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Salivary glands
Core Procedures
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Parotid gland: superficial parotidectomy, total parotidectomy
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Submandibular gland: excision of submandibular gland
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Sublingual gland: excision of sublingual gland, excision of ranula
The salivary glands are exocrine glands that discharge saliva via a duct into the oral cavity. Grossly, these can be divided into three major pairs of glands, including the parotid, submandibular and sublingual glands, and over 400 minor glands, including those in the tongue, palatine tonsils, palate, lips and cheek. The main differentiator between major and minor salivary glands is the location of the glandular portion of the tissue: major glands lie some distance from the oral cavity, connected by their respective ducts, whereas minor glands are often found within the mucosa or submucosa itself.
Saliva functions to lubricate food in anticipation of deglutition, provide vital enzymes necessary for digestion, and confer vital protection to the oral cavity from desiccation. The actions of the salivary glands are largely comparable, though their excretory compositions are not similar. The parotid gland, for example, delivers a largely serous composition, the sublingual gland a predominately mucinous, and the submandibular gland a combination of both, i.e. seromucinous.
Embryology
The development of all salivary glands, both major and minor, is similar and involves interactions between the oral ectoderm and the underlying neural crest mesenchyme. Each gland develops, by branching morphogenesis, into secretory acini with the original epithelial invagination elongating to form a duct. The neural crest mesenchyme differentiates into all of the surrounding connective tissue.
The parotid gland appears first (35–36 days post fertilization), followed by the submandibular gland (after 42 days post fertilization) and the sublingual gland (about 47 days post fertilization). Other cells that secrete saliva develop later in the submucosa of the wall of the oral cavity without significant branching.
As the salivary glands and ducts enlarge, they become interwoven with the local tissues. The parotid gland surrounds the branches of the facial nerve, local lymphoid tissue and the parotid duct (of Stensen). The duct of the submandibular gland (of Wharton) originates lateral to the tongue; its final orifice is below the tip of the tongue close to the median plane. The sublingual glands are made up of a series of anatomically distinct glands that combine to form one structural unit. Their individual ducts are retained and persist in adult life as the ducts of Rivinus, though typically a select few fuse to form a major sublingual duct, the so-called duct of Bartholin.
Surgical surface anatomy
Parotid glands
Of the three major salivary glands, the parotid glands are the largest. The parotid gland is found inferior to the zygomatic arch and directly anteroinferior to the external auditory canal (EAC) (external acoustic meatus) and temporomandibular joint. The surface markings of the parotid duct (Stensen's duct) correspond to the middle third of a line drawn between the lower border of the tragus and a point between the nasal ala and the labial margin.
Although not absolute, the approximate location of the gland can be established by defining its borders: anteriorly, a line extending from the mandibular condyles, via the midpoint of the ipsilateral masseter to a point approximately 2 cm below the angle of the mandible; and posteriorly, a curved line extending from the tragus of the pinna, extending inferiorly below the ear lobule dorsally to the mastoid process.
To palpate the gland clinically, the patient is required to clench the jaw and the physician passes a finger intraorally and in a posteromedial direction, in the area of the vestibule. The masseter can be felt in its tonic state and the parotid gland rolled on top of it by moving the examining finger back and forth.
Submandibular glands
The submandibular gland is a small gland measuring up to 4 cm in length. It is found in the anterior triangle of the neck : specifically, the submandibular triangle in level Ib ( Fig. 12.1 ). The largest part of the gland is usually found between the body of the mandible and mylohyoid in the floor of the mouth and is readily palpable via bimanual examination. The gland is composed of two parts – deep and superficial – joined posteriorly around the free edge of mylohyoid.
Sublingual glands
The sublingual glands are flat, narrow structures, measuring up to 2 cm in length. They are the smallest of the major salivary glands and are found directly beneath the oral mucosa, above both geniohyoid and mylohyoid.
The sublingual glands can be readily identified on intraoral examination by asking the patient to raise the tongue to the palate. Parallel to the line of the inferior dentition is a raised raphe known as the sublingual fold (plica fimbriata), on to which the multiple excretory ducts of the sublingual glands open. The gland itself lies directly below this mucosa, just posterior to the mandibular symphysis and anterior to the deep lobe of the submandibular gland.
Relations of the sublingual glands can be described as follows: superior, the mucosa of the floor of the mouth; inferior, submandibular gland and mylohyoid; anterior, lingual nerve; and posterior, lingual nerve and submandibular duct.
Clinical anatomy
Superficial parotidectomy
The goal of successful parotid surgery is the identification and dissection of the facial nerve and subsequent resection of the parotid lesion. There are several important areas of anatomy for the surgeon and these are detailed as they are encountered during the operation.
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