Chitosan/polyglycolic acid nerve grafts for axon regeneration from prolonged axotomized neurons to chronically denervated segments

Abstract

Peripheral nerve regeneration for long-term delayed injuries is usually unsatisfied. Here we attempted to use a chitosan/polyglycolic acid (PGA) artificial nerve graft to bridge a long-term delayed 10-mm defect in SD rats based on the previous studies on the graft used for immediate repair of 30-mm-long dog sciatic nerve defects and for clinical treatment of a 35-mm-long median nerve defect at elbow of a human patient. In this study, for experimental groups, the rat sciatic nerve had been transected leaving a 10-mm defect, which was maintained for 3 or 6 months before implantation with the chitosan/PGA artificial nerve graft. The animals non-grafted or grafted with autograft served as negative or positive control group. In experiment groups, nerve regeneration with functional recovery was achieved as measured by electrophysiological and histological techniques, although differences in the quantity and the quality of the regenerated nerve were observed between the 3- and 6-month delayed subgroups. The results showed that: (1) a few denervated Schwann cells survived and sustained their ability to myelinate axons at least 6 months, and (2) the atrophic denervated muscle could be reinnervated by regenerated axons through new muscle-nerve connections. These observations provide the possibility of guiding regenerated axons from survived axotomized neurons to distal nerve stump by the chitosan/PGA artificial nerve graft.

Introduction

Compared to the immediate repair of acutely denervated nerves, the capacity of delayed nerve repair to sustain nerve regeneration and functional recovery decrease dramatically as the duration of axotomy and denervation increases in the peripheral nervous system (PNS) [1], [2]. The impaired outcome is generally attributed to prolonged neuron axotomy [3], [4], chronically denervated distal nerve stumps [1], [5] and long-term denervated atrophic muscle target [1], [6]. In orthopedics and neurology, it is generally believed that the long-term delayed repair (at least one year delay) of peripheral nerve injuries cannot be expected to produce satisfactory outcomes [7], [8], [9].

Nevertheless, there has been some disagreement regarding whether it is worth while attempting delayed repair of peripheral nerve injuries. Trail reported a case of a fifteen-year-old child where the delay between divisions and successful repair of the ulnar nerve was nine years [10]. Zook et al. used sural nerve grafts to bridge gaps measuring 2.5–4.5 cm in previously transected but not repaired (the delayed term was 4, 5, and 7 months, respectively) posterior interosseous nerves in three patients [11]. All three patients recovered full function of the involved hand 1 year after grafting. Yin and his colleagues treated 171 cases of delayed peripheral nerve injuries that were thoroughly evaluated by clinical and electrophysiological examinations and 54 out of 115 (47%) major peripheral nerve injuries obtained excellent and good results [12].

Thus the possibility of nerve regeneration after long-term delayed injuries in the PNS is not well understood. It is necessary to develop more basic and clinical experiments to determine whether the delayed repair is effective, especially, when nerve defects occur in the chronic injuries unavoidably.

Delayed peripheral nerve injuries often have nerve defects on account of trauma itself, retraction of nerve ends, trimming scar of distal stump and neuroma of proximal one. When a nerve defect or defect is so long that end-to-end suturing becomes difficult, implantation of a nerve graft is often necessary to bridge the proximal and distal nerve stumps for promoting nerve regeneration if the repair were considered to be triable. Nerve autografting has been the first choice for repairing peripheral nerve defects. However, this recognized ‘gold standard’ technique for peripheral nerve repair has inevitable disadvantages, such as limited supply of available nerve grafts, permanent loss of the donor nerve function and potential differences in tissue structure and size [13]. To overcome these drawbacks, various strategies have been investigated to replace traditional autograft techniques, such as nerve allografts [14], autologous and allogenous organs or tissues [15], [16], [17], many biodegradable [13], [18], [19], [20], [21], [22] or non-biodegradable grafts [23], [24]. In animal experiments, it has been demonstrated that biodegradable artificial nerve grafts may be promising substitutes for autografts. Previously, we reported successful nerve regeneration in dog 30-mm defects of sciatic nerves and a 35-mm-long median nerve defect at elbow of a human patient with a chitosan/polyglycolic acid (PGA) artificial nerve graft [25], [26].

In the present study, we attempted to bridge chronically delayed 10-mm defects with the above-mentioned chitosan/PGA artificial nerve graft to investigate the possibility of nerve regeneration after long-term delayed injuries and evaluated its feasibility for chronic nerve defects by using a variety of histological and electrophysiological techniques.