Neurophysiological Markers of Functional Recovery in Anterior Interosseous Nerve Transfers

Introduction: Nerve transfers to the anterior interosseous nerve (AIN) are a promising intervention for restoring finger flexion in tetraplegic patients. However, the relationship between preoperative donor nerve neurophysiology, and long-term functional outcomes remains poorly understood. This study aims to investigate whether preoperative motor unit counts (MUCs) of the donor correlate with recipient nerve reinnervation and whether neurophysiological markers can serve as reliable predictors of finger flexion recovery.

Methods: Electromyography (EMG) was conducted preoperatively on donor muscles to determine the robustness of axonal innervation. At various postoperative intervals, recipient muscles of target nerves were similarly tested for their MUC. Additionally, finger motor function was assessed using Medical Research Council (MRC) grading, along with maximum active range of motion (AROM) for finger flexion. AROM angles were categorized into ranges: 0°, 1–15°, 16–30°, 31–45°, and >45°. Functional recovery was considered satisfactory if MRC was ≥3 and AROM reached 16–30°.

Results: 11 patients underwent AIN transfers with brachialis-graft-AIN done in 6 patients, extensor carpi radialis brevis (ECRB) nerve branch-AIN done in 3, and pronator teres-AIN done in 2. All patients had ≥3.5 preoperative donor MUCs, as measured through the ECRB, PT, or brachialis (average pre-operative donor MUC across all patients was 5.5+0.8). The average 6- and 18-month AIN (recipient) MUC was 0.9+1.3 and 3.0+1.0, respectively and 8 patients (73%) reached satisfactory functional outcomes at 18-months. Of these 8 patients, 6 (75%) demonstrated ≥3 MUCs on 18-month FPL EMG testing. In 3 patients that did not reach satisfactory functional recovery, 2 (67%) were brach-AIN transfers and 0% reached ≥3 FPL MUCs at 18-months. The average pre-operative donor nerve MUC was 4.8 and 5.8 in patients that reached satisfactory vs. unsatisfactory functional recovery outcomes, respectively.

Conclusion : This study demonstrates that neurophysiological recovery strongly correlates with functional outcomes following AIN transfers. There was also an association between patients reaching ≥3 recipient muscle MUCs and those that achieved satisfactory functional outcomes by 18-months. Lastly, there seems to be a correlation between long-term functional recovery outcomes and preoperative donor nerve MUCs.