Facial Nerve Axonal Analysis and Anatomic Localization in Donor Nerve: Optimizing Axonal Load for Cross Facial Nerve Grafting in Facial Reanimation

PURPOSE:: Donor nerve axonal count over 900 in two-stage reconstructions using cross facial nerve grafts is possibly associated with improved outcomes in facial reanimation. Facial nerve axonal analysis was performed to determine the ideal location for optimizing axonal load. Correlation between axonal number, branch diameter, and age was also assessed. METHODS:: 28 fresh unpreserved cadaveric hemi-faces were dissected exposing the extracranial facial nerve branches. Axonal counts at two-centimeter intervals from pes-anserinus along branches inserting into the zygomaticus major muscle were taken, noting position relative to zygomatic arch, posterior ramus border, lateral border of zygomaticus muscle, and anterior parotid gland border. Nerves were fixed, sectioned, and stained with SMI-31 anti-neurofilament stain for digital axonal analysis. RESULTS:: All specimens had one or more intraparotid zygomatic branches with over 900 axons, and 96% had an extraparotid branch with over 900 axons. The likelihood that a zygomatic branch would have over 900 axons at its last intraparotid point (mean 6mm posterior to parotid border) was 92% (Range: 67-100%) in contrast to 61% (Range: 25-100%) when sampled at the first extraparotid point (mean 14mm anterior to parotid border). Nerve cross-sectional area was positively correlated to its axonal count (R² = 78%, p=<0.0001), with nerve diameter over 0.6mm predicting over 900 axons. Age did not correlate with axonal counts. CONCLUSION:: Branches with adequate axonal load were found in all specimens. The likelihood of adequate branch selection improved from 61% to 92% with short retrograde intraparotid dissection. Nerve diameter correlated with axonal load.