Collagen Nerve Conduits and Processed Nerve Allografts for the Reconstruction of Digital Nerve Gaps: A Single-Institution Case Series and Review of the Literature

Nerve Coaptation in 2023: Adjuncts to Nerve Repair Beyond Suture

Effective nerve coaptation entails tensionless repair of healthy fascicles with intact fascicular architecture and a well-vascularized environment, supportive of the regenerative cellular behaviors of neurons, immune cells, and Schwann cells. Suture coaptations have historically been used to ensure that these criteria are met for end-to-end repair, nerve transfers, and allograft or autograft reconstructions; however, unfortunately, overall restoration of function remains poor. As optimal coaptation is required for return of sensorimotor function, adjunct biomaterials are increasingly being enlisted attempting to optimize these suture-based coaptations. The purpose of this review was to discuss the biological, preclinical, and clinical data for the use of fibrin glue and nerve wraps made of type 1 collagen, porcine small intestine submucosa, chitosan, and human amniotic membrane. This study provides available data on each material's ability to optimize the regenerative potential of nerve repair as well as available outcomes data. Although each biomaterial discussed has benefits to nerve regeneration, at large, data remain heterogeneous, and continued investigation is required to fully understand the specific mechanisms involved and the long-term potential clinical impacts each can provide for improvement of sensorimotor outcomes.

Biological conduits based on spider silk for reconstruction of extended nerve defects

Objectives

The availability of appropriate conduits remains an obstacle for successful reconstruction of long-distance nerve defects. In previous sheep trials, we were able to bridge 6 cm nerve gaps with nerve conduits based on spider silk fibers with full functional outcomes. Here, we describe the first application of spider silk for nerve repair in humans.

Methods

Four patients with extended nerve defects (>20 cm) underwent nerve reconstruction by interposition of conduits that were composed of spider silk fibers contained in autologous veins. The longitudinal luminal fibers (approx. 2500 fibers per graft) consisted of drag line silk from Trichonephila spiders. All patients were evaluated between 2 and 10 years postreconstruction, clinically, and by neurography.

Results

In all patients, primary wound healing and no adverse reactions to the implanted spider silk material were observed. Patients regained the following relevant functions: protective sensibility, full flexor function with near-normal grasp and powerful function after microvascular gracilis muscle transfer, and key grip function and gross finger flexion after additional tenodesis. One patient with sciatic nerve reconstruction developed protective sensibility of the lower leg, foot, and gait, enabling normal walking and jogging. No neuroma formation or neuropathic or chronic pain occurred in any of the patients.

Conclusions

For patients with extended peripheral nerve defects in the extremities, use of conduits based on spider silk fibers offers the possibility of restoring sensory function and protection from neuroma. This kind of nerve bridges provides new perspectives for the reconstruction of complex and long-distance nerve defects.

Bioengineered Nerve Conduits and Wraps

Peripheral nerve injuries are prevalent and their treatments present significant challenges. Among the various reconstructive options, nerve conduits and wraps are popular choices. Advances in bioengineering and regenerative medicine have led to the development of new biocompatible materials and implant designs that offer the potential for enhanced neural recovery. Cost, nerve injury type, and implant size must be considered when deciding on the ideal reconstructive option.

End-to-side neurorrhaphy in the reconstruction of peripheral segmental neural loss: an experimental study

PURPOSE

To evaluate the effects on peripheral neural regeneration of the end-to-side embracing repair technique compared to the autograft repair technique in Wistar rats.

METHODS

Fifteen male Wistar rats were divided into three groups with five animals each: denervated group (GD), autograft group (GA), and embracing group (EG). For the evaluation, the grasping test, electroneuromyography (ENMG), and muscle weight assessment were used.

RESULTS

Muscle weight assessment and ENMG did not show significant neural regeneration at the end of 12 weeks in the DG and GE groups, but only in GA. The grasping test showed an increase in strength between the surgery and the fourth week in all groups, and only the GA maintained this trend until the 12th week.

CONCLUSIONS

The present study indicates that the neural regeneration observed in the end-to-side embracing neurorrhaphy technique, in the repair of segmental neural loss, is inferior to autograft repair in Wistar rats.

Assessment, management, and rehabilitation of traumatic peripheral nerve injuries for non-surgeons

Electrodiagnostic evaluation is often requested for persons with peripheral nerve injuries and plays an important role in their diagnosis, prognosis, and management. Peripheral nerve injuries are common and can have devastating effects on patients' physical, psychological, and socioeconomic well-being; alongside surgeons, electrodiagnostic medicine specialists serve a central function in ensuring patients receive optimal treatment for these injuries. Surgical intervention-nerve grafting, nerve transfers, and tendon transfers-often plays a critical role in the management of these injuries and the restoration of patients' function. Increasingly, nerve transfers are becoming the standard of care for some types of peripheral nerve injury due to two significant advantages: first, they shorten the time to reinnervation of denervated muscles; and second, they confer greater specificity in directing motor and sensory axons toward their respective targets. As the indications for, and use of, nerve transfers expand, so too does the role of the electrodiagnostic medicine specialist in establishing or confirming the diagnosis, determining the injury's prognosis, recommending treatment, aiding in surgical planning, and supporting rehabilitation. Having a working knowledge of nerve and/or tendon transfer options allows the electrodiagnostic medicine specialist to not only arrive at the diagnosis and prognosticate, but also to clarify which nerves and/or muscles might be suitable donors, such as confirming whether the branch to supinator could be a nerve transfer donor to restore distal posterior interosseous nerve function. Moreover, post-operative testing can determine if nerve transfer reinnervation is occurring and progress patients' rehabilitation and/or direct surgeons to consider tendon transfers.

The Medial Antebrachial Cutaneous Nerve Is a Low-Morbidity Alternative to the Standard Sural Nerve Autograft

BACKGROUND

Nerve interposition grafting is an important technique in nerve reconstructive surgery that is used when a primary repair is not feasible without significant tension. This study sought to evaluate the long-term morbidity of the medial antebrachial cutaneous (MABC) nerve as an alternative donor nerve in comparison with sural nerve harvest.

METHODS

A single surgeon and institution retrospective chart review was performed to identify all patients who underwent nerve autografting using the sural and MABC as donor nerves between January 1, 2000 and December 31, 2019. Surveys assessed overall patient satisfaction with surgery, as well as donor and recipient site morbidity, satisfaction, pain, numbness, and cold sensitivity.

RESULTS

Of the 73 patients contacted, 54 agreed to participate, and 43 of 73 (58.9%) ultimately completed the survey: 28 MABC (65.1%) and 15 sural (34.9%). There were no significant differences between the sural and MABC groups in overall satisfaction with surgery, donor and recipient site satisfaction, pain, cold sensitivity, and effect on quality of life. Even though 66.7% of sural donor sites and 75% of MABC donor sites had residual numbness, the effect this had on quality of life was very low (2 and 3, respectively).

CONCLUSION

The MABC is a safe alternative to the traditional sural nerve autograft. A small subset of patients undergoing nerve autograft harvest will experience long-term morbidity in the form of pain. Conversely, the more common presence of numbness is not reported as bothersome.

Non degradation of chitosan and initial degradation of collagen nerve conduits used for protection of nerve coaptations

BACKGROUND

Nerve conduits are either used to bridge nerve gaps of up to 3 cm or to protect nerve coaptations. Biodegradable nerve conduits, which are currently commercially available, include Chitosan or collagen-based ones. As histological aspects of their degradation are highly relevant for the progress of neuronal regeneration, the aim of this study was to report the histopathological signs of such nerve conduits, which were removed during revision surgery.

MATERIALS AND METHODS

Either Chitosan (n = 2) or collagen (n = 2) nerve conduits were implanted after neuroma resection and nerve grafting (n = 2) or traumatic nerve lesion after cut (n = 1) or crush injury (n = 1) in two females and two men, aged between 17 and 57 years. Revision surgery with removal of the nerve conduits was indicated due to persisting neuropathic pain and sensorimotor deficits, limited joint motion, or neurolysis with hardware removal at a median time of 17 months (range: 5.5-48 months). Histopathological analyses of all removed nerve conduits were performed.

RESULTS

A scar neuroma was diagnosed in one out of four patients. Mechanical complication occurred in one patient after nerve conduit implantation bridged over finger joints. Intraoperatively no or only initial signs of degradation of the nerve conduits were observed. Chitosan conduits revealed largely unchanged shape and structure of chitosan, and coating of the conduit by a vascularized fibrous membrane. The latter contained deposits taken up by macrophages, most likely representing dissolved chitosan. Characteristic histopathologic features of the degradation of collagen conduits were a disintegration of the compact collagen into separate fine circular strands, No foreign body reaction was observed in all removed nerve conduits.

CONCLUSIONS

Both Chitosan nerve conduits have not been degraded. The collagen nerve conduits showed a beginning degradation process. Furthermore, wrapping the repaired nerve with a nerve conduit did neither prevent adhesions nor improved nerve gliding. Therefore, biodegradation in time should be particularly addressed in further developments of nerve conduits.

Neuromas cause severe residual problems at long-term despite surgery

Pain, and disabilities after neuroma surgery, using patient reported outcome measurements (PROMs), were evaluated by QuickDASH and a specific Hand Questionnaire (HQ-8). The 69 responding individuals (response rate 61%; 59% women; 41% men; median follow up 51 months) reported high QuickDASH score, pain on load, cold sensitivity, ability to perform daily activities and sleeping difficulties. Individuals reporting impaired ability to perform daily activities and sleeping problems had higher scores for pain, stiffness, weakness, numbness/tingling, cold sensitivity and QuickDASH. Only 17% of individuals reported no limitations at all. No differences were observed between sexes. Surgical methods did not influence outcome. Symptoms and disabilities correlated moderately-strongly to each other and to ability to perform regular daily activities as well as to sleeping difficulties. Pain, cold sensitivity, sleeping difficulties and limitation to perform daily activities were associated to higher QuickDASH. A weak association was found between follow up time and QuickDASH score as well as pain on load, but not cold sensitivity. A major nerve injury was frequent among those with limitations during work/performing other regular daily activities. Despite surgical treatment, neuromas cause residual problems, which affect the capacity to perform daily activities and ability to sleep with limited improvement in long-term.

Treatment options for digital nerve injury: a systematic review and meta-analysis

BACKGROUND

Surgical treatment of finger nerve injury is common for hand trauma. However, there are various surgical options with different functional outcomes. The aims of this study are to compare the outcomes of various finger nerve surgeries and to identify factors associated with the postsurgical outcomes via a systematic review and meta-analysis.

METHODS

The literature related to digital nerve repairs were retrieved comprehensively by searching the online databases of PubMed from January 1, 1965, to August 31, 2021. Data extraction, assessment of bias risk and the quality evaluation were then performed. Meta-analysis was performed using the postoperative static 2-point discrimination (S2PD) value, moving 2-point discrimination (M2PD) value, and Semmes-Weinstein monofilament testing (SWMF) good rate, modified Highet classification of nerve recovery good rate. Statistical analysis was performed using the R (V.3.6.3) software. The random effects model was used for the analysis. A systematic review was also performed on the other influencing factors especially the type of injury and postoperative complications of digital nerve repair.

RESULTS

Sixty-six studies with 2446 cases were included in this study. The polyglycolic acid conduit group has the best S2PD value (6.71 mm), while the neurorrhaphy group has the best M2PD value (4.91 mm). End-to-side coaptation has the highest modified Highet's scoring (98%), and autologous nerve graft has the highest SWMF (91%). Age, the size of the gap, and the type of injury were factors that may affect recovery. The type of injury has an impact on the postoperative outcome of neurorrhaphy. Complications reported in the studies were mainly neuroma, cold sensitivity, paresthesia, postoperative infection, and pain.

CONCLUSION

Our study demonstrated that the results of surgical treatment of digital nerve injury are generally satisfactory; however, no nerve repair method has absolute advantages. When choosing a surgical approach to repair finger nerve injury, we must comprehensively consider various factors, especially the gap size of the nerve defect, and postoperative complications. Type of study/level of evidence Therapeutic IV.

Evaluation of Processed Nerve Allograft in Peripheral Nerve Surgery: A Systematic Review and Critical Appraisal

Peripheral nerve injuries cause substantial problems when not treated properly. A specific problem is reconstruction of nerve defects, which can be treated in different ways. This study aimed to systematically review whether processed nerve allograft (PNA) is justified in reconstruction of a nerve defect in patients after posttraumatic or iatrogenic peripheral nerve injury and to compare PNA with other established methods.

Methods

A systematic review with a focused question, PICO (patient, intervention, comparison, outcome) and constraints, was performed. A structured literature search, including several databases, was done to evaluate the existing evidence for outcomes and postoperative complications related to PNA. The certainty of evidence was classified according to Grading of Recommendations, Assessment, Development and Evaluations.

Results

No conclusions, concerning differences in outcome of nerve reconstruction using PNA compared with the use of nerve autograft or conduits, could be drawn. The level of certainty for all evaluated outcomes was very low (⊕◯◯◯). Most published studies lack a control group to patients treated with PNA; being only descriptive, making it difficult to compare PNA with established methods without substantial risk of bias. For studies including a control group, the scientific evidence was of very low certainty, due to a low number of included patients, and large, undefined loss of patients during follow-up, rendering a high risk of bias. Finally, the authors often had financial disclosures.

Conclusion

Properly conducted randomized controlled trial studies on the use of PNA in reconstruction of peripheral nerve injuries are needed to establish recommendations in clinical practice.

Management of Complications in Vestibular Schwannoma Surgery

Microsurgical removal of acoustic neuroma has advanced tremendously; however, complications still occur. Facial nerve injury is the most common detrimental complication and should take precedence over gross tumor removal in cases where there is an unfavorable tumor-facial nerve interface. Cerebrospinal fluid leakage can occur even with meticulous closure techniques and is generally treatable with either lumbar-subarachnoid drainage or revision wound closure. Meningitis is a serious complication that requires a high index of suspicion in the postoperative period. Other less common complications include intraoperative and postoperative vascular injuries. Early identification and treatment can prevent devastating outcomes.

Acellular nerve xenografts based on supercritical extraction technology for repairing long-distance sciatic nerve defects in rats

Compared to conventional artificial nerve guide conduits (NGCs) prepared using natural polymers or synthetic polymers, acellular nerve grafts (ACNGs) derived from natural nerves with eliminated immune components have natural bionic advantages in composition and structure that polymer materials do not have. To further optimize the repair effect of ACNGs, in this study, we used a composite technology based on supercritical carbon dioxide (scCO₂) extraction to process the peripheral nerve of a large mammal, the Yorkshire pig, and obtained an innovative Acellular nerve xenografts (ANXs, namely, CD + scCO₂ NG). After scCO₂ extraction, the fat and DNA content in CD + scCO₂ NG has been removed to the greatest extent, which can better supported cell adhesion and proliferation, inducing an extremely weak inflammatory response. Interestingly, the protein in the CD + scCO₂ NG was primarily involved in signaling pathways related to axon guidance. Moreover, compared with the pure chemical decellularized nerve graft (CD NG), the DRG axons grew naturally on the CD + scCO₂ NG membrane and extended long distances. In vivo studies further revealed that the regenerated nerve axons had basically crossed the CD + scCO₂ NG 3 weeks after surgery. 12 weeks after surgery, CD + scCO2 NG was similar to autologous nerves in improving the quality of nerve regeneration, target muscle morphology and motor function recovery and was significantly better than hollow NGCs and CD NG. Therefore, we believe that the fully decellularized and fat-free porcine ACNGs may be the most promising “bridge” for repairing human nerve defects at this stage and for some time to come.

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The native adipose tissue inside acellular nerve xenografts hinders regenerated nerve fibers.

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Environmentally friendly scCO2 extraction has natural advantages in reducing fat content.

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Natural three-dimensional nerve basement membrane tube structure guides regenerating axons.

Tissue-engineered grafts exploit axon-facilitated axon regeneration and pathway protection to enable recovery after 5-cm nerve defects in pigs

Functional restoration following major peripheral nerve injury (PNI) is challenging, given slow axon growth rates and eventual regenerative pathway degradation in the absence of axons. We are developing tissue-engineered nerve grafts (TENGs) to simultaneously "bridge" missing nerve segments and "babysit" regenerative capacity by providing living axons to guide host axons and maintain the distal pathway. TENGs were biofabricated using porcine neurons and "stretch-grown" axon tracts. TENG neurons survived and elicited axon-facilitated axon regeneration to accelerate regrowth across both short (1 cm) and long (5 cm) segmental nerve defects in pigs. TENG axons also closely interacted with host Schwann cells to maintain proregenerative capacity. TENGs drove regeneration across 5-cm defects in both motor and mixed motor-sensory nerves, resulting in dense axon regeneration and electrophysiological recovery at levels similar to autograft repairs. This approach of accelerating axon regeneration while maintaining the pathway for long-distance regeneration may achieve recovery after currently unrepairable PNIs.

Promoting Nerve Regeneration: Electrical Stimulation, Gene Therapy, and Beyond

Peripheral nerve injuries often result in life-altering functional deficits even with optimal management. Unlike the central nervous system, peripheral nerves have the ability to regenerate lost axons after injury; however, axonal regeneration does not equate to full restoration of function. To overcome this physiological shortcoming, advances in nerve regeneration and repair are paramount, including electrical stimulation, gene therapy, and surgical technique advancements.

Smart biomaterials and their potential applications in tissue engineering

Smart biomaterials have been rapidly advancing ever since the concept of tissue engineering was proposed. Interacting with human cells, smart biomaterials can play a key role in novel tissue morphogenesis. Various aspects of biomaterials utilized in or being sought for the goal of encouraging bone regeneration, skin graft engineering, and nerve conduits are discussed in this review. Beginning with bone, this study summarizes all the available bioceramics and materials along with their properties used singly or in conjunction with each other to create scaffolds for bone tissue engineering. A quick overview of the skin-based nanocomposite biomaterials possessing antibacterial properties for wound healing is outlined along with skin regeneration therapies using infrared radiation, electrospinning, and piezoelectricity, which aid in wound healing. Furthermore, a brief overview of bioengineered artificial skin grafts made of various natural and synthetic polymers has been presented. Finally, by examining the interactions between natural and synthetic-based biomaterials and the biological environment, their strengths and drawbacks for constructing peripheral nerve conduits are highlighted. The description of the preclinical outcome of nerve regeneration in injury healed with various natural-based conduits receives special attention. The organic and synthetic worlds collide at the interface of nanomaterials and biological systems, producing a new scientific field including nanomaterial design for tissue engineering.

Closing the Gap: Bridging Peripheral Sensory Nerve Defects with a Chitosan-Based Conduit a Randomized Prospective Clinical Trial

Introduction: If tensionless nerve coaptation is not possible, bridging the resulting peripheral nerve defect with an autologous nerve graft is still the current gold standard. The concept of conduits as an alternative with different materials and architectures, such as autologous vein conduits or bioartificial nerve conduits, could not replace the nerve graft until today. Chitosan, as a relatively new biomaterial, has recently demonstrated exceptional biocompatibility and material stability with neural lineage cells. The purpose of this prospective randomized clinical experiment was to determine the efficacy of chitosan-based nerve conduits in regenerating sensory nerves in the hand. Materials and methods: Forty-seven patients with peripheral nerve defects up to 26 mm distal to the carpal tunnel were randomized to receive either a chitosan conduit or an autologous nerve graft with the latter serving as the control group. Fifteen patients from the conduit group and seven patients from the control group were available for a 12-month follow-up examination. The primary outcome parameter was tactile gnosis measured with two-point discrimination. The secondary outcome parameters were Semmens Weinstein Monofilament Testing, self-assessed pain, and patient satisfaction. Results: Significant improvement (in static two-point discrimination) was observed six months after trauma (10.7 ± 1.2 mm; p < 0.05) for chitosan-based nerve conduits, but no further improvement was observed after 12 months of regeneration (10.9 ± 1.3 mm). After six months and twelve months, the autologous nerve graft demonstrated comparable results to the nerve conduit, with a static two-point discrimination of 11.0 ± 2.0 mm and 7.9 ± 1.1 mm. Semmes Weinstein Filament Testing in the nerve conduit group showed a continuous improvement over the regeneration period by reaching from 3.1 ± 0.3 after three months up to 3.7 ± 0.4 after twelve months. Autologous nerve grafts presented similar results: 3.3 ± 0.4 after three months and 3.7 ± 0.5 after twelve months. Patient satisfaction and self-reported pain levels were similar between the chitosan nerve conduit and nerve graft groups. One patient required revision surgery due to complications associated with the chitosan nerve tube. Conclusion: Chitosan-based nerve conduits are safe and suitable for bridging nerve lesions up to 26 mm in the hand. Tactile gnosis improved significantly during the early regeneration period, and functional outcomes were similar to those obtained with an autologous nerve graft. Thus, chitosan appears to be a sufficient substitute for autologous nerve grafts in the treatment of small nerve defects in the hand.

Implantable Biomaterials for Peripheral Nerve Regeneration-Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon's perspective and give an outlook toward future challenges in the field.

Enhanced Nerve Regeneration by Bionic Conductive Nerve Scaffold Under Electrical Stimulation

Repair of peripheral nerve defect (PND) with a poor prognosis is hard to deal with. Neural conduit applied to nerve defect at present could not achieve the effect of autologous nerve transplantation. We prepared bionic conductive neural scaffolds to provide a new strategy for the treatment of PNDs. The highly aligned poly (L-lactic acid) (PLLA) fiber mats and the multi-microchannel conductive scaffolds were combined into bionic conductive nerve scaffolds, which were implanted into rats with sciatic nerve defects. The experimental animals were divided into the scaffold group (S), scaffold with electrical stimulation (ES) group (S&E), and autologous nerve transplantation group (AT). The regenerative effect of bionic conductive nerve scaffolds was analyzed. Compared with aligned PLLA fiber mats (APFMs), highly aligned fiber mats had a higher fiber orientation and did not change the tensile strength, Young’s modulus, degradation rate, elongation at break of the fiber membrane, and biocompatibility. The bionic conductive nerve scaffolds were well matched with the rat sciatic nerve. The evaluations of the sciatic nerve in Group S&E were close to those in Group AT and better than those in Group S. Immunohistochemical results showed that the expression levels of neurofilament heavy polypeptide (NF-H) and protein S100-B (S100-β) in Group S&E were higher than those in Group S, and the expression levels of low-density lipoprotein receptor-related protein 4 (LRP4), mitogen-activated protein kinase (MAPK) p38, extracellular signal-regulated kinase (ERK), and mitogen-activated protein kinase kinase (MEK) in Group AT were higher than those in Group S. Bionic conductive nerve scaffolds combined with ES could enhance peripheral nerve regeneration and achieve satisfactory nerve regeneration close to autologous nerve grafts. ERK, p38 MAPK, MEK, and LRP4 may be involved in peripheral nerve regeneration under ES.

Ultrasound Appearance of In Vitro Nerve Allografts and Conduits for Peripheral Nerve Reconstruction

Ultrasound enables the accurate assessment of traumatic disorders of small peripheral nerves of extremities. Human nerve allografts and nerve conduits are increasingly used for the surgical treatment of nerve trauma but ultrasound reports on this field are scarce in the radiological literature. We present the macroscopic and in vitro ultrasound appearance of human allografts, and synthetic and biological conduits. In addition, we describe the ultrasound findings in some patients operated upon using the same devices. The in vitro ultrasound appearance correlated well with the macroscopic appearance of the devices. Awareness of their appearance in vitro can help sonologists when examining postsurgical patients.

The Role of Biomaterials in Peripheral Nerve and Spinal Cord Injury: A Review

Peripheral nerve and spinal cord injuries are potentially devastating traumatic conditions with major consequences for patients’ lives. Severe cases of these conditions are currently incurable. In both the peripheral nerves and the spinal cord, disruption and degeneration of axons is the main cause of neurological deficits. Biomaterials offer experimental solutions to improve these conditions. They can be engineered as scaffolds that mimic the nerve tissue extracellular matrix and, upon implantation, encourage axonal regeneration. Furthermore, biomaterial scaffolds can be designed to deliver therapeutic agents to the lesion site. This article presents the principles and recent advances in the use of biomaterials for axonal regeneration and nervous system repair.

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.

A Retrospective Case Series of Peripheral Mixed Nerve Reconstruction Failures Using Processed Nerve Allografts

BACKGROUND

Favorable rates of meaningful recovery (≥M3/S3) of processed nerve allografts (PNAs) for mixed and motor nerve injuries have been reported, but there are few reports of patients having complete PNA failure (M0/S0). The purpose of this study was to describe the outcomes, including rate of complete failures, in a case series of patients who underwent PNA for peripheral mixed nerve reconstructions.

METHODS

A retrospective review of outcomes between May 2018 to September 2020 was performed. Consecutive patients who underwent nerve reconstruction (>15 mm) with PNA for a peripheral mixed nerve injury of the upper or lower extremity were eligible. Those who returned to clinic for a 10-month postoperative visit were included in this study. The primary outcome was whether the patient was defined as having a complete failure (M0/S0).

RESULTS

A total of 22 patients underwent a PNA during the time period; 14 patients participated in follow-up and were included (average age: 34.7 years) with a mean follow-up of 11.9 months. The average gap length was 46.4 mm (range 15-110 mm). At their 10-month postoperative visit, no patients had any motor or sensory improvement; all patients were deemed as having complete failure. Four patients underwent or were planned for subsequent revision surgery.

CONCLUSIONS

In this study, we demonstrated a high number of complete failures, with all 14 included patients sustaining a complete failure (100% failure rate) at a minimum 10-month follow-up visit. Failure in this case series was not observed to affect one nerve type, location, or be related to preoperative injury size.

Comparison of two different decellularization methods for processed nerve allograft

The use of processed nerve allografts as an alternative to autologous nerve grafts, the gold standard treatment for peripheral nerve defects, is increasing. However, it is not widely used in Korea due to cost and insurance issues. Moreover, the main detergent used in the conventional Hudson method is unavailable. Therefore, a new nerve allograft decellularization process is needed. We aimed to compare the traditional Hudson method with a novel decellularization process that may remove cellular content more efficiently while preserving the extracellular matrix (ECM) structure using low concentration sodium dodecyl sulfate (SDS) and nuclease. After each decellularization process, DNA content was measured in nerve tissue. Masson's trichrome staining and scanning electron microscopy were performed to determine the state of preservation of the ECM. A significantly greater amount of DNA content was removed in the novel method, and the ECM structure was preserved in both methods. For the in vivo study, a 15-mm long sciatic nerve defect was created in two groups of Sprague-Dawley rats, and processed nerve allografts decellularized using the Hudson or novel method were transplanted. Functional and histological recovery results were measured 12 weeks post-transplantation. Ankle contracture angle, maximal isometric tetanic force of the tibialis anterior (TA), and the TA mass were compared between the groups, as well as the percent neural tissue (100 × neural area/intrafascicular area). There was no significant difference in functional and histological nerve recovery between the methods. The novel method is appropriate for developing a processed nerve allograft.

Modified Hyaluronic Acid-Laminin-Hydrogel as Luminal Filler for Clinically Approved Hollow Nerve Guides in a Rat Critical Defect Size Model

Hollow nerve guidance conduits are approved for clinical use for defect lengths of up to 3 cm. This is because also in pre-clinical evaluation they are less effective in the support of nerve regeneration over critical defect lengths. Hydrogel luminal fillers are thought to improve the regeneration outcome by providing an optimized matrix inside bioartificial nerve grafts. We evaluated here a modified hyaluronic acid-laminin-hydrogel (M-HAL) as luminal filler for two clinically approved hollow nerve guides. Collagen-based and chitosan-based nerve guides were filled with M-HAL in two different concentrations and the regeneration outcome comprehensively studied in the acute repair rat sciatic nerve 15 mm critical defect size model. Autologous nerve graft (ANG) repair served as gold-standard control. At 120 days post-surgery, all ANG rats demonstrated electrodiagnostically detectable motor recovery. Both concentrations of the hydrogel luminal filler induced improved regeneration outcome over empty nerve guides. However, neither combination with collagen- nor chitosan-based nerve guides resulted in functional recovery comparable to the ANG repair. In contrast to our previous studies, we demonstrate here that M-HAL slightly improved the overall performance of either empty nerve guide type in the critical defect size model.

Evidence-Based Approach to Nerve Gap Repair in the Upper Extremity: A Review of the Literature and Current Algorithm for Surgical Management

The upper extremity is the most common site for nerve injuries. In most cases, direct repair can be performed, but when a critical gap occurs, special techniques must be used to enhance nerve regeneration and allow recovery of sensory and motor functions. These techniques include the use of autografts, processed nerve allografts, and conduits. However, surprisingly few studies have compared outcomes from the different methods of nerve gap repair in a rigorous fashion. There is a lack of evidence-based guidelines for the management of digital and motor and mixed nerve injuries with a nerve gap. The purpose of this study is to perform a comprehensive literature review and propose a rational algorithm for management of nerve injuries with a critical gap.

Polymer Scaffolds for Biomedical Applications in Peripheral Nerve Reconstruction

The nervous system is a significant part of the human body, and peripheral nerve injury caused by trauma can cause various functional disorders. When the broken end defect is large and cannot be repaired by direct suture, small gap sutures of nerve conduits can effectively replace nerve transplantation and avoid the side effect of donor area disorders. There are many choices for nerve conduits, and natural materials and synthetic polymers have their advantages. Among them, the nerve scaffold should meet the requirements of good degradability, biocompatibility, promoting axon growth, supporting axon expansion and regeneration, and higher cell adhesion. Polymer biological scaffolds can change some shortcomings of raw materials by using electrospinning filling technology and surface modification technology to make them more suitable for nerve regeneration. Therefore, polymer scaffolds have a substantial prospect in the field of biomedicine in future. This paper reviews the application of nerve conduits in the field of repairing peripheral nerve injury, and we discuss the latest progress of materials and fabrication techniques of these polymer scaffolds.

Type I Collagen-Based Devices to Treat Nerve Injuries after Oral Surgery Procedures. A Systematic Review

The regeneration of nerve injuries after oral surgery procedures is a quite often attempted procedure in dental medicine. Despite several proposed technical approaches, there is still a lack of consensus on which should be considered the gold standard procedure, even-though in the last decades, the use of collagen-based devices allowing a tension-free direct neurorrhaphy has been used. A systematic search of multiple electronic databases and hand searching was conducted to assess the level of evidence behind the use of type I collagen devices to treat nerve injuries after oral surgery procedures. After screening, four articles (one case series and three retrospective studies) including overall 65 patients suffering from inferior alveolar (IAN)/lingual nerve (LN) injury after mandibular wisdom tooth extraction, met the inclusion criteria and could be included. The Oxford Centre for evidence-based medicine (OCEBM) scaling system was used to evaluate the quality of the included studies. Positive clinical results in terms of sensorial improvements were recorded at least 3 months after surgery, even-though the overall level of evidence is low. The use of collagen membranes to enhance nerve regeneration in oral surgery results in promising results. Nevertheless, additional clinical comparative trials with larger sample sizes are needed.

Influence of aging on the peripheral nerve repair process using an artificial nerve conduit

The influence of aging on the induction of nerve regeneration in artificial nerve conduits has yet to be clarified. In the present study, artificial nerve conduit transplantation and histological analysis using the sciatic nerve of young and elderly mice were performed. Using 20 male C57BL/6 mice, an artificial nerve conduit was transplanted to the sciatic nerve at 8 weeks (Young group) or 70 weeks of age (Aged group), and the sciatic nerve was evaluated histologically at 1, 4 and 12 weeks after surgery. Using hematoxylin and eosin staining, the state of induction of nerve regeneration in the artificial nerve conduit was evaluated. Additionally, immunohistochemical staining was used to investigate an angiogenic marker [vascular endothelial growth factor A (VEGFA)], Schwann cell markers [sex determining region Y-box 10 (SOX10) and S100 calcium-binding protein β (S100β)] and a nerve damage marker [nerve growth factor (NGF)]. The results revealed that the induction of nerve regeneration was significantly higher in the Young group than in the Aged group. In addition, VEGFA and SOX10 expression at 1 week, SOX10 expression at 4 weeks and SOX10, S100β and NGF expression at 12 weeks in the proximal stump were significantly higher in the Young group than in the Aged group. At the center of the artificial nerve conduit, S100β and NGF expression at 4 weeks, and VEGFA, SOX10, S100β and NGF expression at 12 weeks were significantly higher in the Young group than in the Aged group. In the distal stump, no significant difference was noted in immunostaining at any week between the two groups. The present study suggested that the nerve regeneration-inducing functions decrease due to aging.

A Multicenter Matched Cohort Study of Processed Nerve Allograft and Conduit in Digital Nerve Reconstruction

PURPOSE

Biomaterials used to restore digital nerve continuity after injury associated with a defect may influence ultimate outcomes. An evaluation of matched cohorts undergoing digital nerve gap reconstruction was conducted to compare processed nerve allograft (PNA) and conduits. Based on scientific evidence and historical controls, we hypothesized that outcomes of PNA would be better than for conduit reconstruction.

METHODS

We identified matched cohorts based on patient characteristics, medical history, mechanism of injury, and time to repair for digital nerve injuries with gaps up to 25 mm. Data were stratified into 2 gap length groups: short gaps of 14 mm or less and long gaps of 15 to 25 mm. Meaningful sensory recovery was defined as a Medical Research Council scale of S3 or greater. Comparisons of meaningful recovery were made by repair method between and across the gap length groups.

RESULTS

Eight institutions contributed matched data sets for 110 subjects with 162 injuries. Outcomes data were available in 113 PNA and 49 conduit repairs. Meaningful recovery was reported in 61% of the conduit group, compared with 88% in the PNA group. In the group with a 14-mm or less gap, conduit and PNA outcomes were 67% and 92% meaningful recovery, respectively. In the 15- to 25-mm gap length group, conduit and PNA outcomes were 45% and 85% meaningful recovery, respectively. There were no reported adverse events in either treatment group.

CONCLUSIONS

Outcomes of digital nerve reconstruction in this study using PNA were consistent and significantly better than those of conduits across all groups. As gap lengths increased, the proportion of patients in the conduit group with meaningful recovery decreased. This study supports the use of PNA for nerve gap reconstruction in digital nerve reconstructions up to 25 mm.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic III.

Natural-Based Biomaterials for Peripheral Nerve Injury Repair

Peripheral nerve injury treatment is a relevant problem because of nerve lesion high incidence and because of unsatisfactory regeneration after severe injuries, thus resulting in a reduced patient's life quality. To repair severe nerve injuries characterized by substance loss and to improve the regeneration outcome at both motor and sensory level, different strategies have been investigated. Although autograft remains the gold standard technique, a growing number of research articles concerning nerve conduit use has been reported in the last years. Nerve conduits aim to overcome autograft disadvantages, but they must satisfy some requirements to be suitable for nerve repair. A universal ideal conduit does not exist, since conduit properties have to be evaluated case by case; nevertheless, because of their high biocompatibility and biodegradability, natural-based biomaterials have great potentiality to be used to produce nerve guides. Although they share many characteristics with synthetic biomaterials, natural-based biomaterials should also be preferable because of their extraction sources; indeed, these biomaterials are obtained from different renewable sources or food waste, thus reducing environmental impact and enhancing sustainability in comparison to synthetic ones. This review reports the strengths and weaknesses of natural-based biomaterials used for manufacturing peripheral nerve conduits, analyzing the interactions between natural-based biomaterials and biological environment. Particular attention was paid to the description of the preclinical outcome of nerve regeneration in injury repaired with the different natural-based conduits.

Fabrication and evaluation of an optimized acellular nerve allograft with multiple axial channels

Acellular nerve allografts are promising alternatives to autologous nerve grafts, but still have many drawbacks which greatly limit their curative effects. Here, we developed an optimized acellular nerve allograft with multiple axial channels by a modified decellularization method. These allografts were confirmed to preserve more extracellular matrix components and factors, and remove cellular components effectively. Meanwhile, macrochannels and microchannels were introduced to optimize internal microstructure of allografts, which increases porosity and water absorption, without significant loss of mechanical strength. The in vitro experiments demonstrated that the multichannel allografts showed superior ability of facilitating proliferation and penetration of Schwann cells. Additionally, in the in vivo experiments, the multichannel allografts were used to bridge 10 mm rat sciatic nerve defects. They exhibited better capacity to guide regenerative nerve fibers through the defective segment and restore innervation of target organs, thus achieving better recovery of muscle and motor function, in comparison with conventional acellular allografts. These findings indicate that this multichannel acellular nerve allograft has great potential for clinical application and provides a new prospective for future investigations of nerve regeneration. STATEMENT OF SIGNIFICANCE: Acellular nerve allografts, with preservation of natural extracellular matrix, are officially approved to repair peripheral nerve injury in some countries. However, bioactive component loss and compact internal structure result in variable clinical effects of conventional acellular allografts. In the present study, we fabricated an optimized acellular nerve allograft with multiple axial channels, which could both enable decellularization to be easily accomplished and reduce the amount of detergents in the preparation process. Characterization of the multichannel acellular allografts was confirmed to have better preservation of ECM bioactive molecules and regenerative factors. Efficiency evaluation showed the multichannel allografts could facilitate Schwann cells to migrate inside them in vitro, and enhance regrowth and myelination of axons as well as recovery of muscle and motor function in vivo.

Restoration of Neurological Function Following Peripheral Nerve Trauma

Following peripheral nerve trauma that damages a length of the nerve, recovery of function is generally limited. This is because no material tested for bridging nerve gaps promotes good axon regeneration across the gap under conditions associated with common nerve traumas. While many materials have been tested, sensory nerve grafts remain the clinical “gold standard” technique. This is despite the significant limitations in the conditions under which they restore function. Thus, they induce reliable and good recovery only for patients < 25 years old, when gaps are <2 cm in length, and when repairs are performed <2–3 months post trauma. Repairs performed when these values are larger result in a precipitous decrease in neurological recovery. Further, when patients have more than one parameter larger than these values, there is normally no functional recovery. Clinically, there has been little progress in developing new techniques that increase the level of functional recovery following peripheral nerve injury. This paper examines the efficacies and limitations of sensory nerve grafts and various other techniques used to induce functional neurological recovery, and how these might be improved to induce more extensive functional recovery. It also discusses preliminary data from the clinical application of a novel technique that restores neurological function across long nerve gaps, when repairs are performed at long times post-trauma, and in older patients, even under all three of these conditions. Thus, it appears that function can be restored under conditions where sensory nerve grafts are not effective.