April 2020 HSC Section 4 - Plastic and Reconstructive Problems

Management of Flaccid Facial Paralysis

Masseteric nerve (V–VII transfer) The motor nerve to the masseter muscle is a valu- able donor nerve that has been increasingly used in facial reanimation procedures over the past 15 years. This nerve is located in close proximity to zygomatic branches of the facial nerve that controls smiling, allowing for primary neurorrhaphy for targeted bite-driven smile reanimation. Furthermore, the close proximity of this nerve and, therefore, short distance for axonal regrowth, allows for expedient clinical recovery and return of dynamic facial function, which is often as soon as 3 to 4 months following reanimation surgery. The masseteric motor nerve has been estimated to provide be- tween 1500 and 2000 myelinated donor axons, which is a sufficient number to power the main trunk of the facial nerve or transferred free muscle tissue. 19 Another major benefit in the use of the motor nerve to masseter is that its sacrifice results in essen- tially no clinical deficit, which may be related to preserved innervation through undis- turbed proximal masseter nerve branches, as well as compensation by other muscles of mastication. 19 Surgical Technique Several reports have described techniques for localizing the motor nerve to masseter. 19–21 The masseteric motor nerve crosses the caudal aspect of the zygo- matic arch at an angle of approximately 50 and, after entering the masseter muscle, continues distally toward the oral commissure. 19 Dissection may be initiated approx- imately 3 cm anterior to the tragus and 1 cm inferior to the caudal aspect of the zygo- matic arch. An alternative approach to nerve localization involves use of what has been termed the subzygomatic triangle. 20 After locating the starting point in both tech- niques, dissection then proceeds in a superficial to deep manner through the masseter muscle. The authors generally find it helpful to have an assistant retract muscle fibers with a Cummings or Ragnell retractor while a fine curved hemostat and bipolar cautery is used by the surgeon to carefully divide the muscle. A dominant branch of the nerve will be located 1.0 to 1.5 cm deep into the parotidomasseteric fascia. 19,20 Very often, a branch-free segment approximately 2 cm long can be located. An electrical nerve stimulator may also aid in localization of the nerve, which generally causes a strong activation of the masseter muscle. Some surgeons prefer to preserve the superome- dial branch of the masseteric nerve to decrease the risk of facial hollowing associated with complete denervation. However, some evidence suggests that the first branch of the motor nerve to the masseter muscle is not visible unless the zygomatic arch is removed, suggesting that sacrifice inferior to the arch likely preserves some innervation. 19 Contralateral facial nerve via cross-face nerve graft The contralateral facial nerve is another important source of donor axons, which can be used in facial reanimation with a cross-face nerve graft (CFNG). The most important benefit of using the contralateral facial nerve is that spontaneous blink and emotive smile are possible. The CFNG technique relies on a contralateral (unaffected) facial nerve having significant redundancy in the distal buccal and zygomatic neural input due to significant arborization of nerve fibers among branches. Therefore, division and utilization of 1 of the distal buccal or zygomatic branches results in little or no clin- ical deficit. There are, however, distinct disadvantages associated with this approach. Due to the requirement for a long interposition nerve graft, there is a significant delay of 6 to 8 months between the time a CFNG procedure is completed and the time that nerve fibers reach the distal aspect of the nerve graft and target muscle. Because many facial nerve patients present for reinnervation at 1 year or greater following the onset

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