Surgical Anatomy of the Whole Facial Nerve for Enabling Craniofacial and Regenerative Medicine Translational Research in Swine

Facial nerve injuries can be significantly morbid with profound functional and psychological impacts on patients.[1] Resulting facial paralysis can derive from a wide and diverse array of causes ranging from penetrating and blunt trauma to genetic, metabolic, infectious, neoplastic, iatrogenic, and idiopathic conditions. The management of facial paralysis continues to evolve and treatment is largely individualized to each patient's unique situation.[1] One aspect of potential interest is the role of functional nerve conduits or artificial nerve grafts when autologous options for neurotization or grafting are limited.[2] [3] Although mouse and rat models have been used to study methods of peripheral nerve injury and allograft use, there exist fundamental differences between the human and rat facial nerve anatomy and function.[4] Due to histological differences, regeneration studies with rat facial nerves may not exactly mimic the human clinical situation where regenerating nerves have less of a choice regarding the direction of growth once they have entered a fascicle.[4] In addition, large animal models permit the creation of longer (> 30 mm) nerve gaps, which more closely resemble the clinical arena.

The experimental findings in large animal models, such as swine, are an important step toward translation into humans and eventual clinical trials. One essential element for fostering such research is an accurate understanding of the anatomy of the large animal models. Unfortunately, the literature in facial nerve regeneration in large animal models, specifically swine, is scarce.[2] [5] [6] Swine have previously been considered to have the most suitable anatomy for trauma studies to the intratemporal portion of the facial nerve.[5] Recent studies such as those from Sasaki et al have provided valuable insight into the surgical anatomy of the swine face[7] and more recently the hypoglossal nerve.[8] However, a major limitation in all of these studies is that the description of the extratemporal facial nerve is incomplete.[5] [6] [7] [8] To our knowledge, there is no study accurately describing the surgical anatomy and branches of the extratemporal facial nerve in swine.

We performed facial dissections in a total of eight female swines (two Göttingen Minipigs and six Yorkshire), with the goal of better appreciating this intricate anatomy. Miniature swine have been used in a myriad of craniofacial and regenerative translational research models given their ease of use and similarities in anatomy to humans.[2] [6] [7] [8] [9] We studied both a commonly used swine in translational research, the Yorkshire, in addition to a miniature swine, the Göttingen Minipig, to compare the extratemporal facial nerve anatomy between the different species of varying sizes (30–50 kg). Details of all swine cadavers are provided in [Table 1]. All dissections took place within 24 hours of euthanasia and the surgical approach was based on descriptions by Ellis and Zide.[10]

Table 1 Detailed characteristics of swine cadavers

Breed; purpose	Weight (kg)	Branches (no.)	Length of main trunk (cm)	Accessibility of main trunk	Reproducibility	Diameter of main trunk (mm)
Göttingen Minipig; cardiology	30	4	0.5	Y	Y	2
Göttingen Minipig; cardiology	32	4	0.5	Y	Y	2
Yorkshire; wound healing	50	4	0.5	Y	Y	4
Yorkshire; wound healing	50	4	1	Y	Y	4
Yorkshire; vendor	30	4	1	Y	Y	3.5
Yorkshire; wound healing	50	4	None visible; two branches	Y	Y	3.5 (from most proximal extratemporal portion)
Yorkshire; IRI	37	4	Left 0.5; right 1	Y	Y	3.5
Yorkshire; IRI	35	4	0.5	Y	Y	3.5

Abbreviations: IRI, ischemia-reperfusion injury; Y, yes.

Bilateral facial dissections were performed through a retromandibular incision one fingerbreadth medial to the angle of the mandible with a submandibular extension. The thick platysma was then exposed by way of skin flaps and was dissected and split to expose the superficial layer of deep cervical fascia. Sasaki et al describes two main branches of the facial nerve, the buccal and the marginal mandibular branch (MMB) with its upper and lower divisions.[7] However, our dissections consistently found four branches emanating from a main facial nerve trunk exiting the stylomastoid foramen ([Fig. 1]). We identified these additional nerves as the cervical and auricular branches. In humans, the course of the facial nerve takes a tortuous path from the pontomedullary junction to the temporal bone through the internal auditory meatus. After transitioning through the facial canal of the temporal bone, it exits through the stylomastoid foramen to become the extratemporal facial nerve. In our study swine, the main facial nerve trunk was easily identified due to a notable stylohyoid tendon that inserted superior to the stylomastoid foramen. The styloid process and stylohyoid tendon served as reference points for all dissections. This is similar to anatomy observed in humans. The nerve branches typically coursed over a large maxillary vein with accompanying tributaries that were ligated for ease of dissection during these studies ([Fig. 2]), but may be preserved for live animal surgery. No attempts were made to explore the intratemporal portion of the facial nerve, and all nerves were sharply divided at the stylomastoid foramen for gross examination.

Fig. 1 Retromandibular view of the right side of the Göttingen Minipig showing all branches of the facial nerve. Au, auricular branch of the facial nerve; BB, buccal branch of the facial nerve; Ce, cervical branch of the facial nerve; MM, masseter muscle; MMB, marginal mandibular branch of the facial nerve; PG, parotid gland; SG, submandibular gland.

Fig. 2 Retromandibular view of the left side of the Yorkshire face after ligation of maxillary veins. Arrow points to the main trunk of the facial nerve. BB, buccal branch of the facial nerve; Ce, cervical branch of the facial nerve; MM, masseter muscle; MMB, marginal mandibular branch of the facial nerve; MPM, medial pterygoid muscle.

There was variation in the diameters of the main facial nerve trunk that correlated to body weight (see [Table 1]). However, the overall geometry of the facial nerve and its branches remained constant in all eight swine ([Fig. 3]). Specifically, the buccal and MMB were at approximately 45 degree angles to the proximal main trunk and the auricular and cervical branches at 90 degree angles to the main trunk due to its natural course from the skull base. The cervical branch was typically of a similar size as the MMB and could be reliably traced and dissected for a distance of about 5 cm as it coursed over the swine's sternocephalic (sternomastoid) muscle. The auricular branch was also of a similar diameter, but in about half the swine there was an additional branch that derived from the proximal portion of the buccal and joined the main auricular branch. In addition, there was one instance in study swine no. 6 ([Table 1]) where there was no evidence of an extratemporal main trunk, but rather two distinct branches exiting the stylomastoid foramen, which raises the possibility that a proximal branching point existed within the temporal bone. Subtleties and more extreme variations are to be expected between animals and different breeds of swine, as they are with humans. This study adds to the work of Sasaki et al[7] [8] with the goal of providing anatomic studies of the swine facial nerve for craniofacial and nerve regeneration translational research in large animal models.

Fig. 3 Right facial nerve of Yorkshire swine. Au, auricular branch of the facial nerve; BB, buccal branch of the facial nerve; Ce, cervical branch of the facial nerve; FN, facial nerve; L, lower division of the MMB; MMB, marginal mandibular branch of the facial nerve; U, upper division of the MMB.

Note

Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the Department of Defense.

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