Virginia Research Day 2025

Medical Student Research Biomedical

04 Landmarking of the Extracranial Facial Nerve Trunk: A Cadaveric Study

Priyanka Gandhi; Kendyl Fahmy; Camden Little; Tony Nguyen; Jonathan Millard; Aaron Beger Corresponding author: pgandhi02@vcom.edu

Edward Via College of Osteopathic Medicine - Virginia Campus

The facial nerve (CNVII) has motor, parasympathetic, and sensory branches that travel from the brainstem to its extracranial destinations. Its fibers traverse the internal auditory meatus and inner ear before emerging from the stylomastoid foramen (SF) as the extratemporal segment. Upon exiting, the CNVII trunk gives off branches that supply adjacent musculature, enters the parotid gland (PG), and ultimately produces five motor branches that innervate the muscles of facial expression. Clinical interventions related to CNVII include parotidectomies and nerve blocks often used in ophthalmologic surgery and treatments for hemifacial spasms. Some nerve blocks target the CNVII trunk as it exists the SF, achieving temporary akinesia of facial muscles. Damage to this nerve can lead to complications such as prolonged facial paralysis, respiratory distress, and vocal cord paralysis. Thus, mapping the extracranial course of CNVII using intraoperative and surface landmarks, such as Borle’s triangle, is critical during procedures. While past studies have examined these landmarks, a comprehensive understanding of the variations in the nerve’s extracranial course in three-dimensional

space is lacking. This study examines the extracranial progression of CNVII after it exits the SF in three dimensional space. Tissue was dissected from 22 formalin-fixed adult cadavers that donated their body to Edward Via College of Osteopathic Medicine. Exclusion criteria included any destruction of the CNVII trunk or PG during previous student laboratory dissections. Four landmarks (LM) were identified: the intersection of the superior border of posterior belly of digastric muscle (PBD) on the mastoid process (LM1), the intersection of the mandibular ramus and superior aspect of PBD (LM2), the SF (LM3), and the entrance of CNVII trunk into PG (LM4). A Microscribe desktop digitizer was used to capture three-dimensional coordinate data on these four landmarks of interest. Generalized Procrustes superimposition was performed to scale, rotate, and translate landmark configurations, while principal component analysis (PCA) was done to assess variation among configurations. Data was collected from 28 sides of 22 donors, comprising of nine male and thirteen female donors, with a mean age of 80.71 years old. PCA revealed five principal components,

with the first principal component (PC1) explaining 35.85% of the variation. PC1 indicated that the greatest variation occurred at LM4. Positive scaling of PC1 showed inferomedial deviation of LM4, which correlated with LM1 and LM2 deviating toward one another, with superolateral deviation of LM3. Negative scaling of PC1 indicated superolateral deviation of LM4, which related to increased distance between LM1 and LM2, and an inferomedial deviation of LM3. These findings suggest that a reduced distance in the intersection of the PBD with mastoid process and mandibular ramus corresponds to inferomedial deviation of the CNVII trunk’s entrance into the PG and a superolateral deviation of the SF. Such extracranial variations must be considered in medical intervention, including parotidectomies and nerve blocks, to minimize severe complications resulting from CNVII damage and nearby tissue injury.

Approved by VCOM IRB, record #2024-060

69

2025 Research Recognition Day