From Pre-op to Post-op: How Neurophysiology Predicts Supinator to Posterior Interosseous Nerve Transfer Success

Introduction: Nerve transfers (NT) are increasingly employed to restore function in patients with spinal cord or peripheral nerve injury. Despite advances in the field, there is limited understanding of the relationship between preoperative donor nerve neurophysiology, and long-term functional outcomes. This study aims to investigate whether preoperative donor nerve motor unit counts (MUC) correlate with recipient nerve reinnervation and whether neurophysiological markers can serve as reliable predictors of finger extension recovery following supinator to posterior interosseous nerve (PIN) transfers (SUP-PIN).

Methods: Electromyography (EMG) was conducted preoperatively on the supinator muscle to determine the robustness of donor nerve axonal innervation. At various postoperative intervals, recipient muscles of target nerves (PIN) were similarly tested for their MUC, and finger motor function was assessed using Medical Research Council (MRC) grading, along with maximum active range of motion (AROM) for finger extension. 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: 9 patients undergoing SUP-PIN were included. All patients had preoperative supinator MUCs >4 (average supinator MUC was 5.2+0.9). The average 6- and 18-month PIN MUC was 2.3+1.3 and 5.2+0.6, respectively with 8 (88.9%) patients reaching satisfactory functional recovery by 18-months. Of these 8 patients, all (100%) reached ≥4.5 MUCs on 18-month PIN EMG testing. Interestingly, the only patient that failed to reach satisfactory finger extension function also had the lowest preoperative supinator (donor) MUC (4), compared to the rest of the patients who reached good outcomes (5.8).

Conclusion : Most patients achieved strong axonal recovery and good functional outcomes by 18-months, suggesting that reaching ≥3 MUCs is associated with successful reinnervation. Additionally, there seems to be a correlation between long-term functional recovery outcomes and preoperative donor nerve MUCs given that the only patient that failed to reach satisfactory motor function outcomes had the lowest pre-operative donor nerve MUC. Future studies should explore these neurophysiological thresholds to optimize surgical outcomes following NTs.