Facial paralysis can be a devastating injury that results in functional impairment of the eyelids, nose, and lips. Impaired facial expression during communication and the associated blunted emotional exchange significantly affects both the patient and the interactive circle and can lead to depression and strained relationships. The surgical transfer of functional muscle units to the face is presently the only effective option for restoring tone and dynamic animation when the facial muscles are irreversibly paralyzed. Irreversible paralysis of the facial muscle may be the result of prolonged atrophy from chronic denervation, primary muscle disease, extensive scarring, congenital paralysis, and radical surgical resection. Common to these causes is the absence of viable motor units that can respond to neural input. Functional muscle units can be transferred to the face as free neuromuscular units that require reestablishment of neurovascular input using microsurgical techniques. Advantages of free functional muscle transfer include the flexibility in selecting the donor muscle, desired vector of muscle excursion, muscle length and tension, and donor nerve. Recruiting the contralateral facial nerve to drive the free functional muscle provides the potential for achieving a voluntary smile. Free functional muscle transfer is, however, technique intensive and does not provide immediate reanimation. An alternative to free functional muscle flaps for the correction of irreversible facial paralysis is the transposition of regional muscle tendon units (MTU) that maintain their original neurovascular supply. An intact MTU with a given function may be repurposed to perform a new function by releasing and reattaching the tendon from its native insertion site to a new target. Candidate muscles for regional muscle transfer to the paralyzed face include the temporalis, masseter, platysma, and digastric muscles. The major advantage for regional MTU transfer is the potential for immediate restoration of dynamic facial movement in a single-stage procedure. The transfer of the temporalis MTU is the most commonly described regional MTU procedure for facial paralysis and is the focus of this chapter. The transfer of the temporalis muscle as an MTU is distinguished from the transfer of the muscle belly over the zygomatic arch.

In 1952, McLaughlin introduced the concept and technique of mobilizing and transposing the temporalis tendon for facial suspension. This technique was later replaced by the temporalis turndown flap popularized by Rubin, Baker, and Conley. This traditional method had several disadvantages including donor site depression, midfacial widening, and nonanatomic contraction of transposed muscle segment. The temporalis MTU transfer later underwent several refinements to improve functionality and aesthetic appearance. Several authors highlighted the advantages of temporalis transfer in an orthodromic manner. Breidahl modified this technique by approaching the tendon externally, and subsequently, Croxson further modified the procedure by accessing the coronoid-tendon complex through the nasolabial fold. Boahene as well as others adopted a minimally invasive approach for transposing the temporalis MTU through the buccal space sublabial incisions.

The successful application of the temporalis MTU for facial reanimation depends on a fine-tuned adoption and application of the principles and biomechanics of MTU transfer. The principles of MTU transfer have
evolved over a century through extensive experience in upper extremity reconstruction following injuries of the median, ulnar, and radial nerves. Even though the complexity of coordinated movements of facial muscles poses several challenges in achieving optimal functional outcome, the fundamental principles of MTU transfer and subsequent rehabilitation are applicable to all muscles including facial muscles. Table 46.1 outlines the fundamental principles of MTU transfer in extremities, the basic tenets of which may be applicable to all types of functional muscle transfer procedures including the temporalis muscle tendon transfer.

Of the fundamental principles of MTU transfer, the insertion of the temporalis muscle at the ideal length and tension for adequate excursion is the most important principle necessary for achieving a dynamic smile instead of a mere static suspension.

When selecting patients for a temporalis tendon transfer, a thorough history is necessary to establish the cause and duration of the paralysis. Causes of facial paralysis that allow for potential spontaneous recovery should prompt close follow-up and conservative measures for eye protection. Paralysis from progressive neurologic diseases that may involve multiple cranial nerves including the trigeminal nerve or muscles group as in muscular dystrophy should prompt careful considerations before suggesting a temporalis MTU.

The history taking should also elicit information about previous treatments. Patients who have underdone the classic temporalis muscle transfer procedure in which a segment of the muscle belly was transferred over the zygomatic arch are still candidates for reversal and transfer of the tendon as an orthodromic MTU transfer.

A history of radiation therapy over the lateral face is important to note as secondary fibrosis may modify tissue glide planes and limit the potential excursion that may result from transfer of the temporalis tendon. While not a contraindication for the temporalis MTU, the effects of prior radiation on outcomes should be discussed with the patient prior to the procedure.