Facial Nerve High-Resolution Microanatomy Analysis of the Distal Intratemporal to Extracranial Pes Segment: Reconstructive Implications.

BACKGROUND: Current knowledge of facial nerve topography between the stylomastoid foramen to the pes anserinus is very limited. Elucidating this segment's intraneural microanatomy may be advantageous in certain clinical settings: the planning of nerve grafts for gaps extending from the proximal facial nerve trunk to distal branches or in determining coaptation sites for hypoglossal jump grafts to provide selective upper and lower facial tone. This study is the first to provide high-definition intraneural topography of the aforementioned segment to optimize reconstructive outcomes. METHODS: Sixteen facial nerves extending from the second genu to the pes anserinus were harvested from eight cadavers en bloc to preserve orientation. Specimens were imaged by micro-computed tomography using a serial 6-µm protocol and digitally reconstructed three-dimensionally to be analyzed using bioinformatic tools. RESULTS: No clinically significant fascicular separation was noted between 14.4 mm proximal to the stylomastoid foramen until 4.4 mm distal to the foramen. Fascicles remained separate throughout the remainder of the specimen and were found to undergo a mean rotation of 45.5 degrees ( P = 0.0002) between 8.9 and 13.7 mm distal to the stylomastoid foramen. This reliable clockwise rotation in left nerves and counterclockwise rotation in right nerves resulted in superficially oriented fascicles entering the upper division of the pes anserinus, whereas deep-oriented fascicles entered the lower division. CONCLUSION: Intraneural facial nerve topography and rotation are consistent from 4 to 14 mm distal to the stylomastoid foramen, enabling surgeons to accurately place grafts targeted to either the upper or lower face, thus optimizing functional accuracy and minimizing synkinesis.

Topographic and Neural Anatomy of the Depressor Anguli Oris Muscle and Implications for Treatment of Synkinetic Facial Paralysis.

BACKGROUND: Synkinetic patients often fail to produce a satisfactory smile because of antagonistic action of a hypertonic depressor anguli oris muscle and concomitantly weak depressor labii inferioris muscle. This study investigated their neurovascular anatomy to partially explain this paradoxical depressor anguli oris hypertonicity and depressor labii inferioris hypotonicity and delineated consistent anatomical landmarks to assist in depressor anguli oris muscle injection and myectomy. METHODS: Ten hemifaces from five fresh human cadavers were dissected to delineate the neurovascular supply of the depressor anguli oris and depressor labii inferioris muscles in addition to the depressor anguli oris muscle relation to consistent anatomical landmarks. RESULTS: The depressor anguli oris muscle received innervation from both lower buccal and marginal mandibular facial nerve branches, whereas the depressor labii inferioris muscle was solely innervated by marginal mandibular branches. The mandibular depressor anguli oris origin was on average 39 mm wide, and its medial and lateral borders were located 17 mm from the symphysis and 41 mm from the mandibular angle, respectively. The depressor anguli oris fibers consistently passed anterior to the first mandibular molar toward their insertion into the modiolus, which was located 10 mm lateral and 10 mm caudal to the oral commissure. CONCLUSIONS: Depressor anguli oris muscle dual innervation versus depressor labii inferioris single innervation may explain why depressor anguli oris hypertonicity and depressor labii inferioris weakness are commonly observed concomitantly in synkinetic patients. Based on treatment goals, diagnostic percutaneous injection with lidocaine can be performed on the depressor anguli oris muscle along a cutaneous line from the modiolus to the mandibular first molar border, and an intraoral depressor anguli oris myectomy can be performed along that same transmucosal line.

The Effect of Depressor Anguli Oris Muscle Block on Facial Symmetry in Synkinetic Facial Paralysis Patients and Its Role in Preoperative Assessment.

BACKGROUND: Depressor anguli oris muscle hypertonicity in synkinetic facial paralysis patients may have an overpowering antagonistic effect on facial symmetry. Depressor anguli oris muscle block is a crucial diagnostic test before any treatment planning. Presented is the largest patient cohort analysis to date on static and dynamic facial symmetry changes after depressor anguli oris muscle block. METHODS: Unilateral synkinetic patients with depressor anguli oris muscle hypertonicity were included. Resting symmetry and smile modiolus angle, excursion, and exposure of teeth were measured on both synkinetic and healthy hemifaces before and after depressor anguli oris muscle block using Emotrics and FaceGram photographic analyses. RESULTS: Thirty-six patients were included. Before depressor anguli oris block, resting modiolus height was elevated on the synkinetic side (p = 0.047). During open-mouth smile, reduced modiolus angle (p < 0.0001), modiolus excursion (p < 0.0001), and exposure of teeth (p < 0.0001) were observed on the synkinetic hemiface. After depressor anguli oris block, resting modiolus height became symmetric (p = 0.64). During open-mouth smile, modiolus angle and exposure of teeth significantly increased (both p < 0.0001); excursion did not improve on the synkinetic side (p = 0.13) but unexpectedly improved in open-mouth smile on the healthy side (p = 0.0068). CONCLUSIONS: Depressor anguli oris muscle block improved resting symmetry and modiolus angle and exposure of teeth during smile, demonstrating the inhibitory mimetic role of a hypertonic depressor anguli oris muscle in synkinesis. It is a critical diagnostic and communication tool in the assessment and treatment planning of depressor anguli oris muscle hypertonicity, suggesting the potential effects of future depressor anguli oris myectomy. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, IV.