Misdirection of regenerating motor axons after nerve injury and repair in the rat sciatic nerve model

Abstract

Misdirection of regenerating axons is one of the factors that can explain the poor results often found after nerve injury and repair. In this study, we quantified the degree of misdirection and the effect on recovery of function after different types of nerve injury and repair in the rat sciatic nerve model; crush injury, direct coaptation, and autograft repair. Sequential tracing with retrograde labeling of the peroneal nerve before and 8 weeks after nerve injury and repair was performed to quantify the accuracy of motor axon regeneration. Digital video analysis of ankle motion was used to investigate the recovery of function. In addition, serial compound action potential recordings and nerve and muscle morphometry were performed. In our study, accuracy of motor axon regeneration was found to be limited; only 71% (+/-4.9%) of the peroneal motoneurons were correctly directed 2 months after sciatic crush injury, 42% (+/-4.2%) after direct coaptation, and 25% (+/-6.6%) after autograft repair. Recovery of ankle motion was incomplete after all types of nerve injury and repair and demonstrated a disturbed balance of ankle plantar and dorsiflexion. The number of motoneurons from which axons had regenerated was not significantly different from normal. The number of myelinated axons was significantly increased distal to the site of injury. Misdirection of regenerating motor axons is a major factor in the poor recovery of nerves that innervate different muscles. The results of this study can be used as basis for developing new nerve repair techniques that may improve the accuracy of regeneration.

Fig. 1

A, The rat sciatic nerve model with distal tibial and peroneal nerve branches that respectively innervate the gastrocnemius-soleus and anterior tibial muscles for ankle plantar and dorsiflexion. The proximal site of injury for all types of nerve injury and repair, the distal site of injury for autograft repair, and the site of tracer application are shown. B, Sequential retrograde tracing technique with injection of the first tracer (diamidino yellow [DY]) into the peroneal nerve 1 week before nerve injury and the second tracer (fast blue [FB]) 8 weeks after nerve injury and repair. C, 2D digital video analysis of the ankle angles at midstance (MSt), toe-off (TO), and midswing (MSw), reported in degrees from the neutral position, with plantar flexion being negative and dorsiflexion being positive. (Used with permission of Mayo Foundation for Medical Education and Research).

Fig. 2

Distribution of differently labeled profiles (diamidino yellow [DY], fast blue [FB], and FB–DY) for the number of profiles per longitudinal section taken from medial to lateral through the anterior horn. A, Normal distribution of tibial (blue) and peroneal (yellow) motoneurons after simultaneous tracing, with FB and DY application to the tibial and peroneal nerve branches, respectively. B, Distribution of profiles labeled by sequential tracing 8 weeks after autograft repair. In this case there were no signs of reuptake of persistent DY tracer because no double-labeled profiles were found in the area of the anterior horn normally occupied exclusively by tibial motoneurons (indicated by brackets, compare to A).

Fig. 3

Recovery of ankle angles at toe-off (TO) (A), midswing (MSw) (B), and midstance (MSt) (C) after sciatic nerve crush injury and direct coaptation repair. Results for the recovery of ankle angles after autograft repair are not shown because these were not different from the results after direct coaptation repair except that the results were more variable after autograft repair (larger SD).

Fig. 4

Recovery of compound muscle action potential (CMAP) amplitude (A, B), area (C, D), and latency (E, F) in the plantar (A, C, E) and dorsal foot muscles (B, D, F). White bars, results for normal animals; light gray bars, results after crush injury; dark gray bars, results after direct coaptation repair; black bars, results after autograft repair.

Fig. 5

Size distribution of myelinated fibers (MF) after crush injury (A), direct coaptation (B), autograft repair (C).