gms | German Medical Science

Objectives/Interrogation: Selective nerve transfers (SNTs) have been used broadly for the past decade to treat slow nerve regeneration, neuroma pain and to improve prosthetic control. SNTs change the motor unit extensively by connecting motor neurons to new functional targets. Good outcomes have been reported but little is known of the structural and functional effects. On a basic scientific level even less is known about how much motor input is required in order to achieve sufficient functional regeneration.

Methods: In 10 male Sprague Dawley rats the ulnar nerve (UN) was selectively transferred to the long head of the biceps after neurotomy of the biceps motor branch. In another experimental group (n=10), the deep branch of the ulnar nerve (DBUN), which is even more selective, was used to restore elbow function. These transfers were compared against a sham group (n=10) and a positive control group (n=10), in which the musculocutaneus nerve was crushed. Additionally, the equivalent operations were conducted in 20 Thy1-GFP rats (n=5 each group) to analyze nerve regeneration and neuromuscular junction formation.

Results and Conclusions: In all animals the biceps muscle was successfully reinnervated. In the UN group tetanic muscle force was 1.23 +0.21 mN, compared against 1.12 +0.17 mN in the DBUN group. Retrograde Labelling and MUNE (Motor Unit Number Estimation) correlated well and showed a statistically equivalent amount of regenerated motor neurons (86 +11 after UN transfer and 81 +10 in the DBUN group). Sensory and motory immunhistochemical axon quantification of the UN and its deep branch offered valuable clues on how much donor nerve is needed to restore motor function. However, only the UN transfer resulted in massive neuroma formation close to the coaptation site, which was visualized in Thy1-GFP rats. Muscle fibre typing and neuromuscular junction staining revealed impressive changes on all levels of the motor unit.

Transferring not only a high capacity donor nerve, but also an even more selective fascicular group and exploring the effects on the motor unit provides us with a deeper understanding of the neuronal plasticity of the peripheral nervous system. Tetanic muscle force, MUNE and retrograde labelling indicate that the motor neurons merely of the deep branch of the ulnar nerve supported sufficient regeneration and a lot of the additional input resulted in neuroma formation. Hence, nerve transfers could be performed even more selectively from this point forward.