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Anatomic and Histologic Evaluation of Brachialis to Anterior Interosseous Nerve Transfer
Mark J Winston, MD; Eliana B Saltzman, BA; Steve K Lee, MD; Scott W Wolfe, MD
Hospital for Special Surgery, New York, NY

Introduction: Brachialis to anterior interosseous nerve (AIN) transfer offers patients improved hand function with the ability to pinch. The AIN fascicle has previously been described topographically within the postero-medial region of the median nerve. We present a new description of the AIN fascicle based on anatomic terms seen intra-operatively to improve fascicular identification. We hypothesize that this nerve transfer can be performed with reliable anatomy, tension free neurorrhaphy, and appropriate donor-to-recipient axon count ratio.

Methods: Six cadaveric specimens were dissected. The median and musculocutaneous nerves were identified in the mid upper arm. The epineurium overlying the median nerve was incised to expose three fascicles (Figure 1). The fascicle location was described in anatomic terms seen intra-operatively with the arm in the abducted position (ventral, dorsal, cranial, and caudal)(Figure 2). The dorso-caudal fascicle was marked. The AIN was then identified in the proximal forearm as it branched from the median nerve and dissected proximally to confirm that the initial prediction of the AIN fascicle was correct. The AIN branching pattern, length, and fascicle location were recorded. All distances were measured from the medial epicondyle (ME). Brachialis branching pattern and length were also measured from the ME. The brachialis nerve was transferred to the AIN and overlap was measured. Each nerve was then sectioned and sent to histology lab for axon counts.

Results: The AIN fascicle was correctly predicted in all six specimens and was identified in the dorso-caudal portion of the median nerve. The AIN exited the median nerve 6.9 cm (SD 1.04) distal to the ME. Total neurolysable distance of the AIN was 13.9 cm (SD 1.46) proximal to the ME. The brachialis nerve branched from the musculocutaneous nerve 14.7 cm (SD 1.15) proximal to the ME. Length of brachialis nerve prior to branching was 5.2 cm (SD 1.15). Total neurolysable distance of the brachialis was 5.1 cm (SD 1.31). All nerve anastomoses overlapped by average of 1.8 cm (SD 0.49). AIN axon counts averaged 2661.2 while brachialis axon counts averaged 1452.5 (donor-to-recipient ratio 1:1.8).

Conclusions: Identifying the AIN fascicle in the median nerve is predictable based on topographic mappings of the median nerve. Describing the AIN fascicle as dorso-caudal, instead of previously described postero-medial, helps identify AIN fascicle with arm in abducted surgical position. Brachialis to AIN transfer is a tension free transfer with appropriate axon count ratio.




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