Morphology after synergistic terminal end-to-side nerve graft repair: Investigation in a nonhuman primate model

Abstract

BACKGROUND: Functional efficacy of synergistic terminal end-to-side nerve graft repair in a median nerve defect model in nonhuman primates was recently published elsewhere. The purpose of this study was to investigate the morphology of the end-to-side repair site and the nerve graft used after synergistic terminal end-to-side nerve graft repair. METHODS: Experiments were carried out in seven adult baboons. Synergistic end-to-side nerve graft repair bridging from the terminal motor branch of deep branch of the ulnar nerve to the thenar motor branch of the median nerve was performed after transection of the median nerve at forearm level. Specimens were harvested three months after surgery. Chromotrope aniline blue was used for staining neural collagenic connective tissue. S100-antibody was used to quantify the number of Schwann cells. Axon counts were performed using NF (neurofilament) antibody staining. To enable profound morphological assessment, specimens were harvested 5 mm proximal to the end-to-side nerve graft repair site (deep branch of ulnar nerve), at the end-to-side nerve graft repair site (proximal graft), and at the distal end-to-end nerve graft suture site. The contralateral terminal deep branch of ulnar nerve served as a control. RESULTS: The area of collagenic connective tissue was 6% larger at the end-to-side repair site and 28% larger at the distal end of the nerve graft. Highest mean values were found at the distal site of the nerve graft. This was due to very strong collagenization of the nerve graft of animal number six. In other words, there was a moderate enhancement of collagenic connective tissue within the nerve graft in six animals. Schwann cell counts showed similar values within all nerve segments. There was no difference when compared to nonoperated control. The number of axons distal to the end-to-side repair site was smaller by 19% on average. Additionally, we found an average 23% decrease of axons at the end-to-end coaptation site compared to data of the deep branch of ulnar nerve. CONCLUSIONS: The results in this morphological study of synergistic end-to-side nerve graft repair at the level of very peripheral terminal motor branches in a nonhuman primate model demonstrate effective distal axonal sprouting and successful Schwann cell survival. As expected, we found a modest increase of neural collagenic connective tissue within the nerve graft in most animals (6 out of 7 animals).