A review of new developments in peripheral nerve regeneration

Abstract

Peripheral nerve injury (PNI) is a significant health concern, affecting millions worldwide. Key neurotrophic factors, including nerve growth factor, brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor, have shown promise in facilitating neural regeneration. The effects of non-steroidal anti-inflammatory drugs (NSAIDs) and corticosteroids have been extensively studied, emphasizing the importance of appropriate timing and duration of administration. Antioxidants such as vitamin E and melatonin have exhibited neuroprotective effects in animal models, but further research is necessary to determine their efficacy, optimal dosage, and administration in humans. Immunosuppressive agents like tacrolimus (FK506) and cyclosporin A have demonstrated substantial potential in enhancing peripheral nerve recovery. Supportive strategies, including physical therapy and neuromodulation techniques such as electrical and transcranial stimulation, have shown effectiveness in promoting nerve regeneration. Advances in bioengineering, including nerve conduits and stem cell transplantation, which mimic natural nerve repair mechanisms, hold considerable promise for improving PNI treatments. In conclusion, PNI therapy is progressing towards an integrative approach, combining surgical techniques with pharmacological interventions, bioengineering, and regenerative medicine to enhance outcomes while minimizing adverse effects. This review explores recent advancements in peripheral nerve regeneration using both natural and synthetic agents, highlighting the shift toward more comprehensive treatment strategies.

Graphical abstract

Highlights

• •Peripheral nerve injuries impact millions, causing functional loss and reduced quality of life.
• •Research focuses on new medications and therapies to boost nerve regeneration.
• •Bioengineering advances such as nerve conduits and stem cells offer promising PNI treatments.
• •This overview highlights recent synthetic compounds for peripheral nerve repair.

1. Introduction

Peripheral nerves are fundamental components of the nervous system, responsible for transmitting sensory and motor signals. They relay sensory information from the body to the brain, enabling perception, and convey motor commands from the brain to muscles, facilitating movement. However, these vital nerves are vulnerable to injury, which can lead to significant functional impairment and chronic pain.

Peripheral nerve injuries (PNIs) trigger a series of biological processes, including axonal degeneration, demyelination, macrophage infiltration, and Schwann cell activation. Following injury, the distal axon segment undergoes Wallerian degeneration, a process that leads to the breakdown of both the axon and myelin sheath. At the same time, Schwann cells proliferate and form Bands of Büngner, which provide a structural guide for axonal regeneration [1,2].

Importantly, the regenerative response varies depending on the type and severity of the injury, which are classified by established systems. For example, Seddon's classification (1943) divides PNIs into three categories: Neuropraxia, a mild and temporary conduction block without disruption of axons or connective tissue, typically resolves within days to weeks; Axonotmesis, in which axonal damage occurs but the connective tissues (endoneurium, perineurium, or epineurium) remain intact, allowing regeneration at approximately 1–3 mm per day; and Neurotmesis, a complete transection of both axons and connective tissues, which necessitates surgical intervention for recovery [3] (Fig. 1).

Fig. 1.

Schematic of Seddon and Sunderland Peripheral Nerve Injury Classifications.

The diagram correlates the degree of anatomical disruption with the expected prognosis and surgical necessity. (Top) Correlation between Seddon's three types (Neurapraxia, Axonotmesis, Neurotmesis) and Sunderland's five grades (I–V). (Bottom) Clinical decision-making algorithm based on spontaneous regeneration potential. Note that Grades I–III often allow for conservative management, whereas Grades IV and V represent barriers to regeneration requiring surgical intervention.

Building on this framework, Sunderland [4] further refined the model into five grades, ranging from mild myelin injury (Grade I) to complete nerve transection (Grade V), with Grades III to V frequently requiring microsurgical repair [4]. Additionally, the pathophysiological response to nerve injury is influenced by factors such as patient age, comorbidities including diabetes and vascular disease, and the timing of intervention [5]. Therefore, a thorough understanding of these classification systems and the associated biological mechanisms is essential for selecting the most appropriate regenerative approach, whether pharmacological, surgical, or bioengineered.

Causes of PNIs include trauma, metabolic disorders, and iatrogenic factors resulting from medical procedures [6]. While peripheral nerves possess some capacity for self-repair, extensive damage disrupts their regenerative ability, often necessitating surgical intervention such as microsuturing or grafting to restore function [7,8]. Peripheral nerve reconstruction surgery is often required following trauma, iatrogenic damage, or the removal of nerve tumors. The ideal surgical repair technique should promote optimal wound healing with minimal scar formation while ensuring proper alignment of regenerating nerve fibers [9]. When direct repair is not feasible due to excessive tension, nerve grafting remains the gold standard [10]. The success of nerve repair depends on several factors, including the nature and extent of the injury, surgical technique, timing of intervention, nerve anatomy, and patient-specific variables [11,12]. In addition to surgical techniques, pharmacological agents can enhance peripheral nerve repair. Studies in rat models suggest that a 12-week observation period is sufficient to evaluate nerve regeneration [13]. The optimal management of PNI is determined by the type and severity of the lesion. Neuropraxia is typically managed with conservative care and physiotherapy. In cases of axonotmesis, spontaneous regeneration may occur, and conservative or supportive pharmacologic interventions are often employed; surgical repair is generally reserved for instances of inadequate recovery. Neurotmesis and segmental defects usually necessitate surgical reconstruction. For short nerve gaps, commonly less than 3 cm in the upper limb (though this varies by nerve and species), autologous nerve grafting or commercially available biodegradable conduits are effective options. In contrast, autografts remain the gold standard for long-gap defects, while nerve guidance conduits and engineered scaffolds are being actively investigated as alternative approaches. Proximal injuries and large-gap defects are associated with a poorer prognosis and often require more complex reconstructive strategies, such as nerve transfers or vascularized grafts [14] (Fig. 2).

Fig. 2.

Comparative properties of entubulation materials.

Silicone tubes: act as inert physical barriers. While historically significant, their non-biodegradable nature often necessitates secondary removal due to nerve compression or fibrosis.

Chitosan-based conduits: are biodegradable and permeable, supporting physiological ion exchange and reducing scar formation without the need for removal. Clinical evidence supports their use for gaps up to 26 mm.

Autografts: remain the gold standard for long-gap repair (>30 mm) due to the presence of native Schwann cells and basal lamina, though they require donor site morbidity.

Functional recovery is commonly assessed using the sciatic function index (SFI), a widely accepted method in nerve regeneration research. However, relying solely on histological evidence of nerve fiber growth is inadequate for determining functional recovery [13]. Information derived from the Basso, Beattie and Bresnahan (BBB) scale could be a more valuable tool for assessing peripheral nerve regeneration. The research on drugs that influence nerve repair has shown promise in improving treatment outcomes.

Perineural adhesions and excessive fibrotic reactions frequently occur after nerve injury or surgical repair. Collagen and extracellular matrix deposition around the nerve sheath restricts natural gliding, thereby impeding axonal regeneration and reinnervation. Disruption of the epineurium triggers inflammatory and fibroblastic cascades, leading to the formation of scar tissue. This tethering of the nerve to adjacent soft tissues increases mechanical tension and reduces neural perfusion. Several biomaterial barriers have been developed to minimize perineural fibrosis. Collagen wraps, chitosan conduits, and hyaluronic acid-based hydrogels have been shown to reduce scar formation and improve nerve conduction in animal models [15]. A systematic review of rat sciatic nerve models by Mayrhofer-Schmid et al. [16] reported that over 70% of interventions using nerve wraps reduced perineural fibrosis (Mayrhofer-Schmid, Klemm et al., 2023). Hybrid anti-adhesion systems that combine mechanical barriers with biological or pharmacological agents, such as anti-fibrotic cytokines or slow-release drugs, are currently under development. Additionally, novel nanofiber membranes and bioresorbable wraps have demonstrated potential for improving gliding while maintaining axonal alignment [17]. Further research into comparative biomaterials, long-term safety, and translational applications is necessary to advance the field of regenerative medicine. Current findings, however, support the potential clinical value of anti-adhesion strategies in peripheral nerve surgery. These advancements have the potential to promote enhanced recovery and regain functionality for individuals suffering from peripheral nerve damage.

This review provides an updated, multidisciplinary overview of recent advances in peripheral nerve regeneration, including pharmacological interventions such as corticosteroids, neurotrophic factors, erythropoietin, and 4-aminopyridine, as well as biomaterial-based and biophysical strategies, including nerve conduits, scaffolds, electrical stimulation, and anti-adhesion barriers. This integrated approach enables detailed examination of how pharmacological therapies and bioengineered materials interact to enhance nerve repair, highlighting both translational opportunities and challenges in achieving functional recovery following peripheral nerve injury.

2. Entubulation neurorrhaphy

Nerve guidance conduits (NGCs) are tubular devices intended to bridge segmental nerve gaps when tension-free end-to-end repair is not possible. The primary functions of NGCs are to isolate the injury site, prevent infiltration by external fibroblasts, guide axonal sprouts toward the distal stump, and create a microenvironment that supports regeneration. NGCs are fabricated from either non-biodegradable materials, such as silicone, or biodegradable substances, including collagen and chitosan-based polymers. Non-biodegradable materials, such as silicone, typically remain in the body unless surgically removed, which can induce chronic foreign-body reactions. In contrast, biodegradable conduits degrade naturally over time, eliminating the need for surgical removal and reducing the risk of long-term tissue response. In entubulation neurorrhaphy, the proximal and distal stumps of a severed nerve are inserted into a hollow conduit composed of natural or synthetic biomaterials, such as collagen, chitosan, or silicone. This technique is particularly advantageous for short-gap nerve injuries where tensionless repair is required [7,18,19]. These conduits offer several advantages. First, they eliminate the need to harvest a donor nerve segment, which avoids potential donor site complications like functional loss and neuroma formation. Second, they provide a microenvironment specifically designed to promote optimal nerve regeneration [20,21].

2.1. Silicone tubes

Silicone tubes are among the earliest artificial nerve conduits. In clinical contexts, they are used selectively as an option for bridging peripheral nerve gaps that cannot be repaired by direct suturing. These devices create a hollow, non-biodegradable chamber for the proximal and distal nerve stumps. This design helps axonal sprouting across the gap and prevents external scar tissue invasion. Successful regeneration with silicone conduits requires adequate vascularity and a stable tissue environment. Evidence from both experimental studies and clinical reports suggests that conduit insertion may be a promising alternative to traditional nerve repair techniques, such as direct end-to-end suturing of nerve stumps or grafting with an autologous nerve segment [7,8].

Autologous nerve grafting is the current standard of care for bridging segmental nerve defects, supported by extensive clinical experience. Alternative graft materials have also been explored, to mimic the natural communication between nerves through interactions with various protein and cellular signaling pathways [22]. Studies suggest that silicone tubes hold promise for bridging nerve gaps due to their inert properties. Silicone implants have been shown to cause minimal scarring and degeneration within the body, making them a potentially favorable material for nerve repair [23,24]. Silicone tubes, with their inert and flexible properties, were among the first and most widely adopted conduits for repairing severed nerves [23]. Clinical studies have shown that silicone rubber tubes are well-tolerated by human patients for extended periods, with documented success even after three years of implantation [25]. This biocompatibility makes silicone chambers a valuable tool in research settings. They serve as a standard experimental model for studying nerve regeneration processes, allowing scientists to investigate how nerves regrow within a controlled environment [26]. Despite significant advancements, the precise physiological and molecular cues that trigger nerve regeneration remain largely elusive. A complex interplay of factors appears to be crucial. For axons to elongate, the induction of transcription factors, adhesion molecules, growth-associated proteins, and structural components is necessary. Additionally, intracellular signaling molecules that regulate the cell cycle and differentiation processes seem to play a key role in nerve regeneration [27]. Because silicone is non-biodegradable, it may induce chronic inflammation or compression over time, often necessitating surgical removal. For this reason, silicone conduits are typically used in research or for selected clinical cases rather than in routine clinical practice.

2.2. Biodegradable chitosan-selenium nanocomposite tube

Chitosan–selenium conduits are biodegradable NGCs that provide structural support from chitosan and antioxidative, neuroprotective effects from selenium. Unlike silicone tubes, which serve only as barriers, chitosan-based conduits support tissue regeneration by promoting Schwann-cell adhesion, modulating inflammation, and degrading as axonal growth advances. Their biodegradability eliminates the need for surgical removal and reduces the risk of long-term foreign-body reactions [28,29]. Current research has directed attention towards creating chitosan-selenium biodegradable nanocomposite tubes aimed at facilitating peripheral nerve regeneration. In an investigation, these conduits were crafted and implanted onto transected sciatic nerves in a rat model, revealing noteworthy enhancement in regenerated axons compared to a control group employing chitosan alone [30]. This implies that incorporating selenium nanoparticles into the chitosan matrix could augment the regenerative potential of the conduit [31]. The mechanisms through which chitosan-selenium nanocomposites facilitate nerve regeneration are diverse. Chitosan, recognized for its capacity to create porous structures conducive to cell growth, osteoconduction, and the stimulation of cell proliferation and neovascularization in vivo, contributes significantly to these processes [32]. The amalgamation of selenium with the nanocomposite demonstrates synergistic effects, enhancing biodegradation and antibacterial attributes. These enhancements are pivotal for averting infections (Such as staphylococcus aureus and coagulase-negative staphylococci) and fostering an optimal milieu for nerve repair [31]. The efficacy of chitosan-selenium nanocomposite conduits in nerve regeneration has been evaluated through diverse criteria, encompassing behavioral, functional, biomechanical, histomorphometric, and immunohistochemical analyses. Research indicates that these conduits promote functional restoration of the sciatic nerve, as demonstrated by enhanced electrophysiological readings and muscle wet weight, signifying effective reinnervation of target muscles ([30]; Zhang et al., 2022). The morphological and ultrastructural alterations observed in regenerated nerves further validate the advantageous impacts of the conduits. Exploration is underway regarding the utilization of chitosan-selenium nanocomposite tubes in clinical settings, particularly for addressing traumatic sensory nerve lesions in the hand [33]. Continuing and forthcoming clinical trials will provide additional insights into the capacity of these conduits to enhance outcomes for individuals afflicted with peripheral nerve injuries. So, chitosan–selenium conduits remain entirely preclinical, with evidence restricted to rodent models and no human trials published to date.

In summary, the biomaterial strategies reviewed here vary widely in their stage of development. Silicone tubes represent older, non-biodegradable conduits with limited clinical adoption; biodegradable chitosan-based conduits are currently in preclinical development and are bioactive; and autografts remain the clinical standard of care. Explicitly distinguishing these stages helps clarify the translational readiness of each approach.

3. Anti-inflammatory steroids

Peripheral nerve injuries may result in neuropathies, which disrupt the normal functioning of peripheral nerves. The management of such conditions frequently entails the administration of medications aimed at alleviating inflammation and facilitating nerve regeneration. Corticosteroids, distinguished by their anti-inflammatory attributes, have emerged as a prospective therapeutic avenue in this regard. Corticosteroids, including prednisone and methylprednisolone, are potent anti-inflammatory medications employed in the treatment of diverse conditions, including peripheral neuropathies [34]. Steroids are not universally recommended for all forms of traumatic nerve injury. They may be beneficial in cases where inflammation or excessive scar formation impedes nerve regeneration. Studies have indicated that corticosteroids can exert a beneficial influence on functional recuperation and neuronal viability after nerve injury. Corticosteroids can be administered either locally or systemically. Localized methods, such as utilizing methylprednisolone-loaded hydrogels or sustained-release membranes, have demonstrated efficacy in promoting nerve recovery and suppressing scar formation at the location of peripheral nerve injuries. For example, the local application of methylprednisolone has been demonstrated to improve the functional recuperation of transected sciatic nerves in rats [35]. Systemic delivery, such as oral administration of prednisone, is additionally employed in treating a broad spectrum of conditions, including those impacting the nervous system [35]. Although corticosteroids provide therapeutic advantages, prolonged administration at high doses can result in various adverse effects. The potential adverse effects of corticosteroids in nerve repair depend on both dose and duration. Short-term use, such as a 24-h perioperative course, shows benefits and tolerability in preclinical models. In contrast, extended or high-dose therapy can increase risks. These include systemic immunosuppression, delayed wound healing, metabolic complications, and nerve toxicity. Recent reviews emphasize the importance of striking a balance between efficacy and safety in future translational research [36]. Currently, there is no established consensus regarding optimal dosing regimens for chronic or repeated corticosteroid administration in peripheral nerve regeneration. The majority of preclinical studies employ short-term perioperative administration, while clinical data are limited. This highlights the necessity for rigorous dose-escalation and safety studies in translational models.

Betamethasone belongs to a class of corticosteroids, distinct from anabolic steroids. In their research, Mohammadi et al. examined the impact of topical application of betamethasone on the regeneration of peripheral nerves and subsequent functional enhancement following sciatic nerve amputation in 45 healthy male Wistar rats. The functional analysis verified a swifter recuperation of regenerated axons in the inside-out vein graft (IOVG)/BETA group compared to the IOVG group (P < 0.05). Additionally, the gastrocnemius muscle mass in the IOVG/BETA group was significantly higher than that in the IOVG group (P < 0.05). The morphometric analysis of the regenerated fibers revealed a notable increase in both the number and diameter of myelinated fibers in the IOVG/BETA group compared to the control group (P < 0.05). Immunohistochemical examination demonstrated a distinctly higher positivity of S-100 reactions in the IOVG/BETA group compared to the IOVG group. IOVG is a modification of the vein-graft technique in which a harvested vein is inverted (turned inside-out) so that the adventitial surface faces the lumen; this approach was proposed to provide a more favorable internal scaffold for axonal guidance and to reduce luminal collapse. Building on this rationale, IOVG has been extensively studied in rodent sciatic nerve models, showing faster axonal regeneration compared to standard vein grafts in several preclinical reports [37]. When incorporated into a vein graft, betamethasone led to enhancements in both functional recovery and quantitative morphometric indices of the sciatic nerve. The topical application of this readily accessible agent provides advantages such as cost savings and the avoidance of complications linked with systemic administration [38]. Extended study durations are necessary to ascertain whether the topical administration of betamethasone yields more advantageous effects on the functional recuperation of the peripheral nerve. The findings concerning the effectiveness of betamethasone in peripheral nerve regeneration vary and occasionally contradict within the literature. One potential reason for the relative inconsistency in study outcomes regarding experimental nerve injuries is the diversity in models and testing methodologies employed [39,40]. Systemic administration of betamethasone accelerates axonal regrowth. In animal models of peripheral nerve injury, a subcutaneous dose of 2 mg/kg/day betamethasone is typically administered for 24 h during the pericrushing period, as demonstrated in a rat sciatic nerve crush study [41]. Nevertheless, an ideal scenario would involve a topical formulation offering equivalent neurotrophic stimulation, primarily due to its lower systemic toxicity [38].

4. Adrenocorticotropic hormone

Adrenocorticotropic hormone (ACTH) is a crucial hormone produced by the anterior pituitary gland, a pea-sized structure located at the base of the brain. The hypothalamic-pituitary axis, a complex network involving the hypothalamus and pituitary, regulates ACTH production. This hormone plays a vital role in the endocrine system by stimulating the adrenal glands to produce cortisol and androgens. Disruptions in ACTH levels can lead to various health conditions, including Addison's disease (characterized by insufficient cortisol production), Cushing syndrome, and Cushing's disease (arising from a pituitary tumor that secretes excess ACTH) [42]. Mohammadi et al. investigated the potential of locally applied ACTH to enhance functional recovery after sciatic nerve injury in a rat model. Their study involved sixty healthy male white rats. The findings revealed that ACTH treatment improved both functional recovery and morphometric indices of the sciatic nerve. These results suggest a potential role for ACTH as a therapeutic strategy following peripheral nerve repair, with possible implications for the clinical management of patients after nerve amputation [43]. Research suggests that peptides derived from ACTH possess neurotrophic properties. These neurotrophic effects on the nervous system can promote improved functional recovery following peripheral nerve crush injuries [44,45]. Studies have shown that Org 2766, a synthetic analog of the ACTH fragment (4-9), promotes enhanced functional recovery in animal models of peripheral nerve crush injury. This neurotrophic effect, demonstrably improving nerve function, has been observed across various levels of analysis. Histological examination, neurophysiological testing, and behavioral assessments all revealed benefits in both sensory and motor function. Notably, these improvements were observed in young, adult, and even aging rats [46].

5. Estrogen

Estrogen stands out as a unique hormone influencing a wide range of cellular processes. It regulates the expression of various genes involved in cell proliferation, growth, function, and even cell death [47]. Within the nervous system, it's noteworthy that estrogen receptors, particularly alpha and beta isoforms, are not limited to areas controlling reproduction. Instead, they are extensively distributed throughout anatomically distinct regions [48]. Estrogen exerts its effects on neuronal cells through a well-established pathway known as the classical genomic pathway. In this pathway, estrogen binds to nuclear receptors belonging to the steroid/thyroid superfamily. This binding triggers the activation of genes regulated by estrogen [47]. Beyond the classical genomic pathway, estrogen might also influence neurons through interactions with membrane-based signaling pathways involving MAP kinases. This suggests additional mechanisms by which estrogen exerts its effects on the nervous system [49]. Estrogen has been shown to promote neuronal regeneration in the hamster's facial motor nucleus, highlighting its potential role in repair processes [50]. Estrogen and progesterone therapy were employed to mitigate scar formation after the transection and suturing of the sciatic nerve, as well as to facilitate the re-innervation of muscles post-crush injury to the sciatic nerve [51,52]. The sensory neurons located in the dorsal root ganglia express both the a and b isoforms of estrogen receptors. Recent studies have indicated the involvement of estrogen in the proliferation of Schwann cells [53,54]. Schwann cells play a crucial role in the initiation of nerve regeneration across a gap, with their proliferation and migration between nerve stumps serving as a significant rate-limiting step. Hence, the creation of a precisely aligned extracellular matrix scaffold is imperative to facilitate Schwann cell proliferation within a conduit. This conduit not only supports the migration of blood vessels and other cell types but also lays the groundwork for the assembly of a new nerve structure [55].

6. 17-Beta estradiol

Nobakhti-Afshar et al. employed silicon conduits to offer a scaffold aiding in Schwann cell migration. The systemic administration of estradiol is linked to various issues, including the promotion of breast carcinogenesis. In the study conducted by Nobakhti-Afshar et al., the localized application of estradiol was selected as a therapeutic approach for nerve injuries to mitigate potential systemic adverse effects [56]. Nobakhti-Afshar et al. observed that the localized delivery of 17-beta-estradiol resulted in enhanced functional recuperation and improved morphometric parameters of the sciatic nerve in female Wistar rats. This finding holds potential clinical significance for the surgical treatment of patients following facial nerve transection. In this investigation, results from assessments encompassing behavioral, functional, biomechanical, electrophysiological parameters, as well as gastrocnemius muscle mass, along with morphometric indices, collectively substantiated a more rapid recovery of regenerated axons in the treatment group compared to the control groups (p < 0.05) [56].

7. Dihydrotestosterone

Dihydrotestosterone (DHT) has been scrutinized for its potential neuroregenerative properties concerning peripheral nerve injuries. An investigation employing a rat sciatic nerve model utilizing allografts revealed that DHT administration could expedite functional recuperation subsequent to nerve allograft procedures. This implies that DHT could hold practical significance in clinical contexts to augment nerve repair [57]. Neuroactive steroids, such as DHT, have been associated with diverse facets of peripheral neuropathy. For example, dihydroprogesterone, another neuroactive steroid, has demonstrated the ability to elevate the expression of myelin basic protein in the spinal cords of diabetic rats [58].

8. Non-steroidal anti-inflammatories

NSAIDs have been investigated for their capacity to facilitate the repair of peripheral nerves.

8.1. Celecoxib

The FDA advocates for the utilization of celecoxib and other NSAIDs, alongside acetaminophen, as the primary choice of analgesics for individuals diagnosed with osteoarthritis and rheumatoid arthritis. Furthermore, celecoxib holds FDA approval for addressing acute pain in adult females and managing primary dysmenorrhea [59]. Mohammadi et al. examined the localized impact of celecoxib on nerve regeneration in a sciatic nerve transection model using forty-five healthy male white rats. Filling the lumen of a 10 mm silicone conduit with 10 μL of celecoxib solution (0.1 g/L) in a rat nerve gap model enhanced axonal regeneration and functional recovery compared to the control [60]. Systemic administration of celecoxib via intraperitoneal injection at a dose of 10 mg/kg once daily for seven days, initiated immediately before injury, has demonstrated significant potential to accelerate nerve regeneration in vivo in the rat sciatic nerve crush model. Nevertheless, a more optimal strategy would utilize a topical formulation that provides equivalent neurotrophic stimulation while minimizing systemic toxicity [61]. Other selective COX-2 inhibitors, such as etoricoxib and meloxicam, are pharmacologically plausible. However, evidence for their efficacy in peripheral nerve regeneration is very limited, and no preclinical studies have directly compared them with celecoxib. Therefore, celecoxib is the most extensively validated COX-2 inhibitor for nerve repair.

8.2. Ibuprofen and indomethacin

Research has demonstrated that NSAIDs such as ibuprofen and indomethacin can surmount obstacles to axon growth by suppressing the RhoA pathway, a signaling cascade downstream of inhibitors like myelin and proteoglycans. These medications promote neurite growth in cultured neurons and, when given systemically, have the capability to counteract active RhoA signaling in the vicinity of the injury site [62]. Significantly, research has revealed that ibuprofen can trigger axon sprouting and enhance locomotor function in cases of spinal cord injuries, even when administration is delayed by up to one week following the trauma [63]. Nevertheless, not all NSAIDs exhibit these effects; for instance, naproxen, which lacks RhoA inhibition, does not stimulate axon growth [62].

8.3. Acetylsalicylic acid

ASA Acetyl salicylic acid (ASA) has demonstrated a neuroprotective impact in an in vitro model simulating neuronal ischemia-reperfusion injury. ASA is extensively employed as an analgesic, antipyretic, and anti-inflammatory medication, exerting these effects primarily through the inhibition of cyclooxygenases. However, emerging evidence suggests novel cyclooxygenase-independent mechanisms of ASA, such as the suppression of excitatory amino acid release, NF-kappa beta (Nfkb) translocation to the nucleus, and the expression of inducible nitric oxide synthase subsequent to cerebral ischemia. These findings position ASA as a promising neuroprotective agent for stroke treatment [64]. The impact of ASA on peripheral nerve injuries is noted to expedite recovery in a mouse nerve crush model [65]. Mohammadi et al. documented that locally administered ASA enhances functional recuperation subsequent to sciatic nerve transection in rats [66]. The potential of ASA to expedite nerve regeneration in vivo shows promise, particularly when administered systemically in rats. ASA promotes axonal regrowth upon systemic administration.

9. Calcium channel blockers

Calcium channel blockers (CCBs) such as Flunarizine and Nimodipine belong to a medication class typically prescribed for managing hypertension and heart ailments. Nonetheless, recent studies suggest they might also hold substantial potential in facilitating the regeneration of peripheral nerves. A pivotal mechanism through which calcium channel blockers facilitate nerve regeneration involves diminishing scarring at the site of nerve injury. Verapamil, a distinct type of calcium channel blocker, has demonstrated efficacy in reducing axonal resistance via anastomosis, which refers to the surgical or natural connection formed between two nerves. This decrease in resistance is paramount for promoting nerve regeneration [67,68]. Moreover, verapamil efficiently suppresses the development of scar tissue resulting from nerve injury, thereby creating an optimal microenvironment conducive to nerve fiber regeneration. Since scar formation can obstruct nerve growth, its inhibition represents a crucial advancement towards recovery [69]. In addition to verapamil, diltiazem has shown promising regenerative properties in neuronal models. In vitro studies have demonstrated that diltiazem significantly promotes neurite outgrowth in adult dorsal root ganglion neurons and in induced human sensory neurons, even under inhibitory conditions provided by chondroitin sulfate proteoglycans. However, evidence for diltiazem in peripheral nerve injury models in vivo is currently lacking, and its translational potential remains to be validated [70]. In a study investigating the impact of calcium channel blockers on nerve recovery, particularly concerning exposure to cigarette smoke, it was observed that these blockers exert a beneficial influence on the sciatic function index, a metric used to assess nerve function. This indicates that CCBs can enhance the functional recuperation of nerves. Furthermore, the detrimental effects of cigarette smoke on nerve healing may be mitigated through the administration of CCBs. This discovery holds significant relevance for individuals who smoke and are susceptible to peripheral nerve injury [71]. Research into employing an implantable micro-osmotic pump for administering medications that enhance calcium absorption has been conducted to enhance nerve regeneration. The investigation determined a significant correlation between nerve regeneration and calcium absorption, with the utilization of calcium-modulating agents like Nifedipine resulting in markedly improved nerve functional recovery. The group treated with Nifedipine exhibited notably higher rates of recovery compared to other groups, including those receiving treatment with Calcitonin or a saline solution. This underscores the importance of calcium channel modulation and subsequent calcium absorption in the process of nerve repair [72].

10. Immune system suppressants

Drugs such as Tacrolimus (FK506) and Cyclosporine A, when administered in low doses, appear to exert beneficial effects beyond their intended immunosuppressive function. These drugs may stimulate the growth of axons, the long fibers responsible for transmitting signals within nerves. This increased axonal regeneration could potentially lead to better restoration of nerve function. Promotion of Myelin Repair: The myelin sheath, a fatty layer that insulates nerves, plays a crucial role in efficient signal conduction. Immunosuppressants might promote the regeneration of myelin, thereby improving nerve communication and signal transmission. Reduced Scar Formation: Scar tissue formation can impede nerve repair and regeneration. These drugs may help minimize scarring, creating a more favorable environment for nerve regrowth [73]. While these findings are exciting, further research is necessary to fully elucidate the mechanisms by which immunosuppressants contribute to peripheral nerve repair and to optimize their use for maximizing therapeutic benefits. Additionally, as with any medication, immunosuppressants can potentially cause side effects, necessitating careful consideration and patient monitoring during their application.

11. Growth factors

Developments in tissue engineering, especially concerning growth factors, have arisen as encouraging alternatives to conventional surgical techniques for augmenting nerve regeneration (Önger et al., 2016). Growth factors are bioactive macromolecules that have received approval from the FDA for clinical application. They play a crucial role in the intricate process of peripheral nerve regeneration, aiding in nerve cell growth and viability, regeneration of axons and myelin sheaths, cell differentiation, and angiogenesis [74].

11.1. Insulin-like growth factor

Insulin-like Growth Factor-I (IGF-I) is a peptide hormone primarily synthesized by the liver in response to growth hormone secretion by the pituitary gland. It plays a pivotal role in various physiological functions and operates through endocrine, paracrine, and autocrine pathways to stimulate growth. The generation of IGF-I signifies the presence of essential nutrients required for its anabolic effects [75]. In the investigation conducted by Mohammadi et al., the impact of IGF-I on peripheral nerve regeneration utilizing allografts was examined in a sciatic nerve model involving 30 male white Wistar rats. Behavioral assessments, functional analysis of the sciatic nerve, and measurement of gastrocnemius muscle mass indicated a swifter regeneration of axons in the ALLO/IGF group compared to the ALLO group (P < 0.05). The administration of IGF-I was observed to expedite functional recuperation following nerve allografting in the sciatic nerve, suggesting potential clinical significance for the surgical treatment of patients following facial nerve transection [76]. In an experimental investigation lacking functional assessments, other researchers concluded that localized infusion of IGF-I at an optimal concentration facilitates the regeneration of a peripheral nerve [77]. Kanje et al. documented a substantial 49% increase in axonal regeneration in a rat model of sciatic nerve crush/transection following treatment with IGF-I. Notably, this effect was exclusive to IGF-I, as treatment with nerve growth factor demonstrated no impact [78]. The infusion of IGF-I can counteract the effects of cycloheximide, a protein synthesis inhibitor. This suggests that locally generated IGF-I is retrogradely transported to the cell body, where it exerts its beneficial effects [79].

11.2. Brain-derived neurotrophic factor with silicone conduit

BDNF is among the neurotrophic factors crucial for promoting the differentiation, maturation, and survival of neurons within the nervous system. Additionally, BDNF exhibits neuroprotective properties in challenging circumstances, including glutamatergic stimulation, cerebral ischemia, hypoglycemia, and neurotoxicity [80,81]. BDNF promotes and regulates the generation of new neurons from neural stem cells, a process known as neurogenesis [82]. In addition to its neuroprotective properties, BDNF also significantly influences energy balance regulation. Administration of BDNF either peripherally or intracerebroventricularly has been shown to inhibit energy intake and decrease body weight [83]. In their study, Mohammadi et al. demonstrated that loading BDNF into a silicone graft resulted in enhanced functional recovery and morphometric indices of the sciatic nerve. This observation underscores the significance of BDNF in promoting peripheral nerve repair [84]. Previous studies have indicated that BDNF protein, released by neurons located in the dorsal root ganglia and the anterior horn of the spinal cord, is transported to the site of regeneration through axonal pathways [85,86]. Additionally, research has shown that BDNF protein, secreted at the neuromuscular junction, is transported to the site of regeneration through axonal pathways [87].

12. Cell transplantation from bone marrow

Stem cells derived from bone marrow and other sources exhibit promising potential in bolstering peripheral nerve regeneration [88]. Stem cell-based therapy emerges as a prospective treatment avenue for enhancing outcomes following peripheral nerve reconstruction. These cells possess the capability to increase Schwann cell populations, sustain their support for regeneration, rescue and replenish neuronal populations post-axotomy, and preserve injured tissues like denervated muscle. The ultimate aim of such therapy is to foster a conducive environment for axonal regeneration while maintaining this support over time [89]. Ideal stem cells for peripheral nerve repair in clinical applications should possess several characteristics, including easy accessibility, rapid expansion potential in culture, in vivo survival, integration into host tissue, stable transfection, and expression of exogenous genes. Stem cells have the capacity to replace lost neurons or augment glial support cells. When differentiated into Schwann cell-like cells, they may integrate into Bands of Büngner to aid axonal guidance and remyelination [90]. Various sources of stem cells hold promise for peripheral nerve regeneration. Bone Marrow-Derived Stem Cells offer accessibility and carry minimal ethical concerns. They exhibit differentiation potential into non-mesodermal lineages, supporting dorsal root ganglion neurite outgrowth and displaying a pro-regenerative effect in nervous system injury models. Adipose-derived stem Cells demonstrate enhanced proliferation and differentiation capabilities compared to Marrow-Derived Stem Cells. Embryonic Stem Cells provide an unlimited source of cells with superior differentiation potential and long-term proliferation capacity, surpassing mesenchymal stem cells. Additionally, other potential sources include neural crest stem cells, skin-derived precursors, hair follicle stem cells, and dental pulp stem cells, all showing promise for peripheral nerve regeneration [89].

12.1. Transplantation of adipose-derived nucleated cell fractions

Mohammadi et al. examined the effects of transplanting adipose-derived nucleated cell fractions (ADNCs) on sciatic nerve regeneration. Following the intervention, animals receiving ADNCs achieved a SFI value of 31.6 ± 3.14, whereas the control group reached 42.5 ± 3.7. The gastrocnemius muscle mass was significantly higher in the ADNCs-transplanted group compared to controls (P = 0.001). Morphometric analysis demonstrated a significant increase in both the number and diameter of myelinated fibers in the ADNCs group relative to controls (P = 0.001). Immunohistochemical staining for S100 was also more extensive in the ADNC-transplant animals. These findings suggest that transplantation of ADNCs into an artery graft is a practical approach that improves functional recovery of the sciatic nerve [91]. The omentum contains growth factors, interleukins, cytokines, and chemokines. Adipocytes, pre-adipocytes, and macrophages contribute to the local synthesis of these proinflammatory proteins [92]. Nerve growth factors have been shown to promote nerve regeneration [93]. These factors stimulate Schwann cell synthesis of nerve growth factors, which, in turn, promote sensory axon regeneration [94]. Differentiated adipose-derived stem cells have been shown to enhance regenerative distance to a degree comparable to that of differentiated bone marrow stem cells [95].

12.2. Local xenotransplantation of Bone Marrow-Derived Mast Cells

Mast cells are found across all vertebrate classes and originated over 500 million years ago, predating the emergence of adaptive immunity. These cells, initially identified by Paul Ehrlich, are bone marrow-derived and were first documented in his doctoral thesis in 1878 [96,97]. In Mohammadi et al.'s investigation, the impact of Bone Marrow-Derived Mast Cells (BMMCs) on the functional restoration of transected sciatic nerves in cats was examined. They found that biomechanical analyses corroborated a swifter recuperation of regenerated axons in animals transplanted with BMMCs compared to the control group (p < 0.05). Morphometric assessments of the regenerated fibers revealed a notable increase in both the number and diameter of myelinated fibers in animals transplanted with BMMCs compared to the control group (p < 0.05). In immunohistochemical analysis, the presence of S-100 reactions in animals transplanted with BMMCs was notably more pronounced compared to the control group. The xenotransplantation of BMMCs offers a readily available source of cells that could enhance the recovery process of transected sciatic nerves [98]. There is a limited number of studies on feline nerve regeneration employing diverse materials across various nerve models, yet few address the biomechanical aspects of the regenerated nerve fibers. However, these studies have examined the electrophysiological, histomorphometric, and immunohistochemical characteristics of the regenerated nerves [[99], [100], [101]]. The regulatory impact of mast cells is further evidenced during the proliferation stage of nerve repair. The modification in mast cell behavior could prove advantageous in cell therapy scenarios requiring readily available and abundant cell sources. This factor should be considered in the burgeoning field of regenerative medicine and surgery.

13. The effect of whole-body exposure to pulsed electromagnetic fields

Peripheral nerve injuries often result in substantial functional impairments, prompting the ongoing quest for effective non-invasive treatments. Pulsed electromagnetic fields (PEMF) have garnered attention for their potential to enhance nerve regeneration. Studies have demonstrated that PEMF can accelerate peripheral nerve regeneration, with axons regenerating nearly twice as fast in PEMF-exposed animals compared to controls, leading to more effective motor function recovery. PEMF treatment has been associated with increased expression of neuronal nitric oxide synthase (nNOS) and phospholipase C-γ1 (PLC-γ1) in regenerated nerves, key molecules in nerve cell signaling and regeneration pathways. Both in vitro and in vivo experiments confirm the efficacy of PEMF in stimulating nerve regeneration, with evidence suggesting a priming effect of PEMF on nerve tissue [102]. Additionally, PEMF treatment influences protein synthesis at both systemic and cellular levels, indicating its broad impact on cellular processes contributing to nerve regeneration [103]. These findings underscore the potential of PEMF as a therapeutic modality for peripheral nerve injuries, warranting further research to optimize protocols and maximize functional recovery in affected individuals. Treatment Strategies for Peripheral Nerve Regeneration are listed at Table 1.

Table 1.

Comparative evaluation of treatment strategies for peripheral nerve regeneration.

Treatment Strategy (Drug/Cell)	Type and number of Animals	Purpose of the Study	Working Method/Device Used	Dosage and method of administration	Study duration	Final Result	Reference
Electrical Stimulation (ES)	Human	To promote axon regeneration and functional reinnervation in clinical settings.	Application of low-frequency electrical stimulation following nerve repair.	Brief, low-frequency ES (commonly ∼20 Hz for ∼1 h) applied soon after repair.	Variable (Review of multiple trials)	Patients receiving ES consistently demonstrated superior recovery compared to controls; accelerated sensorimotor recovery.	[111]
Knitted polyethylene terephthalate (PET) silk prosthesis	12 female Wistar rats	The objective was to assess the suitability of the extracellular matrix developing within a knitted prosthesis for facilitating nerve regeneration.	The transected sciatic nerve stumps were bridged using a knitted PET tube filled with Gelaspon® saturated in PBS.	N/A	6 weeks	Implantation of the knitted prosthesis resulted in successful nerve regeneration free from compression, with the presence of specific ECM molecules suggesting a supportive milieu for axonal growth.	[25]
Autologous Nerve Grafting	Human	To repair segmental nerve defects (Gold Standard).	Harvesting donor nerve (e.g., sural) to bridge the gap.	N/A (Surgical Graft)	Long-term	Remains the gold standard for long-gap defects; superior to conduits for large gaps.	[14]
Chitosan-Selenium Biodegradable Nanocomposite (CSBNC)	Rat	To fabricate and transplant chitosan-selenium biodegradable nanocomposite conduit for peripheral nerve repair	Normal group: sciatic nerve exposed; Transected group: sciatic nerve transected and stumps fixed; Chitosan and CSBNC groups: 10-mm nerve defects bridged with respective conduits	Local transplantation of chitosan-selenium biodegradable nanocomposite conduit	12 weeks	Significant recovery of regenerated axons in CSBNC group; higher number and diameter of myelinated fibers compared to chitosan group; suggests potential for clinical application in nerve repair.	[30]
Nimodipine	60 Male Wistar Rats (n = 15 per group)	To assess the effect of locally administered nimodipine on peripheral nerve regeneration and functional recovery	Transected group: sciatic nerve transected and stumps fixed; Treatment group: defect bridged with inside-out artery graft filled with nimodipine; Sham group: sciatic nerve exposed; Control group: graft filled with phosphate-buffered saline	10 μL nimodipine (100 ng/mL), local administration via inside-out artery graft	12 weeks	Behavioral testing, biomechanical studies, sciatic nerve function, gastrocnemius muscle mass, morphometric indices	[124]
Flunarizine (FNZ)	60 male Wistar rats, divided into 4 groups (n = 15)	To assess the effect of locally administered flunarizine (FNZ) on peripheral nerve regeneration and functional recovery	Transected group: sciatic nerve transected and stumps fixed; Treatment group: defect bridged with inside-out vein graft (IOVG) filled with FNZ; Control group: vein graft filled with phosphate-buffered saline	10 μL FNZ (100 ng/mL), local administration via inside-out vein graft	12 weeks	The FNZ-treated group (IOVG/FNZ) showed quicker recovery of regenerated axons, with more pronounced positive S-100 immunohistochemical reactions. Additionally, there was enhanced functional recovery and improved morphometric indices in the IOVG/FNZ group.	[115]
FK506	54 male white Wistar rats	To examine the effect of FK506-filled vein grafts on the regeneration and reinnervation of the sciatic nerve.	Sciatic nerve defect bridged with inside-out vein graft (IOVG); Control group: vein graft filled with carrier dilution alone	10 μL FK506 (10 ng/mL), local administration via inside-out vein graft	4, 8, and 12 weeks	Faster recovery of regenerated axons in FK506-treated group; improved gastrocnemius muscle weight ratios; larger number and diameter of myelinated fibers; more positive S-100 immunohistochemical reactions.	[116]
Cyclosporine A (CsA)	54 male rats	To evaluate the impact of cyclosporine A (CsA) incorporated into a chitosan conduit on the regeneration of peripheral nerves.	Sciatic nerve defect bridged with chitosan conduit; Control group: conduit filled with carrier dilution alone	10 μL CsA (10 μg/L), local administration via chitosan conduit	4, 8, and 12 weeks	The CsA-treated group exhibited quicker recovery of regenerated axons, improved gastrocnemius muscle weight ratios, and a greater number and larger diameter of myelinated fibers. Additionally, there were more pronounced positive S-100 immunohistochemical reactions in the CsA group.	[117]
Azithromycin	40 male Wistar rats, divided into 4 groups (n = 10)	To determine the effect of azithromycin on functional recovery following sciatic nerve crush	Right sciatic nerve crushed using non-serrated clamp. Experimental groups treated with azithromycin	15 mg/kg/day and 150 mg/kg/day, administered for 7 days	8 weeks	Azithromycin accelerated sensory and motor recovery; upregulated NGF and BDNF gene expression; recovery reached preoperative levels by the 8th week.	[118]
17-beta-estradiol	60 female Wistar rats, divided into 4 groups (n = 15)	To assess the neuroprotective effects of local administration of 17-beta-estradiol on nerve regeneration	Silicon group: defect bridged using a silicon conduit; Sham-surgery group: sciatic nerve exposed and manipulated; Transected group: sciatic nerve transected and stumps fixed; Treatment group: defect bridged using silicon conduit.	10 μL (0.1 mg/mL) 17-beta-estradiol, local administration via silicon conduit	12 weeks	Faster recovery in treatment group; more positive immunohistochemical reaction to S-100; improved functional recovery and morphometric indices in treatment group compared to others.	[56]
Adrenocorticotropic Hormone (ACTH)	60 male Wistar rats, divided into 4 groups (n = 15)	To assess the local effect of ACTH on functional recovery following sciatic nerve transection	Sham-operated group: sciatic nerve exposed; Transected group: nerve transected and ends fixed; Silicone graft group: defect bridged with silicone tube filled with saline; Treatment group: silicone tube filled with ACTH	10 μL ACTH (0.1 mg/mL), local administration via silicone tube	4, 8 and 12 weeks	Earlier regeneration of axons in ACTH-treated group (SIL/ACTH) compared to silicone-only group (SIL); improved functional recovery and morphometric indices.	[43]
Bone Marrow Derived Mast Cells (BMMCs)	12 male cats	To determine the effects of BMMCs on functional recovery of transected sciatic nerve	BMMC group: 20-mm sciatic nerve defect bridged with silicone nerve guide filled with BMMCs; Sham group: sciatic nerve exposed; Control group: nerve gap repaired with silicone nerve guide and phosphate-buffered saline	100 μL BMMCs (2 × 106 cells/100 μL), local administration via silicone nerve guide	6 months	The BMMCs-transplanted group showed a faster recovery of axons, with a greater number and larger diameter of myelinated fibers. Additionally, there were more pronounced positive S-100 immunohistochemical reactions in this group compared to the control.	[97]
Undifferentiated Bone Marrow Stromal Cells (BMSCs)	72 male Wistar rats	To study the effect of undifferentiated BMSCs on sciatic nerve regeneration	Sciatic nerve defect bridged with inside-out vein graft (IOVG) filled with BMSCs; Control group: IOVG filled with phosphate-buffered saline	2 × 107 cells/mL, local administration via inside-out vein graft	12 weeks	Significant recovery of regenerated axons in IOVG/BMSC group; higher number and diameter of myelinated fibers in BMSC group compared to control.	[119]
Platelet Rich Plasma (PRP)	45 male Wistar rats, divided into 3 groups (n = 15)	To determine the effects of local PRP administration on peripheral nerve regeneration	Normal control group: sciatic nerve exposed; Silicone group: sciatic nerve transected and grafted with silicone conduit; PRP group: same procedure with PRP in silicone conduit	20 μL PRP, local administration via silicone conduit	12 weeks	Accelerated functional recovery in PRP-treated group (SIL/PRP); improved functional recovery and gastrocnemius muscle mass compared to the silicone-only group.	[120]
Fibroblast Growth Factor (FGF)	80 male Wistar rats, divided into 4 groups (n = 20)	To assess the effect of local FGF administration on sciatic nerve regeneration	Transected group: sciatic nerve transected; Treatment group: defect bridged with chitosan conduit filled with FGF; CHIT group: chitosan conduit filled with saline; Normal group: sciatic nerve exposed	10 μL FGF, local administration via chitosan conduit	12 weeks	The FGF-treated group (CHIT/FGF) exhibited quicker recovery of regenerated axons, with more pronounced positive S-100 immunohistochemical reactions. Additionally, there was improved functional recovery and better morphometric indices compared to the CHIT group.	[121]
Hepatocyte Growth Factor (HGF)	60 male Wistar rats, divided into 4 groups	To evaluate the local impact of HGF on peripheral nerve repair in a sciatic nerve transection model.	Sham group: sciatic nerve exposed; Transected control group: sciatic nerve transected; Silicone graft group: defect bridged with silicone tube filled with PBS; HGF group: silicone tube filled with HGF	HGF loaded in silicone tube, local administration	4, 8, and 12 weeks	The HGF-treated group (SIL/HGF) showed faster axon regeneration, enhanced sciatic nerve function, increased gastrocnemius muscle mass, and better morphometric indices, along with more pronounced S-100 immunohistochemical reactions compared to the silicone graft-only (SIL) group.	[122]
Insulin-like Growth Factor I (IGF I)	80 male Wistar rats, divided into 4 groups (n = 20)	To evaluate the impact of locally applied IGF I on peripheral nerve regeneration and functional recovery.	Transected group: sciatic nerve transected and stumps fixed in muscle; Treatment group: defect bridged with inside-out artery graft (IOAG) filled with IGF I; Control group: graft filled with phosphate-buffered saline	10 μL IGF I (100 ng/kg), local administration via inside-out artery graft	16 weeks	Faster recovery of regenerated axons in IGF I-treated group (IOAG/IGF); improved sciatic nerve function, gastrocnemius muscle mass, and morphometric indices compared to control group.	[123]

14. Erythropoietin

Erythropoietin (EPO), traditionally used for erythropoiesis, has been extensively studied for its neuroprotective and neuroregenerative properties in models of peripheral nerve injury [[105], [106], [107]]. For example, mice treated with EPO following a sciatic nerve crush demonstrated significantly faster functional recovery (approximately a 60% improvement in sciatic functional index at day 7) compared to controls [105]. In addition to reducing neuronal apoptosis and promoting Schwann-cell proliferation, EPO has been shown to enhance angiogenesis and myelination in injured peripheral nerves in mice models [107,108]. The translational appeal is salient given EPO's established human safety profile; however, robust human clinical trials are still lacking [107].

15. 4-Aminopyridine

4-Aminopyridine (4-AP) is a voltage-gated potassium channel blocker used in the treatment of other neurological disorders and, more recently, in the management of peripheral nerve trauma [109]. 4-AP has demonstrated efficacy primarily in preclinical in vivo models, while human use remains experimental and has not progressed beyond early exploratory studies. In mouse models of sciatic crush injury, early systemic administration of 4-AP increased both the speed and extent of behavioral recovery, with additional improvements in conduction velocity, remyelination, and axonal cross-sectional area [109]. Together, these findings suggest that 4-AP can augment endogenous repair processes and may help distinguish incomplete from complete lesions [109]. Ongoing human equivalent-dose studies aim to explore its diagnostic and therapeutic utility [110].

16. Electrical stimulation

Electrical Stimulation (ES) is a non-pharmacologic modality that promotes axon regeneration and functional reinnervation.8–10 Animal and human studies show that brief, low-frequency ES (commonly ∼20 Hz for ∼1 h applied soon after repair) upregulates regeneration-associated genes (e.g., GAP-43, BDNF), enhances Schwann-cell activation, improves axonal outgrowth, and accelerates sensorimotor recovery [111,112]. In a recent systematic review of clinical applications, patients receiving ES consistently demonstrated superior recovery compared to controls, though protocols varied widely in terms of timing, frequency, intensity, and nerves treated [104]. Further standardization and larger trials in lower-extremity and mixed-nerve injuries are needed [104]. Taken together, EPO, 4-AP and ES represent promising adjunctive treatments that may enhance outcomes beyond traditional surgical repair and biomaterial strategies in peripheral nerve injury. Electrical stimulation has advanced further than many other adjuncts, with both animal evidence and small clinical studies, although standardized clinical protocols are not yet established.

17. Conclusion and future perspective

Despite significant advances in microsurgical reconstruction, biomaterial conduits, cell-based therapies, and adjunctive pharmacologic or biophysical approaches for PNI, achieving complete functional restoration remains a major clinical challenge.

Lopes et al. [113] demonstrated that even with the combination of surgical repair, stem cell strategies, and engineered scaffolds, long-term outcomes are frequently unsatisfactory, particularly in severe or proximal lesions [113]. Modrak et al. [114] further emphasized that unresolved biological barriers, such as delayed myelin clearance, dysregulated Schwann cell responses, and an inhibitory microenvironment, continue to impede regeneration in the peripheral nervous system [114].

This updated review synthesizes emerging therapeutic strategies, including erythropoietin, 4-aminopyridine, and electrical stimulation, as well as advances in anti-adhesion technologies, biomaterial design, and translational interventions. Several key research directions are identified: first, the integration of multimodal treatments that combine surgical repair with targeted pharmacotherapy, scaffold enhancements, and biophysical stimulation to create synergistic regenerative environments; second, the advancement of standardization and harmonization of experimental and clinical protocols, including outcome metrics, intervention timing, and efforts to bridge translational gaps; third, a focus on high-risk injury types, such as large-gap defects or proximal lesions where clinical need is greatest; fourth, optimization of the regeneration microenvironment by addressing myelin debris clearance, fibrosis and adhesion prevention, Schwann cell activation, and vascular support; and finally, the pursuit of personalized regenerative strategies using biomarker profiling, regenerative-medicine analytics, and artificial intelligence to tailor interventions to patient-specific characteristics.

It is important to note that the majority of preclinical studies reviewed here utilized male animal models to minimize hormonal variability. However, sexual dimorphism plays a significant role in nerve regeneration. Female sex steroids, particularly estrogen and progesterone, have been shown to exhibit potent neuroprotective and regenerative properties, potentially leading to faster intrinsic recovery rates in females compared to untreated males. Conversely, androgens influence regeneration in males.

Consequently, results observed in male models may not be entirely extrapolatable to females without accounting for these hormonal variances. Future translational research must prioritize comparative studies involving both sexes to develop sex-specific therapeutic strategies. The successful translation of emerging pharmacologic and biomaterial-based strategies into clinical practice depends on close collaboration among researchers, clinicians, and clinical pharmacists. The expertise of clinical pharmacists in drug safety, formulation, and therapeutic optimization is essential for ensuring safe and effective pharmacotherapy in the treatment of peripheral nerve injuries.

Declaration of AI-assisted technologies in the writing process

We confirm that no generative AI tools (such as ChatGPT or other text-generating models) were used in the preparation of the manuscript text, figures, or tables. The entire content was written, reviewed, and edited manually by the authors.

Funding

Not applicable.

CRediT authorship contribution statement

Sayyed Jafar Hasani: Visualization, Writing – original draft. Rahim Mohammadi: Methodology, Writing – review & editing. Alireza Jafarbeglou: Writing – review & editing. Farzin Abbasi: Writing – review & editing. Ali-Asghar Tehrani: Writing – review & editing. Afra Afshar: Writing – review & editing. Farshid Davoodi: Methodology, Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no competing interest.

Abbreviations & Glossary

ACTH: Adrenocorticotropic hormone, BDNF: Brain-derived neurotrophic factor, EPO: Erythropoietin, ES: Electrical stimulation, IOVG: Inside-Out Vein Graft, NGC: Nerve guidance conduit, NSAIDs: Non-steroidal anti-inflammatory drugs, SFI: Sciatic Functional Index, 4-AP: 4-Aminopyridine, PNIs: Peripheral nerve injuries, BBB: Basso, Beattie and Bresnahan scale, NGCs: Nerve guidance conduits, DHT: Dihydrotestosterone, CCBs: Calcium channel blockers, IGF-I: Insulin-like Growth Factor-I, ADNCs: Adipose-derived nucleated cell fractions, PEMF: Pulsed electromagnetic fields Entubulation neurorrhaphy: surgical technique inserting nerve stumps into a tubular conduit to bridge a gap, IOVG (Inside-out vein graft): inversion of a harvested vein so adventitia faces lumen, used as an autologous conduit for nerve repair.

Data availability

No data was used for the research described in the article.