Motor Branch of Masseter for Innervation of Free Muscle Flap



Motor Branch of Masseter for Innervation of Free Muscle Flap


Michael J. A. Klebuc





ANATOMY



  • The masseter nerve branches from the mandibular nerve and passes above the lateral pterygoid muscle where it runs through the mandibular notch to enter the substance of the masseter muscle.


  • Along its intramuscular path, the main trunk liberates a series of small proximal branches and terminates in a long descending branch that courses obliquely in posteroanterior, proximal-distal trajectory.


  • The main trunk of the masseter nerve can be identified at a point 3 cm in front of the tragus, 1 cm below the zygomatic arch, and 1.5 cm deep to the SMAS (FIG 1).1


  • The masseter muscle has three lobes (superficial, middle, and deep).


  • The motor nerve to the masseter (CN V) lies on the superficial surface of the deep lobe.2


  • The main trunk and descending branches contain approximately 2700 and 1550 myelinated motor fibers, respectively.1,3


  • The descending branch of the masseter nerve is usually selectively transected and employed to innervate free muscle flaps utilized for facial reanimation.






FIG 1 • Topographic landmarks for masseter nerve isolation. Main nerve trunk located 3 cm in front of the tragus, 1 cm below the zygomatic arch, and 1.5 cm deep to the SMAS.


PATHOGENESIS



  • The masseter nerve is frequently selected as a source of innervation in cases of bilateral facial paralysis and when the facial nerve is unavailable as a donor (ie, Moebius syndrome, Lyme disease, bilateral temporal bone fractures, brainstem cavernous malformation, Guillain-Barré).4


  • Often selected over cross face nerve grafts in older patients where the potential for nerve regeneration is diminished.5


PATIENT HISTORY AND PHYSICAL FINDINGS



  • Etiology of the facial paralysis:



    • Developmental


    • Traumatic


    • Post-tumor extirpation


    • Bell palsy


    • Ramsay Hunt syndrome


  • Patient age:



    • Peripheral nerve regeneration declines with advanced age.


    • There is no definitive age cutoff for cross face nerve grafting; however, after the age of 50 to 55 years, the ipsilateral motor nerve branch to masseter or other adjacent cranial nerves often provide a more reliable source of innervation to free muscle flaps with more powerful muscle flap contraction.


  • Comorbidities impairing nerve regeneration:



    • Smoking—higher risk of skin flap necrosis


    • Diabetes


    • Vascular disease


    • Previous radiation


    • Neurologic disorders


  • History of hypercoagulable state:



    • Patient or family history of pulmonary emboli or deep venous thrombosis signals the need for a hematologic workup prior to free, functional muscle flap reconstruction.


  • The patient’s level of motivation and willingness to comply with postoperative physical therapy is also important to ascertain.


  • History of temporomandibular joint dysfunction—consider alternative technique.



IMAGING



  • Imaging studies are seldom required in the presence of a normal physical examination.


SURGICAL MANAGEMENT


Preoperative Planning



  • Smoking cessation and avoidance of nicotine


  • Discontinuation of oral anticoagulants


  • EMG



    • Concomitant activation of the masseter muscle with attempted smiling (may predict development of an effortless smile)6


  • Mark the nasolabial folds and smile vectors prior to entering the operating room.


Positioning

Feb 27, 2020 | Posted by in Pediatric plastic surgery | Comments Off on Motor Branch of Masseter for Innervation of Free Muscle Flap

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