Abductor Digiti Minimi (ADM) and Anterior Interosseous Nerve (AIN) to Ulnar Motor Nerve (UMN) transfer: The "Double SETS" transfer
Blair Robert Peters, MD, Department of Surgery/Section of Plastic Surgery, University of Manitoba, Winnipeg, MB, Canada; Department of Surgery/Section of Plastic Surgery, Washington University School of Medicine, St Louis, MO, Lauren Jacobson, MD, Washington University in St Louis, St Louis, MO and Susan E. Mackinnon, MD, Department of Surgery, Division of Plastic and Reconstructive Surgery, Washington University School of Medicine, St. Louis, MO
Introduction: Anterior interosseous nerve (AIN) to ulnar motor nerve (UMN) supercharge end to side (SETS) nerve transfer to restore intrinsic function is a recently adopted nerve transfer in severe ulnar neuropathy. However, its success is predicated on the critical threshold number of axons innervating the intrinsics. Given the relative expendability of the abductor digiti minimi (ADM) and the critical function of the other intrinsics, we have modified our SETS transfer to redirect axons from the ADM to the UMN to innervate the more critical intrinsic muscles, referring to this procedure as the "Double SETS".
Methods: An estimate of the number of comparable functional motor axons in the ADM as compared to the first dorsal interosseous (FDI) and more distal intrinsics can be obtained by the compound muscle action potential amplitudes (CMAP) on electrodiagnostic (EDX) studies. The ulnar nerve is decompressed through the distal forearm and Guyon's canal with specific release of the deep motor branch. Nerve branch(es) to the ADM are identified by retracting the ulnar neuromuscular bundle laterally. The ADM fascicle, when neurolysed is the ulnar most fascicular group of the deep motor branch and can be neurolysed proximally as needed for a tension free repair. The branch is followed as distally as possible for adequate length. It is transposed proximally to the proximal aspect of Guyon's canal and a SETS transfer is performed to the UMN, airing on the radial aspect. This is done in combination with a traditional AIN to UMN SETS more proximally.
Results: In all cases there has been a single isolated motor branch to the ADM entering the muscle in it's proximal third and originating from the deep motor branch. We have performed the "Double SETS" transfer in 8 patients. There has been no increased hypothenar atrophy, or loss of range of motion or strength of small finger abduction.
Conclusions: While current methods of assessment of re-innervation are likely unable to differentiate between contributions from the AIN versus ADM transfer, this technique follows accepted fundamentals of modern nerve surgery where directing the maximum number of nerve fibers in a timely fashion to target is critical for the best functional recovery. Thus, we suggest the Double SETS technique will optimize outcomes and offer this additional transfer to our previously described SETS AIN to UMN transfer for consideration.
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