The Parotid Gland

EMBRYOLOGY

The parotid gland develops as a thickening of the epithelium in the cheek of the oral cavity in the 15 mm Crown Rump length embryo. This thickening extends backward toward the ear in a plane superficial to the developing facial nerve. The deep aspect of the developing parotid gland produces bud-like projections between the branches of the facial nerve in the third month of intra-uterine life. These projections then merge to form the deep lobe of the parotid gland. By the sixth month of intra-uterine life the gland is completely canalized. Although not embryologically a bilobed structure, the parotid comes to form a larger (80%) superficial lobe and a smaller (20%) deep lobe joined by an isthmus between the two major divisions of the facial nerve. The branches of the nerve lie between these lobes invested in loose connective tissue. This observation is vital in the understanding of the anatomy of the facial nerve and surgery in this region (Berkovitz, Langdon, and Moxham 2003).

ANATOMY

The parotid is the largest of the major salivary glands. It is a compound, tubuloacinar, merocrine, exocrine gland. In the adult, the gland is composed entirely of serous acini.

The gland is situated in the space between the posterior border of the mandibular ramus and the mastoid process of the temporal bone. The external acoustic meatus and the glenoid fossa lie above together with the zygomatic process of the temporal bone (Figure 1.1). On its deep (medial) aspect lies the styloid process of the temporal bone. Inferiorly, the parotid frequently overlaps the angle of the mandible and its deep surface overlies the transverse process of the atlas vertebra.

The shape of the parotid gland is variable. Often it is triangular with the apex directed inferiorly. However, on occasion it is more or less of even width and occasionally it is triangular with the apex superiorly. On average, the gland is 6 cm in length with a maximum of 3.3 cm in width. In 20% of subjects a smaller accessory lobe arises from the upper border of the parotid duct approximately 6 mm in front of the main gland. This accessory lobe overlies the zygomatic arch.

The gland is surrounded by a fibrous capsule previously thought to be formed from the investing layer of deep cervical fascia. This fascia passes up from the neck and was 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 (Berkovitz and Moxham 1988; Ellis 1997; McMinn, Hutchings, and Logan 1984; Williams 1995). Recent investigations suggest that the superficial layer of the parotid capsule is not formed in this way but is part of the superficial musculo-aponeurotic system (SMAS) (Flatau and Mills 1995; Gosain, Yousif, and Madiedo et al. 1993; Jost and Levet 1983; Mitz and Peyronie 1976; Thaller, Kim, and Patterson et al. 1989; Wassef 1987; Zigiotti, Liverani, and Ghibel-lini 1991). 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 r

Figure 1.1. A lateral view of the skull showing some of the bony features related to the bed of the parotid gland. 1: Mandibular fossa; 2: Articular eminence; 3: Tympanic plate; 4: Mandibular condyle; 5: Styloid process; 6: Ramus of mandible; 7: Angle of mandible; 8: Mastoid process; 9: External acoustic meatus. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

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. This observation is important in parotid surgery. When operating on the parotid gland, the skin flap can either be raised in the subcutaneous fat layer or deep to the SMAS layer. The SMAS layer itself can be mobilized as a separate flap and can be used to mask the cosmetic defect following parotidectomy by reattaching it firmly to the anterior border of the sternocleidomastoid muscle as an advancement flap (Meningaud, Bertolus, and Bertrand 2006).

The superior border of the parotid gland (usually the base of the triangle) is closely molded around the external acoustic meatus and the tem-poromandibular joint. An avascular plane exists between the gland capsule and the cartilaginous and bony acoustic meatus (Figure 1.2). The inferior border (usually the apex) is at the angle of the mandible and often extends beyond this to overlap the digastric triangle, where it may lie very close to the posterior pole of the submandibular salivary

Figure 1.2. The parotid gland and associated structures. 1: Auriculotemporal nerve; 2: Superficial temporal vessels; 3: Temporal branch of facial nerve; 4: Zygomatic branch of facial nerve; 5: Buccal branch of facial nerve; 6: Mandibular branch of facial nerve; 7: Cervical branch of facial nerve; 8: Parotid duct; 9: Parotid gland; 10: Masseter muscle; 11: Facial vessels; 12: Platysma muscle; 13: External jugular vein; 14: Sternocleidomastoid muscle; 15: Great auricular nerve. Published with permission, Martin Dunitz, London, Langdon JD, Berkow-itz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

gland. The anterior border just overlaps the posterior border of the masseter muscle and the posterior border overlaps the anterior border of the sternocleidomastoid muscle.

The superficial surface of the gland is covered by skin and platysma muscle. Some terminal branches of the great auricular nerve also lie superficial to the gland. At the superior border of the parotid lie the superficial temporal vessels with the artery in front of the vein. The auriculotemporal branch of the mandibular nerve runs at a deeper level just behind the superficial temporal vessels.

The branches of the facial nerve emerge from the anterior border of the gland. The parotid duct also emerges to run horizontally across the mas-seter muscle before piercing the buccinator muscle anteriorly to end at the parotid papilla. The transverse facial artery (a branch of the superficial temporal artery) runs across the area parallel to and approximately 1 cm above the parotid duct. The anterior and posterior branches of the facial vein emerge from the inferior border.

The deep (medial) surface of the parotid gland lies on those structures forming the parotid bed. Anteriorly the gland lies over the mas-seter muscle and the posterior border of the mandibular ramus from the angle up to the condyle. As the gland wraps itself around the ramus it is related to the medial pterygoid muscle at its insertion on to the deep aspect of the angle. More posteriorly, the parotid is molded around the styloid process and the styloglossus, stylohyoid, and stylopharyngeus muscles from below upward. Behind this, the parotid lies on the posterior belly of the digastric muscle and the sternocleidomas-toid muscle. The digastric and the styloid muscles separate the gland from the underlying internal jugular vein, the external and internal carotid arteries and the glossopharyngeal, vagus, accessory, and hypoglossal nerves and the sympathetic trunk.

The fascia that covers the muscles in the parotid bed thickens to form two named ligaments (Figure 1.3). The stylomandibular ligament passes from the styloid process to the angle of the mandible. The mandibulostylohyoid 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.

Figure 1.3. The mandibulostylohyoid ligament. 1: Styloid process; 2: Stylomandibular ligament; 3: Mandibulostylohyoid ligament; 4: Masseter muscle; 5: Posterior border of ramus; 6: Lateral pterygoid muscle; 7: Medial pterygoid muscle; 8: Superior pharyngeal constrictor muscle; 9: Stylopharyngeus muscle; 10: Middle pharyngeal constrictor muscle; 11: Inferior pharyngeal constrictor muscle; 12: Submandibular gland; 13: Facial vein and artery. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

Figure 1.3. The mandibulostylohyoid ligament. 1: Styloid process; 2: Stylomandibular ligament; 3: Mandibulostylohyoid ligament; 4: Masseter muscle; 5: Posterior border of ramus; 6: Lateral pterygoid muscle; 7: Medial pterygoid muscle; 8: Superior pharyngeal constrictor muscle; 9: Stylopharyngeus muscle; 10: Middle pharyngeal constrictor muscle; 11: Inferior pharyngeal constrictor muscle; 12: Submandibular gland; 13: Facial vein and artery. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

CONTENTS OF THE PAROTID GLAND The Facial Nerve

From superficial to deep, the facial nerve, the auriculotemporal nerve, the retromandibular vein, and the external carotid artery pass through the substance of the parotid gland.

The facial nerve exits the skull base at the stylomastoid foramen. The surgical landmarks are important (Figure 1.4). To expose the trunk of the facial nerve at the stylomastoid foramen the dissection passes down the avascular plane between the parotid gland and the external acoustic canal until the junction of the cartilaginous and bony canals can be palpated. A small triangular extension of the cartilage points toward the facial nerve as it exits

Figure 1.4. Anatomical landmarks of the extratemporal facial nerve. 1: Cartilaginous external acoustic meatus; 2: Parotid gland; 3: Sternocleidomastoid muscle; 4: Tip of the mastoid process; 5: Styloid process; 6: Posterior belly of digastric muscle. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

Figure 1.4. Anatomical landmarks of the extratemporal facial nerve. 1: Cartilaginous external acoustic meatus; 2: Parotid gland; 3: Sternocleidomastoid muscle; 4: Tip of the mastoid process; 5: Styloid process; 6: Posterior belly of digastric muscle. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

the foramen (Langdon 1998b). The nerve lies about 9 mm from the posterior belly of the digastric muscle and 11 mm from the bony external meatus (Holt 1996). The facial nerve then passes downward and forward over the styloid process and associated muscles for about 1.3 cm before entering the substance of the parotid gland (Hawthorn and Flatau 1990). 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.

On entering the parotid gland the facial nerve separates into two divisions, temporofacial and cervicofacial, the former being the larger. The division of the facial nerve is sometimes called the "pes anserinus" due to its resemblance to the foot of a goose. From the temporofacial and cervicofa-cial divisions, the facial nerve gives rise to five named branches—temporal, zygomatic, buccal, mandibular, and cervical (Figure 1.5). The peripheral branches of the facial nerve form anastomotic arcades between adjacent branches to form the parotid plexus. These anastomoses are important during facial nerve dissection, as accidental damage to a small branch often fails to result in any facial

Skoliose Eklig
Figure 1.5. Clinical photograph of dissected facial nerve following superficial parotidectomy.

weakness due to dual innervation from adjacent branches. Davis et al. (1956) studied these patterns following the dissection of 350 facial nerves in cadavers. The anastomotic relationships between adjacent branches fell into six patterns (Figure 1.6). They showed that in only 6% of cases (type VI) is there any anastomosis between the mandibular branch and adjacent branches. This explains why, when transient facial weakness follows facial nerve dissection, it is usually the mandibular branch that is affected.

Auriculotemporal Nerve

The auriculotemporal nerve arises from the posterior division of the mandibular division of the trigeminal nerve in the infratemporal fossa. It runs backward beneath the lateral pterygoid muscle between the medial aspect of the condylar neck and the sphenomandibular ligament. It enters the

Figure 1.6. The branching patterns of the facial nerve. I: Type I, 13%; II: Type II, 20%; III: Type III, 28%; IV: Type IV, 24%; V: Type V, 9%; VI: Type VI, 6%; 1: Temporal branch; 2: Zygomatic branch; 3: Buccal branch; 4: Mandibular branch; 5: Cervical branch. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

Figure 1.6. The branching patterns of the facial nerve. I: Type I, 13%; II: Type II, 20%; III: Type III, 28%; IV: Type IV, 24%; V: Type V, 9%; VI: Type VI, 6%; 1: Temporal branch; 2: Zygomatic branch; 3: Buccal branch; 4: Mandibular branch; 5: Cervical branch. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

anteromedial surface of the parotid gland passing upward and outward to emerge at the superior border of the gland between the temporomandibu-lar joint and the external acoustic meatus. This nerve communicates widely with the temporofa-cial division of the facial nerve and limits the mobility of the facial nerve during surgery (Flatau and Mills 1995). Further communications with the temporal and zygomatic branches loop around the transverse facial and superficial temporal vessels (Bernstein and Nelson 1984).

Retromandibular Vein

The vein is formed within the parotid gland by the union of the superficial temporal vein and the maxillary vein. The retromandibular vein passes downward and close to the lower pole of the parotid, where it often divides into two branches passing out of the gland. The posterior branch passes backward to unite with the posterior auricular vein on the surface of the sternocleidomastoid muscle to form the external jugular vein. The anterior branch passes forward to join the facial vein.

The retromandibular vein is an important landmark during parotid gland surgery. The division of the facial nerve into its temporofacial and cervicofacial divisions occurs just behind the ret-romandibular vein (Figure 1.7). The two divisions lie just superficial to the vein in contact with it. It is all too easy to tear the vein while exposing the division of the facial nerve!

External Carotid Artery

The external carotid artery runs deeply within the parotid gland. It appears from behind the posterior belly of the digastric muscle and grooves the parotid before entering it. It gives off the posterior auricular artery before ascending and dividing into its terminal branches, the superficial temporal and maxillary arteries at the level of the condyle. The superficial temporal artery continues vertically to emerge at the superior border of the gland and crosses the zygomatic arch. Within the substance of the parotid it gives off the transverse facial artery, which emerges at the anterior border of the gland to run across the face above the parotid duct. The maxillary artery emerges from the deep aspect of the gland anteriorly to enter the infratemporal fossa. The maxillary artery gives off the deep auricular artery and the anterior tympanic artery within the substance of the parotid. All these branches from the external carotid also give off numerous

Figure 1.7. The facial nerve and its relationship to the retromandibular vein within the parotid gland. 1: Facial nerve at stylomastoid foramen; 2 Temporofacial branch of facial nerve 3: Cervicofacial branch of facial nerve 4: Temporal branch of facial nerve; 5 Zygomatic branch of facial nerve; 6 Buccal branch of facial nerve; 7: Mandibular branch of facial nerve; 8: Cervical branch of facial nerve; 9: Posterior belly of digastric muscle; 10: Retro-mandibular vein and external carotid artery. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

small branches within the parotid to supply the gland itself.

Parotid Lymph Nodes

Lymph nodes are found within the subcutaneous tissues overlying the parotid to form the preauricu-lar nodes and also within the substance of the gland. There are typically ten nodes within the substance of the gland, the majority being within the superficial lobe and therefore superficial to the plane of the facial nerve. Only one or two nodes lie within the deep lobe (Garetea-Crelgo et al. 1993; Marks 1984; McKean, Lee, and McGregor 1985). All the parotid nodes drain into the upper deep cervical chain.

Parotid Duct

The parotid duct emerges from the anterior border of the parotid gland and passes horizontally across the masseter muscle. The surface markings of the duct are obtained by drawing a line from the lowest point of the alar cartilage to the angle of the mouth (Figure 1.8). This line is bisected and its midpoint is joined with a straight line to the most anterior point of the tragus. This line is divided into three equal parts and the middle section corresponds to the position of the parotid duct. The duct lies approximately 1 cm below the transverse facial vessels. The accessory lobe of the parotid gland, when present, drains into its upper border via one or two tributaries. Anastomosing branches

Parotid Gland
Figure 1.8. The surface markings for the parotid duct.

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 pad of fat and the buccinator muscle at the level of the upper molar teeth. The duct then bends again to pass forward for a short distance before entering the oral cavity at the parotid papilla.

Nerve Supply to the Parotid

The parasympathetic secretomotor nerve supply comes from the inferior salivatory nucleus in the brain stem (Figure 1.9). From there the fibers run in the tympanic branch of the glossopharyngeal nerve contributing to the tympanic plexus in the middle ear. The lesser petrosal nerve arises from the tympanic plexus leaving the middle ear and running in a groove on the petrous temporal bone in the middle cranial fossa. From here it exits through the foramen ovale to the otic ganglion, which lies on the medial aspect of the mandibular branch of the trigeminal nerve. Postsynaptic post-ganglionic fibers leave the ganglion to join the auriculotemporal nerve, which distributes the par-asympathetic secretomotor fibers throughout the parotid gland. Some authorities suggest that there are also some parasympathetic innervations to the parotid from the chorda tympani branch of the facial nerve.

The sympathetic nerve supply to the parotid arises from the superior cervical sympathetic ganglion. The sympathetic fibers reach the gland via the plexus around the middle meningeal artery. They then pass through the otic ganglion without synapsing and innervate the gland through the auriculotemporal nerve. There is also sympathetic innervation to the gland arising from the plexuses that accompany the blood vessels supplying the gland.

Sensory fibers arising from the connective tissue within the parotid gland merge into the auriculotemporal nerve and pass proximally through the otic ganglion without synapsing. From there the fibers join the mandibular division of the trigeminal nerve. The sensory innervation of the parotid capsule is via the great auricular nerve.

Figure 1.9. The parasympathetic innervations of the salivary glands. The parasympathetic fibers are shown as blue lines. Published with permission, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray's Anatomy. 39th edition.

Figure 1.9. The parasympathetic innervations of the salivary glands. The parasympathetic fibers are shown as blue lines. Published with permission, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray's Anatomy. 39th edition.

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  • aamos laiho
    Which ganglion distributes fibers to parotid gland?
    5 months ago

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