66.1 Introduction

Advanced techniques in microsurgery, high-resolution magnification, and sensitive intraoperative electrophysiologic monitoring have all contributed to a high percentage of facial nerve preservation following resection of vestibular schwannoma (VS). An anatomically preserved facial nerve, however, does not always result in normal facial function. Depending on the degree of facial nerve manipulation and tumor effect, postoperative facial function may be completely normal, partially weak, or totally paralyzed. Fortunately, a high percentage of patients with facial nerve paresis and some with complete paralysis after VS resection recover spontaneously and regain satisfactory facial movement. However, some patients do not spontaneously recover facial muscle function and require surgical intervention to restore facial tone and animation. For these patients, timely intervention is critical in minimizing irreversible degeneration of the facial muscles and poor functional outcome after reanimation surgery. In a small set of patients, the anatomical continuity of the facial nerve is lost during tumor dissection and a suitable proximal stump at the brainstem is not available for interposition nerve grafting. These patients have no chance of recovering dynamic facial function without timely reanimation surgery.

The goal of facial reanimation surgery is to restore resting facial symmetry, spontaneous and voluntary facial motion, as well as emotional facial expression. The facial nerve and facial nucleus is unique in its ability to provide normal synchronized facial animation and cannot be adequately replaced by any other cranial nerve. When interrupted, the proximal end of the facial nerve from the brainstem may be coapted either directly or with an interposition graft. When the proximal facial nerve is unavailable for reanimation, alternative donor cranial nerves may be substituted to power the facial muscles. Potential donor sources include the hypoglossal, trigeminal, contralateral facial, spinal accessory, and phrenic nerves. For several years, the hypoglossal nerve was considered the go-to donor nerve for facial reanimation surgery.s. Literatur In the classical description, the hypoglossal nerve is completely sectioned in the neck and coapted to the main facial nerve branch. Coaptation of the entire hypoglossal nerve to the facial nerve effectively restores facial tone and some volitional movement, but is also associated with severe tongue hemiatrophy and dysfunction leading to difficulties with speech, mastication, and swallowing. Modifications to the classic total hypoglossal nerve transfer procedure including the use of the split hypoglossal technique, interposition jump grafts, and transfer of the facial nerve for an end-to-side coaptation to an intact hypoglossal nerve have minimized morbidity of the tongue. Hypoglossal-to-facial nerve transfer is discussed further in Chapter 65.

The masseteric nerve, as a substitute donor source in facial reanimation, is increasing in popularity. As a substitute nerve for facial reanimation, Escat and Viela first reported the use of the masseteric nerve in 1925.s. Literatur Spira wrote a similar report in 1978.s. Literatur Several factors make the masseteric nerve an attractive donor source. These include its anatomic proximity to the facial nerve and mimetic muscles, its axonal density, low morbidity when sacrificed, and potential neurophysiologic and functional synergy with the facial nerve for facial expression.s. Literatur ^,​ s. Literatur

66.2 Anatomy of the Masseteric Nerve

The masseteric nerve is a branch of the trigeminal nerve originating from its motor root in the lateral aspects of the pons. On exiting the cranium through the foramen ovale, V3 divides into anterior and posterior divisions. The masseteric nerve arises from the anterior division of V3 together with the deep temporal, buccal, and the lateral pterygoid nerves. It passes through the posterior aspect of the sigmoid notch accompanied by the masseteric artery posterior to the insertion of the temporalis muscle on the coronoid process.s. Literatur The masseteric neurovascular bundle enters the deep surface of the masseter muscle and arborizes in an anteroinferior trajectory to innervate the muscle (Fig. 66‑1 ). The average diameter of the masseteric nerve is 2 mm at the level of the sigmoid notch. The nerve can be traced inferiorly for an average length of 27 mm before it divides into smaller caliber branches, less than 0.6 mm wide.s. Literatur Direct electrical stimulation of the trigeminal nerve in the retrogasserian region shows a motor conduction velocity of the masseteric nerve of about 54 m/s,s. Literatur which compares favorably to a velocity of 45 to 60 m/s in facial nerves.

66.3 Advantages of the Masseteric Nerve

The close proximity of the masseteric nerve to the facial nerve is a unique advantage, as it allows for mobilization of both nerves in a single surgical field for tension-free coaptation. The masseteric nerve may be coapted to the main trunk of the facial nerve or selected branches without the need for an interposition graft. Direct coaptation allows faster onset of recovery as the regenerating axons have a single barrier to cross to innervate the muscles. The masseteric nerve has a high fiber density which translates to strong neural input when coapted to facial nerve branches. Histologic studies have recorded an average myelinated fiber count from 1,500 to 2,700.s. Literatur ^,​ s. Literatur By comparison, the numbers of axons in normal hypoglossal and facial nerves were estimated at 9,778 ± 1,516 and 7,228 ± 950, respectively.s. Literatur A cross-facial nerve graft has been reported to have 100 to 200 fiber counts.s. Literatur The functional morbidity associated with the sacrifice of the masseteric nerve and subsequent denervation of the masseter muscle is usually minimal due to the overlapping function of the temporalis and medial pterygoid muscles. Additionally, anatomic studies have shown proximal branches of the masseteric nerve that can be preserved during nerve transfer preventing total denervation of the masseter muscle.s. Literatur ^,​ s. Literatur A major drawback that applies to the use of alternative cranial nerves for facial reanimation is the lack of spontaneous and emotional facial movement. Masseteric nerve transfer initially requires teeth clenching for activation; however, several studies have reported development of spontaneous facial motion with time. Manktelow et al reported 85% of patients were able to smile without biting in a review of 45 patients who underwent smile restoration with free flap innervated by masseteric nerve.s. Literatur In a similar study, Wang et al reported an effortless smile in 81% of their 16 patients at 12 months postoperatively.s. Literatur There are several reasons behind this potential cerebral adaptation. Electromyography (EMG) of the masseter muscle during normal smile has shown concurrent electrical activity in a subset of patients, suggesting a connection between the masseteric nucleus and the facial nucleus.s. Literatur Embryologic studies have also demonstrated the presence of trigeminal nerve fibers within the facial nerve branches.s. Literatur This potential connection between the masseteric and facial nerves most likely accounts for the potential to develop effortless smile.

Disadvantages of using the masseteric nerve for facial reanimation are relatively few. Wang et al reported masseter muscle atrophy following nerve transfer in 25% of their series of patients resulting in a concavity of the face.s. Literatur There are also reports of involuntary movements of the face during mastication following masseter nerve substitution.s. Literatur These reports are few and appear to have minimal impact on the patient’s quality of life. Table 66‑1 summarizes the pros and cons of the masseteric nerve and alternative cranial nerves commonly considered for facial reanimation.s. Literatur

66.4 Patient Evaluation

Facial paralysis following VS resection can be classified as reversible or irreversible based on the status of the facial muscles.s. Literatur Reversible paralysis implies the presence of electrophysiological and mechanically viable facial muscle fibers that will respond to reinnervation. In the absence of longstanding preoperative paralysis, most patients who developed facial paralysis after VS resection will have reversible paralysis. Patients who have prolonged preoperative paralysis or prolonged periods of denervation following tumor resection may be classified as having irreversible paralysis as the facial muscles become fibrotic and lose the ability to accept axonal ingrowth. In patients with incomplete paralysis or partial recovery, the facial muscles may remain electrophysiologically viable for many years and respond to further innervation. EMG studies may be helpful in determining the status of the facial muscles. The presence of fibrillation potentials indicates a physiological intact facial muscle despite denervation, whereas electrical silence represents irreversibly paralyzed facial muscles.s. Literatur

Patients deemed to have a reversible paralysis should undergo reinnervation as early as possible using the selected donor nerve or nerves. Patients with irreversible paralysis can only achieve dynamic facial movement after undergoing functional muscle tendon unit transfer or free functional muscle transfer procedures. Given the unique features of the facial muscles, all effort must be made to reestablish innervation to denervated facial muscles long before they become irreversibly paralyzed.