, we describe an overview of the modern sourcing, regulation, processing, and applications of tissue products relevant to the field of plastic and reconstructive surgery.

Mechanism of Danggui Sini underlying the treatment of peripheral nerve injury based on network pharmacology and molecular docking: A review

Danggui Sini is a traditional Chinese medicine prescription for treating peripheral nerve injury (PNI). We studied the mechanisms of this decoction through network pharmacology analysis and molecular docking. Using R language and Perl software, the active components and predicted targets of Danggui Sini, as well as the related gene targets of PNI, were mined through TCMSP, GeneCards, OMIM, TTD, and DrugBank. The network diagram of active components and intersection targets was constructed using Cytoscape software and the STRING database. The CytoNCA plug-in was used to screen out the core compounds and key targets. The genes were analyzed for Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment. AutoDock was used to analyze the molecular docking of key targets and core compounds of diseases. The drug component disease target regulatory network showed that the key components included quercetin, kaempferol, naringenin, and licochalcone A, which play key roles in the whole network and may be the primary compounds associated with the action of Danggui Sini against PNI. PPI network topology analysis showed high degree values for RELA, JUN, MAPK1, RB1, and FOS. Enrichment analysis showed that the core targets of Danggui Sini participated in pathways associated with neurogenesis-multiple diseases. Molecular docking showed that the active ingredients in Danggui Sini had a good binding ability with key targets. We conclude that many active components of Danggui Sini play therapeutic roles in PNI treatment by regulating RELA, JUN, MAPK1, RB1, and FOS, and multiple other targets in inflammation, immunity, and lipid metabolism.

Neuronal cell response on aligned fibroporous electrospun mat generated from silver ion complexed ethylene vinyl alcohol copolymer

Generating electrospun mats with aligned fibers and obtaining neurite extension in the aligned fiber direction could provide hope for fabricating nerve guidance conduits or wraps through an easy method. The growing interest in generating electrospun mats with aligned fibers for tissue engineering is looking for simple methods to generate the same. Here, in this study, ethylene vinyl alcohol copolymer (EVAL) chains were complexed with silver ions (Ag⁺ ) to generate aligned fibers during the electrospinning process. The fibers thus produced were subjected to physico-chemical characterization and biological studies to ensure their properties and to examine whether suitable for neuronal cell attachment and neurite extension that may be useful in making nerve guidance conduits or wraps. The presence of silver ions and its complex formation with -OH of EVAL has been confirmed with EDX and XPS analysis respectively. The alignment of fibers was visualized from SEM analysis and confirmed using directionality analysis using Fiji-ImageJ software. Mechanical properties done with dumbbells punched out in longitudinal and transverse directions also substantiated the alignment of fibers. The results obtained from direct contact, MTT, and live/dead assay showed the cells are viable on the material. From the actin staining and immunostaining assays, it was evident that the PC12 cells could attach and extend their neurites in an aligned manner on the fibers. The maximum neurite extension was up to 200 μm in length. These properties of electrospun EVAL-Ag mat with aligned fibers indicated that it could be developed as a biocompatible nerve guidance conduit or wrap.

Decellularized allografts as an alternative for reconstruction of large inferior alveolar nerve defects: a systematic review

BACKGROUND

Inferior alveolar nerve (IAN) injuries are a clinical problem with devastating consequences, causing temporary or permanent paresthesia, significantly affecting the patient's quality of life. Despite morbidity, side effects and controversy regarding its results, autologous nerve grafting is still the main treatment for these type of lesions. However, due to advances in knowledge about nerve damage and with the aim of preventing the described problems of autografts, new treatment alternatives based on decellularized allografts have emerged. The aim of this systematic review was to evaluate the reported efficacy of decellularized allografts for the treatment of IAN damage.

MATERIAL AND METHODS

We performed a systematic search in Pubmed, Scopus and Web of Science databases following the PRISMA guidelines. Cohort studies, randomized or non-randomized clinical studies, prospective or retrospective studies, without age limits and language restriction that included human subjects who received decellularized allograft as treatment for IAN damage were included.

RESULTS

Six articles met the inclusion criteria and were included for data analysis. In all 6 articles, resolution of IAN damage was observed in more than 85% of patients after a 12-month follow-up period, and in 2 of them, complete resolution was observed in 100% of their patients at longer follow-ups.

CONCLUSIONS

Decellularized allograft appears to be a promising alternative to resolve IAN lesions, without requiring a nerve autograft procedure. However, more randomized clinical trials are needed to validate adequate treatment modalities with decellularized allografts.

Development of 3D-Printed, Biodegradable, Conductive PGSA Composites for Nerve Tissue Regeneration

Nerve conduits are used to reconnect broken nerve bundles and provide protection to facilitate nerve regeneration. However, the low degradation rate and regeneration rate, as well as the requirement for secondary surgery are some of the most criticized drawbacks of existing nerve conduits. With high processing flexibility from the photo-curability, poly (glycerol sebacate) acrylate (PGSA) is a promising material with tunable mechanical properties and biocompatibility for the development of medical devices. Here, polyvinylpyrrolidone (PVP), silver nanoparticles (AgNPs), and graphene are embedded in biodegradable PGSA matrix. The polymer composites are then assessed for their electrical conductivity, biodegradability, three-dimensional-printability (3D-printability), and promotion of cell proliferation. Through the four-probe technique, it is shown that the PGSA composites are identified as highly conductive in swollen state. Furthermore, biodegradability is evaluated through enzymatic degradation and facilitated hydrolysis. Cell proliferation and guidance are significantly promoted by three-dimensional-printed microstructures and electrical stimulation on PGSA composites, especially on PGSA-PVP. Hence, microstructured nerve conduits are 3D-printed with PGSA-PVP. Guided cell growth and promoted proliferation are subsequently demonstrated by Schwann cell culture combined with electrical stimulation. Consequently, 3D-printed nerve conduits fabricated with PGSA composites hold great potential in nerve tissue regeneration through electrical stimulation.

Peripheral Nerve Regeneration: Opportunities and Challenges

Peripheral nerve injury has detrimental effects on the quality of life for patients and is a worldwide issue with high rates of morbidity. Research on the molecular mechanisms of nerve injury, microsurgical techniques, and advances in stem cell research have led to substantial progress in the field of translational neurophysiology. Current research on peripheral nerve regeneration aims to accelerate peripheral nerve development through pluripotent stem cells and potential use of smart exosomes, pharmacological agents, and bioengineering of nerve conduits. In this article critically reviewed and summarized various methods used for peripheral nerve regeneration and highlight the opportunities and challenges that come along with these strategies.

Effects of Altering Magnesium Metal Surfaces on Degradation In Vitro and In Vivo during Peripheral Nerve Regeneration

In vivo use of biodegradable magnesium (Mg) metal can be plagued by too rapid a degradation rate that removes metal support before physiological function is repaired. To advance the use of Mg biomedical implants, the degradation rate may need to be adjusted. We previously demonstrated that pure Mg filaments used in a nerve repair scaffold were compatible with regenerating peripheral nerve tissues, reduced inflammation, and improved axonal numbers across a short-but not long-gap in sciatic nerves in rats. To determine if the repair of longer gaps would be improved by a slower Mg degradation rate, we tested, in vitro and in vivo, the effects of Mg filament polishing followed by anodization using plasma electrolytic oxidation (PEO) with non-toxic electrolytes. Polishing removed oxidation products from the surface of as-received (unpolished) filaments, exposed more Mg on the surface, produced a smoother surface, slowed in vitro Mg degradation over four weeks after immersion in a physiological solution, and improved attachment of cultured epithelial cells. In vivo, treated Mg filaments were used to repair longer (15 mm) injury gaps in adult rat sciatic nerves after placement inside hollow poly (caprolactone) nerve conduits. The addition of single Mg or control titanium filaments was compared to empty conduits (negative control) and isografts (nerves from donor rats, positive control). After six weeks in vivo, live animal imaging with micro computed tomography (micro-CT) showed that Mg metal degradation rates were slowed by polishing vs. as-received Mg, but not by anodization, which introduced greater variability. After 14 weeks in vivo, functional return was seen only with isograft controls. However, within Mg filament groups, the amount of axonal growth across the injury site was improved with slower Mg degradation rates. Thus, anodization slowed degradation in vitro but not in vivo, and degradation rates do affect nerve regeneration.

Silk-in-Silk Nerve Guidance Conduits Enhance Regeneration in a Rat Sciatic Nerve Injury Model

Advanced nerve guidance conduits can provide an off-the-shelf alternative to autografts for the rehabilitation of segmental peripheral nerve injuries. In this study, the excellent processing ability of silk fibroin and the outstanding cell adhesion quality of spider dragline silk are combined to generate a silk-in-silk conduit for nerve repair. Fibroin-based silk conduits (SC) are characterized, and Schwann cells are seeded on the conduits and spider silk. Rat sciatic nerve (10 mm) defects are treated with an autograft (A), an empty SC, or a SC filled with longitudinally aligned spider silk fibers (SSC) for 14 weeks. Functional recovery, axonal re-growth, and re-myelination are assessed. The material characterizations determine a porous nature of the conduit. Schwann cells accept the conduit and spider silk as growth substrate. The in vivo results show a significantly faster functional regeneration of the A and SSC group compared to the SC group. In line with the functional results, the histomorphometrical analysis determines a comparable axon density of the A and SSC groups, which is significantly higher than the SC group. These findings demonstrate that the here introduced silk-in-silk nerve conduit achieves a similar regenerative performance as autografts largely due to the favorable guiding properties of spider dragline silk.

Traumatic neuromas of peripheral nerves: Diagnosis, management and future perspectives

Traumatic neuromas are infrequent in clinical settings but are prevalent following trauma or surgery. A traumatic neuroma is not a true malignancy, rather, it is a hyperplastic, reparative nerve reaction after injury and typically manifests as a nodular mass. The most common clinical manifestations include painful hypersensitivity and the presence of a trigger point that causes neuralgic pain, which could seriously decrease the living standards of patients. While various studies are conducted aiming to improve current diagnosis and management strategies via the induction of emerging imaging tools and surgical or conservative treatment. However, researchers and clinicians have yet to reach a consensus regarding traumatic neuromas. In this review, we aim to start with the possible underlying mechanisms of traumatic neuromas, elaborate on the diagnosis, treatment, and prevention schemes, and discuss the current experiment models and advances in research for the future management of traumatic neuromas.

Additive Manufacturing of Polyhydroxyalkanoate-Based Blends Using Fused Deposition Modelling for the Development of Biomedical Devices

In the last few decades Additive Manufacturing has advanced and is becoming important for biomedical applications. In this study we look at a variety of biomedical devices including, bone implants, tooth implants, osteochondral tissue repair patches, general tissue repair patches, nerve guidance conduits (NGCs) and coronary artery stents to which fused deposition modelling (FDM) can be applied. We have proposed CAD designs for these devices and employed a cost-effective 3D printer to fabricate proof-of-concept prototypes. We highlight issues with current CAD design and slicing and suggest optimisations of more complex designs targeted towards biomedical applications. We demonstrate the ability to print patient specific implants from real CT scans and reconstruct missing structures by means of mirroring and mesh mixing. A blend of Polyhydroxyalkanoates (PHAs), a family of biocompatible and bioresorbable natural polymers and Poly(L-lactic acid) (PLLA), a known bioresorbable medical polymer is used. Our characterisation of the PLA/PHA filament suggest that its tensile properties might be useful to applications such as stents, NGCs, and bone scaffolds. In addition to this, the proof-of-concept work for other applications shows that FDM is very useful for a large variety of other soft tissue applications, however other more elastomeric MCL-PHAs need to be used.

Bioengineered nerve conduits and wraps for peripheral nerve repair of the upper limb

BACKGROUND

Traumatic peripheral nerve injury is common and incurs significant cost to individuals and society. Healing following direct nerve repair or repair with autograft is slow and can be incomplete. Several bioengineered nerve wraps or devices have become available as an alternative to direct repair or autologous nerve graft. Nerve wraps attempt to reduce axonal escape across a direct repair site and nerve devices negate the need for a donor site defect, required by an autologous nerve graft. Comparative evidence to guide clinicians in their potential use is lacking. We collated existing evidence to guide the clinical application of currently available nerve wraps and conduits.

OBJECTIVES

To assess and compare the effects and complication rates of licensed bioengineered nerve conduits or wraps for surgical repair of traumatic peripheral nerve injuries of the upper limb. To compare effects and complications against the current gold surgical standard (direct repair or nerve autograft).

SEARCH METHODS

We used standard, extensive Cochrane search methods. The latest search was 26 January 2022. We searched online and, where not accessible, contacted societies' secretariats to review abstracts from the British Surgical Society of the Hand, International Federation of Surgical Societies of the Hand, Federation of European Surgical Societies of the Hand, and the American Society for Peripheral Nerve from October 2007 to October 2018.

SELECTION CRITERIA

We included parallel group randomised controlled trials (RCTs) and quasi-RCTs of nerve repair in the upper limb using a bioengineered wrap or conduit, with at least 12 months of follow-up.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane procedures. Our primary outcomes were 1. muscle strength and 2. sensory recovery at 24 months or more. Our secondary outcomes were 3. British Medical Research Council (BMRC) grading, 4. integrated functional outcome (Rosén Model Instrument (RMI)), 5. touch threshold, 6. two-point discrimination, 7. cold intolerance, 8. impact on daily living measured using the Disability of Arm Shoulder and Hand Patient-Reported Outcome Measure (DASH-PROM), 9. sensory nerve action potential, 10. cost of the device, and 11. adverse events (any and specific serious adverse events (further surgery)). We used GRADE to assess the certainty of the evidence.

MAIN RESULTS

Five studies involving 213 participants and 257 nerve injuries reconstructed with wraps or conduits (129 participants) or standard repair (128 participants) met the inclusion criteria. Of those in the standard repair group, 119 nerve injuries were managed with direct epineurial repair, and nine autologous nerve grafts were performed. One study excluded the outcome data for the repair using an autologous nerve graft from their analysis, as it was the only autologous nerve graft in the study, so data were available for 127 standard repairs. There was variation in the functional outcome measures reported and the time postoperatively at which they were recorded. Mean sensory recovery, assessed with BMRC sensory grading (range S0 to S4, higher score considered better) was 0.03 points higher in the device group (range 0.43 lower to 0.49 higher; 1 RCT, 28 participants; very low-certainty evidence) than in the standard repair group (mean 2.75 points), which suggested little or no difference between the groups, but the evidence is very uncertain. There may be little or no difference at 24 months in mean touch thresholds between standard repair (0.81) and repair using devices, which was 0.01 higher but this evidence is also very uncertain (95% confidence interval (CI) 0.06 lower to 0.08 higher; 1 trial, 32 participants; very low-certainty evidence). Data were not available to assess BMRC motor grading at 24 months or more. Repair using bioengineered devices may not improve integrated functional outcome scores at 24 months more than standard techniques, as assessed by the Rosén Model Instrument (RMI; range 0 to 3, higher scores better); the CIs allow for both no important difference and a better outcome with standard repair (mean RMI 1.875), compared to the device group (0.17 lower, 95% CI 0.38 lower to 0.05 higher; P = 0.13; 2 trials, 60 participants; low-certainty evidence). Data from one study suggested that the five-year postoperative outcome of RMI may be slightly improved after repair using a device (mean difference (MD) 0.23, 95% CI 0.07 to 0.38; 1 trial, 28 participants; low-certainty evidence). No studies measured impact on daily living using DASH-PROM. The proportion of people with adverse events may be greater with nerve wraps or conduits than with standard techniques, but the evidence is very uncertain (risk ratio (RR) 7.15, 95% CI 1.74 to 29.42; 5 RCTs, 213 participants; very low-certainty evidence). This corresponds to 10 adverse events per 1000 people in the standard repair group and 68 per 1000 (95% CI 17 to 280) in the device group. The use of nerve repair devices may be associated with a greater need for revision surgery but this evidence is also very uncertain (12/129 device repairs required revision surgery (removal) versus 0/127 standard repairs; RR 7.61, 95% CI 1.48 to 39.02; 5 RCTs, 256 nerve repairs; very low-certainty evidence).

AUTHORS' CONCLUSIONS

Based on the available evidence, this review does not support use of currently available nerve repair devices over standard repair. There is significant heterogeneity in participants, injury pattern, repair timing, and outcome measures and their timing across studies of nerve repair using bioengineered devices, which make comparisons unreliable. Studies were generally small and at high or unclear risk of bias. These factors render the overall certainty of evidence for any outcome low or very low. The data reviewed here provide some evidence that more people may experience adverse events with use of currently available bioengineered devices than with standard repair techniques, and the need for revision surgery may also be greater. The evidence for sensory recovery is very uncertain and there are no data for muscle strength at 24 months (our primary outcome measures). We need further trials, adhering to a minimum standard of outcome reporting (with at least 12 months' follow-up, including integrated sensorimotor evaluation and patient-reported outcomes) to provide high-certainty evidence and facilitate more detailed analysis of effectiveness of emerging, increasingly sophisticated, bioengineered repair devices.

Clinical outcomes of different implant types in mandibular bar-retained overdentures: a retrospective analysis with up to 20 years follow-up

PURPOSE

To determine the clinical and radiological outcomes of hybrid-design- (HD) and bone-level (BL) implants for bar-retained mandibular implant-overdentures (IODs).

METHODS

For this retrospective study, edentulous patients who had received maxillary complete dentures and mandibular bar-retained IODs were invited for a follow-up assessment. Implant survival, implant success and health of peri-implant tissues were assessed on an implant level-based analysis. Patient-based parameters served to identify risk factors for peri-implant bone loss, presence of peri-implantitis and success.

RESULTS

Eighty patients (median age 72.72 [67.03; 78.81] years, 46 females) with 180 implants (median follow-up 12.01 [10.82; 21.04] years) were assessed. There was no difference concerning the rate of implant failure (p = 0.26), or peri-implantitis (p = 0.97) between HD and BL implants. Solely in one study group, there was the presence of peri-implant pus. Implant success was higher in BL implants with one group being notably higher than the comparing groups (p = 0.045). For bone loss, a width of keratinized mucosa (KM) ≤ 1 mm (p = 0.0006) and the presence of xerostomia (p = 0.09) were identified as risk factors. Smoking (p = 0.013) and a higher body mass index (BMI) (p = 0.03) were a risk factor for peri-implantitis. As risk factors for reduced implant success, a small width of KM (p = 0.003) and the presence of xerostomia (p = 0.007) were identified.

CONCLUSIONS

For mandibular bar-retained IODs, both BL and HD implants are mostly successful. A minimum of 1 mm KM around implants and normal salivary flow are relevant factors for implant success and stable peri-implant bone levels. Smoking and a high BMI are potential risk factors for peri-implantitis.

Evaluation of muscle atrophy after sciatic nerve defect repair – experimental model

Peripheral nerve injuries are one of the most important causes for disability generating considerable costs around the world. Appropriate assessment of the extent of peripheral nerve lesions and the choice of the right therapeutic protocol remain some of the biggest challenges. Non-invasive neuroimaging approaches are suitable in managing peripheral nerve repairs, providing in the same time accuracy details in structural neural detection, with minimal discomfort at a low cost. Medical imaging technology development has led to progress in examination of peripheral nervous system, using a series of tools and methods, such as: ultrasonography (US), positron emission tomography (PET), magnetic resonance imaging (MRI) and magnetic diffusion tensor imaging (DTI). In this study, we evaluated the regeneration process in Wistar rats after sciatic nerve defects repair with 4 different techniques (i.e. nerve graft, rat aorta used as a nerve conduit, rat aorta filled with platelet rich plasma (PRP) and rat aorta filled with stem cells) by using MRI investigation alongside the clinical evaluation. Our results showed that among the 4 batches, the PRP batch had the best results in muscle atrophy condition (both on MRI scan and on gastrocnemius index); on the second place the stem cell batch, then the nerve graft batch and finally the aortic conduit batch. MRI proved to be a reliable non-invasive monitoring method and showed good result in correlation with the footprint test (sciatic functional index) and the gastrocnemius index

Submicron Topographically Patterned 3D Substrates Enhance Directional Axon Outgrowth of Dorsal Root Ganglia Cultured Ex Vivo

A peripheral nerve injury results in disruption of the fiber that usually protects axons from the surrounding environment. Severed axons from the proximal nerve stump are capable of regenerating, but axons are exposed to a completely new environment. Regeneration recruits cells that produce and deposit key molecules, including growth factor proteins and fibrils in the extracellular matrix (ECM), thus changing the chemical and geometrical environment. The regenerating axons thus surf on a newly remodeled micro-landscape. Strategies to enhance and control axonal regeneration and growth after injury often involve mimicking the extrinsic cues that are found in the natural nerve environment. Indeed, nano- and micropatterned substrates have been generated as tools to guide axons along a defined path. The mechanical cues of the substrate are used as guides to orient growth or change the direction of growth in response to impediments or cell surface topography. However, exactly how axons respond to biophysical information and the dynamics of axonal movement are still poorly understood. Here we use anisotropic, groove-patterned substrate topography to direct and enhance sensory axonal growth of whole mouse dorsal root ganglia (DRG) transplanted ex vivo. Our results show significantly enhanced and directed growth of the DRG sensory fibers on the hemi-3D topographic substrates compared to a 0 nm pitch, flat control surface. By assessing the dynamics of axonal movement in time-lapse microscopy, we found that the enhancement was not due to increases in the speed of axonal growth, but to the efficiency of growth direction, ensuring axons minimize movement in undesired directions. Finally, the directionality of growth was reproduced on topographic patterns fabricated as fully 3D substrates, potentially opening new translational avenues of development incorporating these specific topographic feature sizes in implantable conduits in vivo.

Systematic review of the therapeutic use of Schwann cells in the repair of peripheral nerve injuries: Advancements from animal studies to clinical trials

Objective

To systematically evaluate the literature on the therapeutic use of Schwann cells (SC) in the repair of peripheral nerve injuries.

Methods

The Cochrane Library and PubMed databases were searched using terms [(“peripheral nerve injury” AND “Schwann cell” AND “regeneration”) OR (“peripheral nerve injuries”)]. Studies published from 2008 to 2022 were eligible for inclusion in the present study. Only studies presenting data from in-vivo investigations utilizing SCs in the repair of peripheral nerve injuries qualified for review. Studies attempting repair of a gap of ≥10 mm were included. Lastly, studies needed to have some measure of quantifiable regenerative outcome data such as histomorphometry, immunohistochemical, electrophysiology, or other functional outcomes.

Results

A search of the PubMed and Cochrane databases revealed 328 studies. After screening using the abstracts and methods, 17 studies were found to meet our inclusion criteria. Good SC adherence and survival in conduit tubes across various studies was observed. Improvement in morphological and functional outcomes with the use of SCs in long gap peripheral nerve injuries was observed in nearly all studies.

Conclusion

Based on contemporary literature, SCs have demonstrated clear potential in the repair of peripheral nerve injury in animal studies. It has yet to be determined which nerve conduit or graft will prove superior for delivery and retention of SCs for nerve regeneration. Recent developments in isolation and culturing techniques will enable further translational utilization of SCs in future clinical trials.

Biomaterial-Based Schwann Cell Transplantation and Schwann Cell-Derived Biomaterials for Nerve Regeneration

Schwann cells (SCs) dominate the regenerative behaviors after peripheral nerve injury by supporting axonal regrowth and remyelination. Previous reports also demonstrated that the existence of SCs is beneficial for nerve regeneration after traumatic injuries in central nervous system. Therefore, the transplantation of SCs/SC-like cells serves as a feasible cell therapy to reconstruct the microenvironment and promote nerve functional recovery for both peripheral and central nerve injury repair. However, direct cell transplantation often leads to low efficacy, due to injection induced cell damage and rapid loss in the circulatory system. In recent years, biomaterials have received great attention as functional carriers for effective cell transplantation. To better mimic the extracellular matrix (ECM), many biodegradable materials have been engineered with compositional and/or topological cues to maintain the biological properties of the SCs/SCs-like cells. In addition, ECM components or factors secreted by SCs also actively contribute to nerve regeneration. Such cell-free transplantation approaches may provide great promise in clinical translation. In this review, we first present the current bio-scaffolds engineered for SC transplantation and their achievement in animal models and clinical applications. To this end, we focus on the physical and biological properties of different biomaterials and highlight how these properties affect the biological behaviors of the SCs/SC-like cells. Second, the SC-derived biomaterials are also reviewed and discussed. Finally, the relationship between SCs and functional biomaterials is summarized, and the trends of their future development are predicted toward clinical applications.

A Combined Conduit-Bioactive Hydrogel Approach for Regeneration of Transected Sciatic Nerves

Transected peripheral nerve injury (PNI) affects the quality of life of patients, which leads to socioeconomic burden. Despite the existence of autografts and commercially available nerve guidance conduits (NGCs), the complexity of peripheral nerve regeneration requires further research in bioengineered NGCs to improve surgical outcomes. In this work, we introduce multidomain peptide (MDP) hydrogels, as intraluminal fillers, into electrospun poly(ε-caprolactone) (PCL) conduits to bridge 10 mm rat sciatic nerve defects. The efficacy of treatment groups was evaluated by electromyography and gait analysis to determine their electrical and motor recovery. We then studied the samples' histomorphometry with immunofluorescence staining and automatic axon counting/measurement software. Comparison with negative control group shows that PCL conduits filled with an anionic MDP may improve functional recovery 16 weeks postoperation, displaying higher amplitude of compound muscle action potential, greater gastrocnemius muscle weight retention, and earlier occurrence of flexion contracture. In contrast, PCL conduits filled with a cationic MDP showed the least degree of myelination and poor functional recovery. This phenomenon may be attributed to MDPs' difference in degradation time. Electrospun PCL conduits filled with an anionic MDP may become an attractive tissue engineering strategy for treating transected PNI when supplemented with other bioactive modifications.

Development of a regenerative porous PLCL nerve guidance conduit with swellable hydrogel-based microgrooved surface pattern via 3D printing

Peripheral nerve injury causes severe loss of motor and sensory functions, consequently increasing morbidity in affected patients. An autogenous nerve graft is considered the current gold standard for reconstructing nerve defects and recovering lost neurological functions; however, there are certain limitations to this method, such as limited donor nerve supply. With advances in regenerative medicine, recent research has focused on the fabrication of tissue-engineered nerve grafts as promising alternatives to the autogenous nerve grafts. In this study, we designed a nerve guidance conduit using an electrospun poly(lactide-co-ε-caprolactone) (PLCL) membrane with a visible light-crosslinked gelatin hydrogel. The PLCL nanoporous membrane with permeability served as a flexible and non-collapsible epineurium for the nerve conduit; the inner-aligned gelatin hydrogel paths were fabricated via 3D printing and a photocrosslinking system. The resultant gelatin hydrogel with microgrooved surface pattern was established as a conducting guidance path for the effective regeneration of axons and served as a reservoir that can incorporate and release bioactive molecules. From in vivo performance tests using a rat sciatic nerve defect model, our PLCL/gelatin conduit demonstrated successful axonal regeneration, remyelination capacities and facilitated functional recovery. Hence, the PLCL/gelatin conduit developed in this study is a promising substitute for autogenous nerve grafts. STATEMENT OF SIGNIFICANCE: Nerve guidance conduits (NGCs) are developed as promising recovery techniques for bridging peripheral nerve defects. However, there are still technological limitations including differences in the structures and components between natural peripheral nerve and NGCs. In this study, we designed a NGC composed of an electrospun poly(lactide-co-ε-caprolactone) (PLCL) membrane and 3D printed inner gelatin hydrogel to serve as a flexible and non-collapsible epineurium and a conducting guidance path, respectively, to mimic the fascicular structure of the peripheral nerve. In particular, in vitro cell tests clearly showed that gelatin hydrogel could guide the cells and function as a reservoir that incorporate and release nerve growth factor. From in vivo performance tests, our regenerative conduit successfully led to axonal regeneration with effective functional recovery.

Augmenting Peripheral Nerve Regeneration with Adipose-Derived Stem Cells

Peripheral nerve injuries (PNIs) are common and debilitating, cause significant health care costs for society, and rely predominately on autografts, which necessitate grafting a nerve section non-locally to repair the nerve injury. One possible approach to improving treatment is bolstering endogenous regenerative mechanisms or bioengineering new nervous tissue in the peripheral nervous system. In this review, we discuss critical-sized nerve gaps and nerve regeneration in rats, and summarize the roles of adipose-derived stem cells (ADSCs) in the treatment of PNIs. Several regenerative treatment modalities for PNI are described: ADSCs differentiating into Schwann cells (SCs), ADSCs secreting growth factors to promote peripheral nerve growth, ADSCs promoting myelination growth, and ADSCs treatments with scaffolds. ADSCs' roles in regenerative treatment and features are compared to mesenchymal stem cells, and the administration routes, cell dosages, and cell fates are discussed. ADSCs secrete neurotrophic factors and exosomes and can differentiate into Schwann cell-like cells (SCLCs) that share features with naturally occurring SCs, including the ability to promote nerve regeneration in the PNS. Future clinical applications are also discussed.

Three-dimensional-printed polycaprolactone/polypyrrole conducting scaffolds for differentiation of human olfactory ecto-mesenchymal stem cells into Schwann cell-like phenotypes and promotion of neurite outgrowth

Implantation of a suitable nerve guide conduit (NGC) seeded with sufficient Schwann cells (SCs) is required to improve peripheral nerve regeneration efficiently. Given the limitations of isolating and culturing SCs, using various sources of stem cells, including mesenchymal stem cells (MSCs) obtained from nasal olfactory mucosa, can be desirable. Olfactory ecto-MSCs (OE-MSCs) are a new population of neural crest-derived stem cells that can proliferate and differentiate into SCs and can be considered a promising autologous alternative to produce SCs. Regardless, a biomimetic physicochemical microenvironment in NGC such as electroconductive substrate can affect the fate of transplanted stem cells, including differentiation toward SCs and nerve regeneration. Therefore, in this study, the effect of 3D printed polycaprolactone (PCL)/polypyrrole (PPy) conductive scaffolds on differentiation of human OE-MSCS into functional SC-like phenotypes was investigated. Biological evaluation of 3D printed scaffolds was examined by in vitro culturing the OE-MSCs on samples surfaces, and conductivity showed no effect on increased cell attachment, proliferation rate, viability, and distribution. In contrast, immunocytochemical staining and real-time polymerase chain reaction analysis indicated that 3D structures coated with PPy could provide a favorable microenvironment for OE-MSCs differentiation. In addition, it was found that differentiated OE-MSCs within PCL/PPy could secrete the highest amounts of nerve growth factor and brain-derived neurotrophic factor neurotrophic factors compared to pure PCL and 2D culture. After co-culturing with PC12 cells, a significant increase in neurite outgrowth on PCL/PPy conductive scaffold seeded with differentiated OE-MSCs. These findings indicated that 3D printed PCL/PPy conductive scaffold could support differentiation of OE-MSCs into SC-like phenotypes to promote neurite outgrowth, suggesting their potential for neural tissue engineering applications.

A novel decellularized nerve graft for repairing peripheral nerve long gap injury in the rat

Decellularized nerve allografts are an alternative to autograft for repairing severe nerve injuries, since they have higher availability and do not induce rejection. In this study, we have assessed the regenerative potential of a novel decellularization protocol for human and rat nerves for repairing nerve resections, compared to the gold standard autograft. A 15-mm gap in the sciatic nerve was repaired with decellularized rat allograft (DC-RA), decellularized human xenograft (DC-HX), or fresh autograft (AG). Electrophysiology tests were performed monthly to evaluate muscle reinnervation, whereas histological and immunohistochemical analyses of the grafts were evaluated at 4 months. A short-term study was also performed to compare the differences between the two decellularized grafts (DC-RA and DC-HX) in early phases of regeneration. The decellularization process eliminated cellularity while preserving the ECM and endoneurial tubules of both rat and human nerves. Higher amount of reinnervation was observed in the AG group compared to the DC-RA group, while only half of the animals of the DC-HX showed distal muscle reinnervation. The number of regenerating myelinated axons in the mid-graft was similar between AG and DC-RA and lower in DC-HX graft, but significantly lower in both DC grafts distally. At short term, fibroblasts repopulated the DC-RA graft, supporting regenerated axons, whereas an important fibrotic reaction was observed around DC-HX grafts. In conclusion, the decellularized allograft sustained regeneration through a long gap in the rat although at a slower rate compared to the ideal autograft, whereas regeneration was limited or even failed when using a decellularized xenograft.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Hydrogel, Electrospun and Composite Materials for Bone/Cartilage and Neural Tissue Engineering

Injuries of the bone/cartilage and central nervous system are still a serious socio-economic problem. They are an effect of diversified, difficult-to-access tissue structures as well as complex regeneration mechanisms. Currently, commercially available materials partially solve this problem, but they do not fulfill all of the bone/cartilage and neural tissue engineering requirements such as mechanical properties, biochemical cues or adequate biodegradation. There are still many things to do to provide complete restoration of injured tissues. Recent reports in bone/cartilage and neural tissue engineering give high hopes in designing scaffolds for complete tissue regeneration. This review thoroughly discusses the advantages and disadvantages of currently available commercial scaffolds and sheds new light on the designing of novel polymeric scaffolds composed of hydrogels, electrospun nanofibers, or hydrogels loaded with nano-additives.

Facial Nerve Reconstruction Using Acellular Nerve Allograft

ABSTRACT

When repaired with interposition nerve grafts, segmental facial nerve defects are traditionally treated with nerve autograft. The authors present a patient who presented after partial resection of a basal cell carcinoma over the left zygomatic region with positive deep and peripheral margins and complete loss of frontal and zygomatic facial nerve function. She was subsequently treated with wide resection, superficial parotidectomy, and cervicofacial flap. The parotidectomy was performed for deep margin control and the facial nerve was dissected distally demonstrating the prior resection of a segment of frontal and zygomatic branches consistent with clinical exam. For acute segmental facial nerve defects, nerve autograft has been the gold standard. In our patient, segmental repair using processed nerve allograft demonstrated rapid and complete recovery. This is a viable option for facial nerve reconstruction with various benefits of avoiding donor site morbidity, ease of allograft nerve handling and decreased operative time.

PVDF and P(VDF-TrFE) Electrospun Scaffolds for Nerve Graft Engineering: A Comparative Study on Piezoelectric and Structural Properties, and In Vitro Biocompatibility

Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are considered as promising biomaterials for supporting nerve regeneration because of their proven biocompatibility and piezoelectric properties that could stimulate cell ingrowth due to their electrical activity upon mechanical deformation. For the first time, this study reports on the comparative analysis of PVDF and P(VDF-TrFE) electrospun scaffolds in terms of structural and piezoelectric properties as well as their in vitro performance. A dynamic impact test machine was developed, validated, and utilised, to evaluate the generation of an electrical voltage upon the application of an impact load (varying load magnitude and frequency) onto the electrospun PVDF (15-20 wt%) and P(VDF-TrFE) (10-20 wt%) scaffolds. The cytotoxicity and in vitro performance of the scaffolds was evaluated with neonatal rat (nrSCs) and adult human Schwann cells (ahSCs). The neurite outgrowth behaviour from sensory rat dorsal root ganglion neurons cultured on the scaffolds was analysed qualitatively. The results showed (i) a significant increase of the β-phase content in the PVDF after electrospinning as well as a zeta potential similar to P(VDF-TrFE), (ii) a non-constant behaviour of the longitudinal piezoelectric strain constant d33, depending on the load and the load frequency, and (iii) biocompatibility with cultured Schwann cells and guiding properties for sensory neurite outgrowth. In summary, the electrospun PVDF-based scaffolds, representing piezoelectric activity, can be considered as promising materials for the development of artificial nerve conduits for the peripheral nerve injury repair.

Nerve grafts in head and neck reconstruction

PURPOSE OF REVIEW

This article reviews recent literature on repair of peripheral nerve injuries in the head and neck with a focus on autografts, allografts, nerve conduits, and technical considerations.

RECENT FINDINGS

Contemporary nerve grafting techniques offer the potential to improve peripheral nerve outcomes and reduce donor site morbidity. A variety of donor nerves autografts have been described that offer favorable outcomes for segmental reconstruction of facial nerve defects. Recent studies have demonstrated promising results in repair of inferior alveolar nerve injuries with human allografts. Animal models describe successful reinnervation of small defects with neural conduits. The latest data do not favor protocolled nerve graft polarity or use of a motor versus sensory donor nerves.

SUMMARY

Interposition nerve grafting is the gold standard for repair of peripheral nerve injuries when a tension-free primary neurorrhaphy is not possible. Autografts are the work-horse for the majority of head and neck neural defects, however, can result in some degree of donor site morbidity. Recent developments in allografting and neural conduits have the potential to further diversify the head and neck reconstructive surgeon's armamentarium. It is unclear if nerve graft makeup or polarity affect functional outcome.

Recent Advances in the Application of Two-Dimensional Nanomaterials for Neural Tissue Engineering and Regeneration

The complexity of the nervous system structure and function, and its slow regeneration rate, makes it more difficult to treat compared to other tissues in the human body when an injury occurs. Moreover, the current therapeutic approaches including the use of autografts, allografts, and pharmacological agents have several drawbacks and can not fully restore nervous system injuries. Recently, nanotechnology and tissue engineering approaches have attracted many researchers to guide tissue regeneration in an effective manner. Owing to their remarkable physicochemical and biological properties, two-dimensional (2D) nanomaterials have been extensively studied in the tissue engineering and regenerative medicine field. The great conductivity of these materials makes them a promising candidate for the development of novel scaffolds for neural tissue engineering application. Moreover, the high loading capacity of 2D nanomaterials also has attracted many researchers to utilize them as a drug/gene delivery method to treat various devastating nervous system disorders. This review will first introduce the fundamental physicochemical properties of 2D nanomaterials used in biomedicine and the supporting biological properties of 2D nanomaterials for inducing neuroregeneration, including their biocompatibility on neural cells, the ability to promote the neural differentiation of stem cells, and their immunomodulatory properties which are beneficial for alleviating chronic inflammation at the site of the nervous system injury. It also discusses various types of 2D nanomaterials-based scaffolds for neural tissue engineering applications. Then, the latest progress on the use of 2D nanomaterials for nervous system disorder treatment is summarized. Finally, a discussion of the challenges and prospects of 2D nanomaterials-based applications in neural tissue engineering is provided.