regarding potential complications. One must be familiar with all aspects of cutaneous anatomy including cosmetic subunits, skin tension lines, underlying blood supply, fascial planes, nerves, musculature, and other vital structures to avoid adverse and potentially devastating structural and functional consequences.

Summary: Innervation of the Face and Scalp

•The facial nerve (cranial nerve VII) provides motor innervation to the muscles of facial expression.

•The facial nerve possesses five main branches: the temporal, zygomatic, buccal, mandibular, and cervical rami.

•The temporal and marginal mandibular branches of the facial nerve are particularly susceptible to injury given their superficial locations.

The facial nerve originates in the pons and subsequently enters the internal acoustic meatus, coursing through the temporal bone and emerging through the stylomastoid foramen. Following its exit from the stylomastoid foramen, the facial nerve gives rise to the posterior auricular nerve which innervates the occipitalis and posterior auricular muscles [2, 3]. The site of entry of the facial nerve into the parotid gland may be approximated at the center of a line connecting the superior tragus with the angle of the mandible. After exiting the stylomastoid foramen, the facial nerve is protected by the mastoid process. Children under the age of five do not have a fully developed mastoid process; therefore the facial nerve may be particularly vulnerable to injury at this site [4].

Within the deeper substance of the parotid gland, the facial nerve first divides to form superior (temporofacial) and inferior (cervicofacial) divisions. It then presumes a more superficial location anteriorly within

rior scalp as well as motor innervation to the muscles of mastication.

•Prior to exiting the skull, the trigeminal nerve divides into three main branches: the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions.

•The sensory innervation of the external ear is complex and comprised of contributions from the trigeminal, facial, glossopharyngeal, and vagus nerves as well as the cervical plexus (C2 and C3).

vical rami (Figs. 30.1 and 30.2). A small percentage of patients may present with aberrations in this branching pattern [5]. Within the parotid gland, one must also be mindful of the parotid duct, which drains secretions from the parotid gland into the oral cavity. This structure courses superiorly to the masseter muscle, then deep to the buccinator muscle, opening into the oral cavity in the vicinity of the second upper molar. Its course can be plotted by drawing an imaginary line from the notch of the ear, inferior to the tragus, to the center of a vertical line from the alar rim to the oral commissure. Injury to the parotid gland itself may lead to the formation of a draining sinus which often heals spontaneously. If the parotid duct is injured, however,

30.2.1Motor Innervation of the Face and Scalp

The facial nerve (cranial nerve VII) provides motor innervation to the muscles of facial expression. In addition, it also provides sensory innervation to a portion of the external auditory canal, soft palate, and pharynx as well as taste to the anterior two-thirds of the tongue. Precise knowledge of the anatomy of this structure is critical for the cutaneous surgeon. Permanent injury to this nerve may lead to significant cosmetic and functional deficits [1].

surgical repair [1].

Following their exit from the parotid gland, divisions of the facial nerve are covered by superficial fascia until they reach their respective muscle of facial expression, which they enter at the posterolateral aspect. The most superior branch of the facial nerve is the temporal branch. This branch exits the parotid gland superiorly, coursing over the zygomatic arch and temporal fossa and further divides into four rami innervating the orbicularis oculi, corrugator supercilii, frontalis, and temporoparietalis muscles [4].

The area of greatest risk of injury to the temporal branch can be estimated by connecting the endpoints

of the lines drawn from the earlobe to a point just superiorly and laterally to the highest forehead crease and from the earlobe to the lateral eyebrow (Fig. 30.3) [5]. Damage to the temporal branch of the facial nerve may lead to paralysis of the frontalis muscle and resultant flattening of the forehead and inability to elevate

the brow (Fig. 30.4a, b). Compensatory wrinkling of the contralateral forehead and elevation of the contralateral brow also occurs, leading to quizzical appearance. Muscle atrophy may ensue over time, causing ptosis and redundant eyelid skin, and the visual field may be obstructed. Procedures such as browplasty or

Danger zone of the temporal branch of the facial nerve

Danger zone of the marginal mandibular branch of the facial nerve

Fig. 30.3 Danger zones of the face

blepharoplasty may lessen the functional and cosmetic deficits following injury to this nerve [6].

The zygomatic branch of the facial nerve travels in the direction of the lateral canthus and provides innervation to the lower portion of the orbicularis oculi, procerus, and nasalis muscles. Muscles of the midface and lip elevators, specifically the depressor septi, nasalis, levator labii superioris alaeque nasi, levator labii superioris, zygomaticus major/minor, buccinator, orbicularis oris, and levator anguli oris muscles, are supplied by the buccal division of the facial nerve [2, 4]. It should be noted that the exact muscles supplied by the buccal and zygomatic divisions of the facial nerve may vary among individuals, and these branches arborize and anastomose with one another.

The buccal and zygomatic branches may be more susceptible to injury as they travel over the buccal fat pad. In this location, they run rather superficially and are only covered by a thin fascia and risorius muscle which may be of variable thickness or even absent in some individuals [4]. If the zygomatic branch is disrupted, one may experience difficulty tightly closing the lower eyelid. Weakness of the lip elevators, buccinators, or nasal muscles is less common due to significant anastomoses with the buccal branch of the facial nerve.

Another branch of the facial nerve particularly susceptible to injury is the marginal mandibular ramus due its superficial location as well as its lack of anastomo-

Fig. 30.4 (a) Right facial nerve paralysis (right brow ptosis, right lower lid eversion and ectropion, asymmetric smile). (b) Patient with facial paralysis who developed severe lagophthalmos and Bell’s phenomenon to protect the cornea

ses with other branches. It is responsible for innervating the risorius, orbicularis oris, depressor anguli oris, depressor labii inferioris, mentalis, and platysma muscles. This branch exits the inferior aspect of the parotid gland and maintains a superficial course as it travels over the masseter muscle near the angle of the mandible. In this location, it is only covered by skin, subcutaneous fat, and fascia. The marginal mandibular nerve then courses over the body of the mandible where it is covered superficially by platysma muscle (Fig. 30.3). The thickness of the platysma muscle is relatively unpredictable and may only present as a layer of fascia. In approximately 20% of individuals, the marginal mandibular nerve travels 1–2 cm and rarely up to 4 cm below the mandible [5]. The nerve may also be translocated below the angle of the mandible in surgical positioning if the head is hyperextended and rotated contralaterally [1]. Marginal mandibular nerve injury

Fig. 30.5 (a and b) Marginal mandibular nerve injury (a – at rest, b – full smile)

results in an asymmetric smile (Fig. 30.5a, b). When smiling is attempted, there is an inability to pull down the ipsilateral lip or evert the vermillion border, creating the appearance of a smirk. At rest, there may be little obvious deformity. The final branch of the facial nerve is the cervical division which innervates the platysma muscle and is of little functional significance [4].

Motor nerve injury may lead to permanent anatomic defects, especially if a nerve root or a large, proximal, or non-anastomosing nerve branch is transected. Generally, nerves located medially to a line from the lateral canthus to the angle of the mouth are relatively protected from permanent damage either by overlying musculature or sufficient anastomoses. Temporary

motor nerve paralysis from local anesthesia may last several hours following injection. Neuropraxia, or injury to a nerve secondary to stretching, is reversible but may take up to 6 months to recover. Permanent nerve injury may be differentiated from temporary nerve injury by inability to mount adequate muscle contraction on nerve stimulation test [4].

30.2.2Sensory Innervation of the Face and Scalp

The trigeminal nerve (cranial nerve V) provides sensory innervation to the face and anterior scalp as well as motor innervation to the muscles of mastication. Prior to exiting the skull, it divides into three main branches: the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions (Fig. 30.6). Several branches of the trigeminal nerve exit the skull through foramina located along the midpupillary line, providing specific locations for effective regional nerve blocks. Sensory nerves are located more superficially than the motor branches of the facial nerve; hence, they are more susceptible to trauma [6].

Prior to exiting the orbit, the first branch of the trigeminal nerve, the ophthalmic nerve (V1), divides into three rami: the frontal, nasociliary, and lacrimal nerves. The frontal branch is the largest branch and a continuation of V1. Within the skull, it subdivides, giving off the supratrochlear and supraorbital nerves. The supratrochlear nerve exits the orbit approximately 1 cm lateral to the midline, piercing the corrugator muscle and traveling superiorly. This branch innervates the medial aspects of the forehead, scalp, conjunctiva, and upper eyelid. The supraorbital nerve exits via the supraorbital foramen at the superior orbital rim approximately 2.5 cm lateral to the midline. It provides innervation to the scalp, forehead, conjunctiva, and upper eyelids [4, 6]. The nasociliary branch bifurcates to give rise to the infratrochlear nerve as well as the anterior ethmoidal nerve, which innervates the upper nasal and septal mucosa and terminates as the external nasal nerve. The infratrochlear branch leaves the skull superiorly to the medial canthus and supplies sensation to the nasal root and medial canthal region. The external nasal branch of the anterior ethmoidal nerve exits onto the nose between the nasal bone and superior lateral nasal cartilage and provides sensory innervation to the nasal dorsum, supra-tip, tip, and columella. The lacrimal nerve exits at the superolateral aspect of the orbital

Trigeminal ganglion

1.Ophthalmic nerve: Frontal N.

Supraorbital N.

Supratrochlear N.

Infratrochlear N.

Lacrimal N.

Anterior ethmoidal N.

External nasal branch

Nasociliary N.

2.Maxillary nerve:

Infraorbital N.

Zygomaticofacial N.

Zygomaticotemporal N.

Nasopalatine N.

3.Mandibular nerve: Mental N.

Inferior Alveolar N.

Lingual N.

Buccal N.

Auriculotemporal N.

Fig. 30.6 Branches of the trigeminal nerve

Cervical plexus: Lesser occipital N.

Great auricular N.

Transverse cervical N.

Supraclavicular N.

rim and supplies sensation to the lateral upper lid, conjunctiva, and lateral forehead (Fig. 30.7) [6, 7].

The sensory innervation of the midface is supplied by the maxillary (V2) division of the trigeminal nerve. This nerve exits the middle cranial fossa via the foramen rotundum, giving off two small branches, the zygomaticotemporal and the zygomaticofacial nerves. The zygomaticotemporal nerve exits the skull near the lateral orbital margin and provides sensory innervation to the anterior temple and supratemporal scalp, and the zygomaticofacial nerve emerges from the zygomatic bone, lateral to the infraorbital foramen, to innervate the malar eminence and a portion of the lateral canthus. The maxillary division of the trigeminal nerve then courses through the infraorbital canal at the floor of the orbit, giving off branches to the upper alveolus, gingival mucosa, palate, and nasal floor and finally

exits the skull through the infraorbital foramen. These terminal branches supply sensory innervation to the lower eyelid, conjunctiva, medial cheek, nasal ala, upper lip, and superior labial mucosa (Fig. 30.7) [2, 4]. The infraorbital foramen is located at the midpupillary line 2.5 cm lateral to the midline and 1 cm inferior to the lower orbital rim. A regional nerve block may be performed at this location either through a percutaneous or an intraoral approach.

The mandibular nerve (V3) is the largest and most complex branch of the trigeminal nerve. It not only provides sensory innervation but is also responsible for motor innervation of the muscles of mastication (the temporalis, masseter, and medial and lateral pterygoid muscles), the tensor tympani, and the tensor veli palatini. There are three main branches of the mandibular nerve: the auriculotemporal, buccal, and inferior alveo-

lar branches. The auriculotemporal nerve is a branch of the posterior trunk of the mandibular nerve which emerges inferiorly to the zygomatic arch to course superficially, just deep to the superficial temporal artery. It travels superiorly over the zygomatic arch and anterior to the ear, providing innervation to the posterior temple, temporoparietal scalp, upper one-third of the auricle, tympanic membrane, and the anterior portion of the external ear and external auditory canal (Fig. 30.7). Interestingly, the auriculotemporal nerve also possesses parasympathetic secretomotor fibers which innervate the parotid gland. Disruption and misdirection of these fibers to sweat glands may lead to Frey syndrome [5].

The buccal branch of the mandibular nerve initially courses deep to the parotid gland then travels more superficially over the surface of the buccal fat pad and buccinator muscle. It provides sensation to the skin of the mid-cheek then pierces the buccinator, traveling inferiorly to supply the buccal and gingival mucosa. The inferior alveolar branch terminates as the mental nerve after coursing through the mandible to supply sensation to the lower teeth. The mental nerve exits the mandible through the mental foramen, innervating the skin of the chin, the lower lip, and the inferior labial

mucosa. A mental nerve block may be performed by locating the mental foramen, which is approximately 2.5 cm lateral to the midline in the center of the mandible or inferior to the second premolar if the intraoral approach is preferred [6].

Sensory innervation to the scalp is provided by branches of the trigeminal nerve as well as the cervical plexus. These nerves course through the scalp at the level of the subcutaneous fat and travel centripetally. Caution must be taken in this region as transections of these nerve branches may lead to large areas of permanent hypoesthesia. Several branches of the trigeminal nerve are responsible for sensory innervation to the scalp. The supratrochlear and supraorbital nerves, branches of V1, supply the central scalp extending anteriorly from the forehead as far as the vertex. The temporal scalp is innervated by the zygomaticotemporal nerve, a division of V2, as well as the auriculotemporal branch of V3. The lesser occipital nerve is a branch of the cervical plexus (C2) which emerges posterior to the sternocleidomastoid muscle and travels superiorly to supply sensation to the lateral scalp posterior to the ear. The greater occipital nerve is a branch of C2 and C3 which supplies the occipital scalp to the