April 2020 HSC Section 4 - Plastic and Reconstructive Problems

Joseph & Kim

of paralysis, an additional period of 6 to 8 months of denervation may result in further muscle and distal nerve fibrosis, potentially limiting functional outcomes. However, in this situation, some surgeons may prefer to perform dual innervation and use a so- called babysitter nerve that may provide earlier reinnervation and thus preserve the function of native distal muscle and nerve (see later discussion of dual innervation). Additional disadvantages include the potential for weakness in the unaffected (good) facial nerve, more unpredictable results compared with other donor nerves, and fewer available donor axons compared with hypoglossal or masseteric nerves. Of note, it has been shown that an ideal donor nerve for facial reinnervation proced- ures should have greater than 900 axons because this results in improved functional outcomes. 22 In an elegant anatomic study, Hembd and colleagues group demon- strated that, when sampling facial nerve branches just adjacent to the anterior border of the parotid gland, there is a 90% chance of obtaining a donor buccal or zygomatic nerve branch with more than 900 axons. 23 Surgical Technique We begin by harvest of a sural nerve graft from the lower extremity. This procedure may be performed with or without the use of an endoscope and has been well described elsewhere. 24,25 Following harvest of the sural nerve graft, the distal end is marked with methylene blue. Next, a facelift style preauricular incision is designed on the donor side of the face and 1:100,000 epinephrine without lidocaine is injected into the proposed preauricular incision line and the subcutaneous tissue of the donor midface. A subcutaneous skin flap is raised similar to that performed with a facelift. The superficial musculoaponeurotic system (SMAS) fascia is identified and an inci- sion is sharply made within this layer, 2 to 3 cm anterior to the tragus. Next, a sub-SMAS flap is developed and dissection proceeds in this layer anteriorly until the masseterocutaneous ligaments are identified. Just beyond these ligaments is the buccal fat pad, at which point several buccal branches of the facial nerve may be identified. The facial nerve branch to the zygomaticus major may be identified at the Zuker point, which is located halfway along a line drawn between the root of the helix and the oral commissure. 26 The branches can be followed in a retrograde fashion for a short distance to the edge of the parotid gland to obtain donor nerve segment of larger caliber and greater axonal load. We routinely use a nerve stimu- lator to test several candidate nerve branches. Among candidate branches that have similar caliber, we select nerve branches that have the most specificity for causing zygomaticus major activation. After selection of a donor facial nerve branch, the sural nerve is tunneled in anti- dromic orientation from a stab incision in the gingivolabial sulcus to the location where the donor nerve branch was identified. Next, we use an operating microscope for neu- rorrhaphy, which is performed between the donor facial nerve branch and the sural nerve graft through approximation of the epineurium of the nerves with 9 to 0 nylon suture. Often, we will use a small vein graft for entubulation of the neurorrhaphy sites. Next, the neurorrhaphy is performed for the contralateral side, where a dominant facial nerve branch to the zygomaticus is usually selected. The wounds are irrigated and then closed in a routine fashion. Dual innervation procedures Dual innervation procedures are newer approaches to reinnervation that involve the use of donor nerve input from more than 1 cranial nerve. Each cranial nerve possesses inherent characteristics that may affect its usefulness in individual reconstructive sce- narios. Characteristics that are commonly important to the facial nerve surgeon

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