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Page 4 of 8 Bekisz et al. Plast Aesthet Res 2022;9:61 https://dx.doi.org/10.20517/2347-9264.2022.69

Restoring elbow flexion is the most common indication for functional muscle transfer in the upper
[34]
extremity . While the free gracilis flap and free or pedicled latissimus dorsi flap are overwhelmingly the
most popular options [35,36] , use of rectus femoris and vastus lateralis has also been described. Loss of elbow
extension, although a less common indication for FFMT, has also been addressed with the gracilis muscle
flap . Purposeful movement of the hand and fingers can be addressed via free gracilis muscle transfer,
[37]
which is facilitated by attaching the distal tendinous portion of gracilis to the tendons of the digital flexors
or extensors [38,39] .

With FFMT, the reconstructive surgeon must also carefully select which donor nerve will be used to power
the transferred muscle (a consideration that does not necessarily apply in pedicled functional muscle
transfer). The choice of nerve will depend on the goals of a particular procedure and the options available in
that setting, considering that the donor nerves available will differ substantially between a patient with a
total brachial plexus avulsion and a patient with loss of elbow flexion following an oncologic extirpation. As
described by Mackinnon and Novak in their 1999 seminal paper on nerve transfers, the ideal donor nerve
should be expendable, located in close proximity to its intended target, contain the specific fiber types
desired, and in the case of motor nerves, derive from a donor muscle that is synergistic with its
destination . There are several popular options for upper extremity FFMT, including but not limited to
[40]
intercostal nerves, the spinal accessory nerve, the contralateral C7 root, or spared roots of the ipsilateral
brachial plexus [37,38,41] .

However, while a number of donor nerves have demonstrated success in FFMT, the evidence suggests that
they are not universally interchangeable. Among the factors that are critical to consider when selecting a
donor are whether the nerve originates within the brachial plexus (intra-plexal) or outside of it (extra-
plexal) and the axon count ratio of the donor to recipient nerves. Intra-plexal donors include the ipsilateral
brachial plexus roots as well as the medial pectoral, thoracodorsal, and ulnar nerves, while common extra-
plexal nerves chosen are the contralateral C7 root along with the spinal accessory and intercostal
nerves [42-44] . A 2016 paper by Nicoson et al. that reviewed outcomes using several different donor nerves in
free functional gracilis muscle transfer to restore elbow flexion following brachial plexus injury described
[45]
intra-plexal donors as the best choice for achieving better motor strength . Regarding the proposed
advantages of intra-plexal donors, the authors cited the quality and quantity of axons in these nerves as well
as their proximity to the transferred muscle, which can subvert the need for a nerve graft. However, despite
these theoretical benefits, extra-plexal nerves have also shown promise in FFMT, with a paper by Cho et al.
showing no differences in outcomes pertaining to motor strength across 38 patients undergoing FFMT for
brachial plexus injuries according to whether the spinal accessory or ulnar nerves were used as donors .
[46]
With respect to axon counts in donor and recipient nerves, the 2015 paper by Schreiber et al. has provided
much of the evidence pairing axon count ratios with functional outcomes . In reviewing average axon
[44]
counts for the donor nerves frequently used in upper extremity FFMT, they observed that the axon counts
are generally higher for intra-plexal donors than for extra-plexal donors. They found that increased donor
nerve axon counts tended to increase the likelihood of a meaningful functional outcome, and ultimately
[44]
advised a donor-to-recipient ratio of at least 0.7:1 for the best chance of regaining useful muscle strength .
The complexity of functional muscle transfer derives largely from the numerous variables that must be
carefully considered and addressed to achieve a meaningful functional outcome. Preoperatively, this begins
with obtaining a thorough understanding of a patient’s history of injury and meticulous evaluation of their
anatomy and examination, with attention paid to abilities and deficits. Observation of the patient
performing occupational tasks or other activities of daily living can be informative, and coordination with a
hand and occupational therapy team is essential. Surgical planning must aim to anticipate potential