Microanatomy of the Frontal Branch of the Facial Nerve: The Role of Nerve Caliber and Axonal Capacity

Abstract

Background: A commonly seen issue in facial palsy patients is brow ptosis caused by paralysis of the frontalis muscle powered by the frontal branch of the facial nerve. Predominantly, static methods are used for correction. Functional restoration concepts include the transfer of the deep temporal branch of the trigeminal nerve and cross-facial nerve grafts. Both techniques can neurotize the original mimic muscles in early cases or power muscle transplants in late cases. Because axonal capacity is particularly important in cross-facial nerve graft procedures, the authors investigated the microanatomical features of the frontal branch to provide the basis for its potential use and to ease intraoperative donor nerve selection. Methods: Nerve biopsy specimens from 106 fresh-frozen cadaver facial halves were obtained. Histologic processing and digitalization were followed by nerve morphometric analysis and semiautomated axon quantification. Results:The frontal branch showed a median of three fascicles (n = 100; range, one to nine fascicles). A mean axonal capacity of 1191 ± 668 axons (range, 186 to 3539 axons; n = 88) and an average cross-sectional diameter of 1.01 ± 0.26 mm (range, 0.43 to 1.74 mm; n = 67) were noted. In the linear regression model, diameter and axonal capacity demonstrated a positive relation (n = 57; r² = 0.32; p < 0.001). Based on that equation, a nerve measuring 1 mm is expected to carry 1339 axons. Conclusion:The authors' analysis on the microanatomy of the frontal branch could promote clinical use of cross-facial nerve graft procedures in frontalis muscle neurotization and free muscle transplantations.

Histomorphometry of the Sural Nerve for Use as a CFNG in Facial Reanimation Procedures

“…A comparison between our findings on the diameter of the sural nerve with donor and recipient nerve branches may provide insight into possible size. For the frontal branch of the facial nerve, an average cross-sectional diameter of 1.01 ± 0.26 was found [30]. Revising the extracranial course of the facial nerve, a 1.002 ± 0.4598 mm diameter for the zygomatic branch and a 0.99 ± 0.3962 mm diameter for the buccal branch of the facial nerve have both been published [66].…”

“…In all reanimation procedures, remarkably in CFNG, the axonal load plays a crucial role. To further investigate axon capacity in CFNG procedures, we compared our findings with relevant studies that provided data on donor and recipient nerves (Table 3) [30][31][32]. [32] Gracilis branch, obturator nerve 598 ± 83 [31] For cross-facial nerve grafts, a capacity greater than 900 donor axons was identified as a promising indicator for favorable outcomes in midfacial reanimation using free muscle grafts.…”

“…Whilst non-facial donor nerves can provide greater axonal loads than cross-facial nerve grafts, muscle tone for symmetry at rest can be provided by facial donor nerve branches [30,[40][41][42].…”

Using High-Resolution Ultrasound to Assess Post-Facial Paralysis Synkinesis—Machine Settings and Technical Aspects for Facial Surgeons

“…Research works have shed light on this muscular network in the specific setting of FP [ 14 , 15 , 16 , 17 ]. The key muscles for facial symmetry and physiological smile movement have been carved out in micro- and macroanatomical studies [ 18 , 19 , 20 , 21 ]. As a protagonist muscle in the perioral region, the Depressor Anguli Oris (DAO) muscle displays a linear origin from the mental tubercle and is inserted on average into the modiolus 10 mm lateral and 10 mm caudal to the oral commissure.…”

Reply to Nduka et al. Comment on “Kehrer et al. Using High-Resolution Ultrasound to Assess Post-Facial Paralysis Synkinesis—Machine Settings and Technical Aspects for Facial Surgeons. Diagnostics 2022, 12, 1650”

“…Together with other advancements in clinical facial palsy (FP) diagnostics and basic scientific translational efforts, such as automated grading systems or axon quantification, high-resolution ultrasound (HRUS) enlarges the FP surgeon’s arsenal and understanding of FP disease [ 4 , 5 , 6 , 7 , 8 , 9 ].…”