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Bryan et al. Plast Aesthet Res 2022;9:53 https://dx.doi.org/10.20517/2347-9264.2022.39 Page 3 of 12

AIN injuries can be traumatic or spontaneous, caused by penetrating stab wounds, supracondylar fractures,
orthopedic surgery complications, compartment syndrome, neuritis, or entrapment under the pronator
[12]
teres muscle . Patients will often be unable to make an "OK" sign and will have a positive Pinch Grip Test,
[13]
where a patient will be unable to pinch an object with normal strength .

Radial nerve branches
[14]
The radial nerve originates from the posterior cord (C5-T1) of the brachial plexus . In the proximal
forearm, it gives off branches to the brachioradialis, extensor carpi radialis longus (ECRL) and ECRB before
dividing into a superficial branch and the PIN . In most cases, these branches are found proximal to the
[15]
supinator canal, although the location can be variable. Notably, the nerve to the ECRB is found in the
proximal forearm and can have anatomical variation: originating from the radial nerve before it divides, the
PIN before it pierces through the supinator, or the superficial branch of the radial nerve . The ECRL and
[16]
ECRB are responsible for wrist extension, while the PIN is responsible for finger extension, innervating the
extensor digitorum communis (EDC), extensor digitorum minimi (EDM), extensor carpi ulnaris (ECU),
abductor pollicis (AP), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis
proprius (EIP).

Injury to the radial nerve causes weakness in extension, with an isolated PIN injury resulting in finger
extension weakness. As stated previously, the branches to the ECRL and ECRB typically come off the radial
nerve before it passes through the supinator muscle and branches to form the PIN, so wrist extension is
spared in cases of PIN injury. Additionally, radial deviation is usually present due to the lack of motor input
from the ECU with the preserved function of the ECRL and ECRB .
[17]

REINNERVATION TECHNIQUES
Median nerve: anterior interosseous nerve transfers
Brachialis to AIN
Technique: In this technique, the distal portion of the nerve to the brachialis, coming from the
musculocutaneous nerve, is transferred to the AIN [Figure 1]. The patient is placed supine, and a sharp
incision is made in the medial arm. Subcutaneous tissue is divided with sharp dissection and cautery, paying
particular attention to the medial antebrachial cutaneous nerve. Once the median nerve and
musculocutaneous nerve are exposed, the nerve to the brachialis can be identified, branching off the
musculocutaneous nerve approximately 17 centimeters (cm) from the acromion. The brachialis branch is
more distal than the biceps brachii nerve (13 cm from the acromion). The nerve branches are marked and
protected, while the median nerve is inspected. The AIN fascicle is found within the median nerve proper,
and intraoperative mapping is aided by a nerve stimulator to identify these fascicles. Once identified, the
AIN is dissected proximally, and the remainder of the median nerve fascicles are identified. Following
confirmation of the brachialis nerve with stimulation demonstrating brachialis muscle contraction, the
brachialis nerve is dissected distally to decrease tension during the repair. The brachialis and AIN can then
be coapted end-to-end using microsurgical techniques .
[18]

Outcomes: The nerve to brachialis to AIN transfer is widely reported in the literature, and many reports
have demonstrated favorable results. When assessing outcomes and clinical function, the Medical Research
Council (MRC) scale for muscle strength can be utilized, with a grade 3 sufficient for object release and
hand opening following AIN reinnervation . Mackinnon et al. reported the first case of thumb and finger
[19]
reinnervation after a spinal cord injury with brachialis to AIN transfer 23 months after injury . Fifteen
[20]
months postoperatively, the patient regained MRC grade 3 strength of the FPL and FDP . In a case series
[20]
of 4 patients written by Ray et al., all patients with brachial plexus injuries who received brachialis to AIN