Masseteric Nerve Ultrasound Identification for Dynamic Facial Reanimation Purposes

Background: The masseteric nerve is one of the main options to neurotize free muscle flaps in irreversible long-term facial paralysis. Several preoperative skin-marking techniques for the masseteric nerve have been proposed to limit the surgical dissection area, shorten the surgical time, and enable a safer dissection. However, these have shown variability among them, and cannot visualize the nerve preoperatively. The authors designed an observational study to validate a high-frequency ultrasound (HFUS) nerve identification technique. Methods: A systematic HFUS examination was designed and performed to visualize the masseteric nerve in 64 hemifaces of healthy volunteers. One-third were randomly selected to undergo an additional HFUS-guided needle electrostimulation to validate the HFUS image. Results: The masseteric nerve was identified by HFUS in 96.9% of hemifaces (95% CI, 0.89 to >0.99) and showed almost perfect agreement with direct needle stimulation as calculated with Cohen kappa coefficient (0.95; 95% CI, 0.85 to 1.00). The masseteric nerve was found within the masseter muscle, in between the deeper muscle bellies, at 18.3 mm (SD +/- 2.2) from the skin. Only in 12.9% of cases (95% CI, 0.06 to 0.24) did its course become adjacent to the mandible periosteum. Other important features, such as disposition in relation to the parotid gland, or if the nerve was covered directly by a thick intramuscular aponeurosis, could be well observed by HFUS. Conclusion: HFUS enables masseteric nerve identification and can provide the surgeon with specific information on anatomical relations for each examined individual before surgery.