Intraoperative FCU CMAP Amplitude During Oberlin Nerve Transfer: Association with Reinnervation Timing and Functional Outcomes

Background/Objectives: Selective transfer of an ulnar nerve fascicle to the motor branch of the musculocutaneous nerve (Oberlin technique) is widely used to restore elbow flexion following upper brachial plexus injury. Intraoperative neurophysiological mapping allows quantitative recording of compound muscle action potentials (CMAPs) during donor fascicle selection; however, its prognostic relevance remains unclear. This study evaluated whether intraoperative flexor carpi ulnaris (FCU) CMAP amplitude is associated with time to electromyographic reinnervation of the biceps brachii and with final functional outcomes. Methods: A retrospective observational study was conducted including patients who underwent selective nerve transfer to the biceps brachii between 2006 and 2025 at two tertiary referral centers. Donor fascicles were selected using intraoperative neurophysiological mapping with quantitative CMAP recordings from three ulnar-innervated muscles. Primary outcomes were time to electromyographic evidence of reinnervation and final elbow flexion strength assessed using the British Medical Research Council grading system. Associations were analyzed using nonparametric statistical methods. Results: Twenty patients met the inclusion criteria. Higher intraoperative FCU CMAP amplitudes were associated with a shorter time to electromyographic reinnervation (Spearman ρ = −0.572, p = 0.0106). No association was observed between CMAP amplitude and final elbow flexion strength (Spearman ρ = −0.168, p = 0.479), or between time to reinnervation and final functional outcome (Spearman ρ = −0.276, p = 0.253). A positive association was found between the injury-to-surgery interval and intraoperative CMAP amplitude (Spearman ρ = 0.681, p = 0.000943). Conclusions: The intraoperative FCU CMAP amplitude facilitates objective donor fascicle selection and is associated with earlier electromyographic reinnervation. Nevertheless, it was not associated with final elbow flexion strength in this cohort and should be interpreted as a technical adjunct rather than a standalone prognostic indicator. Functional recovery following nerve transfer appears to reflect multifactorial biological and temporal determinants beyond a single intraoperative neurophysiological measurement. These findings should be interpreted cautiously given the limited sample size.