Nerve regeneration in a bony bed: vascularized versus nonvascularized nerve grafts

Nerve recovery from treatment with a vascularized nerve graft compared to an autologous non-vascularized nerve graft in animal models: A systematic review and meta-analysis

Background

Treatment of nerve injuries proves to be a worldwide clinical challenge. Vascularized nerve grafts are suggested to be a promising alternative for bridging a nerve gap to the current gold standard, an autologous non-vascularized nerve graft. However, there is no adequate clinical evidence for the beneficial effect of vascularized nerve grafts and they are still disputed in clinical practice.

Objective

To systematically review whether vascularized nerve grafts give a superior nerve recovery compared to non-vascularized nerve autografts regarding histological and electrophysiological outcomes in animal models.

Material and methods

PubMed and Embase were systematically searched. The inclusion criteria were as follows: 1) the study was an original full paper which presented unique data; 2) a clear comparison between a vascularized and a non-vascularized autologous nerve transfer was made; 3) the population study were animals of all genders and ages. A standardized mean difference and 95% confidence intervals for each comparison was calculated to estimate the overall effect. Subgroup analyses were conducted on graft length, species and time frames.

Results

Fourteen articles were included in this review and all were included in the meta-analyses. A vascularized nerve graft resulted in a significantly larger diameter, higher nerve conduction velocity and axonal count compared to an autologous non-vascularized nerve graft. However, during sensitivity analysis the effect on axonal count disappeared. No significant difference was observed in muscle weight.

Conclusion

Treating a nerve gap with a vascularized graft results in superior nerve recovery compared to non-vascularized nerve autografts in terms of axon count, diameter and nerve conduction velocity. No difference in muscle weight was seen. However, this conclusion needs to be taken with some caution due to the inherent limitations of this meta-analysis. We recommend future studies to be performed under conditions more closely resembling human circumstances and to use long nerve defects.

A novel loop neurorrhaphy technique to preserve lower lip sensate in mandibular reconstruction using an innervated vascularized iliac bone flap

BACKGROUND

This study aimed to introduce a novel loop neurorrhaphy technique using an innervated vascularized iliac bone flap (VIBF) with vascularized ilioinguinal nerve (IIN) to reconstruct the inferior alveolar nerve (IAN) and preserve lower lip sensation simultaneously with mandibular reconstruction.

METHODS

This study prospectively included patients who underwent mandibular reconstruction using VIBF from May 2018 to April 2020. Subjects were allocated into two groups: (1) Group I; innervated VIBF with loop neurorrhaphy (IIN doubly anastomosed with IAN and mental nerve), (2) Group II (control); conventional VIBF. Evaluation was done with operative time, intraoperative indocyanine green (ICG), lower lip sensory assessment (two-point discrimination [TPD] test and current perception threshold [CPT]), and drooling.

RESULTS

Twelve patients were included; 6 in each group, (7 males and 5 females), age ranging from 18 to 57 years (average: 36.75 years). In all cases, intraoperative perfusion of IIN was confirmed by ICG. Group I showed a statistically significant more flap harvesting time compared with group II (mean difference, 5.67 min; P = 0.0091). There was a significant difference in sensory recovery favoring group I (P < 0.05). The TPD results in group I showed an average of 9.8 ± 6.9 mm and 6.2 ± 5.7 mm on operated and non-operated sides, while Group II showed a poor sensory recovery, and the TPD showed an average of 24.6 ± 6.7 mm and 8.4 ± 2.3 mm on operated and non-operated sides. The CPT results showed a significant difference between both groups. In Group I, the extent of drooling was 3.16 ± 0.75, while in Group II, the score was 1.6 ± 0.81, revealing a significant difference favoring Group I.

CONCLUSIONS

Concurrent mandibular reconstruction using VIBF and loop neurorrhaphy with vascularized IIN to reconstruct IAN successfully restore lower jaw form and preserve lip sensation.

Facial Nerve Repair: Bioengineering Approaches in Preclinical Models

Injury to the facial nerve can occur after different etiologies and range from simple transection of the branches to varying degrees of segmental loss. Management depends on the extent of injury and options include primary repair for simple transections and using autografts, allografts, or conduits for larger gaps. Tissue engineering plays an important role to create artificial materials that are able to mimic the nerve itself without extra morbidity in the patients. The use of neurotrophic factors or stem cells inside the conduits or around the repair site is being increasingly studied to enhance neural recovery to a greater extent. Preclinical studies remain the hallmark for development of these novel approaches and translation into clinical practice. This review will focus on preclinical models of repair after facial nerve injury to help researchers establish an appropriate model to quantify recovery and analyze functional outcomes. Different bioengineered materials, including conduits and nerve grafts, will be discussed based on the experimental animals that were used and the defects introduced. Future directions to extend the applications of processed nerve allografts, bioengineered conduits, and cues inside the conduits to induce neural recovery after facial nerve injury will be highlighted.

Three-Dimensional Engineered Peripheral Nerve: Toward a New Era of Patient-Specific Nerve Repair Solutions

Reconstruction of peripheral nerve injuries (PNIs) with substance loss remains challenging because of limited treatment solutions and unsatisfactory patient outcomes. Currently, nerve autografting is the first-line management choice for bridging critical-sized nerve defects. The procedure, however, is often complicated by donor site morbidity and paucity of nerve tissue, raising a quest for better alternatives. The application of other treatment surrogates, such as nerve guides, remains questionable, and it is inefficient in irreducible nerve gaps. More importantly, these strategies lack customization for personalized patient therapy, which is a significant drawback of these nerve repair options. This negatively impacts the fascicle-to-fascicle regeneration process, critical to restoring the physiological axonal pathway of the disrupted nerve. Recently, the use of additive manufacturing (AM) technologies has offered major advancements to the bioengineering solutions for PNI therapy. These techniques aim at reinstating the native nerve fascicle pathway using biomimetic approaches, thereby augmenting end-organ innervation. AM-based approaches, such as three-dimensional (3D) bioprinting, are capable of biofabricating 3D-engineered nerve graft scaffolds in a patient-specific manner with high precision. Moreover, realistic in vitro models of peripheral nerve tissues that represent the physiologically and functionally relevant environment of human organs could also be developed. However, the technology is still nascent and faces major translational hurdles. In this review, we spotlighted the clinical burden of PNIs and most up-to-date treatment to address nerve gaps. Next, a summarized illustration of the nerve ultrastructure that guides research solutions is discussed. This is followed by a contrast of the existing bioengineering strategies used to repair peripheral nerve discontinuities. In addition, we elaborated on the most recent advances in 3D printing and biofabrication applications in peripheral nerve modeling and engineering. Finally, the major challenges that limit the evolution of the field along with their possible solutions are also critically analyzed.

Medial Sural Perforator “Nerve through Flap”: Anatomical Study and Clinical Application

Background: Nerve recovery after a complex trauma is affected by many factors and a poorly vascularized bed is often the cause of failure and perineural scar. Many techniques have been devised to avoid this problem and the possibility to transfer a nerve with a surrounding viable sliding tissue could help in this purpose; Methods: We performed an anatomic study on 8 injected specimens to investigate the possibility to raise a medial sural artery perforator (MSAP) flap including the sural nerve within its vascularized sheath; Results: In anatomic specimens, a visible direct nerve vascularization was present in 57% of legs (8 out of 14). In 43% a vascular network was visible in the fascia layer. There were no vascular anomalies. In one patient the MSAP flap was raised including the sural nerve with its proximal tibial and peroneal components within the deep sheath. The tibial and peroneal component of the sural nerve were anastomized independently with the common digital nerve of 4th and 5th fingers and with the collateral nerve for the ulnar aspect of the 5th. After 9 months, the patient showed an improving nerve function both clinically and electromyographically without any problem due to nerve adherence; Conclusions: Given the still debated advantage of a vascularized nerve graft versus a non-vascularized one, this flap could be useful in those cases of composite wounds with nerve lesions acting as a “nerve through flap”, in order to reduce nerve adherence with a viable surrounding gliding tissue.

Machine intelligence for nerve conduit design and production

Nerve guidance conduits (NGCs) have emerged from recent advances within tissue engineering as a promising alternative to autografts for peripheral nerve repair. NGCs are tubular structures with engineered biomaterials, which guide axonal regeneration from the injured proximal nerve to the distal stump. NGC design can synergistically combine multiple properties to enhance proliferation of stem and neuronal cells, improve nerve migration, attenuate inflammation and reduce scar tissue formation. The aim of most laboratories fabricating NGCs is the development of an automated process that incorporates patient-specific features and complex tissue blueprints (e.g. neurovascular conduit) that serve as the basis for more complicated muscular and skin grafts. One of the major limitations for tissue engineering is lack of guidance for generating tissue blueprints and the absence of streamlined manufacturing processes. With the rapid expansion of machine intelligence, high dimensional image analysis, and computational scaffold design, optimized tissue templates for 3D bioprinting (3DBP) are feasible. In this review, we examine the translational challenges to peripheral nerve regeneration and where machine intelligence can innovate bottlenecks in neural tissue engineering.

The role of vascularization in nerve regeneration of nerve graft

Vascularization is an important factor in nerve graft survival and function. The specific molecular regulations and patterns of angiogenesis following peripheral nerve injury are in a broad complex of pathways. This review aims to summarize current knowledge on the role of vascularization in nerve regeneration, including the key regulation molecules, and mechanisms and patterns of revascularization after nerve injury. Angiogenesis, the maturation of pre-existing vessels into new areas, is stimulated through angiogenic factors such as vascular endothelial growth factor and precedes the repair of damaged nerves. Vascular endothelial growth factor administration to nerves has demonstrated to increase revascularization after injury in basic science research. In the clinical setting, vascularized nerve grafts could be used in the reconstruction of large segmental peripheral nerve injuries. Vascularized nerve grafts are postulated to accelerate revascularization and enhance nerve regeneration by providing an optimal nutritional environment, especially in scarred beds, and decrease fibroblast infiltration. This could improve functional recovery after nerve grafting, however, conclusive evidence of the superiority of vascularized nerve grafts is lacking in human studies. A well-designed randomized controlled trial comparing vascularized nerve grafts to non-vascularized nerve grafts involving patients with similar injuries, nerve graft repair and follow-up times is necessary to demonstrate the efficacy of vascularized nerve grafts. Due to technical challenges, composite transfer of a nerve graft along with its adipose tissue has been proposed to provide a healthy tissue bed. Basic science research has shown that a vascularized fascial flap containing adipose tissue and a vascular bundle improves revascularization through excreted angiogenic factors, provided by the stem cells in the adipose tissue as well as by the blood supply and environmental support. While it was previously believed that revascularization occurred from both nerve ends, recent studies propose that revascularization occurs primarily from the proximal nerve coaptation. Fascial flaps or vascularized nerve grafts have limited applicability and future directions could lead towards off-the-shelf alternatives to autografting, such as biodegradable nerve scaffolds which include capillary-like networks to enable vascularization and avoid graft necrosis and ischemia.

The Surgical Management of Nerve Gaps: Present and Future

Peripheral nerve injuries can result in significant morbidity, including motor and/or sensory loss, which can affect significantly the life of the patient. Nowadays, the gold standard for the treatment of nerve section is end-to-end neurorrhaphy. Unfortunately, in some cases, there is segmental loss of the nerve trunk. Nerve mobilization allows primary repair of the sectioned nerve by end-to-end neurorrhaphy if the gap is less than 1 cm. When the nerve gap exceeds 1 cm, autologous nerve grafting is the gold standard of treatment. To overcome the limited availability and the donor site morbidity, other techniques have been used: vascularized nerve grafts, cellular and acellular allografts, nerve conduits, nerve transfers, and end-to-side neurorrhaphy. The purpose of this review is to present an overview of the literature on the applications of these techniques in peripheral nerve repair. Furthermore, preoperative evaluation, timing of repair, and future perspectives are also discussed.

A Nerve Conduit Containing a Vascular Bundle and Implanted With Bone Marrow Stromal Cells and Decellularized Allogenic Nerve Matrix

Cells, scaffolds, growth factors, and vascularity are essential for nerve regeneration. Previously, we reported that the insertion of a vascular bundle and the implantation of bone marrow-derived mesenchymal stem cells (BM-MSCs) into a nerve conduit promoted peripheral nerve regeneration. In this study, the efficacy of nerve conduits containing a vascular bundle, BM-MSCs, and thermally decellularized allogenic nerve matrix (DANM) was investigated using a rat sciatic nerve model with a 20-mm defect. Lewis rats were used as the sciatic nerve model and for the preparation of BM-MSCs, and Dark Agouti rats were used for the preparation of the DANM. The revascularization and the immunogenicity of the DANM were investigated histologically. The regeneration of nerves through nerve conduits containing vessels, BM-MSCs, and DANM (VBD group) was evaluated based on electrophysiological, morphometric, and reinnervated muscle weight measurements and compared with that of vessel-containing conduits that were implanted with BM-MSCs (VB group). The DANM that was implanted into vessel-containing tubes (VCTs) was revascularized by neovascular vessels that originated from the inserted vascular bundle 5-7 days after surgery. The number of CD8+ cells found in the DANM in the VCT was significantly smaller than that detected in the untreated allogenic nerve segment. The regenerated nerve in the VBD group was significantly superior to that in the VB group with regard to the amplitude of the compound muscle action potential detected in the pedal adductor muscle; the number, diameter, and myelin thickness of the myelinated axons; and the tibialis anterior muscle weight at 12 and 24 weeks. The additional implantation of the DANM into the BM-MSC-implanted VCT optimized the axonal regeneration through the conduit. Nerve conduits constructed with vascularity, cells, and scaffolds could be an effective strategy for the treatment of peripheral nerve injuries with significant segmental defects.

Vascularized versus nonvascularized facial nerve grafts using a new rabbit model

BACKGROUND

The use of vascularized nerve graft models has been limited because of the complexity of the operation. The authors sought to develop a simple and effective rabbit model for facial nerve repair and evaluated its advantages over conventional nerve grafts.

METHODS

Rabbits were divided into three groups consisting of six rabbits each. The central auricular nerve and its nutrient vessels were used as a vascularized graft. Rabbits were grafted with a vascularized facial nerve graft (vascularized nerve graft group), with a free nerve graft (free nerve graft group), or with a vascularized nerve graft and a free nerve graft on each side of the face (vascularized nerve graft/free nerve graft group). Four months after surgery, facial performance and electrophysiologic monitoring were evaluated. The rabbits were then killed to prepare the nerve specimens for histologic, immunohistochemical, and transmission electron microscope study.

RESULTS

At 4 months after the facial nerve repair, the functional recovery of the facial nerve was observed and analyzed. The side grafted with vascularized nerve graft was superior to the side grafted with free nerve graft. Regenerated nerve fibers were observed in all groups, and rabbits grafted with vascularized nerve grafts had more regenerated axons than those that underwent free nerve grafting, although the regenerated nerves were not as good as the natural nerves.

CONCLUSIONS

This study demonstrates that it is feasible to establish a vascularized nerve graft model in rabbits. The model offers the obvious advantages of operability and reliability. The vascularized nerve graft is demonstrated to have a superior value for facial nerve repair.

The vascularized sural nerve graft based on a peroneal artery perforator for reconstruction of the inferior alveolar nerve defect

The sural nerve has been described for nerve reconstruction of the maxillofacial region since it provides many advantages. We report a case of a vascularized sural nerve graft based on a peroneal artery perforator for immediate reconstruction after the removal of intraosseous neuroma originating in the inferior alveolar nerve. The patient had a neuroma caused by iatrogenic injury to the inferior alveolar nerve. A 4-cm long neuroma existed in the inferior alveolar nerve and was resected. A peroneal perforator was chosen as the pedicle of the vascularized sural nerve graft for the nerve gap. The graft including the skin paddle for monitoring the perfusion supplied by this perforator was transferred to the lesion. The nerve gap between the two stumps of the inferior alveolar nerve was repaired using the 6-cm long vascularized sural nerve. The perforator of the peroneal artery was anastomosed to the branch of the facial artery in a perforator-to-perforator fashion. There was no need to sacrifice any main arteries. The skin paddle with 1 cm × 3 cm in size was inset into the incised medial neck. Perceptual function tests with a Semmes-Weinstein pressure esthesiometer and two-point discrimination in the lower lip and chin at 10 months after surgery showed recovery almost to the level of the normal side. This free vascularized sural nerve graft based on a peroneal artery perforator may be a good alternative for reconstruction of inferior alveolar nerve defects.

Timing of applying electrical stimulation is an important factor deciding the success rate and maturity of regenerating rat sciatic nerves

BACKGROUND

The timing of electrical stimulation (ES) after peripheral nerve transection may enhance axonal regeneration and functional recovery.

OBJECTIVE

The authors examined whether percutaneous ES at 1 mA and 2 Hz affects regeneration between the proximal and distal nerve stumps.

METHODS

Four groups of adult rats were subjected to sciatic nerve section followed by repair using silicone rubber conduits across a 10-mm gap. All groups received ES for 15 minutes every other day for 2 weeks. Stimulation was initiated on day 1 following the nerve repair for group A, day 8 for group B, and day 15 for group C. The control group D received no ES.

RESULTS

At 6 weeks after surgery in groups B and C, histological evaluations showed a significantly higher number of regenerated myelinated fibers in the sciatic nerve, and the electrophysiological results showed higher levels of reinnervation with relatively larger mean values of amplitudes, durations, and areas of compound muscle action potentials compared with A and D.

CONCLUSION

A short delay in the onset of ES may improve the recovery of a severe peripheral nerve injury, which should be considered as a way of augmenting rehabilitative approaches.

Reconstruction of accessory nerve defects with sternocleidomastoid muscle—great auricular nerve flap

We introduced a new method for reconstruction of accessory nerve defects by using the sternocleidomastoid muscle (SCM)-great auricular nerve flap. Thirty-four patients receiving traditional radical neck dissection were divided into two groups: the accessory nerve not-reconstructed group (Group A, N = 19) and the accessory nerve reconstructed group (Group B, n = 15). The surgical procedure of the reconstruction is described in detail. Postoperative shoulder functions were compared between the two groups. We found that Group B experienced much better shoulder function recovery than Group A. Both groups had good wound healing. It is concluded that reconstruction of accessory nerve defects with SCM-great auricular nerve flap is simple, effective and complication-free and worthwhile.

Turnover epineural sheath tube in primary repair of peripheral nerves

The epineural repair technique, which is the gold standard of peripheral nerve injuries, is still far from being ideal. The purpose of this study was to investigate the effects of the turnover epineural sheath tube (TEST) when used over the primary nerve repair site to improve nerve regeneration. Twenty-five Wistar rats were divided into three groups and were operated. In the sham control group, the sciatic nerve was dissected from the sciatic notch to its bifurcation and was left intact. In the primary epineural repair group an incision was made on the nerve and it was repaired using six epineural sutures. In the TEST group, after the incision was made the nerve ends were approximated with two epineural sutures. A proximal circular epineural incision was then made to enable the epineurium to be turned and slid over the repair site. Functional recovery was evaluated by walking tract analysis, and the sciatic functional index was calculated. Histomorphometric studies of the sciatic nerves and gastrocnemius muscles were also performed 3 months postoperatively. Three months postoperatively, functional analysis and nerve and muscle histomorphometric studies revealed similar results in the primary repair and TEST groups. There was no significant difference (p > 0.05) between the results of the TEST and the primary nerve repair groups. However, during the microscopic examination, a decrease in both foreign material reaction and an inflammatory response with less fibrosis were observed in the TEST group. The TEST has a nerve-healing property similar to primary epineural repair, with the advantage of a reduced number of sutures, which decreases the fibrosis around the repair site. The TEST is an alternative treatment modality among other techniques, especially for polyfascicular peripheral nerves.

Vascular endothelial growth factor promotion of neoangiogenesis in conventional nerve grafts

The effect of vascular endothelial growth factor (VEGF) on angiogenesis and neovascularization of conventional nerve grafts was evaluated in 48 rabbits. A 2.5-cm segment of right sciatic nerve was removed and orthotopically repaired. This graft was wrapped in dialysis tubing to prevent vessel ingrowth from adjacent tissue. An osmotic pump delivered either VEGF (100 ng/h for 3 days) or control solution. Evaluation methods included angiography, vessel density, and nerve blood flow measurement at 3, 7, and 14 days. On day 3, 42% of the control nerves and 100% of VEGF-treated nerves had partial longitudinal neovascularization. Vessel density (0.84 +/- 0.22 vessel/mm(2)) and nerve blood flow [25.34 +/- 7.62 mL/(min.100 g)] in VEGF-treated nerves were significantly higher than control group values [0.23 +/- 0.13 vessel/mm(2) and 5.35 +/- 0.99 mL/(min.100 g)]. Progressive improvement in parameters was seen at 7 and 14 days. Vascular endothelial growth factor infusion accelerates longitudinal neoangiogenesis and shortens the nerve ischemic time to 3 days in this model. The revascularization of VEGF-treated conventional nerve grafts in a poorly vascularized bed is identical to that of grafts in a healthy bed.

Effects of percutaneous electrical stimulation on peripheral nerve regeneration using silicone rubber chambers

The purpose of this study was to determine whether 0.8-1 mA, 2 Hz of percutaneous electrical stimulation could affect the regeneration of a 10-mm gap of rat sciatic nerve created between the proximal and distal nerve stumps, which were sutured into silicone rubber tubes. Six weeks after implantation, though the group receiving the electrical stimulation had a lower success percentage of regeneration (57%) compared with the controls receiving no stimulation (70%), quantitative histology of the successfully regenerated nerves revealed that the mean values of the axon density, blood vessel number, blood vessel area, and percentage of blood vessel area in total nerve area in the group with the electrical stimulation were all significantly larger than those in the controls (p < 0.05). These results showed that the electrical stimulation could elicit rehabilitating effects on the regenerated nerves.

Peripheral nerve regeneration using silicone rubber chambers filled with collagen, laminin and fibronectin

A 10 mm gap of rat sciatic nerve was created between the proximal and distal nerve stumps, which were sutured into silicone rubber tubes filled with an extracellular gel containing collagen, laminin and fibronectin. Empty silicone rubber tubes were used as controls. Six weeks after implantation, all extracellular elements were completely degraded and absorbed, and 90% of the animals from the extracellular gel group exhibited regeneration across the nerve gaps, whereas only 60% in the control group. Both qualitative and quantitative histology of the regenerated nerves revealed a more mature ultrastructural organization with 28% larger cross-sectional area and 28% higher number of myelinated axons in the extracellular gel group than the controls. These results showed that the gel mixture of collagen, laminin and fibronectin could offer a suitable growth medium for the regeneration of axons.

Vascularized transplantation of the long thoracic nerve for sensory reinnervation of the lower lip

Microsurgical techniques have improved functional and morphological reconstruction of the face in recent years. An important factor is the re-establishment of neuronal function. The aim of this study was a follow-up of the regeneration of sensation in the inferior alveolar nerve after partial resection of a tumour and reconstruction with a vascularized long thoracic nerve graft. Five patients were examined in monthly intervals to assess the degree of re-establishment of sensation. Pressure and pain responses were elicited as early as three months postoperatively, sense of touch and vibration were found after five months, and sensitivity to temperature after seven months postoperatively. In four patients nine months postoperatively, sensory qualities in the region of the mental nerve were identical on both sides. The vascularized long thoracic nerve is therefore an adequate nerve graft for covering defects as a result of resection of the inferior alveolar nerve patients with tumours.