Neurorrhaphy

Often in microvascular surgery it is necessary to perform neural anastomoses to reconstitute motor or sensory function to the transferred flap tissue. Essentially, a nerve trunk is a collection of axons (nerve fibers) ensheathed in endoneurium, peri-neurium, and epineurium. Endoneurium surrounds the individual axons, perineur-ium encases groups of axons, and epineurium surrounds the perineurium and ensheaths the entire nerve trunk (Fig. 25). A nerve fascicle is a group of axons and its perineurium and it is the smallest component of a nerve trunk that can be used for microneural anastomosis. A monofascicular nerve (e.g., facial nerve trunk) consists of a single large fascicle ensheathed by epifascicular epineurium. A polyfas-cicular nerve (e.g., inferior alveolar nerve and lingual nerve) consists of many small fascicles of varying size with intervening interfascicular perineurium all ensheathed by epifascicular epineurium. In cases of microvascular transplantation of muscle for facial paralysis, fascicular (perineural) repairs are done to match precisely the unique donor motor fascicles to the appropriate division of the facial nerve. For example, the obturator nerve trunk consists of nerve fascicles that innervate discrete areas of the gracilis muscle. The epifascicular epineurium is opened and the individual fascicles are electrically stimulated to define two independent areas of muscular movement and innervation. The fascicle supplying the part of the muscle used for the oral commissure is anastomosed to the lower division of the facial nerve. The fascicle supplying the upper part of the muscle that is used for the eyelids is anastomosed to the upper division of the facial nerve (or cross-facial nerve graft from the upper division of the opposite facial nerve).

Epineurium-to-epineurium microneural anastomosis is sufficient in the majority of cases involving microvascular surgery. Unlike peripheral nerve coapta-tion after traumatic neurolysis, the topographical precision provided by a perineural repair is not needed when purely sensory or purely motor nerves are being coapted. Cutaneous sensory reinnervation is possible using the radial forearm flap and the fibular osteocutaneous flap. The greatest benefit of sensory reinnervation is found

Figure 25 Layers of a typical peripheral nerve. The nerve fascicles are enveloped in perineurium that is in turn enveloped in epineurium. Microneural anastomosis is usually performed at the level of the perineurium.

in the perioral area to prevent drooling. Sensation around the lips and anterior oral cavity improves articulation, oral competence, and food bolus manipulation. Sensory reinnervation using the lingual nerve or inferior alveolar nerve results in the return of all aspects of normal sensation including touch, pressure, temperature, and pain. An interesting finding is that sensory discrimination may actually be improved at the recipient site because of the greater cortical representation of the cranial nerves. Motor reinnervation is possible using the rectus, latissimus, and gra-cilis muscles. Reinnervation is useful to maintain muscle bulk in cases of tongue reconstruction and to achieve movement in cases of facial paralysis and tongue base reconstruction. The ipsilateral facial nerve or a cross-facial nerve graft is used for cases of microvascular muscle transfer to correct facial paralysis.

The technique of microneurorrhaphy is a precise method of nerve anastomosis ideally resulting in the accurate alignment of nerve sheaths composed of viable Schwann's cells to allow free axonal regrowth across the site of coaptation. A ten-sionless epineural repair is important because the epineurium will stretch imperceptibly, separating the underlying nerve fascicles. An epineural repair that results in prolapsing fascicles may be too tight and should be loosened to allow the fascicles to lie end to end. Excess nerve fascicles extruding through the suture line should be trimmed so that regenerating nerve fibers are not directed outside the anastomosis. If the individual fascicles are sufficiently large, one or two 10-0 approximating perineural sutures can be used prior to suture coaptation of the epineurium.

Excess adventitia is trimmed from the end of the donor and recipient nerve trunks under the microscope. Two stay sutures (10-0 or 9-0 nylon) are placed 180 degrees apart through the epineurium of each nerve trunk, with care taken not to penetrate the underlying nerve fascicles. Subsequent epineural sutures are placed to allow adequate coaptation. Small nerves may only require four sutures while larger nerve trunks may require up to eight sutures. Factors associated with successful neural anastomosis include optimal nerve coaptation, minimal surgical trauma, and survival of the nerve graft. Survival of the nerve graft is important because viable Schwann cells are needed for axonal propagation along the nerve graft. This is usually not an issue with microvascular transfer since the nerves are revascularized with the rest of the transplant. Schwann cells in nonvascularized nerve grafts depend upon a healthy vascularized bed for survival. A nonvascular-ized nerve graft undergoes inosculation and ingrowth of blood vessels in a process similar to revascularization of a skin graft. Nonvascularized nerve grafts may not perform well if they are placed on an avascular (e.g., previously irradiated) bed. In these cases, a vascularized nerve graft should be considered to maximize neural regeneration (51).

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