Utility of Chitosan-Based Devices in the Treatment of Peripheral Nerve Injuries: A Literature Review

Abstract

Chitosan is a resorbable cationic polysaccharide known for its biodegradability and electrostatic and self-aggregation properties. Chitosan has been shown to influence Schwann cell proliferation, reduce scarring, support axon growth, and provide superior peripheral nerve regenerative outcomes compared to nerve injuries without chitosan. This article reviews preclinical studies to collectively determine whether the presence of chitosan enhances neuroregenerative outcomes following nerve injury as compared to settings without chitosan. The most consistent outcome measure reported across studies was functional analysis, followed by histomorphometry. Most animal studies showed no significant differences in functional recovery, electrophysiology metrics, and histomorphometry parameters between chitosan-based conduit repairs, reconstruction using autografts, or direct nerve repairs. A subset of studies reported superior outcomes with chitosan conduits for nerve reconstruction, while others indicated inferior results compared to conventional repair. The two human studies focused on digital nerve repair with sensory gaps ≤ 26 mm and demonstrated significantly improved 2-point discrimination at 6 months and equivalent function by 12 months with chitosan conduits compared to standard direct repair. The introduction of chitosan into nerve repair and reconstructions provides a potentially beneficial biological augmentation to the nerve microenvironment that enhances cellular, electrophysiological, and functional outcomes. However, heterogeneous approaches to functional, electrodiagnostic, and histological assessments in addition to varying control groups create a significant deficiency in understanding the true utility of chitosan-based devices within the field of nerve regeneration. Further needs for standardization in the study and comparison of biomaterials for effective clinical translation is needed. Nonetheless, this study highlights papers that are effective in achieving a strong propensity towards the utility of chitosan within biomaterial development for nerve reconstruction.

Impact Statement

This review investigating the use of chitosan-based devices as compared to repairs and reconstructions without chitosan in preclinical and clinical studies found greater neuroregenerative outcomes in the presence of chitosan compared to environments without chitosan in some studies. However, with the heterogeneity of postimplantation assessments and varying control groups, comparisons within and across studies remains difficult. With the growing availability of chitosan-based devices for clinical use, this study provides insights into the potential benefits of chitosan but also the shortcomings of current research and need for more rigorous approaches to determine true impact of chitosan and other novel biomaterials.

Keywords

chitosan nerve regeneration nerve repair peripheral nerve repair

Introduction

Peripheral nerve injuries often result in devastating functional disability due to permanent loss of sensorimotor function.¹ Recovery from these injuries relies on sufficient regeneration of the nerve from the site of injury to denervating muscles and sensory receptors, a process that frequently progresses slowly and incompletely in many patients.² As a result, incomplete nerve regeneration can result in paresthesia, dysesthesias, chronic neuropathic pain, paralysis, psychological distress, and deformity.³ Thus, there is a need for more effective treatment options to enhance regeneration and thereby improve consistency and progression for reinnervation.

Currently, repair for nerve injuries without segmental loss involves tension-free end-to-end repair.^4,5 For segmental nerve defects, intervention choice depends on gap length and may involve autograft, allograft, or for smaller gaps, conduit reconstruction.^4,6,7 Currently, there exists several chitosan nerve conduits and wraps on the market, including Food and Drug Administration-approved chitosan-based conduit (Reaxxon Plus, Medovent GmbH, Germany) and a chitosan nerve wrap (Neuroshield, Checkpoint Surgical, Cleveland, OH).^8,9 While these interventions aim to support nerve regeneration, each option has associated risks and limitations. Autologous nerve grafts (ANGs) face multiple challenges, such as donor site morbidity and painful neuroma formation.^10,11 Allografts, taken from donated nerve tissue, require the removal of immunogenic components and proteins that can dampen the graft’s regenerative potential.¹² Lastly, there are synthetic nerve conduits that have unique drawbacks, such as the need for removal of nondegradable materials and present difficulties in maintaining shape and strength.¹³

To address these challenges, researchers have aimed to optimize peripheral nerve repair by developing biomaterials that may enhance the regenerative microenvironment that is not present in current materials. One naturally derived polymer garnering interest is chitosan, which has demonstrated affinity for nerve cells.^14–16 A linear polysaccharide, chitosan possesses a functional amine group with tunable deacetylation profiles and positive charges that provide the potential for enhanced rates of degradation and increased biological activity.¹⁴ The nontoxic, biodegradable, biocompatible, and bioactive nature of chitosan has positioned it as a promising biomaterial.¹⁴ Preliminary investigations surrounding in vitro, ex vivo, and in vivo models highlight the material’s ability to support axonal regeneration, promote Schwann cell (SC) differentiation, and reduce fibroblastic activity.^3,17,18 Additionally, the degradation products of chitosan, chito-oligosaccharides, have demonstrated positive effects on nerve regeneration.^19,20

This study aimed to review existing animal and human literature to discuss current work with chitosan in (1) functional outcomes, (2) muscle mass restoration, (3) electrophysiology, and (4) axonal and cellular regeneration between chitosan-based devices through conventional techniques of ANGs or direct nerve repair. This review of preclinical and clinical assessments with chitosan will help to inform research approaches to further translation of this material towards impactful clinical use.

Methods

Search strategy and article eligibility

A literature search was conducted using the following electronic medical databases: PUBMED, EMBASE, and Cochrane Database of Systematic Reviews. The following key search terms were used for article identification: (chitosan conduit AND (peripheral nerve OR nerve repair OR peripheral nerve regeneration)). Three independent reviewers (E.E., E.O., and C.J.) screened the titles and abstracts of retrieved articles published between July 1991 and July 2025 to assess their potential relevance. Subsequently, 56 full texts of the selected articles were reviewed. Articles were included if they met the strict inclusion and exclusion criteria (Table 1). Disagreements among reviewers were evaluated by a third reviewer (N.L.) to reach a consensus on the potential relevance of an article.

Table 1.

Inclusion and Exclusion Criteria

Inclusion criteria
1. Study population consisting of animal models or human subjects with peripheral nerve injury
2. Reconstruction of peripheral nerve defects using unmodified chitosan-based conduits
3. Treatment groups include comparison to autografts, isografts or primary end-to-end neurorrhaphy
4. Assessment of quantitative outcome measures including functional assessments, electrodiagnostic studies, muscle weight analysis, or histomorphometry
5. Analysis of outcome measures between groups chitosan-based conduits and treatment groups of interest

Exclusion Criteria
1. Review articles, conference abstracts/letters, in vitro experiments
2. Non-English articles
3. Studies not evaluating outcomes of peripheral nerve repair with chitosan-based conduits
4. Studies evaluating central nervous system or nonperipheral nerve injury models
5. Chitosan conduits enhanced with cells, growth factors, biomaterials, or molecular compounds
6. Studies reporting only qualitative outcome data
7. Lack of comparison to autograft/isograft/ primary end-to-end neurorrhaphy

Classification and outcome measures

The included articles were categorized depending on comparisons made in their studies: (I) comparison of pure chitosan conduits to controls or (II) comparison of pure chitosan conduits to modified or enhanced chitosan conduits. The assessed outcomes were subsequently grouped into four primary categories: (1) functional analysis, (2) muscle mass measurements, (3) electrodiagnostic studies, and (4) histology/histomorphometry. Functional analysis measures included metrics such as sciatic functional index (SFI), gait analysis, sensory discrimination, withdrawal reflexes, and behavioral tests including the Grasping, Staircase, Plantar, Von Frey, and Static Toe Spread Factor tests (Table 2). Muscle mass measurements were also examined from muscles innervated by the sciatic nerve, such as the tibialis anterior and gastrocnemius muscles. Electrodiagnostic studies involved examination of nerve conduction velocities and compound muscle action potential (CMAP) latency and amplitude values. Finally, nerve histomorphometry investigated parameters such as axon/fiber number, diameter and density, number of myelinated fibers, and myelin thickness.

Table 2.

Summary of Functional Analysis Tests Described

Test utilized	Description	Outcome measured
Basso, Beattie, and Bresnahan Locomotor Rating Scale	Scores are given on a scale of 0 to 21 depending on degree of spontaneous movement in a 4-minute exposure period in an open area of mental circular enclosure.	Recovery of hindlimb motor function
Sciatic Functional Index	Footprints of subject’s hind paws are measured, specifically length of foot, distance between the first and fifth toes, and distance between second and fourth toes.	Recovery of sciatic nerve function
Extensor Postural Thrust Test	Subject is held above force plate and extends a hind limb, anticipating contact with the plate. Once contact is made, force generated by the limb is measured.	Recovery of motor function
Withdrawal Reflex Latency Test	Subject is lowered onto hot plate until a limb makes contact with the plate. The time the subject takes to remove the limb is measured.	Recovery of sensory function
Grasping Test	Subject is held near a grasping bar and evaluated in their ability to flex their fingers around the bar and suspend themselves from it.	Recovery of finger flexion and grip force
Von Frey Hair Test	A thin wire filament is applied to the subject’s skin. The response time taken to remove their limb from the stimulus is measured.	Recovery of sensory function via a mechanical stimulus
Plantar Test	A projection lamp is focused on the subject’s limb. The response time taken for the subject to withdraw their limb from the heat stimulus is measured.	Recovery of sensory function via a thermal stimulus
Staircase Test	Subject sits between two staircase apparatuses and reaches out with each forelimb in an attempt to grab food pellets.	Skilled forelimb reaching
Static Toe Spread Factor	The distance between subject’s first and fifth toe is measured; greater distance indicates functional nerve control of the muscles in the foot.	Nerve regeneration in the lower limbs

Data extraction and analysis

Data was extracted from each primary article and used for descriptive comparisons. Information that was collected included author, year, sample size, population, treatment groups, nerve, nerve defect size, chitosan conduit size, length of study, follow up, and study outcomes. All data were summarized descriptively (Tables 3 and 4).

Table 3.

Included Study Characteristics

Study	Sample size (n)	Population	Treatment groups	Degree of deacetylation	Peripheral nerve transected	Defect size	Size of chitosan conduit	Assessment modality	Follow up time
Ao, 2011²¹	36	Sprague-Dawley rats	Chitosan conduits with BMSC-derived Schwann cells; Chitosan conduits with sciatic nerve-derived Schwann cells; Chitosan conduits filled with PBS; Autograft	85%	Sciatic nerve	12 mm	16 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	12 weeks
Crosio, 2019²²	15	Wistar rats	Chitosan tube; Chitosan tube with longitudinal piece of pectoralis major muscle; Autograft	N/A	Median nerve	8 mm/10 mm	10 mm	Functional analysis and histomorphometry	16 weeks
Deng, 2022²³	40	Sprague-Dawley rats	Autograft; Chitosan conduit; Chitosan conduit filled with conductive hydrogel; Chitosan conduit with conductive hydrogel loaded with 7,8-dihydroxyflavone	N/A	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	12 weeks
Dietzmeyer, 2019²⁴	16	Lewis rats	Autograft; Hollow chitosan nerve guide; Chitosan nerve guide with perforated chitosan film insert; Corrugated chitosan nerve guide with perforated chitosan film inserts.	95%	Median nerve	10 mm	14 mm	Functional analysis, electrodiagnostic studies, histomorphometry	16 weeks
Farahpour, 2014	20	Wistar rats	Transected control; Sham-surgery chitosan control; acetyl-L-carnitine loaded chitosan	85%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	16 weeks
Gonzalez -Perez, 2015²⁵	28	Wistar rats	Silicone tube; Low degree acetylation (2%) chitosan tube (DAI); Medium degree acetylation (5%) chitosan tube (DAII); Autograft	98% (low degree), 95% (medium degree)	Sciatic nerve	15 mm	19 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	16 weeks
Haastert-Talini, 2013²⁶	83**	Wistar rats	Chitosan tubes with low degree of acetylation (∼2%); Chitosan tubes with medium degree of acetylation (∼5%); Chitosan tubes with high degree of acetylation (∼20%); Autograft	98%, 95%, 80%	Sciatic nerve	10 mm	14mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	13 weeks
Hsu, 2013²⁷	30	Sprague-Dawley rats	Empty silicone conduit; LN-modified chitosan scaffold in silicone conduit (multiwalled); LN-modified chitosan scaffold with BMSCs in silicone conduit (multichannel plus BMSC conduit)	75%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies, histomorphometry	16 weeks
Ilkhanizadeh, 2017²⁸	60	Sprague-Dawley rats	Positive control (Pos)-chitosan conduit only; Negative control (Neg)-sciatic nerve defect with stumps sutured to adjacent muscle; Sham-surgery group-siactic nerve exposed and manipulated; Chitosan + bone-marrow-derived mast cells	N/A	Sciatic nerve	10 mm	10 mm	Functional analysis	12 weeks
Kusaba, 2016²⁹	15	Wistar rats	Decellularized Sciatic nerve graft; Chitosan mesh tube; Autograft	90%	Sciatic nerve	10 mm	15 mm	Functional analysis, electrodiagnostic studies, Histomorphometry	25 weeks
Lima, 2021³⁰	25	New Zealand rabbits	Unrepaired control group; Autograft; Chitosan nerve conduit; Chitosan + NFC; Chitosan + NFC + polyvinyl alcohol	N/A	Sciatic nerve	5 mm	N/A	Functional analysis, histomorphometry	8 weeks
Mohammadi 2014 (PMID: 25448645)	60	Wistar rats	Transected group; Chitosan group; Chitosan + pulsed electromagnetic fields	85%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, electrodiagnostic studies	12 weeks
Mohammadi 2014 (PMID: 24239939)	18	Diabetic Wistar rats	Sham-operated group; Transected group; Control group; Chitosan + stromal vascular fraction group	85%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement
Najafpour, 2017	45	Wistar rats	Sham-operation;Chitosan only;Chitosan + prostaglandin	85%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, histomorphometry	12 weeks
Nasiri, 2015	60	Wistar rats	Sham operation;Transected control;Chitosan conduit;Laminin-fibronectin treated group	85%	Sciatic nerve	10 mm	10 mm	Functional analysis, muscle mass measurement, histomorphometry	12 weeks
Patel, 2006³¹	24	Lewis rats	Unrepaired group;Chitosan Conduit;Autograft	80%	Sciatic nerve	10 mm	12 mm	Functional analysis, muscle mass measurement, histomorphometry	12 weeks
Raisi 2010³²	60	Wistar rats	Sham-operation;Transected control;Silicone conduit positive control;Chitosan conduit	85%	Sciatic nerve	10 mm	N/A	Functional analysis, muscle mass measurement, electrodiagnostic studies	12 weeks
Shapira, 2015³³	21	Wistar rats	Chitosan nerve conduit;Autograft	N/A	Sciatic nerve	10 mm	12 mm	Functional analysis, muscle mass measurement, histomorphometry, electrodiagnostic studies	12 weeks
Simões, 2010³⁴	41	Sprague-Dawley rats	End-to-end repair with chitosan;Autograft + chitosan;Chitosan conduit;Autograft;PLGA conduitEnd-to-end repair control	N/A	Sciatic nerve	10 mm	16 mm	Functional analysis, histomorphometry	20 weeks
Stößel, 2018³⁵	32	Lewis rats	Chitosan nerve guides;Corrugated chitosan nerve guides);Autografts	95%	Sciatic nerve	15 mm	19 mm	Electrodiagnostic studies, mass measurement, histomorphometry	16 weeks
G. Wang, 2010³⁶	24	Wistar rats	Normal nerve control;Autograft;Chitosan conduit;Carboxymethyl-chitosan conduit	93.5%	Sciatic nerve	10 mm	N/A	Histomorphometry	12 weeks
W. Wang, 2010¹⁸	32	Wistar rats	Chitosan mesh tube;Chitosan mesh tube with nonpolarized β-TCP particles;Chitosan mesh tube with electrically polarized β-TCP particles;Isograft	N/A	Sciatic nerve	10 mm	15 mm	Functional analysis, electrodiagnostic studies, histomorphometry	12 weeks
Yin, 2018	4	Lewis rats	CBD;ANG	N/A	Sciatic nerve	10 mm	12 mm	Histomorphometry	12 weeks
Zhang, 2008	36	Sprague-Dawley rats	Group A (nerve graft in situ);Group B (biological conduit bridging nerve defect);Group C (biological conduit bridging peripheral nerve defect with nerve fibers in conduits)	N/A	Sciatic nerve	5 mm	12 mm	Electrodiagnostic studies, histomorphometry	12 weeks
Zhao, 2014¹⁹	160	Wistar rats	Chitosan-only;Chitosan+FK506 injection;FK506-loaded chitosan	93%	Sciatic nerve	N/A	7 mm	Electrodiagnostic studies, histomorphometry	8 weeks

ANG, autologous nerve graft; β-TCP, beta-tricalcium phosphate; BMSC, bone marrow stem cells; CBD, chitosan-based device; FK506, tacrolimus/fujimycin; N/A, not available; NFC, nano-fibrillated cellulose.

Table 4.

Results of Included Studies

Study	Functional analysis	Muscle mass measurement	Electrodiagnostic studies	Histomorphometry	Summary
I: Comparison of pure chitosan conduits to control
Ao, 2011²¹	No significant differences in SFI between groups.	Muscle mass restoration significantly lower in chitosan group (p < 0.001)	ANGs significantly higher conduction velocities. (p < 0.001).	Cross-sectional area of muscle fibers innervated by chitosan significantly smaller than ANGs (p < 0.05).
Crosio, 2019²²	No significant difference in Grasping test between groups.	Not reported.	Not reported.	No significant difference between total number of myelinated fibers, axon/fiber diameter, or myelin thickness between groups.
Deng, 2022²³	ANG displayed greater SFI at 12 weeks than chitosan (p < 0.001).	Gastrocnemius weight recovery ratio significantly lower in chitosan (p < 0.001).	Chitosan repair significantly less CMAP than ANG (p < 0.001).	Significantly greater muscle fiber area with ANG compared to chitosan (p < 0.001).
Dietzmeyer, 2019²⁴	ANG group displayed superior performance on Grasping and Staircase test (p < 0.05).	Not reported.	ANG group exhibited a significantly higher amplitude area of CMAPs compared to the chitosan group (p < 0.01).	No significant differences between number of myelinated fibers, axon/fiber diameter, and myelin thickness between ANGs and Chitosan groups.
Gonzalez -Perez, 2015²⁵	ANG group demonstrated superior performance over the chitosan groups in the Von Frey test (mechanical) (p < 0.05).No significance between groups for Plantar test (thermal).	ANG groups exhibited significantly higher weights in the Tibialis anterior and gastrocnemius muscles compared to the chitosan groups (p < 0.05).	ANG had significantly higher mean CMAP amplitude than the chitosan groups (p < 0.01).Chitosan groups performed better than control silicone tubes (p < 0.01).	ANG group had greater number of myelinated fibers compared to chitosan groups (p < 0.05).No significant difference in axon diameter and myelin thickness between groups.Chitosan groups demonstrated greater proportion of regenerated nerve than control silicone tubes, which all failed regeneration.	ANGs were superior to chitosan tubes. At low acetylation, there was 43% regeneration and at medium acetylation, there was 57% regeneration. All silicone tube-treated nerves failed regeneration.
Haastert-Talini, 2013²⁶	No significant differences in postsurgery SSI performances between groups.	No significant differences among groups.	No significant difference in amplitudes of evocable CMAPs between groups.	No difference between groups in axon/fiber diameters, myelin thickness. groups. DAI group exhibited a significantly lower total number of myelinated fibers compared to the ANG (p < 0.05).	Deacetylation at 95% demonstrated optimal support for peripheral regeneration. At 98% deacetylation, there were limitations with high speeds of degradation and at 80%, there were issues with low mechanical stability.
Ilkhanizadeh, 2017²⁸	SFI: BMMC significantly superior SFI than Pos; both Pos and BMMC superior to Neg. Similar results for maximum pull force, tensile strength, and ultimate strain.	Not reported.	Not reported.	Not reported.	BMMC-treated is superior to chitosan alone. Both BMMC and chitosan alone superior to negative control.
Kusaba, 2016²⁹	No significant difference in Static Toe Spread Factor and Von Frey hair test between autograft and chitosan groups.	Not reported.	No significance was detected in the amplitude and latency values of CMAPs among the between autograft and chitosan groups.	Mean axon area in ANG group significantly larger than that in the chitosan mesh tube group (p < 0.05).No significant difference between mean axon density.
Lima, 2021³⁰	Pure chitosan in a 5 mm gap does not provide functional recovery of the nerves. However, chitosan group promoted regeneration of nerve guides, compared to nerves without biomaterial and with ANG.	Not reported.	Not reported.	No significant difference in connective tissue between groups; cell type and distribution also showed no significant differences between groups.
Raisi, 2010³²	SFI: At 4 weeks, improvement in Chit group better than SIL group (p < 0.05). At 9 weeks, improvement in both Chit and Sil groups, whereas no improvement observed in TC group. At 12 weeks, Chit improved 72% and SIL improved 52% from initial injury SFI values.	There was significant difference between muscle weight ratios of Chit and SIL groups with TC group (p < 0.05).	Not reported.	At 4 weeks, Chit and SIL groups showed nerve fibers at distal stumps.Both Chit and SIL showed regeneration patterns. The number of nerve fibers in Chit and SIL were significantly higher than in TC. Mean diameter of nerve fibers in Chit (4.86 ± 0.19) was significantly greater than that of TC (4.11 ± 0.22) (p < 0.05). Diameter of axon in Chit was significantly greater than in TC (p < 0.05). Morphometric indices in SIL group significantly greater than that of Chit group.	This study showed Chit was significantly better than SIL regarding functional gait analysis. Nerve conduction velocity demonstrated similar results between Chit and SIL.Histomorphometric methods also show similar results between Chit and SIL. Thus, this study supports that walking track-analysis may be more comprehensive in assessment of nerve regeneration.
Shapira, 2015³³	No significant difference in SFI between groups.	No significant difference in gastrocnemius muscle weight between groups.	No significant difference in CMAP latency and amplitude between groups.	No significant difference in myelinated fiber diameter and myelin thickness. ANG greater number of myelinated fibers (p < 0.005).
Simões, 2010³⁴	Recovery of hindlimb extensor response was comparable between chitosan conduits, autograft, and end-to-end repair groups.ANG group exhibited superior performance in withdrawal reflex latency compared to the chitosan group (p < 0.001). Thermal nociception function and motor function demonstrated similar degree of recovery across all experimental groups at 20 weeks.	Not reported.	Not reported.	Nerve regeneration was successful in all chitosan experimental and control groups (neurorrhaphy alone—negative control, ANG—positive control).Chitosan type III membranes demonstrated significantly better nerve regeneration in comparison to control (p < 0.05).
Stößel, 2018³⁵	Not reported.	Significantly higher muscle-weight ratios in ANGs group (p < 0.05).	Significantly higher CMAP amplitudes in ANGs group (p < 0.001)	No significant difference in axon/fiber diameters, myelin thickness or total fiber number or fiber density between groups.
G. Wang, 2010³⁶	Not reported.	Not reported.	Not reported.	CM-chitosan demonstrated better nerve regeneration performance than chitosan tube (control) and equivalent regeneration as ANG.CM-chitosan demonstrated similar average fiber density to ANG and significantly higher than that of common chitosan tubes (control) (p < 0.05).
W. Wang, 2010¹⁸	There were no significant differences in function recovery among these groups.	Not reported.	There were no significant differences in amplitude and latency among the groups.	No significant difference in average axon diameter of myelinated axons among the Groups. Greater axon density and area in ANG (p < 0.01).
II: Comparison of chitosan conduits to enhanced/modified chitosan conduits
Farahpour, 2014³⁷	BBB locomotor rating demonstrated greatest degree of deficits 1 week after surgery (except in sham group). ALC group showed significant improvement in BBB of operated limb compared to CHIT control group during study period.SFI showed that recovery of nerve function was significantly faster in CHIT/ALC as compared to CHIT (p < 0.05).	Mean ratios of gastrocnemius muscles weight showed statistically significant differences between CHIT/ALC and CHIT groups (p < 0.05); CHIT/ALC ratios were larger than CHIT. Weight loss of gastrocnemius was ameliorated with local administration of ALC.	Nerve conduction velocity along regenerated nerves in ALC group were significantly higher than in other experimental groups.	At 4 weeks, CHIT/ALC had significantly greater nerve fiber, axon diameter, and myelin sheath thickness compared to CHIT animals (p < 0.05). However, from 8 weeks onward, there were no significant differences between mean myelin sheath thickness between groups.
Hsu, 2013²⁷	Multiwalled scaffold + BMSC group demonstrated superior SFI scoring as compared to other two groups with significant improvement at 1-month compared to the empty conduit group. SFI values began to decline at 12 weeks in the empty and modified chitosan groups. Recovery levels were maintained in multiwalled scaffolds + BMSC group at 12 weeks.	No statistically significant differences in contralateral atrophy among groups.	Motor nervous system assessment demonstrated that modified conduit was not ideal. Sensory nervous system assessment demonstrated that chitosan conduit group similar to conduit + BMSC group and greater than empty group.	At 16 weeks, nerves had regenerated in empty conduit and multichannel plus BMSC conduit groups. In the multichannel without BMSC, there was hyperplasia around conduit and a nerve fiber-like tissue growing from fibrous tissue outside conduit.H&E cross sections demonstrated angiogenesis and perineurium wrapping in the multichannel plus BMSC conduit group; this was not evident in the other two groups.	Addition of BMSC significantly enhanced nerve regeneration, best demonstrated on histomorphometry.
Mohammadi, 2014	BBB recovery: All experimental groups (except sham) showed greatest degree of functional deficit 1 week after surgery. Local transplantation of SVF+chitosan conduit had superior scores compared to CHIT group.SFI: At 12 weeks, SVF-transplanted group achieved mean SFI of −34.4 ± −3.16 compared with −46 ± 02.2 of CHIT. These values can be compared with −100 after initial nerve transection and 0 prior to nerve transection.	CHIT/SVF demonstrated significantly greater muscle weight ratios (gastronemius) as compared to CHIT group (p < 0.05).	Not reported.	At 4 weeks, CHIT/SVF had significantly greater nerve fiber, axon diameter, and myelin sheath thickness compared to CHIT (p < 0.05). At 8 and 12 weeks, CHIT/SVF had significantly greater nerve fiber and axon diameter than CHIT but similar myelin sheath thickness.
Mohammadi, 2014	BBB recovery: CHIT/PEMF showed significant improvement compared with CHIT (p < 0.05).SFI: At 12 weeks, CHIT/PEMF group achieved mean value of −36.6 ± −3.19 compared to CHIT with mean value of −44.2 ± −4.11 (p < 0.05).	CHIT/PEMF had a significantly greater muscle weight ratio (gastrocnemius) as compared to CHIT group.	Nerve conduction velocity was significantly higher in CHIT/PEMF than in CHIT (p < 0.05).	PEMF added to chitosan allowed for hastened functional recovery of the sciatic nerve.
Najafpour, 2017	BBB: Administration of PGE improved locomotion of injured limb compared with control (p < 0.05).SFI: Recovery of nerve function was significantly greater in CHIT/PGE than in CHIT (p < 0.05).	CHIT/PGE was significantly higher than CHIT group (p < 0.05).	Not reported.	At 4 weeks, CHIT/PGE had significantly greater nerve fiber, axon diameter, and myelin sheath thickness compared to CHIT (p < 0.05).At 8 and 12 weeks, CHIT/PGE did not significantly different changes in axon diameters. There were no significant increases in mean thickness of myelin sheath for both CHIT/PGE and CHIT groups.
Nasiri, 2015	BBB: Laminin-fibronectin treated group showed significant improvement in locomotion of operated limb compared with CHIT (p < 0.05).SFI: CHIT/LF significantly accelerated functional recovery compared to CHIT (p < 0.05).SSI: Similar results to SFI.	Mean ratio of gastrocnemius muscle weight: Significant difference between CHIT/LF and CHIT groups (p < 0.05).	Not reported.	Both CHIT and CHIT/LF showed lower mean diameter of nerve fibers, number of fibers, and diameter of axons than SHAM group (p < 0.05).
Patel, 2006³¹	Chitosan group showed increased functional improvement compared with unrepaired nerves (p < 0.05).	No significant difference in gastrocnemius muscle weight between groups.	Not reported.	No significant differences in axon count between chitosan and ANG.Higher number of myelinated axons and axon diameter in chitosan compared to ANG (p < 0.05).
Yin, 2018	Not reported.	Not reported.	Not reported.	Angiogenesis observed in lumen of conduit. Nano-fibrillated-staining revealed regenerating peripheral nerve longitudinally oriented in the proximal, middle, and distal sections of the conduit. Nerve was noticeably smaller in its transverse diameter in the middle section when compared to proximal.
Zhang, 2008	Not reported.	Not reported.	MNCV at the 12th week demonstrated nerve graft in situ group was superior to other groups.	At 12 weeks, the number of remyelinated nerve axons in Group B and Group C were greater than at 6 weeks; however, Group A (nerve graft) demonstrated less remyelinated axons compared to at 6 weeks.At 12 weeks, cross sectional area was greater than 6 weeks for all 3 groups. However, Group A (nerve graft) was significantly greater than the other two groups at both 6 and 12 weeks.
Zhao, 2014¹⁹	Not reported.	Not reported.	CMAP and motor nerve conduction velocity of injured sciatic nerves were significantly higher in rats who received FK506-loaded chitosan, compared to chitosan-only conduits or empty chitosan conduits with FK506 injection.	With toluidine blue staining at 8 weeks, sciatic nerves in chitosan-only group were obviously damaged; myelin sheath was weak, immature, and loosely packed. In the chitosan + FK506 injection, there were more regenerated nerve fibers, myelin sheath was thicker and more mature, closely packed. In the FK506-loaded chitosan group, higher density of more mature regenerated nerve fibers + myelin sheath was thick and closely packed.

ALC, acetyl-L-carnitine; BBB, Basso, Beattie, and Bresnahan; BMMC, bone-marrow-derived mast cell; BMSC, bone marrow stem cells; CM, Carboxymethyl; CMAP, compound muscle action potential; CHIT group, chitosan control group; MNCV, Motor nerve conduction velocity; PGE, prostaglandin; SFI, sciatic functional index; SSI, Static Sciatic Index; SVF, stromal vascular fraction group.

Experiment

Comparison of pure Chitosan-Based devices to controls

Included studies either compared pure chitosan-based devices (CBDs) with ANGs or pure CBDs with negative controls, such as silicone tubes. Each study analyzed a variety of parameters, some of which included functional analyses, muscle mass measurements, electrodiagnostic studies, and histomorphometry (Table 4). In many studies, statistical significance was demonstrated in some outcome parameters but not others.

Functional outcomes were evaluated in multiple ways with tests targeted to assess sensory deficits, motor deficits, and/or gait patterns (Table 2). Some studies demonstrated comparable results between CBDs and ANGs, while others favored ANGs, and a few suggesting improved outcomes with CBDs. The most common functional tests included the SFI, grasping tests, von Frey hair tests, and various gait analysis methods (Table 2). Among studies assessing SFI, Ao et al. and Shapira et al. reported no significant differences in values between CBDs and ANGs.^21,33 Ao et al. compared chitosan conduits enhanced with bone marrow stromal cell-derived SCs to autograft.²¹ At 4-, 8- and 12-weeks post nerve reconstruction, they found consistent improvements in SFI in both studies with no significant difference in function across all three time points. Similarly, Shapira et al. in a 10-mm rat sciatic nerve defect model found that SFI demonstrated no statistically significant difference between the chitosan tube reconstruction and ANG groups at 4 and 12 weeks.³³ In contrast, Deng et al. reported significantly superior SFI scores at 12 weeks in ANG-treated groups as compared to both pure CBDs and modified CBDs—a hollow chitosan conduit, chitosan conduit filled with a conductive hydrogel, and a chitosan conduit filled with a hydrogel that had been loaded with dihydroxyflavone.²³

Although Ao et al. noted no significant differences in the functional analysis outcomes, they did report significant findings for all other outcome measures, including muscle mass measurement, electrodiagnostic studies, and histomorphometry.²¹ Ao et al. evaluated muscle mass restoration at 12 weeks postoperatively by measuring the leg circumference as a percentage of the contralateral unaffected side, comparing chitosan with ANGs.²¹ Muscles innervated by ANGs resulted in 80% muscle mass restoration (MMR), which was significantly higher than the 55% MMR observed in muscles innervated with chitosan phosphate-buffered-saline-bridged conduit nerve repair and 75% MMR observed in chitosan SC-seeded conduits. Electrodiagnostic studies demonstrated ANGs had significantly higher conduction velocities and histomorphometry showed that CBDs had significantly smaller cross-sectional area of innervated muscle fibers as compared to ANGs.²¹ Conversely, Shapira et al. did not identify any significant differences in the muscle mass and electrodiagnostic studies; however, they did note that there were a greater mean number of myelinated fibers present in ANG-treated group as compared to CBD-treated group (p < 0.005).³³

In studies comparing CBDs to negative controls, CBDs demonstrated superior SFI values consistently, as reported by Ilkhanizadeh et al. and Raisi et al.^28,32 In these studies, other reported outcome parameters reflected the superiority of CBDs to negative controls (Table 4).

Other metrics to assess functional outcomes included Grasping test, Staircase test, von Frey test, Plantar test, Static Sciatic Index (SSI), and Withdrawal reflex (Table 2). Among studies that analyzed results of the Grasping test, Crosio et al. showed no significant differences between pure CBD, modified CBDs, and ANGs.²² Crosio et al. investigated a hollow chitosan tube, a chitosan tube enriched with pectoral muscle, and an ANG in a 10 mm rat median nerve model.²² At 16 weeks postreconstruction, grasping test analysis measured by the function of finger flexor muscles, innervated by the median nerve, was recovered in the experimental groups with no significant differences among the groups. Similarly, the study by Kusaba et al. comparing chitosan mesh tubes with ANGs demonstrated similar functional outcomes with the von Frey hair test and static toe spread factor.²⁹ Simões et al. found comparable results from the Extensor Postural Thrusttest for motor function, the Withdrawal Reflex Latency test for sensory function, and kinematic analyses between ANG and chitosan type III tube-guidegroups.³⁴ However, sensory function assessment demonstrated significantly better recovery in the ANG group compared to the CBD group, with ANG group exhibiting faster and more complete recovery of noxious thermal sensation.³⁴

However, other studies demonstrated that ANG-treated groups had significantly improved functional outcomes as compared to CBDs. With ANG being the gold standard, these results are not unexpected. Studies that demonstrated ANG superiority over CBD regarding functional outcomes included Dietzmeyer et al. and Gonzalez-Perez et al. Dietzmeyer et al. reported that ANG-treated group had superior performance on the Grasping and Staircase test than CBD-treated group (p < 0.05).²⁴ Dietzmeyer et al. also reported that ANG-treated group had significantly higher amplitude area in recorded CMAPs when compared to CBDs. They reported no significant differences in histomorphometry, regarding mean number of myelinated fibers, axon diameter, and myelin thickness.

Gonzalez-Perez et al. compared chitosan tubes of two different degrees of acetylation to autografts, demonstrating the ability of chitosan tubes to bridge a critical 15-mm sciatic nerve gap in rats when compared to standard silicone tubes.³⁸ With the Von Frey test, all rats in the autograft group showed withdrawal responses at 16 weeks postoperatively, whereas the chitosan groups yielded higher mean values due to failed regeneration in some animals. Differences in the Von Frey test results between autograft and chitosan-treated mice were statistically significant. Similarly, when Gonzalez-Perez et al. utilized the plantar test, a measured withdrawal response to heat stimulation, they found that the autograft group displayed withdrawal latencies (12.75 ± 1.38 s) comparable to the contralateral control paw (12.36 ± 0.76 s) without demonstrating statistical significance, while chitosan-treated groups performed inferiorly when compared with the contralateral control paw.³⁸ In addition, Gonzalez-Perez reported significantly greater muscle mass ratios in both tibialis anterior and gastrocnemius muscles for ANG group compared to CBD group. They also reported superior outcomes for CMAP amplitude and greater number of myelinated fibers in the ANG group compared to CBD. There were no significant differences in axon diameter and myelin thickness between ANG and CBD groups. CBDs demonstrated significantly better CMAP and higher proportion of regenerated nerves than negative control silicone tubes.³⁸

Interestingly, there were two studies suggesting improved functional outcomes with CBDs compared to autografts. One study by de Lima et al. assessed functional recovery using a comprehensive clinical-neurological evaluation conducted every 15 days over a 60-day postsurgery period; this study evaluated hollow chitosan nerve conduits, chitosan with nano-fibrillated cellulose (NFC), and chitosan with NFC and polyvinyl alcohol against autografts.³⁰ This evaluation encompassed multiple aspects of limb function, including walking gait analysis, proprioception integrity, jumping ability, tactile positioning, and pain sensitivity. Key findings revealed that autografts failed to show signs of functional recovery throughout the study period. In contrast, chitosan-only conduits demonstrated improvement in functional recovery, albeit limited to enhanced deep pain perception, without recovery of other functional aspects. However, de Lima et al. did not report significant differences in histological and histomorphometric analyses between treatment groups. The second study demonstrating superiority of CBDs in functional outcomes was conducted by Patel et al., who reported on video gait analyses, evaluating ankle angle measurements taken at midstance, terminal stance, and midswing phases.³¹ At 12 weeks following surgery, the chitosan group showed significantly higher terminal stance angles compared to the autograft group (p < 0.05).³¹ Additionally, the chitosan group showed statistically significant greater midstance angles compared to the autograft group at weeks 2, 10, and 12.³¹ No statistically significant differences were observed between groups for midswing angles. Patel et al. conducted muscle mass measurements and did not find differences in gastrocnemius muscle weight ratios between CBD- and ANG-treated groups. Upon histomorphometry, there were no significant differences in axon count between CBD- and ANG-treated groups; however, there were a significantly higher number of myelinated axons and greater axon diameter in CBDs compared to ANGs (p < 0.05).³¹

Although functional outcomes vary across included studies, when reported and significant, muscle mass restoration ratios were always greater in ANG-treated groups compared to CBD-treated groups.^21,23,26,35 CBD-treated groups always demonstrated greater ratios than negative control groups.³²

Like muscle mass restoration outcomes, electrodiagnostic studies consistently demonstrated ANG-treated groups performing significantly better than CBD-treated groups.

Ao et al. at 12 weeks postnerve reconstruction calculated conduction velocity along the bridged sciatic nerve and the contralateral intact side. They reported statistically significant faster conduction velocity with autograft-bridged nerve defects as compared to CBD-bridged nerve defects (60% vs. 30%, p < 0.001). Similarly, at 12 weeks postnerve reconstruction, Deng et al., calculated the CMAP amplitude between the two experimental groups and found that chitosan repair had significantly lower CMAP amplitudes compared to ANGs.²¹ Similarly, Dietzmeyer et al. found that at 12 weeks postsurgery, chitosan had significantly lower amplitude area of CMAP compared to the ANG group.²⁴ In addition, they found that at 16 weeks postsurgery compared to 12 weeks postsurgery, the CMAP amplitude areas of CBD treatment groups that were modified (a chitosan nerve guide with perforated chitosan film insert and a corrugated chitosan nerve guide with perforated chitosan film inserts) continued to increase, while the hollow chitosan nerve guide continued to perform inferiorly to ANGs. These three studies did not compare CBDs to a negative control. One study, Mu et al., demonstrated that CBDs exhibited smaller CMAP amplitude and greater latency when compared to ANG.³⁹

Some studies compared CBD treatment groups to a negative control, providing additional insight into the performance of chitosan. Gonzalez-Perez et al. found that CBDs demonstrated significantly lower CMAP amplitude than ANG but higher than silicone tube (p < 0.01). After 4 months follow-up, the CMAP amplitude in ANG group was 2.65 ± 0.59 mV, in chitosan tubes with 2% acetylation was 0.212 ± 0.10 mV, in chitosan tubes with 5% acetylation was 0.716 ± 0.53 mV, and in silicone tubes 0.0 ± 0.0 mV. There were no observed differences between the two different acetylation levels in the chitosan group.³⁸

Many studies reported no significant difference in the amplitude and latency values of CMAP among chitosan groups and comparison material.^26,29,33 Kusaba et al. cut, reversed, and resutured in situ the sciatic nerve as a negative control.²⁹ Shapira et al. and Haastert-Talini et al. did not have a negative control.^26,33 Electroconductive assessment for axon number as well as myelination maturity, based on amplitude and latency respectively, was variable across these studies, compounded with difficulty in interpretation due to deficient control groups within certain studies.

Histomorphometry outcomes measured across studies included axon/fiber number, diameter, density, and number of myelinated fibers and myelin thickness (Table 3). Many animal studies reported no significant difference between total number of myelinated fibers, axon/fiber diameter, or myelin thickness between groups.^{22,24,26,30,35}

Many studies only compared CBDs with ANGs and demonstrated that chitosan performs inferiorly to autografts. Shapira et al. and Mu et al., found that ANGs had a greater number of myelinated fibers, myelin thickness, and fiber diameter as compared to chitosan.^33,39 Similarly, Ao et al. found that the cross-sectional area of muscle fibers innervated by chitosan were significantly smaller than ANGs.²¹ Deng et al.²³ demonstrated that the ANG-treated group had significantly greater muscle fiber area as compared to those treated with CBDs. They found that there was no significant difference in collagen fiber and muscle fiber density between one chitosan conduit with hydrogel (CS-D-CP₄F₈-c) and ANG. Deng et al. also noted that in all three chitosan experimental groups, which consisted of a chitosan conduit and two other chitosan conduits filled with conductive hydrogel, the density of collagen fibers decreased but the muscle fiber density increased. Likewise, Wang et al.³⁶ demonstrated that chitosan had significantly lower fiber density and fewer fibers compared to autograft. Kusaba et al.²⁹ also found that mean axon area in the autograft group was significantly larger than that in the chitosan mesh tube group. In contrast, Patel et al., reported a higher number of myelinated axons and axon diameter in chitosan as compared to ANG.³¹

Other studies compared CBDs to a negative control and reported superior histomorphometry results in chitosan-treated animals. For example, de Lima et al. compared chitosan nerve conduits to an unrepaired nerve, which served as a negative control.³⁰ They found that the cell type and distribution demonstrated a significantly higher difference in the chitosan-treated nerves as compared to the control. Chitosan accelerated the nerve repair process with increased activity of SCs and decreased Wallerian degeneration.³⁰ Simões et al. also compared chitosan-treated end-to-end nerve repair to an end-to-end nerve repair group without chitosan.³⁴ They found that recovery of hindlimb extensor response was similar between groups. They found that one of the chitosan-treated groups demonstrated significantly superior nerve regeneration as compared to the negative control.³⁴ When compared to autografts, Gonzalez-Perez et al. found that the autograft group had a greater number of myelinated fibers compared to chitosan groups.³⁸ They also found no difference in axon diameter and myelin thickness between all groups. However, when comparing chitosan groups to a negative control silicone tube, chitosan presented a greater proportion of regenerated nerve than silicone tubes, which had all failed regeneration.

Comparison of modified Chitosan-Based devices to pure Chitosan-Based devices

A number of existing studies compare modified or enhanced CBDs to empty CBDs, which have been outlined in Table 4. Enhanced chitosan conduits generally resulted in higher postoperative Basso, Beattie, and Bresnahan ratings and SFI scores, as well as greater preservation of the gastrocnemius muscle when compared to empty CBDs. Additionally, electrodiagnostic data from modified conduits demonstrated superior CMAP amplitudes and conduction velocities while histomorphometry analysis revealed increases in axon diameter, myelin thickness, and fiber count.

Degree of deacetylation

Although not directly assessed in each study, degree of deacetylation remains an important characteristic to consider when utilizing chitosan. Among included studies that reported degree of deacetylation, values ranged from 75%²⁷ to 98%.^26,38 Haasert-Talini et al. investigated varying degrees of acetylation in CBD compared to ANGs at 1-, 4-, 9- and 12-weeks post nerve reconstruction.²⁶ They found no significant differences in functional outcomes (SSI), muscle mass restoration, CMAPs, axon/fiber diameters, and myelin thickness between ANG and CBD groups. However, they did note that 98% deacetylation group exhibited significantly lower number of myelinated fibers as compared to ANG. Haastert-Talini et al. concluded that deacetylation at 95% demonstrated optimal support for peripheral nerve regeneration, whereas at higher 98%, there appeared to be limitations with high speeds of degradation and at 80%, there were limitations with low mechanical stability.²⁶ Gonzalez-Perez et al. found that at 98% deacetylation (low degree), there was 43% regeneration, and at 95% deacetylation (medium degree), there was 57% regeneration. All silicone tube-treated nerves failed regeneration in their study.³⁸

Unmodified versus modified Chitosan-Based devices

These conduits can exist as pure chitosan or augmented with various modifications or cell additives. Examples of additives utilized for augmentation of CBDs include stem cells,^21,40–42 nerve growth factor (NGF),^43,44 chondroitin sulfate,⁴⁵ collagen,⁴⁶ and hyaluronic acid.⁴⁷ Some studies have demonstrated use of altered chitosan with carboxymethyl chitosan.^36,48 Stößel et al. reported that both corrugated and uncorrugated CBDs performed significantly worse than an autograft in functional and electrophysiological analyses; however histomorphometry was similar.³⁵ Dietzmeyer et al. and Stößel et al. utilized corrugated CBDs, in which the conduit’s structure had a wave-like texture believed to help reinforce the durability and flexibility.^24,35 In the effort to develop a physically durable conduit, Jiang et al. devised a woven structure using a warp knitting technique; they concluded their product demonstrated successful nerve regeneration in a 10 mm nerve gap with comparable outcomes to autograft.⁴⁹ Additives, especially extracellular matrix components, and structural alterations provide potential for enhanced augmentation of chitosan.

Takeya et al. assessed SC-encapsulated chitosan-collagen hydrogel nerve conduits and demonstrated superior motor functional recovery, axonal regrowth, and myelination as compared to ANGs in a 10 mm sciatic nerve defect model in rats.⁴⁶ Similarly, Zhang et al. reported that use of carboxymethyl CBDs provided comparable axonal regeneration and functional outcomes to ANGs.⁴⁸ Wei et al. studied NGF CBDs and reported that at 9 weeks postsurgery, histology, axonal morphology, axonal diameters, and functional recovery was comparable to ANGs.⁴⁴

Consideration of gap lengths

Gap length refers to the nerve injury defect size that is resected during surgery to allow for direct repair of healthy nerve fascicles. Small defects may be treated with end-to-end neurorrhaphy and conduits, such as CBDs. Critical defect sizes, often defined as larger than 30 mm in humans, generally require autograft or allograft.^25,50,51 Gonzalez-Perez et al. studied CBDs in long nerve gaps of 15 mm in a sciatic nerve rat model and found that addition of collagen and fibronectin, common extracellular matrix proteins, to CBDs enhanced the nerve regeneration process.⁵² Additionally, they demonstrated incorporation of extracellular matrix additives, such as fibronectin or laminin combined with mesenchymal stem cells (MSC) or SC, to CBDs demonstrated improved regenerations success in large gap length defect of 15 mm in a rat sciatic nerve model. Nerves repaired with SC CBDs demonstrated 100% regeneration, MSC CBDs demonstrated 90% regeneration, and acellular CBDs 75% regeneration.⁵³ Shen et al. worked to look at larger gap lengths with a 25 mm-long canine tibial nerve defect model; they compared poly(lactide-coglycolide) (PLGA)-chitosan-ciliary neurotrophic factor (CNTF) conduits and PLGA/chitosan conduits, noting that the PLGA-chitosan-CNTF demonstrated superior results to the PLGA/chitosan group and similar results to nerves treated with autografts.⁵⁴ From these preclinical studies, gap length consideration is important, and the use of additives may serve to optimize nerve regeneration in critical-size defects.

Discussion

Peripheral nerve injuries are debilitating, causing functional loss and impairments. Biomaterials, such as chitosan, offer the potential to optimize management of these injuries. Chitosan has emerged as a promising biomaterial for peripheral nerve repair owing to its natural abundance, amenability to modifications, biocompatible and biodegradable features, and capacity to facilitate axonal regeneration.^26,55 The purpose of this review was to analyze the current preclinical studies on chitosan nerve conduits in peripheral nerve injury models.

In this review, most studies reported CBDs to perform similarly or inferiorly in comparison to ANGs, the gold-standard method for peripheral nerve reconstruction.^{21,23,24,35,38,39,56} However, the utilization of additives often demonstrated superior regeneration results to unmodified, acellular CBDs.^36,40,42 Metrics assessed across studies included sciatic functional index, sensory discrimination, and muscle mass ratios between CBD and autograft repair. Outcomes serving as indicators of nerve regeneration included nerve conduction velocities, CMAP latencies and amplitudes, and histomorphometry parameters.

Across most studies, chitosan performed inferiorly to autografts; however, this should not detract from the proposed efficacy of chitosan, as autografts are the gold standard for peripheral nerve repair. Histomorphometry results showed superiority of ANGs as compared to CBDs in nine out of the 15 in vivo studies, with one study demonstrating superior outcomes in chitosan³¹ and the remaining studies indicating no difference in results between groups. Electrodiagnostic studies demonstrated greater CMAP amplitudes for ANG treatment groups in a fraction of studies, while other studies showed no significant difference between groups. Results were similar for muscle mass restoration and functional analyses assessments. Patel et al. was the only study that showed increased functional recovery in the chitosan group as compared to ANG;³¹ the other studies either did not report functional outcomes or noted no significant differences between groups. Given the biological nature of ANGs, the number of studies that did not demonstrate a significant difference in functional, electrophysiologic, and histological differences with chitosan is supportive of the biomimetic effect this material has within the nerve microenvironment to facilitate regeneration. Muscle mass restoration and electrodiagnostic studies consistently favored ANGs over CBDs and CBDs over negative controls, while functional gait analyses and histomorphometry, at times, differed from this trend. In prior literature, some have argued in favor of functional gait analyses as the best predictor of nerve regeneration due to its nuanced reflection of sensory and motor recovery.³² Munro et al. conducted a study analyzing various variables related to outcome measures of electrophysiology, morphometry, and functional testing and reported that there were not significant correlations between classes.⁵⁷ In a similar light, Kanaya et al. reported that although SFI is the current gold standard for functional analysis, there did not exist statistically significant correlations with other commonly utilized parameters in nerve regeneration studies.⁵⁸

In addition, in studies that utilized additives to CBDs, these modified CBDs often demonstrated superior regeneration outcomes as compared to unmodified CBDs and in some studies demonstrated comparable results to ANGs.^46,48,49

Most consistently, several studies included a negative control (i.e., silicone tubes, different chitosan acetylation levels, unrepaired nerves, or uninjured normal nerves), from which chitosan performed better than these negative controls.^30,31,34,38 This suggests that although chitosan does not consistently perform superiorly to ANGs, chitosan does benefit the neuroregenerative environment when compared to environments without chitosan. The efficacy of CBDs in a clinical application setting was reinforced by two human studies included in this review reporting comparable sensory recovery as compared to ANGs.^59,60

Nonetheless, the comparable and occasionally superior regeneration with CBDs versus conventional techniques highlights the potential of CBDs as a viable material to investigate readily available nerve reconstruction options that may be able to replace the morbidity associated with autografts and the cost and biological impedance of allografts in critical size defects. Advantages of using CBDs include no donor site morbidity, customizable dimensions, prevention of neuroma formation, and potential for local delivery of neurotrophic factors;^3,59,61,62 these are promising as traditional ANGs are largely limited by donor availability and size. In addition, demonstration of preclinical studies that suggest benefits with utilization of CBD combined with other materials to promote its effectiveness, such as addition of internal filaments, seeding with autologous bone marrow mononuclear cells, SC seeding, and combination of native extracellular matrix proteins.^46,63–66 The transition to large animal models can also better inform clinical translation.

Limitations in this review include the heterogeneity across studies, such as variability in defect sizes, conduit dimensions, degradation times, supplemented molecules, and outcome reporting. For example, analysis of functional outcomes varied greatly as each study utilized different tests and assessments for evaluation. Furthermore, control groups varied or were absent between studies. In addition, the limited number of human studies restricts clinical generalizability. Moreover, significant heterogeneity existed regarding conduit fabrication methods, supplemented bioactive molecules, measures, and timing of assessments. This constrained the ability to conduct a meta-analysis. However, this qualitative synthesis offers greatly valuable insight into the existing literature on chitosan utilization in peripheral nerve injury and repair and highlights the need for enhanced rigor in conducting assessments of novel biomaterials through strategic and standard functional, electrodiagnostic, and histological measures alongside critical control groups. Further studies with standardized protocols will permit more robust quantitative analyses.

This review’s findings indicate that the presence of CBDs in nerve repair may lead to favorable functional and nerve regeneration outcomes, comparable to those of conventional techniques for tension-free reconstruction of peripheral nerve defects, across animal models and preliminary human studies. This underscores the potential of chitosan as a biological, customizable material to augment tension-free nerve repair or enhance nerve reconstruction to ultimately support regenerative cellular behavior and axonal regrowth after peripheral nerve injuries. Optimization of CBDs through biomolecular and structural enhancements will further potentiate their efficacy in future research endeavors. Lastly, structured prospective clinical data with multicenter translational outcomes are needed to conclusively determine therapeutic benefit of CBDs for wider clinical adoption.

Authors’ Contributions

C.J., E.O., and E.O.E. participated in conception, design, literature review, data acquisition, data analysis, and drafting of the initial article. H.S., M.A.F.V., A.S.M., and N.Y.L. participated in revising of the article. N.Y.L. supervised the project and provided final approval of the version to be published. All the authors read and approved the final article.

Footnotes

Author Disclosure Statement

None of the authors had any commercial association that might pose or create a conflict of interest with information presented in this article. N.Y.L. is a paid consultant for Checkpoint Surgical. All other authors had no disclosures or conflicts of interest.

Funding Information

No funding was acquired or utilized in the conception and creation of this project.

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