Advances in Small Gap Sleeve Bridging Peripheral Nerve Injury

Abstract:

Nerve regeneration and re-innervation are usually difficult after peripheral nerve injury. Epineurium neurorrhaphy to recover the nerve continuity was the traditional choice of peripheral nerve mutilation without nerve defects, whereas the functional recovery was not quite satisfactory. In this article, the authors review the literature focused on peripheral nerve injury research and possible clinical application, including introducing peripheral nerve selective regeneration theory, small gap sleeve bridging nerve methodepineurium neurorrhaphy, kinds of biological conduit, and microenvironment research between nerve stumps.

INTRODUCTION

Different from the central nerve system, the peripheral nerve has a strong potential for regeneration; when an appropriate microenvironment is provided, the regenerating axons extend their processes into the distal Bungner bands to achieve regeneration and rehabilitate normal function [1]. Epineurium neurorrhaphy to recover nerve continuity was the traditional choice of peripheral nerve mutilation without sufficient nerve defects, whereas this technique is put out of its power to ensure exact coaptation of the millions of nerve ends. Therefore dissatisfactory rehabilitation after the traditional neurorrhaphy is predictable. Owing to the nerve selective regeneration theory, which has been repeated and confirmed, a novel neurorrhaphy method was advanced to provide an appropriate gap between the distal and proximal severed stumps to facilitate nerve spontaneous selection when it regenerates across this space. This small gap sleeve bridging peripheral nerve method has obtained good effects in an animal experiment study, and might take the place of epineurium neurorrhaphy to be the recommended method to repair peripheral nerve injury [2].

PERIPHERAL NERVE SELECTIVE REGENERATION THEORY

The sensory and motor tracts will regenerate across a gap toward the distal stumps and selectively find their counter-parts, which are the corresponding tracts, if an appropriate space is provided between the distal and proximal severed mixed nerve stumps, known as peripheral nerve selective regeneration theory. Forssman postulated the existence of such a directive influence and Chiu interpreted it as chemotactic in nature [3]. Epineurium neurorrhaphy induces the regeneration by contact, which means enforced coaptation, and always leads to the inapposite nerve fascicles coaptation in individual function, which is the determinant factor of the final effect. Chiu's theory and lots of correlative studies provide a new way to improve the nerve regeneration effect. This theory has been confirmed by repeated studies; sufficient and valuable evidence have been provided by correlative animal experiment studies.

SMALL GAP SLEEVE BRIDGING NERVE METHOD AND EPINEURIUM NEURORRHAPHY

Because of the technical limitations of the epineurium neurorrhaphy in the nerve coaptation, the surgeons found a new neurorrhaphy: small gap sleeve bridging nerve method, based on nerve selective regeneration theory, which has been confirmed repeatedly. The experiment to compare small gap sleeve bridging with epineurium neurorrhaphy was reported by Jiang in 1994, in which the common peroneal nerves of rats were cut and repaired by either epineurium neurorrhaphy in situ or epineurium neurorrhaphy with distal stump turning 180°, and the parallel approaches were operated by small gap artery conduit bridging (with 3mm gap between the two stumps) simultaneously. Electrophysiology examination and histological observation were carried out at postoperation 12^th week. There were no statistical differences among the small gap artery conduit bridging nerve in situ group, small gap artery conduit bridging nerve with distal stump turning 180° group, and epineurium neurorrhaphy in situ group, while the recovery in the epineurium neurorrhaphy with distal stump turning 180° group was worse than others. The recovery speed is approximately equal between small gap artery conduit bridging nerve and epineurium neurorrhaphy in the histological observation [5, 6].

The process of nerve regeneration across a long gap occurs in 4 phases:

1) The hematoma phase, from several hours to 1 day after nerve cutoff; the lumen of the vein conduit is filled with liquid secreted by ruptured stumps, which provide nutrition and support for the regenerating axons.

2) Cellular migration phase; fibroblasts migrate into the vital space of the vessel lumen from the proximal and distal nerve stumps to construct the fibroblastic matrix bridging the severed nerve stumps. This process lasts about 1 week, then Schwann cells migrate into this gap from both stumps, where the major cellular components are composed of fibroblasts and macrophages.

3) Axonal advancement phase; axonal growth into the conduit lags behind Schwann-cell migration. The proximal axons extend their processes at 2 weeks; a part of the regenerating axons can cover 8∼10 mms at 8 week; at 12 weeks regenerating axons reach 10mms, but can not transcend that distance.

4) Myelination and maturation phase; some nonmy-elinated fibers and myelinated fibers can enter the conduit at 2 weeks, and then Schwann cells migrate to the regenerating axons to form mature nodes of Ranvier when the distal stump is present. Some regenerating axons can touch the opposite stumps to form the myelinated fiber tracts at 4 weeks; a series of maturations are observed in the nerve fibers, without obvious inflammatory or immune reaction at 5 weeks; 86% of cellular components in the conduit are composed of nerve fibers [7, 8].

The process described above shows that the nerves regenerate across a long gap, widely different from the one across a small gap. In that situation 2 weeks is needed for the nerve to regenerate across 2 mms, and once the nerve passes through the gap, it will recover the regenerating speed equal to the one treated by epineurium neurorrhaphy. Furthermore, more regenerating axons can extend toward the distal stumps, thanks to the protection of the conduit. The present study indicates that the diameter and myelination of the regenerating nerve are inferior to the normal ones at 8 weeks; hence the longer observation will be needed to confirm the effect of the small gap sleeve bridging nerve [9].

Compared to epineurium neurorrhaphy, conduit not only provides the space for the nerve to selectively regenerate, but also prevents the regenerating nerve from leaking from the stitch site, which may develop the neuroma. Therefore the small gap sleeve bridging never assures effective regeneration, drives more regenerating nerve fibers into the distal stumps than epineurium neurorrhaphy, and provides a relatively close microenvironment to facilitate regeneration in which the cells are prevented from invasion, neurotrophic factors are enriched for the regenerating nerve, and scars could be avoided. Furthermore, it can provide space for the magnifying effect of nerve fibers of broken peripheral nerves, which may participate in the regeneration [10, 11]. The silica gel canal, biomembrane, epineurium, etc., have been used as conduit in the bridging nerve study, and have all got approximately the same effect in repairing sciatic nerves of rats or rabbits. With the development of detecting and materials, more studies have confirmed that the small gap sleeve bridging never results in better regeneration effect than epineurium neu-rorrhaphy, if there is some turning between both stumps [2]. Since the nerve selective regeneration is actually the neurotropism or chemotaxis effect of the distal nerve stumps, which induces the proximal nerve regeneration, the fact that the concentration of chemotaxis factors decreases with the distance increase limits the effective distance of this phenomena. Moreover, a certain distance is also needed for the nerve to complete its selection, so there must be an appropriate gap range for the small gap bridging. It is generally believed that the suitable gap is 1∼4 mms for the common peroneal nerve of rats, and 2∼5 mms for the sciatic nerve of rats. These data are useful for the studies in this small gap field [12].

AUTOGENEIC TISSUE CONDUITS

Today, the common autogenic tissue conduits include autogenic vein, artery, and epineurium. The vein is unable to construct an effective support for the nerve pass due to its weak vessel wall, which is prone to collapse and embarrass the regeneration; the artery can provide suitable support and elasticity, and is competent for animal model to observe small gap bridging, but can not be widely utilized in clinical application; some scholars have described epineurium as conduits to bridge nerve, among which the epineural sleeve technique is relatively mature, for picking up the free edge of the epineurium at the distal stump gently and rolling it back distally, resecting a nerve segment of the distal nerve stump, then pulling over the epineurium and suture to the proximal counterpart to bridge the nerve. This technique can provide a small gap between both stumps, but does not construct an integrity conduit-bridge, so there is a lack of enough protection for the severed nerve stumps. These methods above all need to harvest autogenic tissue, which can destroy the local blood supply. In particular, the artery harvest is a really tough choice. Therefore the limited resource of au-togeneic tissues confines their possible clinical applicable feasibility.

MANUFACTURED CONDUITS

Silica gel canal is the earliest artificial conduit. It avoided the harvest of autogeneci tissues, resulted in postitive regeneration effect, but was verified inferior to the vein, and need a second operation to take out to avoid chronic nerve compress syndrome or inflammatory reaction [13]. Hence, a biodegradable and bioabsorbable artificial conduit is required. The common biodegradable materials include collagen, chi-tin, PLA, PGA, PLGA, PCL-LA, PCA, etc. Time-controlled biodegeradable material is a good choice. This material can provide support for the nerve at the early term, and degrades to innocuous product after the nerve completes the regeneration. Studies showed de-acetyl chitin had good histocom-patibility and bioabsorbability. The conduits were made of this material had revealed good nerve histocompatibility and manipuility in the small gap bridging animal nerve studies [13].

ADVANCEMENT OF CONDUIT MICROENVIRONMENT

Exogenous Neurotrophic Factor

To improve the nerve regeneration effect, neurotrophic factors were added into the conduits. To add exogenous NGF locally has been adopted in studies. This method seems to be useful in repairing long nerve defect and improve the nerve regeneration effect [14]. In the small gap ones, studies showed that better results had only been observed in the number of the fibers, with no difference in the quality of the regenerated nerve or the motor nerve conduction velocity after the addition of NGF; that is plausible to figure out that NGF can not obviously improve the regeneration effect of the motor nerve [10]. Analogously, to fill the conduit with matrix, improve the configuration of the conduit, or culture Schwann cell in the conduit all resulted in positive effects, but these methods focused on repairing the long nerve defect with the supposition that it can provide the axons nutrition persistently to support the nerves to extend a longer distance [17]. As for the small gap bridging nerve, whether these methods above can also improve the regeneration effect is still uncertain.

APPLICATION OF THE CELLS

Schwann cells that serve several important roles in nerve regeneration can secrete neurotrophic factors and extracellular matrix, also form endoneurial sheaths, express ECM molecules and cell-adhesion molecules to enhance nerve regeneration and synthesis diffusability neurotrophic factors to improve the nerve regeneration, and prevent the neuron from apoptosis. Therefore the composite conduit with Schwann cells becomes the focus of the current study. The bioactivity conduit with Schwann cells or segments of short nerve can lead to a better electrophysiology and histology result, and facilitate the nerve regeneration and function recovery. Some studies about the xenogenic SCs showed that they could improve the nerve regeneration without harmful immunological reactions, which indicated that xenogenic SCs might be utilized to improve the therapeutic effect in clinical application to save the time of culturing autogeneic SCs. Some scholars had reported that SCs could be induced from MSCs,and the MSCs were abundant and could be collected easily. This solved the problem of the SCs source, and made the wide clinical application of SCs possible [15]. Furthermore, small gap bridging nerves of rat using conduits with MSCs resulted in a better effect than using simple conduits [16.

FUTURE OUTLOOK

The relatively concentrated studies have confirmed that it is possible to use conduit small gap sleeve bridging on the base of nerve selective regeneration theory to substitute for the traditional epineurium neurorrhaphy. The conduit not only provides both stumps with support, but also prevents the nerve from leakage, and enriches the neurotrophic factor secreted from the distal stumps, which imposes effects on the proximal stumps to construct a microenvironment beneficial to the regeneration of mixed nerve fibers. The imposing effect from distal stumps is very important for regeneration, but has its influence range. If the gap exceeds 10 mms in rat, the effect of the simple conduit reaches its culmination of the neurotrophic factor diffusibility, and can not impose effects on the proximal stumps for regeneration. As for small gap bridging, the short distance avoids the problem of the neurotrophic factor diffusibility. A series of studies have confirmed that the therapeutic effect of small gap sleeve bridging is better than the epineurium neuror-rhaphy with distal stump turning, and approximately equal to epineurium neurorrhaphy in situ. Since the epineurium neurorrhaphy in situ that is performed in the experimental animal model is impossible in clinics, the results have already provided sufficient evidence for using small gap sleeve bridging as a new neurorrhaphy technique in clinics. Further study to confirm the possibility of using this technique should be performed in primates, following normative clinical studies, to ensure the early clinical application of this convenient and effective technique.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This paper was first published online as an Early Online article on 12 January 2010.