Extracranial Course and Terminal Branches of the Facial Nerve
Overview
After exiting the skull through the stylomastoid foramen, the facial nerve courses through the parotid gland and divides into five terminal branches: temporal, zygomatic, buccal, marginal mandibular, and cervical. 1
Extracranial Course
Exit from Temporal Bone
- The facial nerve exits the temporal bone at the stylomastoid foramen after completing its intratemporal course through the labyrinthine, tympanic, and mastoid segments. 2, 1
- Upon exiting, the nerve trunk is located at a depth of 1 to 2 cm from the skin in the vagino-mastoid angle. 3
Course Through Parotid Gland
- The extracranial segment courses through the parotid gland, representing the intraparotid portion of the nerve. 2, 1
- The nerve can be visualized on MRI using CISS sequences from the stylomastoid foramen to the temporofacial and cervicofacial trunks, proximal to the parotid plexus. 4
Primary Division
- The facial nerve bifurcates at the posterior border of the ramus of the mandible into two main divisions: the temporofacial (superior) trunk and the cervicofacial (inferior) trunk. 5
- The mean distance of bifurcation from the angle of the mandible is approximately 28 mm, with 81% occurring within 21 to 35 mm. 3
- In 3.8% of cases, trifurcation rather than bifurcation may occur. 3
Terminal Branches
Five Main Branches (Cranial to Caudal)
The facial nerve divides into five terminal branches that innervate the muscles of facial expression: 1, 5
Temporal branch: Can be outlined between two diverging lines from the earlobe to the lateral ends of the eyebrow and the highest frontal crease. 3
Zygomatic branch: Innervates muscles of the midface and periorbital region. 5
Buccal branch: Supplies muscles around the mouth and cheek. 5
Marginal mandibular branch: Passes below the inferior border of the mandible in 20% of cases posterior to the facial artery; anterior to the artery, it divides into one to four branches supplying the mentalis and depressor labii inferioris muscles. 3
Cervical branch: Innervates the platysma muscle. 5
Branching Pattern Variability
- The branching patterns are highly variable and have been classified into 12 distinct types, with the most frequent being Type 3 (21.05%) showing connections between temporal, zygomatic, and buccal branches, and Type 8 (21.05%) demonstrating a complex network between temporal, zygomatic, buccal, and mandibular branches. 5
- The classical textbook pattern (Type I) is actually one of the least common patterns, occurring in only 11.39% of cases. 3
- Interconnections between branches are common and clinically significant for surgical planning. 5
Clinical Implications
Surgical Considerations
- Detailed knowledge of the number of terminal branches and connections between branches is more clinically useful for surgeons than complex classification systems when attempting to avoid iatrogenic injuries during surgery. 5
- The multiple branches and variable anatomy increase the risk of iatrogenic injury during procedures involving the parotid region, temporal bone, or face. 6
Pathology Along Extracranial Course
- The extracranial facial nerve may be affected by inflammation, parotid tumors, conditions of neighboring anatomic spaces, and skull base pathology including carcinomas, sarcomas, trauma, and inflammatory disease. 2
- Perineural tumor spread can result in facial nerve palsy along the extracranial course. 2
Functional Fiber Distribution
- General sensory fibers provide surface innervation to a small area of the external ear and tympanic membrane. 1
- Special sensory fibers convey taste sensation from the anterior two-thirds of the tongue via the chorda tympani branch. 1
- Parasympathetic fibers innervate the lacrimal and salivary glands. 2