Background

Transoral robotic surgery (TORS) radical tonsillectomy, first described by Weinstein et al in 2007, is a modified variation of the transoral lateral oropharyngectomy described by Holsinger in 2005. ¹ Preoperative evaluation and planning with crosssectional imaging for this procedure is paramount in order to ensure the safety of nearby important vascular structures, such as the carotid arterial system, and also to help preserve functional outcomes. As will be shown, the anatomical relationships in the tonsillar region with its many vital structures require a strong knowledge of the anatomy of this region as well as familiarity with the TORS perspective. The dissection will involve the parapharyngeal and retropharyngeal spaces, the lateral base of the tongue, and the soft palate.

Indications

The indications for TORS radical tonsillectomy are well described in the literature, though as surgeons gain more experience, some of what formerly were absolute contraindications are now becoming relative contraindications. The main indication for TORS is primary resection of squamous cell carcinoma of the tonsillar fossa, specifically T1 and T2 tumors with selected T3 and T4a neoplasms. ² Contraindications for TORS radical tonsillectomy include stage IVC cancer (with exception for curable, solitary, distant metastasis), T4a cancer (with exception of those involving only the extrinsic muscles of the tongue or minimal involvement of the medial pterygoid muscle), radiologic evidence of tumor adjacent to the common or internal carotid artery, any T-stage tumor with fixed invasion of tissues lateral to the pharyngeal constrictor muscles or posteriorly of the prevertebral fascia, unresectable node involvement, and dermal metastasis. ² Other non–tumor-related contraindications include trismus that prevents adequate exposure, a retropharyngeal internal carotid artery, and any medical comorbidities precluding the patient from general anesthesia or surgery. ²

The boundaries of the palatine tonsils are as follows:

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  Anterior pillar

  
  
  
  
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    Palatoglossal arch

    
    
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    Palatoglossus muscle

  
  
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  Posterior arch

  
  
  
  
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    Palatopharyngeal fold

    
    
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    Palatopharyngeus muscle

  
  
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  Lateral border, fibrous capsule

  
  
  
  
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    Superior constrictor

    
    
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    Styloglossus

    
    
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    Anterior fibers of palatopharyngeus muscle

With the exception of the mesial free surface, the tonsil is surrounded by a distinct fibrous capsule, and external to this capsule is found the pharyngeal aponeurosis.

Surgical Anatomy

The TORS radical tonsillectomy starts with proper exposure. The robotic view will display the oral cavity in a vantage point familiar to otolaryngologists. Fig. 1.1a shows the overview of the oropharynx from the viewpoint of the surgical robot endoscope. The uvula should be seen at the bottom of the visual field with the tongue base at the superior aspect of the visual field. The mucosal landmarks of the anterior and posterior tonsillar pillars should initially be identified. These structures are created by the mucosa overlying the palatoglossus and palatopharyngeus muscles, respectively. The anterior and posterior tonsillar pillars create the anterior and posterior boundaries of the tonsillar bed. The tonsillar bed is bounded laterally by the superior pharyngeal constrictor muscle, superiorly by the soft palate, and inferiorly by the base of the tongue.

The operation is initiated with a vertical incision along the line of the pterygomandibular raphe, just anterior to the anterior tonsillar pillar, where the buccinator muscle intersects the superior pharyngeal constrictor. The incision should be carried through the superior pharyngeal constrictor to encounter the buccopharyngeal fascia. The superior pharyngeal constrictor should be retracted medially to expose the parapharyngeal space. In the area of this incision there may be terminal branches of the lesser palatine artery and nerve, specifically near the superior portion of the anterior tonsillar pillar. The parapharyngeal space is an inverted pyramid–shaped space that is bounded laterally by the medial pterygoid muscle and its superficial layer of deep cervical fascia and bounded medially by the superior pharyngeal constrictor muscle and buccopharyngeal fascia. The superior boundary is the skull base, where the fascial layers surrounding the medial pterygoid and superior pharyngeal constrictor muscles meet the base of the skull. Inferiorly these fascial layers converge upon the greater cornu of the hyoid. The anterior border is the pterygomandibular raphe, and the posterior boundary consists of the carotid sheath and prevertebral fascia. Within the parapharyngeal space, there is fatty connective tissue, which can be dissected with blunt dissection alone. With careful dissection, the medial pterygoid muscle should be identified laterally. Care should be taken because the lingual nerve lies between the medial pterygoid muscle and the mandible and runs inferoanteriorly, crossing over the anterior border of the medial pterygoid muscle, where it becomes exposed for potential injury.

Using blunt dissection, the plane along the buccopharyngeal fascia in the parapharyngeal space should be followed posteriorly. As this is performed, a number of arteries from the external carotid and facial arterial systems can be identified. In addition, deep in this space, the buccal fat pad (or the parapharyngeal fat pad) can be identified, which is depicted in Fig. 1.2 . Running superiorly, the tonsillar branch of the facial artery, the ascending palatine artery, and the ascending pharyngeal artery lie in the parapharyngeal space. The tonsillar artery along with smaller branches of the ascending palatine and ascending pharyngeal arteries may be visualized piercing through the superior pharyngeal constrictor muscle en route to supplying the tonsillar bed. In addition, the descending palatine artery, coming from the soft palate, may be encountered in this space. Inferiorly, a branch from the lingual artery may be present. Several of these vessels can be identified in Fig. 1.3 . The major arterial supply to this region is the tonsillar branch of the facial artery, which runs superiorly from its origin to reach the tonsillar bed. Careful blunt dissection should be employed to prevent injury to the major vessels in the parapharyngeal space.

The dissection should be carried posteriorly until the styloglossus and stylopharyngeus muscles are identified. At the level of the tonsillar bed, these muscles should lie directly superficial to the carotid sheath and are useful landmarks for preventing injury to the carotid arterial system. The internal carotid artery enters the carotid canal medial to the styloid process, and the styloglossus and stylopharyngeus muscles cross over the internal carotid artery on their way to the lateral base of the tongue and pharyngeal constrictor apparatus, respectively. The styloglossus lies slightly anterior and lateral to the stylopharyngeus as they both travel inferoanteriorly from the styloid process, with the stylopharyngeus having a more vertical course. Between them, the stylohyoid ligament may be present as it traverses from the styloid process to the greater cornu of the hyoid bone. The styloglossus and the stylopharyngeus muscle should be isolated from surrounding tissue. Fig. 1.4 shows this anatomy in axial, coronal, and sagittal planes. Careful blunt dissection should be carried out in this region due to proximity to the carotid arterial system. Variations in the carotid artery have been reported. Specifically, in a cadaver anatomical study, the internal carotid artery was noted on 30% of cases to have a medial curvature or kink, which brought it closer to the posterior pharyngeal and more exposed to injury. ³ During this procedure, careful attention to arterial pulsations in the parapharyngeal space tissue may enable identification of such a variation and prevent injury to the internal carotid artery.

Inferiorly in the parapharyngeal space, following the styloglossus and stylopharyngeus muscles as they diverge to their respective insertions, the lingual branch of the glossopharyngeal nerve runs anteriorly toward the lateral tongue base. The glossopharyngeal nerve exits the jugular foramen and traverses inferiorly with the carotid sheath posterior to the stylopharyngeus muscle. When the glossopharyngeal nerve turns anteriorly toward the lateral tongue base, it can be identified medial to the stylohyoid ligament between the superior and middle pharyngeal constrictor muscles en route to the base of the tongue. There is some variation in the course of the glossopharyngeal nerve noted in some cadaveric studies. In the majority of cases, the glossopharyngeal nerve is separated from the tonsillar bed by either the superior pharyngeal constrictor, the styloglossus, or the stylopharyngeus muscle; however, in about 20% of cases the glossopharyngeal nerve was adherent to the tonsillar capsule as it traveled to the base of the tongue. ⁴ Due to this variation, careful dissection of the inferior portion of the tonsillar bed should be performed in attempt to prevent injury to the lingual branches of the glossopharyngeal nerve.

The blood supply to the palatine tonsils is as follows:

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  Blood supply from external carotid artery

  
  
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  Lower tonsil pole

  
  
  
  
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    Tonsillar artery branch of facial (sometimes ascending palatine) artery

    
    
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    Dorsal lingual branch of lingual artery

    
    
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    Branch of ascending palatine artery

  
  
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  Upper tonsil pole

  
  
  
  
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    Ascending pharyngeal artery

    
    
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    Descending palatine artery

    
    
    
    
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      Greater palatine artery

      
      
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      Lesser palatine arteries

Next, attention should be directed to the soft palate. The soft palate resection should include the portion of the soft tissue into which the palatoglossus and palatopharyngeus muscles insert. The posterior portion of the soft palate is made up largely of fibers of the levator veli palatini muscles. During dissection of the soft palate, small branches of the descending palatine artery may be encountered as they supply the posterior soft palate, the uvula, and the superior portion of the tonsillar fossa. The lesser palatine artery enters the palate region via the greater palatine foramen at the bifurcation of the descending palatine artery. Terminal branches of the lesser palatine nerve may also be identified in the soft palate, as that nerve provides sensory innervation to the soft palate and uvula. The amount of soft palate requiring resection depends upon the extent of tumor involvement. From the soft palate, the incision should extend posteriorly past the posterior tonsillar pillar, and the superior aspect of the superior pharyngeal constrictor muscle should be resected with the tumor. The constrictor muscles should be carefully removed medially from the buccopharyngeal and alar fascia. As mentioned previously, due to anatomical variations of the internal carotid artery with possible medial curves and kinks, the dissection should be performed with caution.

After resection of the soft palate is complete, the lateral tongue base can be addressed. Again, the extent of dissection and resection of the tongue base is largely dependent on the extent of tumor involvement. With proper preoperative planning, the tongue base resection should not exceed 50% of the tongue base to help prevent poor functional outcomes. ² The base of tongue resection should extend to the vallecula posteriorly. The extrinsic muscles of the tongue that are encountered in the lateral tongue base include the palatoglossus and styloglossus muscles. If a larger dissection of the tongue base is required due to tumor involvement, the lingual artery, the lingual nerve, and possibly the hypoglossal nerve may be encountered in the lateral tongue base. The lingual artery is depicted in Fig. 1.5 .

The lingual artery approaches the tongue base from its origin through the lateral pharyngeal wall, entering the base of the tongue medial to the hyoglossus muscle and lateral to the stylopharyngeus, superior pharyngeal constrictor, and stylohyoid ligament. As it approaches the tongue, the lingual artery gives off several branches medially to the vallecula and a branch laterally to the tonsillar bed. Within the tongue, the artery gives off the dorsal lingual arteries superiorly at the posterior border of the hyoglossus muscle and further distally branches into its terminal branches: the deep lingual artery and the sublingual artery. Deep and lateral to the lingual artery in the lateral base of the tongue, the hypoglossal nerve can be seen coursing anteriorly to the tongue. The hypoglossal nerve, unlike the lingual artery, enters lateral to the hyoglossus muscle. The lingual nerve enters the tongue base from the parapharyngeal space just lateral to both the styloglossus and the hyoglossus muscles. The lingual nerve continues anteroinferiorly to innervate the floor of the mouth and the submandibular gland. The relationship of the lingual nerve to the lingual artery can be seen in Fig. 1.5 .

At this point of the procedure, the resection is completed with the transection of the extrinsic tongue muscles from the base of the tongue. The final step is to resect the posterior pharyngeal wall from the vallecula to the soft palate.

The innervation of the palatine tonsils is as follows:

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  Tonsillar plexus: the middle and posterior palatine nerves (branches of the Meckel′s ganglion) join with the tonsillar branches of the glossopharyngeal nerve to form a plexus around the tonsils (this is known as “circulus tonsillaris”)

  
  
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  Maxillary nerve

  
  
  
  
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    Passes through (not synapses) sphenopalatine ganglion

    
    
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    Distributed via lesser palatine nerves

  
  
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  Tonsillar branches from glossopharyngeal nerve

  
  
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  Referred otalgia from tympanic nerve (branch of glossopharyngeal nerve; Jacobson′s nerve)

Fig. 1.6 demonstrates the steps of the radical tonsillectomy as noted above.