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 amongst them and cannot preoperatively visualize the nerve. Thus, we aim to design 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's kappa coefficient; 0.95 (CI 0.85 to 1.00). It 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) its course became adjacent to the mandible periosteum. Other important features, such as disposition in relation to the parotid gland or whether the nerve was directly covered by a thick intramuscular aponeurosis, could be well observed by HFUS.
Conclusions: HFUS enables masseteric nerve identification and can give the surgeon specific information on anatomical relations for each examined individual prior to surgery.
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