Reply: Supercharge End-to-Side Anterior Interosseous-to-Ulnar Motor Nerve Transfer Restores Intrinsic Function in Cubital Tunnel Syndrome.

Abstract

We thank the authors for their comments and thoughtful questions in response to our article, “Supercharge End-to-Side Anterior Interosseous–to–Ulnar Motor Nerve Transfer Restores Intrinsic Function in Cubital Tunnel Syndrome.”1 We appreciate the opportunity to expand our discussion of the supercharge end-to-side transfer. Hull et al. noted that it takes, on average, 17 years for just 14 percent of medical research to influence patient care.2 We have recently discussed the challenge of introducing new surgical innovations, highlighting the important field of dissemination and implementation science, which promises to speed up the safe translation of spread and adoption of evidence-based interventions into clinical practice.3 Based on extensive research from our translational nerve research laboratory, we performed the first “supercharge” end-to-side nerve transfer of the terminal branch of the anterior interosseous nerve to the motor component of the ulnar nerve in the distal forearm over 12 years ago4; in the United States, we have yet to reach the “tipping point” of adoption,5,6 although there continue to be reports from early adopters.7–11 In an effort to safely translate this procedure to eligible patients, we have published previously on our technique,4,12 preoperative assessment,13 indications,14 outcomes,1,15,16 and educational17 studies. This operation is not for novice surgeons; it is for experienced hand and nerve surgeons to consider for their patients when preoperative electrodiagnostic studies show (1) a normal donor muscle (pronator quadratus), (2) a recipient nerve that would benefit from transfer (severe ulnar intrinsic axonal loss, evidenced by decreased compound muscle action potential amplitude and decreased motor unit recruitment), and (3) a recipient muscle with sufficient receptive motor endplates available for reinnervation (increased insertional activity, fibrillations and positive sharp waves). Our article on refining indications for the use of this nerve transfer in the setting of cubital tunnel syndrome lays this out well.14 The current article on outcomes after supercharge end-to-side distal anterior interosseous nerve transfer to restore intrinsic function included only patients with isolated cubital tunnel syndrome. Those with concomitant palsies were excluded from this study. Moreover, patients with high median nerve palsies would not be candidates for this nerve transfer because of the presence of an abnormal donor. The authors noted astutely that 21 of 39 patients demonstrated improved muscle function before the 3-month time point. We agree that this cannot be due to reinnervation from the nerve transfer, and timing of improvement is important to consider when interpreting results of a nerve transfer. In our article, we believe that those with functional improvement before 3 months were a result of reversal of ischemia (first month) or remyelination (up to 3 months). We discuss this in detail both in our article (Fig. 2 and Discussion section) and in the article by Davidge et al.15 With regard to concerns about a double crush, it is important to consider what the primary site of compression is, which can be determined with a hierarchical sensory (scratch) collapse test.18 Typically, the cubital tunnel is the primary compression site and the Guyon canal is secondary. There can also be other issues, including increased neural tension secondary to scapular abduction from weak middle trapezius function and supraclavicular lower trunk irritation. However, this is secondary and addressed with physiotherapy in addition to the main procedures that address the cubital tunnel.16,19 In the rare case of neurogenic thoracic outlet syndrome, where the primary compression is proximal to the cubital tunnel, an anterior interosseous nerve–to–ulnar motor nerve transfer may also be indicated in cases of severe axonal loss, but these patients were not included in the current study. In fact, the more proximal the compression leading to axonal loss, the more reason to use this nerve transfer to restore intrinsic function, as there is less hope for regeneration to a distal target. We highly encourage publication and dissemination of additional retrospective and prospective studies with positive outcomes of the supercharge end-to-side anterior interosseous nerve–to–ulnar motor nerve transfer. We continue to refine our educational outreach delimitating the electrodiagnostic workup for when to use this nerve transfer. DISCLOSURE None of the authors has a financial interest to declare in relation to the content of this communication. Jana Dengler, M.D.Division of Plastic and Reconstructive SurgerySunnybrook Health Sciences CentreUniversity of TorontoToronto, Ontario, Canada Utku Dolen, M.D.Division of Plastic and Reconstructive SurgeryWashington University School of MedicineSt. Louis, Mo. Jennifer M. M. Patterson, M.D.Department of OrthopedicsUniversity of North CarolinaChapel Hill, N.C. Kristen M. Davidge, M.D.Division of Plastic and Reconstructive SurgeryThe Hospital for Sick ChildrenToronto, Ontario, Canada Andrew Yee, Ph.D.Susan E. Mackinnon, M.D.Division of Plastic and Reconstructive SurgeryWashington University School of MedicineSt. Louis, Mo.