A 3D Modeling Study of the Intramuscular Course of the Masseteric Nerve

Facial nerve paralysis has serious functional, cosmetic and psychosocial consequences.  The standard of care is facial reanimation surgery.  The distal masseteric nerve(MN) is a common donor motor nerve utilized as part of the masseter transposition flap.  This flap has been described as including the anterior one-third to two-thirds of the muscle belly.  A vertical incision is required for rotation of the anterior flap.  Correira et. al.(1976) suggested that the vertical incision should be a maximum of 3.5-4cm to prevent transecting the pedicle.  Romeo et. al.(2014) reported the distance from gonion to masseter neurovascular pedicle as 41.6mm(range 35-49mm), the maximum length of a vertical incision.  To avoid injury to MN during harvest, its distribution in the muscle should be thoroughly understood. No study was found that investigated MN intramuscular course volumetrically in 3D.  The purpose was to investigate the intramuscular innervation of masseter throughout its volume using in situ digitization and 3D modeling.  Eight(M6/F2) embalmed cadaveric specimens(mean age 84.9±12.2 years) were used.  After V3 was exposed at foramen ovale, the extramuscular MN was identified and its course digitized in 1-2mm segments using a Microscribe™ digitzer.  The masseter muscle volume and each intramuscular branch with its distribution was digitized until it could no longer be seen with a dissection microscope.  Photographs were taken throughout the dissection and digitization process.  The data were reconstructed into 3D models in Autodesk® Maya®.  Each model included the extramuscular and intramuscular MN, the fiber bundles of the deep and superficial head of masseter and the surface of the mandible and zygomatic arch as in situ.  The 3D models were used to determine the course and branching pattern of MN throughout the entire muscle volume in each specimen.  Branching patterns were compared between specimens and their frequency determined.  The distance from the gonion to the primary intramuscular MN was measured.  Two or three extramuscular nerves were found to enter the muscle belly.  In all specimens there was one main nerve with either one(n=4) or two(n=4) smaller secondary branches.  The main nerve gave rise to the smaller secondary branches on average 11.9±8.0mm from the nerve entry point.  After the main nerve entered the medial aspect of the deep head it continued obliquely in an anterior-inferior direction through the deep head.  On emerging from the lateral aspect of the deep head the main branch coursed obliquely between the deep and superficial heads and penetrated the belly of the superficial head.  The main nerve gave off numerous branches that continued to travel intramuscularly before terminating.  The secondary branches, whether one or two, innervated the superior aspect of the deep head and did not supply the superficial head.  During its course between the superficial and deep heads the main nerve gave off numerous small anterior(2-3) and posterior(1-2) branches that supplied the superficial head.  The distance from the gonion to the main nerve was 43.6mm(range 40-47mm).  At this point MN was coursing between the superficial and deep heads in all specimens.  All of the 3D models could be viewed from any angle thus providing a unique platform to trace the course of the MN intramuscularly through masseter.  Comprehensive mapping of the intramuscular innervation of masseter throughout its volume in 3D revealed a distinct pattern of innervation.  These results and 3D models capturing the precise intramuscular innervation as in situ provide a foundation for further surgical cadaveric studies and simulations.  The gonion provides an accessible landmark from which to locate and harvest the distal MN.  Correia et.al. Masseter muscle rotation in the treatment of inferior facial paralysis. Anatomical and clinical observations.  Plast.Reconstr.Surg.1973:Oct;52(4):370-3.  Romeo et.al. Segmental masseteric flap for dynamic reanimation of facial palsy.  J.Craniofac.Surg.2014:Mar;25(2):630-2.