Objective
Our goals were to compare the objective and subjective outcomes of anterior interosseous nerve (AIN) transfer for cases with high ulnar neuropathy, including traumatic cases and chronic compressive neuropathies.

Material and Methods
From June 2015 to January 2021, patients with high ulnar nerve palsy related to trauma (neurotmesis) or severe compression (cubital tunnel syndrome) were recruited prospectively from a single center. All of the traumatic patients received direct ulnar nerve repair with coaptation initially. Surgical treatment with ulnar nerve neurolysis, ulnar tunnel release, cubital tunnel release, ulnar nerve transposition, and AIN transfer to deep motor branch of ulnar nerve were done in all included patients. Patients were followed up at five time points: pre-operation, post-operation 6 months, 12 months, 18 months, and 24 months. Objective outcome measurements, clinical signs, and subjective patient-reported outcomes were collected. Statistical analysis was done with Mann-Whitney U test and generalized estimating equation.

Results
A total of 18 compressive and 22 traumatic patients were included in our study. Patients in the compression group were significantly older. Compressive patients had significantly better two-point discrimination (2PD) in ring fingers, larger grip strength ratio to normal side, higher compound muscle action potential (CMAP) of abductor digiti minimi (ADM) and first dorsal interosseous muscle (FDI), and higher sensory nerve action potential (SNAP) pre-operatively, but they generally had comparable results with traumatic patients during the 24-months-follow-up. No differences in subjective measurements between the two groups were found. In addition, baseline ADM CMAP higher than 1 mV and FDI CMAP higher than 0.5 mV were positive predicting factors for FDI CMAP at 24-month follow-up in all patients. Patients with early nerve transfer (mean 92.0 days) had a worse baseline condition in most objective outcome measurements than patients with late nerve transfer (mean 684.1 days), but they yielded similar results at post-operation 24 months. Notably, those with early nerve transfer had a better result of FDI CMAP during the 24-months-follow-up, and baseline FDI CMAP larger than 0.5 mV was predictive of a better recovery of FDI CMAP and ADM CMAP.

Conclusions
Traumatic patients had a worse initial presentation than compressive patients, but they had similar outcomes during the 24-months-follow-up after AIN transfer. Traumatic patients who received surgery earlier exhibited a worse baseline performance, but they had a comparable outcome with those who received surgery later, and even showed greater FDI CMAP. Finally, a better baseline ADM CMAP and FDI CMAP was predictive of a better recovery of post-operation ADM or FDI CMAP.