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Brachialis to Anterior Interosseous Nerve Transfer: An Anatomic Study
Kirsty Usher Boyd, MD, FRCSC; Gurpreet Dhaliwal, MD, FRCSC, FACS; Susan E. Mackinnon, MD
Washington University, St. Louis, MO, USA
Background: Nerve transfers have resulted in improved outcomes in upper level brachial plexus injuries, especially restoration of shoulder and elbow function. Achieving hand function following lower plexus injuries has remained a challenge. Our intra-operative observations suggested that fibers of the anterior interosseous nerve (AIN) exist as a distinct fascicular group on the postero-medial aspect of the median nerve in the upper arm. The brachialis is the terminal branch of the musculocutaneous nerve.
Purpose: The purpose of this study was to evaluate the anatomic feasibility of a brachialis to AIN transfer in a cadaveric model and to establish anatomical landmarks so that this procedure might have broader clinical utility.
Methods: Nine fresh frozen, cadaveric complete upper limbs were dissected. External markers were placed on the acromion, the medial and lateral epicondyles, and the radial styloid. Dissection of the musculocutaneous and median nerves was conducted. The AIN branching pattern was recorded, including the total proximal “neurolysable” distance for potential transfer. The location, number, and length of the brachialis branches were recorded.
Results: Nine arms were dissected. Mean humeral length was 34.0 +/- 1.5 cm and mean forearm length was 28.4 +/- 1.8 cm. The AIN exited from the radial side of the median nerve in all specimens at a mean distance of 6.1 +/- 1.5 cm distal to the medial epicondyle. The branching pattern of the AIN was variable, with the first branch to FPL in 5 of 9 arms. Proximal dissection of the AIN reveals that although the nerve exits the median nerve from its radial side, the fascicles take origin from the postero-medial side of the median nerve in the arm. The mean neurolysable distance above the medial epicondyle was 13.0 +/- 2.4 cm. The motor branch to the brachialis muscle, most frequently a single branch (55.6%), departs from the musculocutaneous nerve at a distance of 11.5 +/- 1.6 cm proximal to the medial epicondyle with the mean terminus at 7.7 +/- 2.0 cm.
Conclusions: The AIN exists as a neurolysable group of fibers on the postero-medial aspect of the median nerve up to 13.0 cm proximal to the medial epicondyle. The brachialis motor terminus is located 7.7 cm proximal to the medial epicondyle. Transfer between these two nerves is anatomically feasible and may provide an option for restoration of finger and thumb flexion in a lower brachial plexus injury.
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