Chapter 56: Salivary Glands - Anatomy

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Chapter 56: Anatomy
Daniel O. Graney, John R. Jacobs, Robert Kern
Parotid Gland
The parotid gland, the largest of the salivary glands, is located on the face and is
palpable between the ramus of the mandible and the mastoid process (Fig. 56-1). The lateral
surface of the gland is covered only by the skin and dermis of the face: thus the gland is
vulnerable to injury even with superficial lacerations of the face. The medial relations of the
gland (Fig. 56-2) or deep surface are buttressed by the styloid process and its associated
muscles (styloglossus, stylohyoid, and stylopharyngeal) as well as the carotid sheath and its
contents (internal carotid artery, internal jugular vein, and cranial nerves (CN) IX, X, and
XII). The superior limit of the gland is the zygomatic arch, and the inferior boundary of the
gland is the oblique anterior border of the sternocleidomastoid muscle.
The gland is often described as having a superficial and deep lobe, although the exact
pattern of lobulation has been the subject of substantial debate (Davis et al, 1956). A
simplified view is to think of the gland as an asymmetric dumbbell with the larger end
representing the superficial lobe and the smaller end the deep lobe, with the glandular isthmus
being the connection between the two. When viewed from above, the groove between the two
is filled by the ramus of the mandible and its muscle coverings. The posterior groove is filled
by the mastoid and posterior belly of the digastric muscle. Much of the superficial lobe lies
close to the skin and is obvious when the gland is swollen, as happens with parotitis. Because
of the close anatomic relationships and the fact that the gland is well encapsulated within a
fascial envelope, swelling of the gland during parotitis (as in mumps) can be quite painful,
particularly when the patient opens his mouth. The hinge action that occurs when the
mandible is depressed compresses the swollen parotid against the mastoid process and the
walls of its inelastic capsule and results in a painful masticatory process.
The main portion of the deep lobe lies adjacent to the styloid and carotid sheath
structures, as just described, but when enlarged by tumor it can contact the lateral pharyngeal
wall in the region of the palatine tonsil and appear clinically as a peritonsillar mass.
Clinical considerations
The surgical concept of the parotid gland is of a structure that contains two lobes:
superficial and deep. The plane between the two lobes is defined by the facial nerve as it exits
the stylomastoid foramen and courses anteriorly through the gland to innervate the fascial
muscles of expression. Surgically removing the superficial lobe is often the recommended
approach for diagnosis and treatment of tumors of the parotid gland. Crucial to successful
removal of the superficial lobe is the accurate identification and preservation of the facial
nerve.
The only constant landmark to the location of the facial nerve is the stylomastoid
foramen from which the nerve exists the skull base. Although the nerve can and often is
displaced by large tumors of the parotid gland, it usually can be identified without dissecting
the mastoid tip. The key to identifying the facial nerve successfully is good exposure. The
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incision starts anteriorly in the pretragal skin creases and extends inferiorly down to the lobule
where is is gently curved posteriorly, approximately a centimeter over the mastoid tip before
tracing the anterior border of the sternocleidomastoid muscle. It is then extended anteriorly
approximately two fingers' breadth beneath the inferior ramus of the mandible. The skin flap
is elevated to expose the entire gland. This usually results in an exposure from the zygomatic
arch to the midportion of the posterior belly of the digastric muscle. Careful dissection down
the cartilaginous external auditory canal medially is then performed. Care should be taken to
avoid dissecting into a "hole". The anterior border of the sternocleidomastoid muscle insertion
onto the mastoid tip should be developed to increase exposure. Eventually the cartilaginous
pointer will be identified; the facial nerve lies inferior and slightly deeper to the pointer. It
is helpful to use an operating headlight and low-power magnifying loops for this portion of
the procedure. Once the nerve is identified visually it is important that confirmation be
obtained with a single low-power electrical stimulation. The nerve trunk is then traced
anteriorly with removal of the so-called superficial lobe. Using a small, pointed hemostat,
tunnels are created over the nerve branches and then connected, allowing preservation of the
nerve. It is critical that the gland be placed under traction by the surgical assistant.
Occasionally the facial nerve cannot be identified via the pointer. It then becomes
necessary to use an alternative method. All of the alternatives with the exception of
localization within the mastoid tip via a mastoidectomy require identification of a branch of
the facial nerve and retrograde dissection back to the main trunk. Perhaps the most reliable
alternative is identifying mandibularis branch as it crosses the facial vein. The ophthalmic and
buccal branches are also potential candidates for identification and subsequent retrograde
tracing.
Parotid duct
Although the parotid duct usually appears to originate from the superficial lobe, its
origin is in fact frequently more complicated. Davis et al (1956) have described the formation
of the parotid duct as arising from a varied pattern of extraglandular ductules. These ductules
may arise from the superficial lobe, the deep lobe, or both, before fusing to form the
substance of the parotid duct. The duct traverses the surface of the masseter muscle and
crosses the anterior aspect of the mandible approximately a fingers' breadth below the
zygomatic process. At this point the duct abruptly turns medially to pierce the buccinator
muscle and the buccal fat pad to enter the oral mucosa, which it penetrates at approximately
the level of the second maxillary molar.
Surface relationships of the parotid gland
The lateral surface of the gland or its superficial lobe has a number of important
anatomic relationships pertaining to various nerves, arteries, and veins that either emerge or
enter the substance of the gland at varying points around its perimeter. If the gland is viewed
as a clock face with the superior pole representing the twelve o'clock position and the inferior
pole representing the six o'clock position, various structures can be seen to enter or leave the
gland at these points (Fig. 56-1). At the twelve o'clock position are three structures: the
auriculotemporal nerve and the superficial temporal artery and vein. At the inferior pole, or
six o'clock position, the structures are the retromandibular vein and its connections with the
external and internal jugular veins. Along the anterior margin of the parotid are the various
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branches of CN VII, which emerge from the substance of the parotid to enter the submuscular
plane of the face. These include the temporal, zygomatic, buccal, mandibular, and cervical
branches. A transverse facial artery and vein are also related to the anterior aspect of the
parotid, located slightly superior to the parotid duct and paralleling the course into the cheek
and infraorbital region. At the posterior margin of the gland the posterior auricular and
occipital arteries can sometimes be identified, but more often they are buried in the parotid
tissue. Superficially located at this posterior aspect of the parotid, however, is the great
auricular nerve. This is a peripheral nerve made up of fibers from the roots of cervical nerves
C2 and C3 and is one of the major branches of the cervical plexus. This nerve serves to
supply cutaneous innfervation over the region of the mastoid and the submandibular triangle
of the neck.
The contents of the parotid gland include, in addition to CN VII, the external carotid
artery and the retromandibular vein. Each of these structures is discussed here.
Origin and course of the intraparotid portion of CN VII
CN VII emerges from the stylomastoid foramen to enter the substance of the parotid
gland on its posteromedial surface. The site of entry into the gland is an important surgical
landmark and can be found using the "tragal" pointer. If the tip of the index finger is placed
at the base of the tragal cartilage and is pushed firmly against the lateral aspect of the skull,
the pad of the finger tip lies between the external acoustic meatus and the mastoid process.
This in effect represents the site of the stylomastoid foramen and the point at which CN VII
emerges from the skull. During parotid surgery, an incision is made along the inferior margin
of the external acoustic meatus. The skin is reflected to visualize the cartilage of the meatus
and the bone of the mastoid tip.
If one follows the interval between the bone and cartilage medially, that is, deep into
the dissection, one eventually reaches the stylomastoid foramen. The depth of the foramen
from the level of the skin is approximately 25 mm. As CN VII leaves the stylomastoid
foramen, it simultaneously enters the substance of the parotid gland and divides into two
major trunks, the temporofacial and the cervicofacial. The branching pattern of these nerves
has been studied extensively by Davis et al (1956). Their study describes six patterns of the
facial nerve based on the anastomosis of individual branches. Type I occurs in approximately
13% of cases and represents no anastomosis between the five branches of CN VII: the
temporal, buccal, mandibular, and cervical. The other five types show varying patterns of
anastomosis between the individual branches. Although to recognize the potential for
variations in anatomic patterns is important for the surgeon, committing the percentages of
these to memory seems fruitless because the surgeon cannot select patients on the basis of
anatomic variants. For the surgeon, Geraldine's law of anatomy prevails: "what you see is
what you get" (with apologies to Flip Wilson).
The temporofacial usually is composed of the temporal, zygomatic, and buccal
branches, whereas the cervicofacial comprises the mandibular and cervical branches. The
trunks divide into their branches within the main body of the gland and then course distally
within the superficial lobe to their respective territories on the face and neck. None of the
branches traverse the deep lobe.
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External carotid artery
Although the external carotid artery can be visualized at the inferior pole of the parotid
gland, it may travel superiorly a few millimeters before entering the substance of the gland.
The posterior auricular artery is usually given off from its posterior surface before the vessel
divides into its two terminal branches, the maxillary artery and the superficial temporal artery.
The maxillary artery exits the deep surface of the gland and supplies the various structures
of the infratemporal fossa before entering the pterygopalatine fossa. The branches of the
terminal part of the maxillary artery supply the maxillary teeth and the palatine and nasal
mucosae. Chapter 35 discusses the details of these vessels. Before the superficial temporal
artery leaves the superior pole of the parotid, it gives off the transverse facial artery, described
previously as following the course of the parotid duct supplying the upper quadrant of the
face. Chapter 18 describes the distribution and pattern of anastomosis of the superficial
temporal artery in the scalp.
Venous pattern within the parotid gland
The venous structures within the parotid gland fundamentally parallel those of the
parotid arteries; they consist of a superficial temporal vein and a series of veins emerging
from the infratemporal fossa and the pterygoid venous plexus. The maxillary vein may be
somewhat short, since it is formed from a series of tributaries rather than a single large vein
crossing the infratemporal fossa, as in the case of the maxillary artery. The fusion of the
maxillary and superficial temporal veins forms the retromandibular vein within the substance
of the parotid. The veins usually occupy the plane between the nerve (superficially located)
and the artery (located in the deep lobe). The retromandibular vein exits from the parotid near
its inferior pole, where anastomosis with the external jugular vein occurs. Frequently a
posterior facial vein connects with the anterior facial vein, the confluence of the two forming
a common facial vein that drains into the internal jugular vein (Figs. 56-1 and 56-3). The
pattern of venous anastomosis at this point varies in different individuals as well as from side
to side in a single individual.
Innervation of the parotid gland
The ninth cranial nerve (CN IX) provides secretomotor function to the parotid gland
after a complicated course through the temporal bone and infratemporal fossa (Fig. 56-4). The
secretomotor fibers begin in the inferior salivary nucleus of the brainstem and travel with CN
IX as it emerges from the jugular foramen. Here a recurrent branch, the tympanic branch
(Jacobson's nerve), turns back into the skull by entering a small canal, the inferior tympanic
canaliculus, which opens onto the floor of the middle ear or tympanic cavity (Fig. 56-4). The
path of this nerve can be identified on most skulls by the passing of a fine bristle through the
tympanic canaliculus, located in the bony septum lying between the jugular and carotid
foramina, as the middle ear space through the external acoustic meatus is observed.
From the floor of the tympanic cavity, the nerve fibers enter the mucosa over the
promontory and course anterosuperiorly to penetrate the bony roof of the tympanic cavity and
enter the subdural space of the middle cranial fossa. It should be noted that the tympanic
nerve also contains sensory fibers that are distributed to the mucosa of the middle ear.
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After penetrating the roof of the tympanic cavity, the secretomotor fibers descend on
the petrous ridge in the middle cranial fossa as a delicate nerve bundle, termed the lesser
petrosal nerve. The fibers are directed toward the area of the forament spinosum and the
foramen ovale and usually exit from the skull via the latter in proximity to the third division
of the trigeminal nerve. Because these fibers are preganglionic parasympathetic secretomotor
fibers, they synapse with the cell body of a second neuron before innervating the parotid. The
accumulated cell bodies of these second-order neurons form the otic ganglion; they are the
source of the postganglionic fibers for innervation of the gland. The ganglion is not always
an identifiable structure attached to the mandibular division of the trigeminal nerve, but may
be formed by scattered cell bodies trapped within a plexus of trigeminal nerve fibers.
Postganglionic fibers from the ganglion do not form a new nerve but joint with the
auriculotemporal branch of the mandibular division of the trigeminal nerve, which crosses the
posterior wall of the infratemporal fossa to the region of the parotid. Most of the
secretomotory fibers leave the auriculotemporal nerve and disperse within the parotid
substance, so that by the time the nerve crosses the temporomandibular joint it contains only
sensor fibers from the scalp.
Frey's syndrome (gustatory sweating, or auriculotemporal nerve syndrome) is familiar
to most surgeons who perform parotidectomies. Following parotidectomy the regeneration of
these secretomotor fibers becomes misdirected, and the fibers innervate the sweat glands of
the skin. The result of these misdirected fibers is facial sweating when CN IX is stimulated
during the course of eating. Ross (1970) observed increased parotid secretion after electrical
stimulation of the tympanic branch of the glossopharyngeal nerve in stapedectomy patients.
In a second patient population with Frey's syndrome, Ross resected the tympanic plexus and
obtained a mixed result with respect to relief of the patient's symptoms. Although 2 patients
had permanent cures, other patients had a return of symptoms within 30 days. This return
resulted from incomplete section of the typmanic plexus with recovery of the remaining nerve
fibers several days after surgery. These fibers arise as a hypotympanic nerve from the
tympanic plexus near the floor of the tympanic cavity and ascent the anterior aspect of the
promontory either within the mucosa or covered by thin bone. A hypotympanic branch has
been described in 50% of specimens (Porto et al, 1978). Parotid atrophy has been reported
in humans (Dishell, 1971) and rabbits (Wallenborn, 1968) after interruption of the tympanic
plexus.
It appears clear from the current evidence that parasympathetic secretomotor fibers
innervate the parotid gland solely through CN IX and that denervation of the gland results in
atrophy. The parotid gland, then, contrasts with other salivary glands that appear to be more
diffusely innervated.
Submandibular Gland
The submandibular gland fills the major portion of the digastric or submandibular
triangle. The gland rests against the structures forming the floor of the triangle, the mylohyoid
and hyoglossus muscles (Fig. 56-3). The gland has two portions: (1) a superficial lobe lying
superficial to the mylohyoid and (2) a deep lobe wrapping around the posterior border of the
mylohyoid muscle. The deep lobe cannot be palpated superficially in the neck but can be
palpated in the floor of the mouth by a gloved finger while the other hand supports the
superficial lobe. The duct of the submandibular gland (Wharton's duct) emerges from the deep
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lobe and courses anteriorly in the plane between the hyoglossus and mylohyoid muscles. It
terminates as an elevated papilla on the floor of the mouth near the frenulum of the tongue,
adjacent to the duct from the opposite side. While occupying this plane the duct is crossed
twice by the lingual nerve, once on its lateral aspect near its origin and again on the medial
aspect of the duct near its termination at the frenulum.
Clinical considerations
The surgical approach to removing the submandibular gland is through an incision
placed approximately two fingers' breadth beneath the inferior border of the ramus of the
mandible. The incision is carefully dissected down to the level of the platysmal muscle and
the fascia lying immediately over the actual gland. At this level the mandibularis branch of
the facial nerve is identified lying on the undersurface of the platysmal muscle and superficial
on the fascia overlying the submandibular gland proper. Damage to this nerve will result in
drooping of the corner of the lip. The facial vein, which lies beneath the nerve, can be used
to raise the nerve out of harm's way by ligation inferior to the nerve and then subsequent
dissection of the vein superiorly with the nerve trapped above the vein.
Once the facial vein is safely out of the operative field, the dissection then can proceed
to identification of the posterior border of the mylohyoid muscle. Placement of a Richardson
retractor on the posterior border of this muscle along with inferior traction on the gland
proper brings into view the important structures that lie on the medial aspect of the gland, the
lingual nerve and submandibular gland duct. With the lingual nerve usually identified as it
loops into the field, the duct can safely be isolated, divided, and ligated. The facial artery is
also in the field and relatively easy to identify by palpation. It is usually ligated twice during
the procedure as it arises from the depths of the wound through the undersurface of the gland
and over the mandible.
Sublingual Gland
The sublingual gland is a flat, oblong structure that accompanies the distal half of the
submandibular duct, so the gland occupies the same plane; that is, between the mylohyoid and
hyoglossus muscles (Fig. 56-3). It lies superficially in the floor of the mouth, covered only
by the oral mucosa. The gland does not have a single large excretory duct but a series of
ductules that open either into the floor of the mouth directly or into the submandibular duct.
The submandibular duct thus serves both glands.
Innervation of the Submandibular and Sublingual Glands
The submandibular and sublingual glands are innervated by secretomotor fibers of CN
VII, which are derived from the superior salivary nucleus. These fibers exit from the
brainstem within the intermediary nerve rather than the motor portion of CN VII. The
intermediary nerve joints CN VII within the internal acoustic meatus, and its fibers parallel
the course of CN VII through the temporal bone (Fig. 56-5). On the posterior wall of the
middle ear the chorda tympani branch of CN VII arises from the vertical portion of CN VII
and crosses the lateral wall of the middle ear. Its course is constant as it parallels the
tympanic membrane, lying between the long process of the incus and the manubrium of the
malleus. Anteriorly it pierces the petrotympanic fissue of the middle ear and enters the
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infratemporal fossa. Here, the nerve has a short independent course before joining the lingual
nerve, a branch of the mandibular division of the trigeminal nerve. The fibers follow the
course of the lingual nerve until it reaches the floor of the mouth, where the fibers leave the
lingual nerve to synapse in the submandibular ganglion, a parasympathetic ganglion. The
ganglion is usually a prominent structure attached to the lingual nerve by a plexus of nerve
fibers (Fig. 56-5). Postganglionic secretomotor fibers course directly to the submandibular
gland from the ganglion, but some postganglionic fibers return to the lingual nerve and travel
anteriorly in the floor of the mouth before supplying the sublingual gland.
Although the innervation of the parotid gland was seen to be quite specific denervation resuling in glandular atrophy - the same is not true for the submandibular and
sublingual glands. Sectioning of the chorda tympani does not produce atrophy of these glands
and results in only partial diminution of secretion, even when combined with sectioning of
the tympanic branch of the glossopharyngeal nerve.
Innervation of the Tongue
Although initially this topic may not seem to fit into the context of this chapter, it is
actually quite appropriate when one considers that the tongue receives sensory innervation
from the chorda tympani branch of the facial nerve, the lingual branch of the trigeminal
nerve, and the terminal branch of the glossopharyngeal nerve.
The twelfth cranial nerve supplies al of the tongue's musculature by fibers beginning
in the hypoglossal nucleus of the medulla. However, the sensory innervation of the tongue
is topographically divided into the posterior one third supplied by the glossopharyngeal nerve
(CN IX) and the anterior two thirds supplied by the lingual and chorda tympani nerves. In the
posterior third the glossopharyngeal nerve supplies both special visceral afferents responsible
for taste and general visceral afferents concerned with touch and the gag reflex. The majority
of the taste buds in the posterior third of the tongue are located in the epithelium lining the
circumvallate papillae, although others can be found in the mucosa of the valleculae and the
epiglottis. The peripheral processes of both modalities, taste and touch, have the cell bodies
of their neurons in the inferior ganglion of the glossopharyngeal nerve, which is located on
the nerve at the level of the jugular foramen. The central processes for both taste and touch
project to the nucleus solitarius in the pons.
In the anterior two thirds of the tongue, touch and taste sensations are carried by two
different nerves, the lingual and the chorda tympani, respectively. The general sensations of
touch, pain, and temperature are carried by fibers via the lingual nerve; the fibers ascent the
infratemporal fossa to the foramen ovale, where they join with other fibers of the mandibular
division of the trigeminal nerve. The peripheral processes of these neurons have their cell
bodies located in the semilunar ganglion of the trigeminal nerve. Central processes project to
the nucleus of the spinal tract of CN V in the case of pain and temperature fibers, whereas
touch fibers project to the main sensory nucleus of CN V. Taste buds in the anterior two
thirds of the tongue are locate either in the foliate papillae along the lateral aspect of the
tongue or in fungiform papillae, which are more diffusely distributed in the mucosa. Afferent
fibers from the taste buds travel posteriorly along the lingual nerve until the junction with the
chorda tympani near the roof of the infratemporal fossa. The fibers follow the course of the
chorda tympani through the middle ear and join the facial nerve on the posterior wall of the
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tympanic cavity. Following the facial canal, the fibers reach their cell bodies in the geniculate
ganglion before projecting centrally via the intermediary nerve to the nucleus solitarius in the
pons.
The importance of neuroanatomic pathways is obvious in understanding the various
clinical deficits exhibited by patients after either a central pontomedullary lesion or a
peripheral trauma affecting the pathways of CN V, VII, or IX.
Anatomic Planes in the Floor of the Mouth
The surgical approaches to the salivary glands in the floor of the mouth may involve
either an intraoral route or an external approach through the submandibular triangle of the
neck. Regardless of the approach chosen, the anatomic relationships are fundamentally the
same. The surgeon must anticipate anatomic structure during surgical dissection. Memorizing
anatomic detail seems tedious, but sometimes simple organizational techniques can facilitate
learning. In the floor of the mouth the keys to these relationships are the mylohyoid and the
hyoglossus muscles, which form vertical planes for the passage of the neurovascular bundle
that is related either superficial or deep to these muscles (Fig. 56-6). In this manner, three
planes can be described: the first, superficial to the mylohyoid; the second, superficial to the
hyoglossus; and the third, deep to the hyoglossus.
Plane that is superficial to the mylohyoid muscle
The relationships of the mylohyoid muscle are in fact the contents of the
submandibular triangle of the neck. Bounded by the two bellies of the digastric and the body
of the mandible, the triangle forms a three-dimensional space rather than a flat triangle. The
skin and platysma close the roof (lateral wall) of the triangle, whereas the mylohyoid and
hyoglossus muscles form the floor (medial wall). Superficial to the mylohyoid are the
submandibular gland, associated lymph nodes, the facial artery and vein, and the motor nerve
to both the mylohyoid muscle and the anterior belly of the digastric muscle. One of the most
superficial structures in the space is the mandibular branch of the facial nerve, which after
emerging from the parotid usually arcs below the mandible before ascending onto the face
again near the anterior part of the triangle. Chapter 18 discusses this nerve's importance to
the innervation of the muscles of the lower lip.
Plane that is superficial to the hyoglossus muscle
The structures related to the hyoglossus muscle might also be described as lying deep
to the mylohyoid; in effect, they occupy the plane between the two muscles. The deep lobe
of the submandibular gland separates these muscles by folding around the posterior border
of the mylohyoid before sending the submandibular duct anteriorly to the frenulum of the
tongue. The sublingual gland surrounds the duct as it lies against the anterior part of the
hyoglossus muscle. In addition to the duct, two nerves occupy this plane, the lingual and CN
XII (the hypoglossal nerve). The hypoglossal nerve always courses along the most inferior
part of the plane between the two muscles. The lingual nerve enters the floor of the mouth,
superior to the submandibular duct, and crosses it laterally before ascending on the medial
surface of the duct adjacent to the hyoglossus muscle.
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Plane that is deep to the hyoglossus muscle
Three structures occupy the plane that is deep to the hyoglossus muscle: the lingual
artery, the stylohyoid ligament attaching to the lesser cornu of the hyoid bone, and CN IX
(glossopharyngeal nerve) (Fig. 56-6). The nerve and ligament are not seen in Fig. 56-6
because they have terminated posterior to the plane of the section.
The three relationships just described provide an inventory of structures the surgeon
should be able to predict during surgical dissection. Whether the challenge is a simple
exploration of the floor of the mouth to remove a ductal occlusion or a complicated
hemiglossectomy, the anatomy is the same.
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