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reanimation. In cases of long-standing facial paralysis, typically greater than 12 months, atrophy of the
native mimetic facial muscles precludes nerve transposition, which relies on reinnervation of the native
facial musculature. This applies similarly to congenital facial nerve palsy cases in which the facial nerve and
muscles may be absent. In such cases, gracilis free muscle transfer (GFMT) is widely accepted as the gold
standard in dynamic smile restoration [Figure 1] .
[2]
One critical aspect of this procedure is the selection of a motor nerve or nerves to innervate the GFMT. In
its original description by Harii et al. in 1976, the deep temporal nerve was used as the donor source of
[2]
innervation. Since then, various other donor nerves have been described in the literature, including the
cross facial nerve graft (CFNG) , masseteric nerve [7-11] , hypoglossal nerve [12,13] , spinal accessory nerve
[3-6]
(SAN) [6,13] , and combinations [14-17] of the above. Each donor nerve option has unique benefits and drawbacks,
and there is no universal consensus on one option that is ideal for all scenarios. The CFNG and the
masseteric nerve are currently the most frequently utilized options. The CFNG, which utilizes midfacial
branches of the contralateral facial nerve, has the potential of creating a spontaneous and synchronous
smile. However, it has lower reliability and less oral commissure excursion than other donor nerves [8,13,18] . In
contrast, the masseteric nerve, a branch of the trigeminal nerve, contains a greater axonal load and more
reliable oral commissure excursion, but at the expense of spontaneity [8,19] . Additional considerations when
comparing CFNG and masseteric nerves are the potential need for multi-stage surgery and the time needed
to achieve movement [20,21] . Other principles that guide the choice of donor nerve are availability and donor
site morbidity. In this review, we will summarize the current literature on options for GFMT innervation
and discuss the advantages and disadvantages in the context of smile excursion, spontaneity, reliability,
timing, and other pertinent outcomes in adults and children. There are other important surgical
considerations for GFMT that affect smile outcomes that are outside the scope of this review, such as
muscle bulk, orientation, and vector at inset, as well as length and tension of the muscle at inset. Relevant
publications were identified from the Medline database using the following search terms: gracilis, gracilis
innervation, facial reanimation, smile reanimation. Additional articles were then identified by cross-
referencing initial search results.

MASSETERIC NERVE TRANSFER
The masseteric nerve has gained popularity in recent decades as one of the preferred sources of innervation
for GFMT due to its reliability, ability to achieve oral commissure excursion, need for only a single stage,
and fast onset of movement [8-10,18,22] . This technique was first described by Zuker et al. in 2000, who utilized
[11]
the masseteric nerve as the donor source for bilateral GFMT in 10 children with Moebius syndrome causing
bilateral facial paralysis. The masseteric nerve arises from the anterior division of the mandibular branch of
the trigeminal nerve, providing motor supply to the masseter. Surgically, the nerve can be identified in the
sub-zygomatic triangle, bounded by the sigmoid notch of the mandible inferiorly and the zygomatic arch
superiorly. The nerve can usually be found within the masseter muscle about 1 cm inferior to the arch and
3 cm anterior to the tragus . It typically courses obliquely or parallel to the zygomatic arch and can be
[20]
located in the middle of the muscle about 10 to 15 mm deep to the parotidomasseteric fascia [7,20] . Once
identified, it can be traced proximally and distally before being divided just before it branches within the
muscle. It can then be transposed superficially for coaptation to a recipient nerve. Notably, the masseteric
nerve is often spared in Moebius syndrome, making it a good option for a nerve donor, whereas the facial
and hypoglossal nerves can be compromised.

One of the main advantages of the masseteric nerve as a donor source for the GFMT lies in the power that it
provides, yielding greater oral commissure excursion when compared to the CFNG [8,18,19,22] . Bhama et al.
[18]
assessed 154 GFMTs performed for facial reanimation in adults and children between ages 8 and 60 and