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Anatomic Analysis of a Masseteric to Zygomatic Nerve Transfer in Rat and Pig Models
Elena Millesi, cand.med.1; Marissa Suchyta, BA1; Huan Wang, MD, PhD1; Mardini Samir, MD2
1Mayo Clinic, Rochester, MN; 2Division of Plastic Surgery, Mayo Clinic, Rochester, MN

Introduction A commonly performed technique for facial nerve reconstruction is a nerve transfer from the masseteric branch of the trigeminal nerve. This procedure allows nerve fibers of the fifth cranial nerve to re-innervate the facial muscles. However, despite multiple treatment options for patients with facial paralysis, none achieve fully satisfying results. Translational research with preclinical animal models is invaluable to establishing new therapeutic approaches to improve outcomes. The anatomical dissections in this study aimed to establish the feasibility of masseteric to zygomatic nerve transfer in rats and pigs.
Material & Methods The masseteric branch of the trigeminal nerve and the largest zygomatic branch of the facial nerve, responsible for eye closure, were dissected in 30 swine hemifaces and 30 rat hemifaces. Both nerves were fully mobilized and their branching patterns documented. The zygomatic nerve was cut most proximally, whereas the masseteric branch was cut most distally, directly beneath the zygomatic bone. Both nerves were re-routed such that they were approximated to achieve the most overlapping between the nerve ends. The overlapping distances and diameters of both nerves at the overlap were measured. Lastly, the regeneration distance, starting from the distal overlap to the intramuscular branches of the zygomatic nerve, was measured.
Results Tension-free coaptation was possible with a minimum 0.5 mm overlap between the masseteric and zygomatic nerves without cutting any tethering branches in all the rat dissections and by cutting an average of four (+/- 2.20) branches of the zygomatic nerve in pig dissections. In rats, the masseteric branch had a diameter of 0.36 mm (+/- 0.06) and the zygomatic branch of 0.46 mm (+/- 0.13), whereas in pigs, the masseteric branch measured 0.52 mm (+/- 0.16) and the zygomatic branch 0.61 mm (+/- 0.12). In regards to branching quantity, a significant difference was determined between both nerves in pigs (P < 0.001), with the masseteric branch having three (+/- 1.35) branches in total and the zygomatic branch 11 (+/- 4.43). However no significant difference was found in rats (P = 0.1723), as both presented with two branches. The regeneration distance measured 5.79 cm (+/-1.04) in pigs and 0.84 cm (+/- 0.23) in rats.
Conclusions Our study demonstrated the feasibility of direct masseteric to zygomatic nerve transfer in both rats and pigs. It further provided general knowledge of the anatomic courses of the zygomatic branch of the facial nerve and the masseteric branch of the trigeminal nerve.


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