Facial Nerve
In the neck, the superficial cervical fascia and platysma are continuous with the SMAS, and the deep cervical fascia is found on the superficial surface of the strap muscles, superior to the hyoid bone. The deep cervical fascia overlies the myelohyoid muscle and extends superiorly over the body of the mandible, continuing as the parotideomasseteric fascia. The facial nerve lies deep to this thin layer in the lower face. Above the zygomatic arch in the temporal region, the parotideomasseteric fascia is continuous with the deep temporal fascia, and the temporal (frontal) branch of the facial nerve lies superficial to this fascial layer. The transition of the temporal branch of the facial nerve from deep to superficial occurs as the nerve crosses over the zygomatic arch. When biopsy of the superficial temporal artery is performed, care is taken to avoid injury to the temporal branch of the facial nerve passing just inferior to the artery, both of which lie in the temporoparietal fascial plane (Fig 9-4).
The facial nerve, cranial nerve VII (CN VII), innervates the mimetic muscles and divides into 5 major branches within or deep to the parotid gland (Fig 9-5): temporal (frontal), zygomatic, buccal, marginal mandibular, and cervical (Fig 9-6). Two surgical planes help surgeons avoid CN VII when operating: dissection on top of the deep temporal fascia (see Fig 9-4), which is deep to the SMAS and deep to CN VII, in the upper face and temporal region; and dissection superficial to the SMAS and CN VII branches in the lower face.
In the temporal area, the temporal branch of CN VII (see Figs 9-5, 9-6) crosses the zygomatic arch and courses superomedially in the deep layers of the temporoparietal fascia. The temporoparietal fascia is continuous with the SMAS of the lower face and the galea aponeurosis of the upper face. Deep to the temporoparietal fascia is the previously mentioned deep temporal fascia, a dense, immobile fascia that overlies the temporalis muscle and is continuous with the frontal periosteum (see Fig 9-1C). Dissection along this fascia allows mobilization of the temporal forehead while avoiding the overlying temporal branch of the facial nerve. This anatomic principle is important when performing brow-lifting and forehead-lifting procedures. When performing a rotation flap in the lateral canthus and temporal region, the safety zone is within 2 cm (on average) from the lateral canthal angle to avoid the frontal branch of the facial nerve as it crosses over the zygomatic arch (Fig 9-7).
In the lower face, the facial nerve branches, sensory nerves, vascular networks, and parotid gland and duct are deep to the SMAS (see Figs 9-1A, 9-6). Dissection just superficial to the SMAS, parotid gland, and parotideomasseteric fascia in the lower face avoids injury to these structures. The face receives sensory innervation from the 3 branches of CN V: V1, ophthalmic; V2, maxillary; and V3, mandibular (Fig 9-8). Damage to these nerves causes facial numbness and paresthesia. Fortunately, overlap of the distal branches makes permanent sensation loss unusual, unless injury occurs at the proximal neurovascular bundles or with extensive distal disruption, as can be seen with a coronal incision.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.