Early and Late Intrinsic Hand Muscle Reinnervation after End-to-Side AIN to Ulnar Motor Nerve Transfer
Eric C Mitchell, MD, MSc1, Mehran Nejad Mansouri, MD1, Katie Garland, MD, MSc1, Thomas Miller, MD FRCPC1, Joshua A Gillis, MD2 and Douglas C. Ross, MD, MEd, FRCSC3, 1Western University, London, ON, Canada, 2Division of Plastic Surgery, Eastern Health, St John's, NF, Canada, 3Plastic Surgery/ Hand and Upper limb Centre, Division of Plastic Surgery, Roth|McFarlane Hand & Upper Limb Centre/Co-Director, The Peripheral Nerve Clinic, Western University, London, ON, Canada
Introduction: The “supercharge” end-to-side (SETS) anterior interosseous nerve to ulnar-motor nerve (AIN-to-UMN) transfer is a technique used with the intention of improving intrinsic muscle recovery in cases of severe ulnar nerve compression and proximal axonotmetic injuries. Previous work has suggested differing intrinsic muscle recovery after AIN-to-UM transfer. The objective of this study was to examine the clinical and electrodiagnostic patterns of recovery in first dorsal interossei (FDI) and abductor digiti minimi (ADM), both in terms of timing and overall extent of recovery, and the potential impact of AIN transfer to a specific fascicular location on the ulnar motor nerve.
Materials & Methods: A retrospective review of one peripheral nerve and hand fellowship-trained surgeon’s consecutive patients at a single centre from December 2019 to September 2021 was conducted. All patients who had an AIN-to-UMN transfer performed for any indication were included. Patients were excluded if they had less than five months of follow-up. Specific fascicular targeting during AIN-to-UMN transfer was either volar-ulnar, direct ulnar or radial, based on the clinical presentation. Demographic, surgical data, and pre- to post-operative changes in clinical data were evaluated using statistical analysis.
Results: Twenty-four patients underwent an AIN-to-UMN transfer during the study period. Twenty patients met inclusion criteria. Average age at surgery was 55.3±14.1 years and 80% were male. At early follow-up (mean 6.8±1.0 months) there were no significant differences in CMAP amplitudes for either ADM (p=0.359) or FDI (p=0.799). At late follow-up (mean 17.3±3.6 months), compound muscle action potential (CMAP) amplitude for ADM significantly increased from pre-op (p<0.05), while FDI amplitude did not. The proportion of patients with functional ADM strength (BMRC≥3) did not significantly increase pre-op to post-op at either early (p=1.00) or late (p=0.125) follow-up. The proportion of patients with BMRC≥3 FDI strength significantly increased from pre-op to late follow-up (p<0.05), but not early follow-up (p=0.625). In the sub-group of patients with targeted AIN insertion in the volar-ulnar position of the motor nerve (n=11), average BMRC significantly increased from pre-op to early follow-up for FDI (p<0.05), but not ADM. Pre-op to late follow-up average BMRC significantly increased for both ADM (p<0.05) and FDI (p<0.05).
Conclusions: We conclude that the end-to-side AIN-to-UMN transfer demonstrates clinical and electrophysiologic evidence of intrinsic muscle recovery and reinnervation, with differing recovery in outcomes. Volar-ulnar location of coaptation during transfer may target FDI more, based on established ulnar-motor nerve topography and the results of this study.
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