In Reply: Brachialis to Anterior Interosseous Nerve Transfer: Comprehensive Anatomic Rationale

To the Editor: We thank the author Dr Hassan1 for his thoughtful and constructive comments on our recent publication, “Brachialis to Anterior Interosseous Nerve Transfer: Comprehensive Anatomic Rationale.”2 We appreciate the opportunity to address the issues raised and to clarify the purpose, methodology, and clinical relevance of our study. As stated in both the Introduction and Discussion sections, the primary aim of our study was to investigate anatomic factors that may underlie the inconsistent clinical outcomes observed after the brachialis to anterior interosseous nerve (BrAIN) transfer. Clinical results vary widely in the literature, even within the same surgical team, and the anatomic underpinnings of this variability remain underexplored.3,4 To address this, we conducted a focused anatomic study examining the following: (1) the intraneural topography of the anterior interosseous nerve (AIN) within the median nerve at the level of the BrAIN transfer and (2) the presence and consistency of intraneural fascicular interconnections that may compromise selective reinnervation by allowing donor axons to disperse into adjacent, nontarget fascicles. Our study was not designed to assess outcomes or techniques clinically but to provide foundational anatomic insights for surgical planning and future translational research. The sample size of 30 fresh cadaveric limbs represents a relatively large cohort for anatomic studies of this type and enabled consistent identification of key anatomic patterns. Our observation of variability in AIN location—dorsomedial in one-third, dorsolateral in another third, and dorsal in the remainder—emphasizes that assumptions of constant topography are unfounded. Our findings are consistent with early observations by Sunderland and more recent imaging studies of median nerve fascicular anatomy.5-7 Although our study was purely anatomic, we believe it has important clinical implications. Bertelli et al, for instance, reported that direct BrAIN transfers yielded limited motor recovery, while patients in whom the AIN was first identified distally and dissected proximally—thereby sacrificing fascicular interconnections—achieved better functional outcomes, including M3 to M4 grip strength.4 These clinical observations support our anatomic findings and suggest that intraneural interconnections may contribute to axonal dispersion and reduced selectivity, resulting in suboptimal recovery. We recognize the potential risks associated with extensive intraneural dissection, including intraneural scar formation and inadvertent fascicular injury, and we explicitly discussed these drawbacks in our manuscript. However, we also note that selective neurolysis of specific fascicular groups to gain length or improve coaptation angle is a well-established technique in nerve surgery.8-10 When performed judiciously, neurolysis of specific fascicular groups allows for optimized donor-recipient alignment and better targeting without compromising neural integrity. Furthermore, the need for intraneural dissection may depend on the pathophysiology. In upper motor neuron syndromes (eg, spinal cord injury with preserved lower motor neurons or traumatic brain injury), intraoperative stimulation of the AIN is feasible, allowing precise targeting. By contrast, in lower motor neuron injuries, where the AIN is no longer stimulable, a distal-to-proximal dissection may be the only reliable way to identify and target the correct fascicle group.11 Regarding axon quantification, we acknowledge the technical limitations of axon quantification due to embedding issues in a minority of specimens. Nonetheless, our donor-to-recipient axon ratio of 0.55 ± 0.27 closely matches published values from Sananpanich et al and Ziaziaris et al.12,13 Consistency across methodologies supports the reliability of this parameter. We also appreciate the suggestion to consider adjunctive strategies such as nerve grafts or conduits when direct coaptation is not feasible. Although such approaches may be necessary in select cases, we highlight that compared with direct nerve transfers, grafts introduce additional regeneration distance, greater risk of axonal dispersion, and a potential reduction in axon density at the target site.14 Studies have explored the use of vascularized grafts and conduits in BrAIN transfer, though outcomes remain variable.15 In summary, we appreciate the author's comments, which highlight important aspects of our work and affirm its relevance to clinical decision making in nerve transfer surgery. Our goal was not to promote a fixed technique but to provide detailed anatomic data that may inform better decision making in nerve transfer surgery. We hope this work contributes to a more personalized approach to BrAIN transfers.