Surgical Management

Benign salivary gland neoplasms are relatively common and can manifest in both the major and minor salivary gland sites of the head and neck region. The parotid gland is the most frequent site, and pleomorphic adenoma is the most common histopathological type, representing 80% of all benign neoplasms, with an estimated incidence of 60 per million population per year in the United Kingdom. The diagnosis is usually established by clinical examination and fine-needle aspiration cytology (FNAC). Additional radiological imaging is sometimes required. The treatment of choice is complete surgical excision.

Benign salivary gland tumors are common, and they may grow over time and can create a cosmetically unattractive or unsightly mass on the face. As the tumor increases in size, it can affect swallowing and breathing, depending on its location. If left untreated, a small proportion undergo malignant transformation. Incomplete removal is associated with recurrent disease, with the resulting clinical findings and increasing difficulties with management (Chapter 24).

Surgical Anatomy of the Parotid Gland and Parapharyngeal Space

Precise knowledge of the anatomy of major and minor salivary glands is essential when performing exploratory and excision surgery.

Parotid Location

The parotid gland is located between the posterior border of the mandible and the mastoid process of the temporal bone (Fig. 22.1). The external auditory meatus, the temporomandibular joint, and the zygomatic process of the temporal bone all lie posterior and superior to the gland. The tail of the parotid is frequently located inferiorly in the neck, as low as the hyoid. On its deep (medial) surface lies the styloid process of the temporal bone, and it overlies the angle of the mandible and the transverse process of the atlas (C1) vertebra.

Fig. 22.1 The lateral head, illustrating the location of the parotid gland in the preauricular area and the local nerves (great auricular nerve and facial nerve, cranial nerve VII) and blood vessels.

Fibrous Capsule

The parotid gland is surrounded by a fibrous capsule. This passes up from the neck and was formerly thought to split to enclose the gland. The deep layer is attached to the mandible and the temporal bone at the tympanic plate and styloid and mastoid processes. This superficial layer is now considered to be part of the superficial musculoaponeurotic system (SMAS). Anteriorly, the superficial layer of the parotid capsule is thick and fibrous, but more posteriorly it becomes a thin, translucent membrane. Within this fascia are scant muscle fibers running parallel with those of the platysma. This superficial layer of the parotid capsule appears to be continuous with the fascia overlying the platysma muscle. Anteriorly, it forms a separate layer overlying the masseteric fascia, which is itself an extension of the deep cervical fascia. The peripheral branches of the facial nerve and the parotid duct lie within a loose cellular layer between these two sheets of fascia.

The Superficial Lobe

The superficial surface of the parotid gland is covered with skin and platysma muscle. Some terminal branches of the great auricular nerve lie superficial to the gland tissue. At the superior border of the parotid lie the superficial temporal vessels, with the artery anterior to the vein (Fig. 22.2). The auriculotemporal branch of the mandibular nerve runs just deep to the superficial temporal vessels.

The Deep or Medial Lobe of the Parotid Gland

The deep (medial) surface of the parotid gland lies on structures called the parotid bed. Anteriorly, the gland lies over the masseter muscle and the posterior border of the mandible from the angle up to the condyle. As the gland wraps around the ramus, it is related to the medial pterygoid muscle at its insertion onto the deep aspect of the angle. Posteriorly, the parotid is molded around the styloid process and the styloglossus, stylohyoid, and stylopharyngeus muscles from below upwards. Behind this, the parotid lies on the posterior belly of the digastric muscle and the sternocleidomastoid muscle. The digastric and the styloid muscles separate the gland from the underlying internal jugular vein, the external and internal carotid arteries, the glossopharyngeal, vagus, accessory, and hypoglossal nerves, and the sympathetic chain.

Fig. 22.2 The parotid bed, illustrating the facial nerve trunk and the upper and lower divisions.

Local Ligaments from the Styloid Process

The fascia that covers the muscles in the parotid bed thickens to form two named ligaments. The stylomandibular ligament passes from the styloid process to the angle of the mandible. The mandibulostyloid ligament (the angular tract) passes between the angle of the mandible and the stylohyoid ligament. Inferiorly, it usually extends down to the hyoid bone. These ligaments are all that separates the parotid gland anteriorly from the posterior pole of the superficial lobe of the submandibular gland.

The Parotid Duct

The parotid duct emerges from the anterior border of the parotid gland and passes horizontally across the masseter muscle. It can be found by drawing a line from the tragal cartilage of the pinna to bisect a line drawn from the alar base of the nose to the commissure of the lip. The middle third of this line is the surface marking of the parotid duct. Branches between the buccal and zygomatic branches of the facial nerve cross the duct. At the anterior border of the masseter, the duct bends sharply to perforate the buccal fat pad and the buccinator muscle at the level of the upper second molar teeth. The duct then bends again to pass forward for a short distance before entering the oral cavity at the parotid papilla.

The Facial Nerve

The facial nerve exits the skull base at the stylomastoid foramen of the temporal bone. The nerve lies about 9 mm superior to the posterior belly of the digastric muscle and 11 mm from the bony external meatus. The facial nerve trunk then passes downwards and forward over the styloid process and the associated muscles for about 13 mm before entering the substance of the parotid gland. The first part of the facial nerve gives off the posterior auricular nerve supplying the auricular muscles and also branches to the posterior belly of the digastric and stylohyoid muscles.

The extratemporal facial nerve trunk (Table 22.1) can be identified proximally using an antegrade technique, most commonly taught,¹ or with a retrograde technique.² The two most reliable surgical landmarks used to identify the facial nerve trunk are the tympanomastoid suture line and the posterior belly of the digastric muscle.³ If the main trunk is not identifiable or cannot be safely approached proximally at the stylomastoid foramen, alternative means of identifying the trunk must be used. One approach is to locate a branch of the facial nerve distally using a retrograde technique. The marginal mandibular branch usually passes superficial to the posterior facial vein, which has been encountered and divided when elevating the parotid off the sternocleidomastoid muscle. The buccal branch can be identified at the anterior border of the parotid gland, at a point horizontal from the nasal alar rim to the inferior rim of the external auditory canal. The zygomatic branches can be identified as the nerve crosses the zygomatic arch, usually at its midportion, between the outer canthus of the eye and the temporomandibular joint.⁴ If all of the above approaches fail, the transmastoid approach can be used, drilling down the mastoid to identify the facial nerve in the middle ear and following it inferiorly to reach the tympanomastoid suture.

**Table 22.1** Extratemporal location of the main trunk of the facial nerve
1. The tympanomastoid (TM) suture line: the facial nerve trunk is usually located approximately 2–4 mm deep to the inferior end of the tympanomastoid suture line. This landmark may not be visualized and can only be detected on palpation
2. The posterior belly of the digastric muscle: the facial nerve trunk is located 0.5–1.5 cm deep and above the muscle as it attaches the digastric groove
3. The tragal pointer: the facial nerve trunk is located approximately 1–2 cm deep and slightly anteroinferiorly. The tragal pointer has a blunt and variable tip that can change with retraction. It is probably the least consistent landmark to use, as it does not point in the appropriate direction in some 80% of cases
4. The styloid process: the facial nerve trunk is located in the inferior and superficial soft tissue. If the styloid process is used to identify the nerve, the surgeon is likely to become lost and there is a risk of injury to the nerve

On entering the parotid gland, the facial nerve separates into two divisions—the temporofacial and cervicofacial trunks. The division of the nerve is known as the pes anserinus (“goose’s foot”). From these two divisions, the facial nerve further divides into five named branches—the temporal and zygomatic, usually from the upper division; and the buccal, mandibular, and cervical branches from the lower division, but the exact configuration is variable. The peripheral branches of the facial nerve form anastomoses between adjacent branches to form the parotid plexus. Davis et al.5 studied these patterns in 350 cadaveric facial nerves and described six patterns (Fig. 22.3). They demonstrated that in only 6% of cases was there any anastomosis between the mandibular branch and the adjacent branches. Miehlke et al.⁶ described eight branching patterns in which no anastomosis between the branches was seen in 48% of patients.

The Parapharyngeal Space

The parapharyngeal space (PPS) is a potential space located in the upper cervical region. The PPS is not readily accessible to routine clinical examination and remains silent until affected by pathological processes such as infections or tumors. The potentially wide range of benign and malignant neoplasms encountered in this complex and deep anatomic region contributes to the challenge of surgical treatment.

Boundaries

The PPS is described as an inverted five-sided pyramid, with its base at the skull base and the apex at the greater cornu of the hyoid bone (Fig. 22.4a).

The superior boundary is a small area of the temporal and sphenoid bones that includes the carotid canal, the jugular foramen, and the hypoglossal foramen. The fascia covering the medial pterygoid muscle borders this region of the skull base laterally; medially lies the attachment of the pharyngobasilar fascia and posteriorly the prevertebral fascia. Anteriorly, the medial and lateral borders converge.

The inferior boundary is formed by the greater horn of the hyoid, the facial attachments of the posterior belly of the digastric muscle, and the sheath of the submandibular gland.

The posterior boundary is the prevertebral fascia.

The medial boundary is formed by the pharyngobasilar fascia overlying the superior constrictor muscle. Further medially lies the tonsil.

Fig. 22.3 The chief types of branching of the facial nerve, as described by Davis et al.⁵

Fig. 22.4 a, b The parapharyngeal space (PPS).
a The prestyloid (A) and post-styloid (B) compartments of the PPS
b The inverted pyramid of the prestyloid compartment, with the and their close proximity to the retropharyngeal space (C). base at the skull base (superiorly) and the apex at the greater cornu of the hyoid bone (inferiorly).

The lateral boundary in the upper part of the PPS, from anterior to posterior, is formed by the ramus of the mandible, the fascia of the medial pterygoid muscle, and the retromandibular portion of the deep lobe of the parotid gland. Below the mandible, the lateral boundary is formed by the fascia overlying the posterior belly of the digastric muscle.

The anterior boundary is the pterygomandibular raphe, and below is the submandibular space.

Spaces or Compartments

The prestyloid space, the anterior space of the PPS, contains fat and parotid salivary tissue, whereas the retrostyloid space posteriorly contains the carotid sheath and is continuous with the upper mediastinum (Fig. 22.4 b). These spaces are separated by the tensor veli palatini muscle and fascia that arises from the styloid process and crosses posteriorly into the parapharyngeal fat.

The Prestyloid Space

The prestyloid space extends superiorly into a blind pouch formed by the joining of the medial pterygoid fascia to the tensor veli palatini fascia. Most of the space is occupied by fat. It contains the inferior alveolar, lingual, and auriculotemporal nerves and maxillary artery—resulting in neoplasms that are limited to the salivary gland, lipomas, and rarely neurogenic tumors (Table 22.2).

**Table 22.2** Contents of the compartments of the parapharyngeal space
Structure	Prestyloid	Post-styloid or retrostyloid
Arteries	Internal maxillary	Internal carotid
		Ascending pharyngeal
Veins		Internal jugular
Nerves	Auriculotemporal	Cranial nerves IX, X, XI, XII
		Cervical sympathetic chain
Lymph nodes		Present
Glomus tissue		Present
Salivary tissue	Parotid tissue

The Retrostyloid or Post-Styloid Space

The retrostyloid or post-styloid space contains the internal carotid artery with the internal jugular vein postero-lateral to the artery at the skull base. Cranial nerves IX, X, XI, and XII and the sympathetic chain also occupy this space.

Deep Lobe of the Parotid

The deep lobe of the parotid gland is defined as the parotid tissue that lies medial to the plane of the facial nerve. Less than 20% by volume of parotid salivary tissue is located deep to the facial nerve, and tumors located here are therefore uncommon. Those located in proximity to the lateral lobe present either in the retromandibular area or as a parapharyngeal space mass.

Accessory Parotid Tissue

Accessory parotid tissue is most commonly located anterior to the parotid gland, along the course of the parotid or Stensen duct (see Fig. 22.2). As judged by anatomic dissection, about 20% of human parotid glands manifest accessory tissue. Accessory parotid glands have been described as either round or triangular in shape, and almost all contain anastomoses of the zygomatic and buccal branches of the facial nerve within their substance. Although this tissue is susceptible to most salivary pathology, tumors of the accessory parotid gland are more frequently malignant than those of the parotid gland.

Surgical Anatomy of the Submandibular Gland

The submandibular gland consists of a large superficial lobe lying within the digastric triangle in the neck and a smaller deep lobe lying within the floor of the mouth posteriorly. The two lobes are continuous with each other around the posterior border of the mylohyoid muscle. The two lobes are not true lobes, as they are embryologically the same gland arising from a single epithelial outgrowth. The gland is a mixed seromucinous gland.