Anatomy of the oropharynx: The robotic perspective

Transoral robotic surgery uses robotic surgical tools in a minimally invasive fashion to remove lesions of the oral cavity, oropharynx, nasopharynx, hypopharynx, and larynx through the open mouth. This method has resulted in head and neck surgeries that avoid the scars and morbidities associated with the traditional approaches to tumors in this region. O'Malley et al. first described the technique in 2005 at the University of Pennsylvania, and since then significant advances have allowed for more refined techniques, as well as increased indications for robotic surgery in otolaryngology.^{,  ,} 

Compared with the traditional open surgical approach, robotic surgery requires a different perspective when approaching an oral cavity, pharyngeal, or laryngeal lesion or tumor. The view is often rotated and angled depending on the type of endoscope utilized. This change in perspective can initially increase the technical difficulty of the surgery. Although similar to functional endoscopic sinus surgery, the use of a 3-dimensional endoscope avoids the flattening effect seen with traditional endoscopes.

We used the da Vinci Surgical System (Intuitive Surgical Inc, Sunnyvale, California) to conduct an examination of the vasculature of the oropharynx, hypopharynx, and supraglottis. This research highlights the anatomy of the head and neck as shown using a robotic surgical system.

The structures of the head and neck arise from the pharyngeal (or branchial) arches, with the ectoderm making the skin, the mesoderm accounting for the musculoskeletal structures and neurovasculature, and the endoderm lining the mucosal surfaces. There are a series of 5 pharyngeal grooves and pouches that separate 6 arches. The grooves are invaginations of ectoderm and the pouches represent corresponding indentations of endoderm, whereas the arches represent thickening of mesoderm.

The first branchial arch eventually develops into the lower lip, mandible, the muscles of mastication, the oral tongue, malleus, sphenomandibular ligament, as well as the maxillary process that forms the incus, lateral aspects of the upper lip and the cheek, as well as the tensor tympani and tensor veli palatini. This arch also forms the floor of the mouth musculature: the mylohyoid and anterior belly of the digastric. The second arch forms the stapes as well as the anterior and lateral neck including the styloid process, and lesser cornu of the hyoid. The stylohyoid, platysma, buccinator, muscles of facial expression, stapedius, and posterior belly of the digastrics also arise from the second arch. The third arch develops into the greater cornu of the hyoid and stylopharyngeus, whereas the fourth and fifth arches make the thyroid and epiglottic cartilage. The fourth arch also forms the pharyngeal constrictors (superior, middle, and inferior), cricothyroid, and levator veli palatini. The sixth arch forms the cricoid, arytenoid, corniculate, and tracheal cartilages as well as the intrinsic muscles of the larynx.

The first arch is innervated by the trigeminal nerve, whereas the second arch follows with branches of the facial nerve; the third arch is supplied by the glossopharyngeal nerve and the fourth and sixth arch is innervated by the vagus nerve. With each of the pharyngeal arches, there is a corresponding primitive aortic arch. The second arch gives rise to the stapedial artery and its branches. Although the stapedial arch routinely atrophies, its branches form the internal maxillary artery and branches of this artery. The third arch forms into the internal carotid artery and the bifurcation between the internal and external carotid. The common carotid and external carotid arise from the paired ventral aorta. The fourth arch develops into the aortic arch and right subclavian artery.

The oropharynx is composed of the soft palate, anterior and posterior tonsillar pillars, tonsillar fossa, posterior pharyngeal wall, and the base of tongue (Figure 1). The neurovascular and muscular relationships are important in this area when performing base of tongue resections and radical tonsillectomies for oncologic cure.

There is a rich vascular supply to this region with numerous anastomoses of the branches of the external carotid artery systems. The tonsillar fossa and pillars are supplied by branches of the lingual, facial, ascending pharyngeal, and internal maxillary arteries. Although the terminal branches of these arteries are often unnamed, the intermediate branches are worth noting. The ascending pharyngeal divides into a pharyngeal and a neuromeningeal branch with the pharyngeal branch dividing into superior, middle, and inferior pharyngeal arteries. The middle and inferior pharyngeal arteries have branches that supply the tonsillar fossa. Additionally, the internal maxillary artery supplies a descending palatine artery that travels through a bony canal into the tonsillar fossa. Occasionally, this artery arises from the ascending palatine, and often an anastomosis between the internal maxillary and ascending palatine systems exists. The facial artery branches with a tonsillar branch and an ascending palatine branch. The ascending palatine often branches near the levator veli palatini into a branch that supplies the soft palate and anastomoses with the contralateral artery. The other branch pierces the superior constrictor and supplies the tonsillar fossa and often anastomoses with both the ascending pharyngeal as well as the tonsillar branch of the facial artery. The venous supply for the tonsillar fossa is from a plexus of tonsillar veins that then drain into the retromandibular vein or the posterior facial vein eventually joining the internal jugular. The view obtained from the robot will be very similar to that noted when performing a traditional tonsillectomy.

The muscle groups can be divided into the constrictors, muscles of mastication, palatine muscles, and tongue musculature. As reviewed in the embryology section, branchial arches 2 through 4 develop into these structures. The medial pterygoid attaches at the medial aspect of the mandible and travels to the medial aspect of the lateral pterygoid plate. This muscle lies just lateral to the superior constrictor. Both the superior and middle constrictors are visible posteriorly in the oropharynx. Laterally, the superior constrictor inserts into the pterygomandibular raphe, which is also the posterior insertion of the buccinator muscle. It also serves as the lateral border of the tonsillar fossa. The anterior and posterior tonsillar pillars are defined by the palatoglossus and palatopharyngeus muscles, respectively (Figure 1). The palatine musculature includes the tensor and levator veli palatini muscles. Lateral to the superior constrictor, the styloglossus, stylohyoid, and stylopharyngeus travel from the styloid process inserting into the tongue, hyoid, and thyroid cartilages, respectively.

In approaching the tonsillar fossa, often a cut is made through the mucosa and palatoglossus. In radical tonsillectomies, the superior constrictor is medialized and much of the vasculature is appreciated perforating the constrictor and entering the tonsillar fossa. As described earlier, the descending palatine and branches of the ascending pharyngeal artery can be seen supplying the superior pole, whereas the facial artery supplies the midpole and also supplies the inferior pole alongside branches of the lingual artery. Lateral to this plane, the medial pterygoid can be visualized, and often the styloglossus lies between the superior constrictor and medial pterygoid (Figure 2). Deep and lateral to the styloglossus by the inferior pole, the submandibular gland can be accessed intraorally.

Using the transoral robotic surgery approach, a view similar to Figure 1 can be obtained. This shows the palatal arch, uvula, and tonsillar fossa with atrophied tonsils in a cadaveric model. Figure 2 displays the same cadaver after dissection with exposure of the superior constrictor, medial pterygoid, and styloglossus muscles. Additionally, the vascular supply to this area is clearly delineated with the major branches of the external carotid easily visible. The submandibular gland can also be appreciated at the inferior aspect of the dissection.

The tongue receives its arterial supply from the lingual artery, with the major branch toward the base of tongue being the dorsal lingual artery or arteries. These branch out of the lingual artery and travel superiorly off the main trunk to supply the base of the tongue. Associated veins travel with the artery and drain into the internal jugular vein.

The tongue consists of both extrinsic and intrinsic muscles. The extrinsic muscles include the styloglossus and palatoglossus as well as the genioglossus and hyoglossus. The genioglossus comprises a significant portion of the body of the tongue and attaches to the mental spine of the mandible as well as the hyoid bone and dorsum of the tongue. The hyoglossus lies medial to the stylohyoid and inserts from the hyoid bone to medial to the styloglossus muscle. The intrinsic muscles represent thin sheaths of muscle that provide shape to the tongue. The base of tongue ends posteriorly at the vallecula and meets the epiglottis. Between the tongue and epiglottis are the median and lateral glossepiglottic ligaments; and deep to these structures lies the hyoepiglottic ligament.

Approaching the base of tongue requires resection of the overlying lymphoid tissue. The muscle fibers of the intrinsic muscles are often difficult to delineate, but an extensive vascular network from the dorsal lingual and lingual arteries is easily visualized. The main trunk of the lingual artery can often be identified just medial to the insertion of the palatoglossus (Figure 3). The lingual nerve lies lateral to the hyoglossus muscle, whereas the lingual artery is often found medial to this muscle (Figure 4). At the level of the vallecula, it is often easy to identify the hyoid laterally through the mucosa (Figure 3). A branch of the lingual artery can also be identified running toward the lingual surface of the epiglottis (Figure 4). Figure 4 further exposes the base of tongue, exposing the hyoglossus and lingual nerve.

The hyoid bone can be easily visualized through the mucosa, lateral to the epiglottis at the level of the vallecula. Dissecting through the mucosa would reveal the suprahyoid muscular attachments to the hyoid. Dissecting through these muscles to the bone would often reveal the suprahyoid artery. The tendon of the digastric can be found just lateral to the greater cornu, alongside the hypoglossal nerve. Deep to the hyoid, the thyrohyoid membrane can be visualized laterally. Figure 5 displays a dissection lateral and inferior to the hyoid bone, displaying the superior laryngeal neurovascular bundle, the tendon of the digastrics, as well as the superior thyroid artery and vein.

A surgical robotic system provides another tool available to the head and neck surgeon in approaching tumors of the oropharynx, nasopharynx, hypopharynx, and supraglottic larynx. Although the anatomical relationships are readily understood and appreciated when approaching these regions directly or externally, it is important to maintain these spatial relationships when approaching the region endoscopically.