SP46. Anatomic Analysis of a Masseteric to Zygomatic Nerve Transfer in Rat and Pig Models

Abstract

PURPOSE: Facial reanimation is frequently achieved by performing a masseteric to zygomatic nerve transfer. This procedure enables nerve fibers of the fifth cranial nerve to re-innervate the facial muscles. Even though re-innervation is usually successful, the clinical outcome is commonly unsatisfying. This nerve transfer is one of multiple surgical techniques to treat facial paralysis. However, none of the current treatment options achieve fully satisfying results. In order to improve outcomes and establish new therapeutic approaches, translational research with preclinical animal models is inevitable. This anatomical study aimed to demonstrate the feasibility of a masseteric to zygomatic nerve transfer in rats and pigs. METHODS: In 30 rat hemifaces and 30 swine hemifaces, the masseteric branch of the trigeminal nerve was found directly below the zygomatic bone and was further dissected until a suitable length was reached. Starting from the stylomastoid foramen, the anatomical course of the zygomatic branch of the facial nerve, was dissected and the largest branch was chosen for the nerve transfer. The branching pattern and the anatomical course of both nerves were documented. The masseteric branch was cut most distally, whereas the zygomatic nerve was cut most proximally. In order to achieve an overlap of both branches, the nerves were fully mobilized and approximated by re-routing. Measurements of the overlapping distance and of the diameter of both branches at the overlapping point were taken. 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. CONCLUSION: This study proves the feasibility of a masseteric to zygomatic nerve transfer in rats as well as pigs, as an overlap was possible in all dissections. Both models have proven as useful preclinical animal model, which can be used for future facial nerve studies. General knowledge of the anatomical courses of the branch to masseter of the trigeminal nerve and of the zygomatic branch of the facial nerve should serve as a baseline for facial paralysis research.