Behavioral recovery and motor reinnervation outcomes after peripheral nerve transection with microsuture or nerve tape repair

Abstract

Background

Recovery after peripheral nerve injury (PNI), even with surgical intervention, is often poor. The current standard in surgical repair of PNIs, microsuture of the severed epineurium is technically demanding, time-consuming, and routinely results in fascicular misalignment.

New Method

Nerve Tape is a coaptation device that circumvents the need for microsuture. Nerve Tape repairs improve fascicular alignment, nerve conduction velocities, as well as muscle and axon morphologies compared to microsuture repairs. However, the efficacy of Nerve Tape for small diameter nerve repairs is understudied, and no reports exist directly comparing sexes. We compared sensory and locomotor recovery as well as the pattern of motor reinnervation after sciatic nerve transection and repair with either microsuture or Nerve Tape in rats of both sexes.

Young adult male and female Lewis rats received a sciatic nerve transection repaired with either microsuture or Nerve Tape. Locomotor and sensory testing was performed weekly post-injury. At either 8- or 12-weeks post-repair, motoneurons whose axons had regenerated into the anterior tibialis muscle were retrogradely labeled, allowing for assessment of motoneuron number and spinal location.

Results and Comparison with existing methods

Nerve Tape repairs took significantly less time than microsuture. Behavioral recovery outcomes were comparable after microsuture and Nerve Tape repairs and did not differ between sexes. Retrograde labeling of motoneurons demonstrated that motor axons successfully regenerated to appropriate and inappropriate muscles regardless of repair type or sex.

Conclusion

Our data supports the efficacy of Nerve Tape to repair small diameter nerve injuries in both males and females.

Graphical abstract

Introduction

Peripheral nerve injuries (PNIs) are common injuries to the nervous system. Approximately 5% of all traumatic injuries are associated with damage to major peripheral nerves (Noble et al., 1998). Injuries to small diameter (≤ 2 mm) nerves, e.g., digital nerves, account for approximately 41% of all upper extremity nerve injuries (Aman et al., 2022, 2024).

PNIs can result in permanent loss of motor and sensory function as well as chronic pain. Long-lasting dysfunction in locomotion is due to a variety of factors, e.g., neuromuscular junction degradation, muscle atrophy, and misdirection of regenerating motor axons (Brushart, 2011; Ghergherehchi et al., 2019). Acute sensory deficits include anesthesia and hypalgesia, but plasticity in intact sensory neurons adjacent to dennervated areas contribute to the later development of allodynia (Devor et al., 1979; Socolovsky et al., 2017; Waxman et al., 1999). The functional and neuroanatomical sequalae of PNIs are also sex dependent. Compared to males, females exhibit faster rates of axonal regeneration (Kovacic et al., 2004; Kujawa et al., 1991), remyelination (Li et al., 2006), and an earlier onset of injury-induced nociceptive behaviors (Ahlström et al., 2021; Vacca et al., 2014).

Over half of all incidences of PNIs require surgical intervention (Noble et al., 1998; Ruijs et al., 2005). The standard of clinical repair for PNIs, neurorrhaphy, typically involves microsuturing the epineurium of the severed nerve ends to bring them into close approximation, promoting accurate regeneration. However, functional recovery after PNIs even with surgical intervention, is often unsatisfactory (Roganovic & Pavlicevic, 2006; Ruijs et al., 2005). For example, 50–60% of patients failed to regain useful recovery of function after upper extremity surgical nerve repairs (Jongen & Van Twisk, 1988; Sakellarides, 1962). A number of factors contribute to poor recovery including surgical errors in nerve end alignment due to the technical challenge of neurorrhaphy. In fact, approximately 40% of microsuture repairs result in unacceptable fascicular misalignment (Bernstein et al., 2013; Isaacs et al., 2016).

Nerve Tape is a novel coaptation device that circumvents the need for neurorrhaphy. Nerve Tape is made from decellularized porcine small intestinal submucosa, and has embedded Nitinol microhooks for stabilizing severed nerve stumps (Bendale et al., 2023). Importantly, 87% of experienced surgeons achieved satisfactory nerve alignment with Nerve Tape (Eberlin et al., 2024; Isaacs et al., 2016). In vivo investigations have reported that nerve conduction velocities after Nerve Tape repair are significantly better than those achieved with neurorrhaphy; moreover, muscle and axon morphology are comparable after both types of repairs (Bendale et al., 2023).

Current investigations on the efficacy of Nerve Tape to repair small diameter nerves is limited. Using the rat sciatic nerve transection model, a commonly used surrogate for human digital and other small diameter nerve injuries, Bendale et al. (2025) report comparable muscle strength/size, axon counts, g-ratios, and locomotor function at 12 weeks postoperative in male rats that received either microsuture or Nerve Tape repair.

In the current manuscript, we used the rat sciatic nerve transection model to further investigate the behavioral and neuroanatomical outcomes of Nerve Tape repair. In both sexes, we assessed locomotor recovery as well as two modalities of sensory recovery after sciatic nerve transection and repair with either microsuture or Nerve Tape. We also assessed motor reinnervation of the anterior tibialis muscle using motoneuron retrograde labeling. Our data demonstrates that behavioral recovery outcomes are comparable between repair types, and do not differ between sexes. Both repair types and sexes also exhibited a comparable degree of appropriate motor axon regeneration; moreover, motoneurons projecting to an inappropriate muscle were present in all animals. Overall, the data supports Nerve Tape repair for small diameter nerve injuries in both sexes when compared to traditional neurorrhaphy.

1. Materials and methods

2.1. Animals

All surgical interventions and post-operative (PO) animal care were performed in accordance with the Guide for the Care and Use of Laboratory Animals (National Research Council) and were approved by the Indiana University Institutional Animal Care and Use Committee (23-030-5 approved January 1^st, 2024).

Young adult male and female Lewis rats (Envigo, Indianapolis, IN; Charles River, Wilmington, MA) approximately 100 days old were maintained on a 12:12 light:dark cycle, with ad libitum access to food and water. Lewis rats were used in this study as opposed to an outbred strain, e.g., Sprague-Dawley, to avoid early euthanasia and confounding injuries due to autophagy (Carr et al., 1992; Kingery & Vallin, 1989). Animals were also provided with enrichment (soft bedding, tubes, cardboard, and wooden blocks) immediately after surgery to further dissuade autophagy. Importantly, no autotomy behaviors were observed in any animal included in this study.

2.2. Injury

Rats were anesthetized with isoflurane by inhalation. The lateral aspect of the left hindlimb was shaved and disinfected with betadine and 70% ethanol. The sciatic nerve was exposed and isolated by making a 2–3 cm incision between the knee joint and the ischial tuberosity through the skin and the biceps femoris, bluntly dissecting the biceps femoris muscle fibers, and removing the connective tissue anchoring the sciatic nerve to the surrounding muscle (Fig. 1A). While exposed, the nerve was kept moist by periodically applying normal mammalian saline (0.9% NaCl). The nerve was then lifted using a pair of curved forceps, then completely transected with a pair of sharp microscissors (Fig. 1B). Surgical repair of the transected nerve was then immediately performed using either microsuture (n= 18 males, 20 females), i.e., neurorrhaphy (Fig. 1C), or through application of Nerve Tape (n= 18 males, 18 females; BioCircuit Technologies, Atlanta, GA, USA; Fig. 1D).

Figure 1.

Digital micrographs of a rat sciatic nerve (A) before transection, (B) after transection, (C) after microsuture repair, i.e., neurorrhaphy, and (D) after Nerve Tape repair. Inset: Illustration of Nerve Tape device. Scale bar = 1.5 mm.

1.2.1. Neurorrhaphy

Three to five non-absorbable sutures (size 10–0) were placed equidistant from each other into the epineurium of the severed nerve stumps. Proper realignment of the severed nerve stumps was attempted by using vasculature present within the epineurium of both stumps as fiducial marks (Fig. 1C).

1.2.2. Nerve Tape

One column of microhooks were trimmed from both ends of a 1.5 mm Nerve Tape device in order to fit within the surgical site, then rehydrated using normal saline until soft and flexible. The device was placed underneath the injury site before placing the proximal nerve stump across the central row of Nitinol microhooks and applying gentle reverse force on the nerve to engage the hooks into the epineurium. The procedure was then repeated for the distal nerve stump, ensuring the two ends were in proper alignment with close approximation. The nerve repair was completed by wrapping the device around the nerve to fully entubulate the injury site (Fig. 1D).

After the nerve transection was surgically repaired, the muscle and skin were then closed in layers using 5–0 suture and stainless-steel wound clips, respectively. The duration of surgery was recorded as the total time the animal spent under anesthesia, i.e., shaving/cleaning the surgical site, isolating, transecting, and repairing the sciatic nerve, and closing the surgical site. The animals were allowed to recover for 8- or 12-weeks PO.

2.3. Behavioral Testing

Animals underwent two trials of locomotor (Sciatic Functional Index, SFI) and sensory (Acetone and Von Frey filament test) behavioral testing prior to injury in order to establish baseline values. After injury and repair, behavioral testing was performed at 3 days PO, to ensure loss of function due to injury, then weekly until their designated PO timepoint. All testing was performed blind to experimental condition.

2.3.1. Sciatic Functional Index

Sciatic Functional Index (SFI) was used to assess the function and return of behaviors mediated by the sciatic nerve in rodents (Carlton, 1986; de Medinaceli et al., 1982). The SFI score is highly dependent upon foot flexion and toe spread, both movements controlled by lower leg and foot muscles innervated by the sciatic nerve, e.g., anterior tibialis, soleus, and gastrocnemius muscles (de Medinaceli et al., 1982). As described previously (Carlton, 1986; Ghergherehchi et al., 2019; Hibbard et al., 2025), after traversing an inclined board, footprints from the inked hindpaws ipsilateral and contralateral to the injury were assessed for print length, total toe spread, and intermediate toe spread to compute an SFI score. For each timepoint, trials were performed in duplicate and an average SFI score was calculated.

2.3.2. Von Frey

Thresholds of mechanical stimulation to induce hindpaw withdrawal were assessed using a series of ten Von Frey filaments with logarithmically incremental stiffness (Stoelting Co., Wood Dale, IL) in a manner similar to that described by Chaplan et al. (1994). Briefly, animals were placed in elevated observation chambers on wire mesh tables. For each trial, stimulation was applied to the lateral aspect of the plantar surface of the hindpaw through the wire mesh platform. A positive response to the stimulation was recorded if the animal either withdrew their hindpaw quickly, immediately moved away from the stimulus, and/or attended to the stimulated hindpaw for at least two seconds post-stimulation. The mechanical stimulation withdrawal threshold for an animal was defined as the amount of force in grams (g) applied by the Von Frey filament that evoked two subsequent positive responses from the animal. If the animal showed no positive response to the stiffest Von Frey filament (180g), an arbitrarily high withdrawal threshold of 400g was assigned. Trials for the hindpaw ipsilateral to the injury were performed in duplicate and an average withdrawal threshold was calculated.

2.3.3. Acetone Test

Reactions of animals to punctate thermal stimulation were assessed using evaporative cooling by application of acetone in a manner similar to that described by Yoon et al. (1994). Briefly, animals were placed in elevated observation chambers on wire mesh tables. Acetone was extruded from a 25-G blunt needle (~3 μl) and applied to the plantar aspect of the animal’s foot. For 1-minute post-application, the time the animal spent exhibiting a response to the acetone, specifically foot flutter and/or biting/licking the hindpaw, was recorded. Trials for the hindpaw ipsilateral to the injury were performed in duplicate and an average response duration was calculated.

2.4. Retrograde Labeling

We have previously reported that after sciatic nerve transection and microsuture repair, the earliest PO timepoint to observe successful retrograde labeling of lumbar motoneurons projecting to the anterior tibialis muscle is 6 weeks, but with only faintly labeled somata and proximal dendrites (Ghergherehchi et al., 2019). Accordingly, in order to ensure adequate labeling for the assessment of motoneuron peripheral connectivity we used two different post-injury survival periods: an early recovery period (8 weeks) sufficient to permit axon regeneration and muscle reinnervation and a late recovery period (12 weeks) to assess potential longer-term consequences.

At either 8 or 12 weeks after injury, animals were re-anesthetized, and the anterior tibialis muscle was exposed and bilaterally injected with horseradish peroxidase conjugated to the cholera toxin β subunit (BHRP; 1 μL/side, 0.2%; Invitrogen, Carlsbad, CA) in order to retrogradely label innervating motoneurons. Forty-eight hours after BHRP injection, a period that ensures optimal labeling of motoneurons (Goldstein et al., 1990; Kurz et al., 1986), animals were re-anesthetized and perfused intracardially with saline followed by cold fixative (4% paraformaldehyde). Following perfusion, the lumbar portion of the spinal cord of each animal was removed, post-fixed for 5 h in the same fixative, then transferred to sucrose phosphate buffer (10% w/v, pH 7.4), overnight for cryoprotection. Spinal cords were then frozen and sectioned horizontally at 40 μm. For visualization of BHRP, sections were immediately reacted using a modified tetramethyl benzidine protocol (Mesulam, 1982), mounted on gelatin-coated slides, and counterstained with thionin. A group of uninjured rats (males, n = 4; females, n = 4) was included as intact controls.

2.4.1. Motoneuron labeling

For each animal, the entire dorsoventral and rostrocaudal range of spinal segments contributing to the sciatic nerve was initially examined under darkfield illumination to maximize the detectability of BHRP-labeled motoneurons. Spinal segment boundaries were determined using the midpoints between dorsal root entrances (Molander et al., 1984). In uninjured animals, intramuscular injection of BHRP into the anterior tibialis results in labeling of motoneurons in only the L3 spinal segment (Nicolopoulos-Stournaras & Iles, 1983; Fig 2A). Motoneurons innervating the anterior tibialis muscle do not form a discrete nucleus but instead are contained within the large continuous population of motoneurons located within the lateral motor column (Nicolopoulos-Stournaras & Iles, 1983); therefore, for each injured animal, a template of BHRP-labeled motoneurons on the right side of the spinal cord, i.e., contralateral to the injury, was made to determine the rostrocaudal boundaries for somata of intact, normally projecting anterior tibialis motoneurons. This template was then used to classify BHRP-labeled motoneurons on the left side of the spinal cord, i.e., ipsilateral to the injury, as falling within or outside of these boundaries. Labeled motoneurons within these boundaries were defined as potentially correctly projecting anterior tibialis motoneurons (Fig. 2B), whereas labeled motoneurons outside of these boundaries were classified as motoneurons whose axons had incorrectly regenerated to the anterior tibialis muscle. It is important to note that motoneurons innervating other muscles can also be found within the range occupied by anterior tibialis motoneurons (Nicolopoulos-Stournaras & Iles, 1983) and thus labeled motoneurons in this area in injured animals could also be aberrantly projecting to the anterior tibialis muscle. Thus, our categorization methodology potentially underestimates the degree of aberrant regeneration of injured axons.

Figure 2.

Darkfield digital micrographs of horizontal hemisections through the L3-L6 spinal segments showing the location of retrogradely labeled motoneurons after injection of horseradish peroxidase conjugated to the cholera toxin β subunit (BHRP) into the anterior tibialis (TA) muscle with an intact sciatic nerve (A) and an injured sciatic nerve, 8 weeks after transection and repair with microsuture (B). The TA muscle is normally innervated by motoneurons in the L3 spinal segment (A, bracket), located at the rostral end of the range for spinal motoneurons contributing to the sciatic nerve. Injection of BHRP into the TA muscle in injured animals also results in labeling of motoneurons located in L3 (B, bracket) as well as aberrantly labeled motoneurons throughout the sciatic range, extending into the more caudal spinal segments. Scale bar = 500 μm.

Counts of BHRP-labeled motoneurons were made at 250x under brightfield illumination where somata could be visualized and cytoplasmic inclusion of BHRP reaction product confirmed. Motoneuron counts were derived from sections spaced 80 μm apart and distributed uniformly through the entire dorsoventral extent of the spinal cord. For each animal, the total number of BHRP-labeled motoneurons located ipsilateral to the injury as well as their classification as either within or outside of the anterior tibialis motoneuron range is reported.

2.5. Statistical Analysis

The assumptions for parametric tests were evaluated for all data sets using the Shapiro-Wilk test for normality and Levene’s test for equality of variances. Behavioral (SFI, Von Frey, and Acetone test) data and motoneuron labeling data met the assumption of normality (all Ws < 0.98, ns) and equality of variances (all Fs < 3.31, ns). Surgical duration data showed equality of variance [F(1,72) = 0.88, ns] but a significant departure from normality was present [W(74) = 0.93, p < 0.001]. As a consequence, surgical duration was assessed with a Man-Whitney U test. All remaining data were analyzed by t-test and analyses of variance (ANOVAs; one-, two-, or three-way with repeated measures as appropriate). Post hoc pairwise comparisons were made with ttests and significance levels were adjusted with the Bonferoni correction. Values are reported as median or mean ± SEM, as appropriate.

3. Results

3.1. Surgery Duration

Neurorrhaphy repairs with microsuture (n = 38) lasted a median of 34.5 minutes under anesthesia, whereas repairs with Nerve Tape (n = 36) took significantly less time (44.9%; U = 59.50, z = −6.75, p < 0.0001), lasting only a median of 19.0 minutes.

3.2. Behavioral Recovery

3.2.1. Locomotor

Sciatic nerve injury immediately reduced locomotor performance, followed by a gradual and partial recovery in all groups (Fig. 3). Compared to baseline (mean overall, 0.39 ± 0.87), SFI scores in all injured animals were significantly reduced at 3 days PO (Females, microsuture, −102.04 ± 2.83, Nerve Tape, −96.62 ± 3.55; Males, microsuture, −96.24 ± 2.11, Nerve Tape, −93.87 ± 3.54; p < 0.0001, Table 1.A) and did not differ by repair type or sex; no interactions were present. SFI scores improved during the early recovery period (3d-8w PO; p < 0.0001; Table 1.B) and did not differ by repair type or sex; no interactions were present. Compared to 3 days PO, all groups exhibited significantly improved SFI scores at 8 weeks PO (Females, microsuture, −80.86 ± 3.63, Nerve Tape, −70.19 ± 4.00; Males, microsuture, −64.64 ± 4.72, Nerve Tape, −70.36 ± 4.97; p < 0.0001; Table 1.C) and did not differ by repair type or sex; no interactions were present. No further changes in SFI scores were seen during the late recovery period (9w-12w PO), with no differences by PO time, repair type, or sex; no interactions were present (Table 1.D).

Figure 3.

As measured by Sciatic Functional Index (SFI) testing, sciatic nerve injury immediately reduced locomotor performance, followed by a gradual and partial recovery in all groups. Compared to baseline, SFI scores in all injured animals were significantly reduced at 3 days PO. At 8 weeks postoperative (PO), all groups exhibited significantly improved SFI scores compared to 3 days PO. At all PO timepoints, locomotor recovery outcomes did not differ by repair type or sex. Points and error bars represent means ± SEM; * indicates significantly different from baseline; † indicates significantly different from 3 days PO; significance determined using three-way ANOVA with and without repeated measures (time x repair type x sex), and post hoc pairwise comparisons (t-tests), as appropriate, see text and Table 1 for details; filled/solid line, microsuture (MS); unfilled/dashed line, Nerve Tape (NT).

Table 1.

Sciatic Functional Index (SFI)

A	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (baseline vs. 3d PO)	1,70	3116.99	< 0.0001
		repair type (MS vs. NT)	1,70	0.85	ns
		sex (female vs. male)	1,70	0.10	ns
		All Interactions	1,70	< 3.25	ns
B	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (3d-8w PO)	8,560	44.64	< 0.0001
		repair type (MS vs. NT)	1,560	2.93	ns
		sex (female vs. male)	1,560	2.54	ns
		All Interactions	8,560	< 1.47	ns
C	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (3d vs. 8w PO)	1,70	120.93	< 0.0001
		repair type (MS vs. NT)	1,70	1.32	ns
		sex (female vs. male)	1,70	2.63	ns
		All Interactions	1,70	< 1.54	ns
D	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (9w-12w PO)	3,90	1.06	ns
		repair type (MS vs. NT)	1,90	0.06	ns
		sex (female vs. male)	1,90	0.08	ns
		All Interactions	3,90	< 2.21	ns

3.2.2. Mechanical Sensitivity

Sciatic nerve injury immediately elevated withdrawal thresholds to mechanical stimuli, indicating acute mechanical anesthesia, followed by an early and complete recovery in all groups (Fig. 4). Compared to baseline (overall mean, 84.47 ± 5.24g), withdrawal thresholds were significantly elevated in all injured animals at 3 days PO (Females, microsuture, 400.00 ± 0.00g, Nerve Tape, 387.78 ± 12.22g; Males, microsuture, 400.00 ± 0.00g, Nerve Tape, 384.21 ± 8.59g; p < 0.0001; Table 2.A) and did not differ by repair type or sex; no interactions were present. Recovery from acute mechanical anesthesia occurred during the early recovery period (3d-8w PO; p < 0.0001; Table 2.B) and did not differ by repair type or sex; no interactions were present. Compared to 3 days PO, all groups exhibited significantly decreased withdrawal thresholds at 3 weeks PO (Females, microsuture 166.00 ± 39.55g, Nerve Tape, 256.67 ± 37.09g; Males, microsuture, 193.67 ± 36.10g, Nerve Tape, 234.74 ± 30.51g; p < 0.0001; Table 2.C) and did not differ by repair type or sex; no interactions were present. By 4 weeks PO, all animals exhibited withdrawal thresholds that were no different than baseline (Females, microsuture 98.80 ± 25.30g, Nerve Tape, 95.89 ± 11.93g; Males, microsuture, 118.78 ± 26.25g, Nerve Tape, 114.00 ± 19.39g; Table 2.D) and did not differ by repair type or sex; no interactions were present. No further changes in withdrawal thresholds were seen during the late recovery period (9w-12w PO) with no differences by PO time, repair type, or sex (Table 2.E).

Figure 4.

As measured by Von Frey filament testing, sciatic nerve injury immediately elevated withdrawal thresholds to mechanical stimuli, indicating acute mechanical anesthesia, followed by an early and complete recovery in all groups. Compared to baseline, withdrawal thresholds were significantly elevated in all injured animals at 3 days postoperative (PO). For all groups, withdrawal thresholds were significantly decreased and were no longer different than baseline by 3 weeks and 4 weeks PO, respectively. Points and error bars represent means ± SEM; * indicates significantly different from baseline; † indicates significantly different from 3 days PO; significance determined using three-way ANOVA with and without repeated measures (time x repair type x sex), and post hoc pairwise comparisons (t-tests), as appropriate, see text and Table 2 for details; filled/solid line, microsuture (MS); unfilled/dashed line, Nerve Tape (NT).

Table 2.

Mechanical Sensitivity (Von Frey)

A	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (baseline vs. 3d PO)	1,70	2611.34	< 0.0001
		repair type (MS vs. NT)	1,70	0.097	ns
		sex (female vs. male)	1,70	0.13	ns
		All Interactions	1,70	< 3.10	ns
B	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (3d-8w PO)	8,560	173.17	< 0.0001
		repair type (MS vs. NT)	1,560	0.27	ns
		sex (female vs. male)	1,560	1.29	ns
		All Interactions	8,560	< 1.91	ns
C	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (3d vs. 3w PO)	1,70	90.33	< 0.0001
		repair type (MS vs. NT)	1,70	2.11	ns
		sex (female vs. male)	1,70	0.002	ns
		All Interactions	1,70	< 3.90	ns
D	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (baseline vs. 4w PO)	1,70	2.44	ns
		repair type (MS vs. NT)	1,70	0.28	ns
		sex (female vs. male)	1,70	0.22	ns
		All Interactions	1,70	< 2.06	ns
E	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (9w-12w PO)	3,90	2.33	ns
		repair type (MS vs. NT)	1,90	1.29	ns
		sex (female vs. male)	1,90	0.79	ns
		All Interactions	3,90	< 1.06	ns

3.2.3. Thermal Sensitivity

Sciatic nerve injury quickly elevated the time spent exhibiting a response subsequent to the application of acetone, which did not improve over the recovery period in all groups (Fig. 5). Compared to baseline (overall mean, 3.26 ± 0.46s), response durations were significantly elevated for all groups except Nerve Tape-repaired males (3.86 ± 1.18s) at 3 days PO (Females, microsuture, 13.28 ± 1.72s, Nerve Tape, 9.33 ± 1.56s; Males, microsuture, 16.96 ± 3.18s; p < 0.0001; Table 3.A) and did not differ by sex. A significant effect of repair type was also present (p < 0.0001), as well as significant PO time x repair type (p = 0.0048) and repair type x sex interactions (p = 0.0348). At 3 days PO, response durations in females and males with microsuture repairs did not differ (Table 3.B), but males with Nerve Tape repairs had significantly shorter response durations than females with the same repair (p = 0.0124; Table 3.C).

Figure 5.

As measured by acetone testing, sciatic nerve injury quickly elevated response durations, which did not improve over the recovery period in all groups. At 3 days postoperative (PO), response durations were significantly elevated compared to baseline for all groups besides males with Nerve Tape repairs, having significantly shorter response durations than females with the same repair. This shorter response duration in Nerve Tape males was present through 2 weeks PO. Response durations remained elevated throughout the PO period regardless of repair type or sex. Points and error bars represent means ± SEM; * indicates significantly different from baseline; † indicates significantly different from 3 days PO; X indicates significant differences across groups within a PO timepoint; significance determined using one-, two-, and three-way ANOVA with and without repeated measures, and post hoc pairwise comparisons (t-tests), as appropriate, see text and Table 3 for details; filled/solid line, microsuture (MS); unfilled/dashed line, Nerve Tape (NT).

Table 3.

Thermal Sensitivity (Acetone test)

A	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (baseline vs. 3d PO)	1,70	76.62	< 0.0001
		repair type (MS vs. NT)	1,70	20.34	< 0.0001
		sex (female vs. male)	1,70	0.12	ns
		Interaction (repair type x sex)	1,70	4.63	0.0348
		Interaction (PO time x sex)	1,70	0.14	ns
		Interaction (PO time x repair type)	1,70	8.49	0.0048
		Interaction (PO time x repair type x sex)	1,70	3.62	ns
B	1-way ANOVA	sex (MS at 3d PO)	df	F	p
		sex (female vs. male)	1,36	1.10	ns
C	1-way ANOVA	sex (NT at 3d PO)	df	F	p
		sex (female vs. male)	1,34	6.97	0.0124
D	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (3d-8w PO)	8,560	12.17	< 0.0001
		repair type (MS vs. NT)	1,560	4.51	0.0372
		sex (female vs. male)	1,560	21.68	< 0.0001
		Interaction (repair type x sex)	1,560	5.59	0.0208
		Interaction (PO time x sex)	8,560	2.00	0.0449
		Interaction (PO time x repair type)	8,560	4.02	0.0001
		Interaction (PO time x repair type x sex)	8,560	2.79	0.0049
E	2-way ANOVA with repeated measures	time x repair type (Females only)	df	F	p
		PO time (3d-8w PO)	8,288	11.26	< 0.0001
		repair type (MS vs. NT)	1,288	0.04	ns
		Interaction (PO time x repair type)	8,288	3.21	0.0016
F	2-way ANOVA with repeated measures	time x repair type (Males only)	df	F	p
		PO time (3d-8w PO)	8,272	3.02	0.0029
		repair type (MS vs. NT)	1,272	8.10	0.0075
		Interaction (time x repair type)	8,272	3.60	0.0005
G	1-way ANOVA with repeated measures	time (microsuture males)	df	F	p
		time (3d-8w PO)	8,136	1.04	ns
H	1-way ANOVA with repeated measures	time (Nerve Tape males)	df	F	p
		time (3d-8w PO)	8,136	7.37	< 0.0001
I	3-way ANOVA with repeated measures	time x repair type x sex	df	F	p
		PO time (9w-12w PO)	3,90	2.15	ns
		repair type (MS vs. NT)	1,90	0.01	ns
		sex (female vs. male)	1,90	3.58	ns
		All Interactions	3,90	< 1.69	ns

Response durations continued to be elevated through the early recovery period for all injured animals (3d-8w PO; p < 0.0001; Table 3.D), with significant effects of repair type (p = 0.0372), sex (p < 0.0001), and significant interactions (repair type x sex, p = 0.0208; PO time x sex, p = 0.0449; PO time x repair type, p = 0.0001; PO time x repair type x sex, p = 0.0049). In females, response durations increased through 8 weeks PO regardless of repair type (Microsuture, 25.83 ± 1.54s; Nerve Tape, 20.39 ± 2.20s; p < 0.0001; Table 3.E); a signification interaction (PO time x repair type, p = 0.0016) was also present, but no pairwise comparisons (repair type within timepoint) reached statistical significance. Response durations also increased in males through 8 weeks PO (p = 0.0029; Table 3.F), with an effect of repair type (Microsuture, 13.12 ± 2.05s; Nerve Tape, 16.98 ± 1.89s; p = 0.0075); a significant interaction (PO time x repair type, p = 0.0005) was also present due to significant differences between repair types at 3d and 2w PO (Fig. 5). In microsuture-repaired males, response durations were consistently elevated through 8 weeks PO and did not differ from those at 3 days PO (Table 3.G). In contrast, response durations in Nerve Tape-repaired males gradually increased through 8 weeks PO relative to those at 3 days PO (p < 0.0001; Table 3.H), with significant differences starting at 4 weeks PO (Fig. 5).

Response durations remained elevated throughout the late recovery period (9w-12w PO) for all injured animals, with no differences by PO time, repair type, or sex; no interactions were present (Table 3.I).

3.3. Motoneuron Labeling

In uninjured animals, injection of BHRP into the anterior tibialis muscle labeled ipsilateral anterior tibialis motoneurons in the lateral motor column of spinal segment L3 (Figs. 2A, 6A). Consistent with previous reports using similar tracer volumes and concentrations (Ghergherehchi et al., 2019; Meadows et al., 2023), an average of 68.50 ± 4.35 motoneurons per animal was labeled with BHRP and did not differ by sex (Females, 65.00 ± 7.90; Males, 72.00 ± 4.24; Table 4.A; Figs. 7A–D).

Figure 6.

Brightfield digital micrographs of horizontal sections through the lumbar spinal cord showing retrogradely labeled motoneurons after injection of horseradish peroxidase conjugated to the cholera toxin β subunit (BHRP) into the anterior tibialis muscle. (Top) Labeled motoneurons in the L3 spinal segment of an (A) intact female and (B) injured/microsuture (MS) repaired female at 8 weeks postoperative (PO). (Bottom) Aberrantly labeled motoneurons in the caudal aspect of the sciatic range (specifically, L5/L6 spinal segments) of an (C) injured/microsuture repaired male and (D) injured/Nerve Tape (NT) repaired male at 8 weeks PO. Scale bar = 250 μm.

Table 4.

Motoneuron Number

Total Motoneurons
			df	t	p
A	unpaired t-test	uninjured females vs. uninjured males	6	−0.78	ns
B	3-way ANOVA	time x repair type x sex	df	F	p
		PO time (8w vs. 12w PO)	1,65	5.24	0.0254
		repair type (MS vs. NT)	1,65	1.88	ns
		sex (female vs. male)	1,65	20.13	< 0.0001
		All Interactions	1, 65	< 1.16	ns
C	1-way ANOVA	treatment (females at 8w PO)	df	F	p
		treatment (intact vs. injured)	2,22	0.55	ns
D	1-way ANOVA	treatment (males at 8w PO)	df	F	p
		treatment (intact vs. injured)	2,19	4.55	0.0243
			df	t	p
E	unpaired t-test	intact vs. microsuture (males at 8w PO)	11	3.30	0.0071
		intact vs. Nerve Tape (males at 8w PO)	11	2.55	0.0273
		microsuture vs. Nerve Tape (males at 8w PO)	16	1.14	ns
F	2-way ANOVA	repair type x sex (at 12w PO)	df	F	p
		repair type (MS vs. NT)	1,30	1.24	ns
		sex (female vs. male)	1,30	4.57	0.0409
		Interaction (repair type x sex)	1,30	0.84	ns
		treatment (intact vs. injured)	2,18	1.48	ns
G	1-way ANOVA	treatment (FEmales at 12w PO)	df	F	p
		treatment (intact vs. injured)	2,18	1.48	ns
H	1-way ANOVA	treatment (males at 12w PO)	df	F	p
		treatment (intact vs. injured)	2,18	0.20	ns
Motoneuron Location (inside TA range)
I	2-way ANOVA	treatment x sex (at 8w PO)	df	F	p
		treatment (intact vs. injured)	2,42	10.47	0.0002
		sex (female vs. male)	1,42	0.82	ns
		Interaction (treatment x sex)	2,42	0.89	ns
			df	t	p
J	unpaired t-test	intact vs. microsuture (8w PO)	27	4.82	< 0.0001
		intact vs. Nerve Tape (8w PO)	24	3.80	0.0009
		microsuture vs. Nerve Tape (8w PO)	37	−0.96	ns
K	2-way ANOVA	repair type x sex (at 8w PO)	df	F	p
		repair type (MS vs. NT)	1,35	1.11	ns
		sex (female vs. male)	1,35	2.87	ns
		Interaction (repair type x sex)	1,35	0.31	ns
L	2-way ANOVA	treatment x sex (at 12w PO)	df	F	p
		treatment (intact vs. injured)	2,36	4.08	0.0253
		sex (female vs. male)	1,36	0.30	ns
		Interaction (treatment x sex)	2,36	0.05	ns
			df	t	p
M	unpaired t-test	intact vs. microsuture (12w PO)	23	3.69	0.0012
		intact vs. Nerve Tape (12w PO)	23	2.40	0.0249
		microsuture vs. Nerve Tape (12w PO)	32	−0.09	ns
N	2-way ANOVA	repair type x sex (at 12w PO)	df	F	p
		repair type (MS vs. NT)	1,30	0.01	ns
		sex (female vs. male)	1,30	0.09	ns
		Interaction (repair type x sex)	1,30	0.02	ns
Motoneuron Location (outside TA range)
O	2-way ANOVA	repair type x sex (at 8w PO)	df	F	p
		repair type (MS vs. NT)	1,35	4.76	0.036
		sex (female vs. male)	1,35	15.6	0.0004
		Interaction (repair type x sex)	1,35	0.01	ns
P	1-way ANOVA	repair type (females at 8w PO)	df	F	p
		repair type (MS vs. NT)	1,19	1.46	ns
Q	1-way ANOVA	repair type (males at 8w PO)	df	F	p
		repair type (MS vs. NT)	1,16	8.11	0.0117
R	2-way ANOVA	repair type x sex (at 12w PO)	df	F	p
		repair type (MS vs. NT)	1,30	1.98	ns
		sex (female vs. male)	1,30	7.82	0.0089
		Interaction (repair type x sex)	1,30	0.91	ns
S	1-way ANOVA	repair type (females at 12w PO)	df	F	p
		repair type (MS vs. NT)	1,15	1.63	ns
T	1-way ANOVA	repair type (males at 12w PO)	df	F	p
		repair type (MS vs. NT)	1,15	0.35	ns

Figure 7.

Motoneuron labeling results after injection of horseradish peroxidase conjugated to the cholera toxin β subunit (BHRP) into the anterior tibialis (TA) muscle at 8- (A, C, & E) and 12- (B, D, & F) weeks after sciatic nerve transection and repair with microsuture (MS) or Nerve Tape (NT) in female (filled bars) and male (open bars) rats. (A & B) The total number of BHRP-labeled motoneurons found in the sciatic range of the lumbar spinal cord was reduced in injured males regardless of repair type at 8 weeks postoperative (PO) but no longer differed from intact values at 12 weeks PO. For females, the total number of BHRP-labeled motoneurons did not differ between intact and injured animals regardless of repair type or PO timepoint. (C & D) At both PO timepoints, the total number of BHRP-labeled motoneurons within the typical anterior tibialis range is reduced in injured animals regardless of sex or repair type. (E & F) At 8 weeks PO, the number of aberrantly projecting motoneurons differed between microsuture- and Nerve Tape-repaired animals. Specifically, injured MS-repaired males had more aberrantly projecting motoneurons than injured NT-repaired males at 8 weeks PO; however, this difference was no longer significant at 12 weeks PO. At both PO timepoints, injured females had more aberrantly-projecting, BHRP-labeled motoneurons than injured males. Bar heights and error bars represent means ± SEM; points indicate data from individual animals. * indicates significantly different from intact animals. † indicates significant differences by sex; X indicates significant differences between repair types; significance determined with one- and two-way ANOVA and post hoc pairwise comparisons (t-tests), as appropriate, see text and Table 4 for details.

Consistent with that seen in uninjured animals, injection of BHRP into the anterior tibialis muscle in injured animals resulted in labeling of motoneurons located in the lateral motor column of spinal segment L3 (Figs. 2B, 6B); in addition, injured animals also exhibited aberrantly labeled motoneurons extending into the more caudal spinal segments (L4-L6) of the sciatic range (Figs. 2B, 6C & D). These more caudal motoneuron populations normally innervate other muscles of the lower leg and foot (Nicolopoulos-Stournaras & Iles, 1983). Thus, this aberrant BHRP labeling after anterior tibialis injection in injured animals indicates motoneuron axon regeneration into inappropriate muscles. One injured animal was excluded from this analysis due to the absence of BHRP labeling in either injured or intact motoneurons, likely due to histochemical processing failure.

3.3.1. Motoneuron Number

In injured animals, the total number of labeled motoneurons did not differ by repair type (Microsuture, 73.32 ± 7.03; Nerve Tape, 65.26 ± 4.87; Table 4.B); however, animals did show significant effects of both sex (p < 0.0001) and PO time (p = 0.0254); no interaction was present. At 8 weeks PO, the total number of BHRP-labeled motoneurons throughout the L3-L6 spinal segments of females did not differ between intact and injured females (overall mean = 79.14 ± 5.57; Table 4.C; Fig. 7A). In contrast, the total number of BHRP-labeled motoneurons in injured males (overall mean = 46.00 ± 4.33) was reduced by 36.1% on average compared to intact values (p = 0.0243; Table 4.D; Fig. 7A). This reduction in total motoneuron number was present in males with either repair type (Table 4.E) and did not differ between repair types (Table 4.E).

At 12 weeks PO, the total number of BHRP-labeled motoneurons did not differ by repair type (Microsuture, 80.59 ± 7.08; Nerve Tape, 71.18 ± 7.09; Table 4.F), but did differ by sex (p = 0.0409); no interaction was present. For females, the total number of labeled motoneurons continued to not differ between intact and injured animals with either repair type (overall mean = 85.76 ± 8.35; Table 4.G; Fig. 7B); however, the number of BHRP-labeled motoneurons in injured males with either repair type at 12 weeks PO had increased (an average of 43.5%) from 8-week PO values and no longer differed from intact values (overall mean = 66.00 ± 4.61; Table 4.H; Fig. 7B).

3.3.2. Motoneuron Location

The number of BHRP-labeled motoneurons in injured animals was further assessed by the location of labeling, either within the area normally occupied by motoneurons projecting to the anterior tibialis muscle or outside of it throughout the sciatic range. Within the typical anterior tibialis motoneuron range (L3; Figs. 2A, 6A, 6B), the number of BHRP-labeled motoneurons in all injured animals was reduced relative to intact values at 8 weeks PO (an average of 35.33 ± 4.51; 47.7%; p = 0.0002; Table 4.I; Fig. 7C); this reduction from intact values was present in animals with either microsuture or Nerve Tape repairs (Table 4.J; Fig. 7C). There was no effect of sex or interaction present (Table 4.I). Within injured animals, the number of BHRP-labeled motoneurons within the TA range did not differ by repair type (Microsuture, 32.76 ± 4.24; Nerve Tape, 38.89 ± 4.79) or sex (Females, 40.19 ± 4.66; Males, 30.22 ± 3.98); no interaction was present (Table 4.K).

At 12 weeks PO, the number of BHRP-labeled motoneurons within the typical anterior tibialis motoneuron range continued to be reduced relative to intact values in all injured animals (an average of 44.12 ± 7.58; 35.6%; p = 0.0253; Table 4.L; Fig. 7D); this reduction from intact values was present in animals with either microsuture or Nerve Tape repairs (Table 4.M; Fig. 7D). There was no effect of sex or interaction present (Table 4.L). Within injured animals, the number of BHRP-labeled motoneurons within the TA range did not differ by repair type (Microsuture, 43.76 ± 4.10; Nerve Tape, 44.47 ± 6.50) or sex (Females, 42.94 ± 5.77; Males, 45.29 ± 5.05); no interaction was present (Table 4.N).

Outside of the typical anterior tibialis motoneuron range (Figs. 2B, 6C, 6D), the number of aberrantly projecting motoneurons differed between microsuture- and Nerve Tape-repaired animals at 8 weeks PO (p = 0.036; Table 4.O; Fig. 7E), with more aberrantly projecting motoneurons present in microsuture-repaired animals (50.6%) than in Nerve Tape-repaired animals. A sex difference was also present, with more (146.8%) aberrantly projecting motoneurons in females (p = 0.0004; Table 4.O; Fig. 7E). The number of aberrantly projecting motoneurons in injured females did not differ by repair type (Microsuture, 44.00 ± 6.90; Nerve Tape, 32.22 ± 6.43; Table 4.P). In addition to having fewer labeled motoneurons, the number of aberrantly projecting motoneurons in injured males differed by repair type, with significantly more misprojecting motoneurons in microsuture males than in Nerve Tape males (Microsuture, 22.22 ± 3.84; Nerve Tape, 9.33 ± 2.40; p = 0.0117; Table 4.Q; Fig. 7E).

At 12 weeks PO, the number of aberrantly projecting motoneurons no longer differed between microsuture- and Nerve Tape-repaired animals (Table 4.R; Fig. 7F). The sex difference in the number of aberrantly projecting motoneurons continued to be greater (106.8%) in females than in males (p = 0.0089; Table 4.R; Fig. 7F). The number of aberrantly-projecting motoneurons in injured females continued to not differ by repair type (Microsuture, 53.00 ± 14.75; Nerve Tape, 33.78 ± 5.58; Table 4.S). However, the difference in the number of aberrantly-projecting motoneurons in injured males by repair type seen at 8 weeks PO was no longer present (Microsuture, 22.44 ± 3.69; Nerve Tape, 18.75 ± 5.13; Table 4.T), due to an increase (101.0%) in Nerve Tape males (Fig. 7F).

4. Discussion

Traumatic injuries to peripheral nerves and their consequent sequalae are oftentimes debilitating for patients. Unfortunately, the current standard of clinical repair for PNIs, neurorrhaphy, often does not result in good clinical outcomes (Lundborg, 2000, 2003; Lundborg & Rosén, 2007; Robinson, 2000; Siemionow & Brzezicki, 2009). Compared to microsuture repairs, an innovation in nerve repair methodology, Nerve Tape, has been shown to result in comparable locomotor recovery, as well as axon and muscle morphologies post-sciatic nerve injury in male rats (Bendale et al., 2025). Expanding upon the findings of Bendale et al. (2025), in this study, we assessed behavioral recovery, both locomotor and sensory, and patterns of motor axon reinnervation after sciatic nerve transection and repair with either microsuture or Nerve Tape in rats of both sexes. We found that microsuture and Nerve Tape repair resulted in comparable behavioral recovery outcomes that did not differ between sexes, thus supporting the efficacy of Nerve Tape for both males and females. In parallel, patterns of motoneuron labeling were also comparable between repair types, suggesting no impediment on axon regeneration due to Nerve Tape devices. Moreover, Nerve Tape repair also saves the animal a considerable amount of exposure to anesthesia, which is an important consideration clinically, as duration of surgery is frequently cited as a major risk factor for postoperative complications (Lone et al., 2021). These results support the continued use of Nerve Tape devices for small diameter nerve injury repair in both sexes.

4.1. Behavioral Analyses

4.1.1. Locomotor Function

Asymmetric gait patterns, as measured by SFI testing, are a consequence of sciatic nerve injuries and the resultant denervation of the lower hindpaw (Bain et al., 1989; Carlton, 1986). Regardless of repair type, all animals in this study exhibited a substantial loss of locomotor function immediately after injury. Due to the gradual reinnervation of distal muscles through the slow regeneration of axons from the proximal nerve stump (Robinson, 2000; Sunderland, 1968), SFI scores improved significantly during the early recovery period compared to those at 3 days PO. Despite this recovery, animals still exhibited a consistent plateau in recovery around 9 weeks PO likely due to degeneration in the periphery, i.e., muscle atrophy and neuromuscular junction degeneration, that limits recovery outcomes (Germinario et al., 2002; Sakuma et al., 2016; Yin et al., 2021).

Bendale et al. (2025) reported comparable locomotor recovery between repair types at 12 weeks post-injury in male rats. Consistent with Bendale et al. (2025), the results of this study also demonstrate similar SFI scores between repair types at 12 weeks PO. Expanding beyond previous results, we also demonstrated through weekly SFI testing that the timeline of locomotor recovery does not differ between microsuture- and Nerve Tape-repaired animals. Moreover, our data is the first to report a direct comparison of recovery outcomes after application of Nerve Tape between sexes. Despite having sciatic nerves with smaller diameters than males (Kegoye et al., 2025; Xu et al., 2023), female animals repaired with the same size Nerve Tape devices as used in males exhibited similar levels of locomotor recovery compared to males of either repair type or microsutured females.

4.1.2. Mechanical Sensitivity

Acutely, PNI induces several deficits in sensory function including mechanical anesthesia (Socolovsky et al., 2017). Regardless of repair type, all animals in this study exhibited a substantial loss of mechanical sensitivity immediately after injury. In contrast to investigations of mechanical hyperesthesia, the results reported here, including the large amounts of force required for withdrawal, are consistent with other evaluations of mechanical anesthesia (e.g., Kingery & Vallin, 1989; Zhou et al., 2025). Similar to locomotor recovery outcomes, the gradual reinnervation of distal targets during the recovery period restores sensory function compared to 3 days PO. Cobianchi et al. (2014) report regenerating sciatic nerve fibers reach the epidermis of the affected hindpaw around 3 to 4 weeks post-injury, which is consistent with the timeline of recovery we observed. Animals recovered normal degrees of mechanical sensitivity by 4 weeks PO regardless of sex or repair type; moreover, prior studies report similar timelines of recovery after PNI (Cobianchi et al., 2014; Kingery & Vallin, 1989; Zhou et al., 2025). Lastly, withdrawal thresholds did not differ between sexes or repair types at any PO timepoint. These results suggest that injury-induced changes to mechanical sensitivity are parallel between sexes and repair types.

4.1.3. Thermal Sensitivity

After sciatic nerve injury, thermal allodynia, i.e., pain associated with otherwise non-painful stimuli, is attributed to the progressive upregulation of peptidergic nociceptors and collateral sprouting of the neighboring saphenous nerve (Duraku et al., 2013; Kingery et al., 1994; Ma & Bisby, 2000; Vallin & Kingery, 1991). Such plasticity likely explains the early and continuous increase in response durations to cold stimuli exhibited throughout the PO period by all injured animals in this study. Interestingly, males repaired with Nerve Tape exhibited significantly less elevation in thermal sensitivity than microsuture-repaired males for several weeks in the early PO period. This early protection from thermal sensitivity in Nerve Tape animals may be due to the significant difference in surgical duration, and consequently, exposure to anesthesia, between repair types. In fact, sensory neurons located in lumbar dorsal root ganglion have been reported to exhibit negative side effects, e.g., neuroinflammation and hyperalgesia, when exposed to anesthetics (Puljak et al., 2009; Zhang et al., 2001). However, females repaired with Nerve Tape did not exhibit the same early protection from increased thermal sensitivity. Females have been reported to exhibit an earlier onset of nociceptive behaviors including thermal allodynia, compared to males (Ahlström et al., 2021; Vacca et al., 2014). In general, these results support the use of Nerve Tape by establishing the devices did not augment thermal allodynia beyond that that seen in microsuture-repaired animals.

4.2. Motoneuron Labeling

We have previously demonstrated that anterior tibialis motoneurons can be successfully labeled with the retrograde tracer, BHRP, as early as 6 weeks post-sciatic nerve transection and microsuture repair (Ghergherehchi et al., 2019). This suggests that motor axons require at least 6 weeks in order to regenerate to the most proximal, sciatic-mediated muscle. In this study, we evaluated motor reinnervation of the anterior tibialis muscle at 8- and 12-weeks PO. Consistent with the results of our previous study (Ghergherehchi et al., 2019), all injured animals at both PO timepoints exhibited successful retrograde labeling of spinal motoneurons. A sex difference was observed at 8 weeks PO, showing males had significantly fewer BHRP-labeled motoneurons than females. This is likely due to inherent differences in axon regeneration rates between sexes. As demonstrated in multiple injury models, females exhibit faster axonal regeneration rates than males (Kovacic et al., 2004; Kujawa et al., 1991; Li et al., 2006). In this study, by 8 weeks PO, females had already achieved substantial motor axon regeneration to the anterior tibialis, resulting in motoneuron labeling that did not differ from intact females; however, males require more time to regenerate, at least 12 weeks PO, in order to achieve similar degrees of motoneuron labeling seen in intact animals. Importantly, the focal intramuscular BHRP injection used here only labels a subset of motoneurons innervating the muscle of interest; therefore, no conclusions can be made concerning the completeness of motor reinnervation. However, the results of this study suggest that this method is still sensitive to differences in regeneration.

Importantly, the total number of motoneurons labeled did not differ between microsuture- and Nerve Tape-repaired animals. This suggests that the Nerve Tape device does not impede motor axon regeneration or retrograde transport. This supports the results of previous studies that have demonstrated a lack of intraneural fascicule disruption due to the microhooks in Nerve Tape (Bendale et al., 2023). Histological assessments of axon regeneration, i.e., axon counts and g-ratios, have been reported to be comparable between repair types (Bendale et al., 2025). The comparable motoneuron labeling between repair types reported here expands upon this previous finding by providing evidence that motor axons reach distal muscles as early as 8 weeks PO and are capable of executing integral cellular processes such as retrograde transport.

After PNIs, proximal axons slowly regenerate back to distal targets. Surgical repair of PNIs with either microsuture or Nerve Tape devices provides some guidance to regenerating axons by bringing the severed nerve stumps into close approximation. This prevents axons from severe and disadvantageous misdirection, but the interfascicular architecture of nerves must still be maintained by properly aligning nerve stumps during surgical repair in order to guide axons to their appropriate targets (Hibbard & Sengelaub, 2022). Nerve Tape has been previously reported to result in better nerve stump alignment outcomes than microsuture repairs (Eberlin et al., 2024; Isaacs et al., 2016). In this study, Nerve Tape repairs did result in fewer misprojecting motor axons in males at 8 weeks PO; however, this effect was no longer significant at 12 weeks PO and there were no significant differences between repair types in the number of motoneurons within the anterior tibialis range for either PO timepoint or sex. This suggests that there may be a ceiling effect on the degree to which the surgical alignment of nerve stumps can prevent axon misdirection. Within intraneural fascicles, axons that project to individual muscles are discretely organized, but are not isolated from axons projecting to other muscles within the same fascicle (Hibbard & Sengelaub, 2022); therefore, even with excellent surgical alignment, adjacent axons serve as opportunities for misdirection that cannot be avoided. This difficulty in surgical alignment is likely further compounded by the size of the nerve being repaired. The smaller diameter sciatic nerves of females compared to males (Kegoye et al., 2025; Xu et al., 2023) could underlie our observation that regardless of repair type, injured females consistently had more aberrantly projecting motoneurons than injured males. Interventions that augment cellular processes or signaling pathways that direct regenerating axons back to appropriate targets warrant future investigation.

5. Conclusion

For several years it was generally accepted that surgical techniques for PNI repair had reached a plateau (Lundborg, 2003); however, technology and microengineering techniques have advanced substantially, leading to novel innovations in nerve repair such as Nerve Tape. The results of this study suggest that Nerve Tape repair of small diameter nerves produces behavioral recovery outcomes, both locomotor and sensory, and motor reinnervation outcomes that are equivalent to nerve repairs with standard neurorrhaphy in both sexes. The improvement on a number of limitations to surgical nerve repair with neurorrhaphy observed with Nerve Tape repair, e.g., successful nerve alignment and substantial reduction in surgical anesthesia duration, clearly support the continued use of this novel repair technique.

Highlights.

• Recovery after peripheral nerve injury, even with surgical repair, is often poor.

• Microsuturing, the standard in nerve repair, is challenging and time-consuming.

• Repair with Nerve Tape or microsuture results in similar behavioral outcomes.

• Repair with Nerve Tape or microsuture results in similar neuroanatomical outcomes.

• Recovery outcomes did not differ between sexes.

Data availability

Data will be made available on request.