A novel contralateral ulnar nerve transfer model for selective muscle reinnervation in upper motor neuron syndrome

Journal article, 2026

Stroke and traumatic brain injury lead to upper motor neuron syndrome, which is characterized by muscle spasticity or paresis of varying severity depending on the lesion's location and extent. Current treatments are mostly symptomatic with limited efficacy and significant side effects. Nerve transfer techniques, such as the contralateral L4 ventral root transfer in animal models and C7 root transfer in both animal and clinical studies, have been shown to reduce spasticity and improve function in upper motor neuron syndrome; however, they lack selectivity. Our hypothesis is that using a selective peripheral donor nerve from the contralateral side, rather than the entire nerve root, may represent an effective nerve transfer and provide a robust basis for future research on selective muscle reinnervation in upper motor neuron syndrome. Ten rats underwent a contralateral ulnar-to-ulnar nerve transfer procedure. Electrophysiological measurements were conducted twelve weeks post-surgery to assess successful reinnervation of the contralateral flexor carpi ulnaris muscle. Additionally, muscle biopsies of the reinnervated flexor carpi ulnaris were harvested to examine the muscle fiber type composition, cross-sectional area, and collagen content as well as compare them to naive counterparts. Axon quantification of the reinnervated nerves was also performed. All rats recovered uneventfully, maintaining the use of both paws post-surgery. Electrophysiological tests confirmed the successful reinnervation of the flexor carpi ulnaris muscle. Muscle fiber type composition, cross-sectional area, and collagen content did not show statistically significant changes. Axon counts indicated successful nerve regeneration without architectural disruption. In conclusion, we were able to demonstrate this novel contralateral nerve transfer model's feasibility, reproducibility, and safety as well as achieve effective muscle reinnervation. This model provides a valuable tool for further research on selective muscle reinnervation and treatment of upper motor neuron syndrome, with potential implications for improving clinical outcomes in stroke and traumatic brain injury patients.