Time course of histomorphometric alterations in nerve grafts without connection to a muscle target organ: an experimental study in sheep.

In 15 adult sheep, the saphenous nerve (28 +/- 1.8 cm) was used for ipsilateral or for cross-nerve grafting and was sutured to the proximal stump of the cut, motor-nerve branch of the vastus muscle. The distal end of the nerve graft was left without a target organ. Semi-thin cross sections of normal vastus nerves and saphenous grafts and of the distal ends of the grafts were analyzed by computer-assisted planimetry, 3, 6, 9, 12, and 18 months after the nerve-grafting procedure. Electronmicroscopy was also performed on specimens from the distal ends of the nerve grafts. Comparing the total number of myelinated nerve fibers in the distal end of the graft, the ipsilateral group showed an increase with time elapsed since nerve grafting, while the cross-over group showed a maximum after 3 and 6 months, and fewer fibers after longer periods of regeneration. Independent of the time passed since nerve grafting, the diameters of the myelinated nerve fibers were homogeneously thin in both experimental groups. Most interesting, the cross-nerve grafts did more poorly than the ipsilateral ones, even before they were influenced by the target muscle. With ultrastructural investigation, the ends of the grafts containing only a few myelinated fibers also showed a low number of unmyelinated fibers, but an increase of collagen fibers. The results have consequences for the clinical application of cross-nerve grafting.