Leopold Harnoncourt ¹ Martin Schmoll ² Christopher Festin ¹ Laurenz Pflaum ¹ Markus Breuss ² Johanna Klepetko ¹ Dominik C Dotzauer ¹ Florian J Jaklin ¹ Udo Maierhofer ¹ Philipp Tratnig-Frankl ¹ Oskar C Aszmann ^1,3*

• ¹ Clinical Laboratory for Bionic Extremity Reconstruction, Department of Plastic, Reconstructive and Aesthetic Surgery, Medical University of Vienna, Vienna, Austria
• ² Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Vienna, Austria
• ³ Department of Plastic, Reconstructive and Aesthetic Surgery, Medical University of Vienna, Vienna, Austria

Peripheral nerve injuries often only result in incomplete recovery, particularly after proximal lesions due to extended reinnervation time and thus reduction in regeneration-supportive factors and muscle recovery potential. In these cases, supercharge end-to-side (SETS) transfers preserve the original nerve's continuity, while facilitating additional axonal support thus mitigating muscle atrophy. This approach aims to enhance functional recovery and has been demonstrated to be effective in both preclinical models and clinical settings. In this study a novel SETS nerve transfer model in the upper extremity of the rat is presented to assess the impact on muscle function, innervation ratio and motor neuron regeneration, investigating its potential to enhance recovery of motor function. Surgical interventions included transection and end-to-end repair of the musculocutaneous nerve (MCN) in Group A, transferring the ulnar nerve (UN) to the side of the MCN in Group B and a combination of both in Group C. The biceps muscle's function was assessed twelve weeks post-surgery using electrical stimulation, revealing no significant differences in force between the experimental groups. UN related muscle reinnervation was only observed in Group C after transfer to a regenerating nerve. Retrograde labeling demonstrated motor neuron regeneration of both the MCN and UN in a distal direction towards the muscle, however, tracer uptake of the UN motor neurons following intramuscular tracer application was only detected in Group C. In contrast, stained pseudounipolar cells in the dorsal root ganglia (DRG) associated with the UN and MCN revealed afferent muscle innervation in Groups B and C. This novel SETS nerve transfer model enables isolated electrophysiological as well as histological evaluation of all nerve sections and thus exact determination of muscle innervation ratio. Our findings indicate that substantial functional efferent muscle innervation by the donor nerve is found exclusively in a regenerating environment.