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Oral surgical and dental procedures are routinely performed in an
outpatient setting. Regional anesthesia is the most common method of
anesthetizing the patient before office-based procedures. Many techniques
can be used to achieve anesthesia of the dentition and surrounding hard and
soft tissues of the maxilla and mandible. The type of procedure to be
performed as well as the location of the procedure determines the technique
of anesthesia to be used. Orofacial anesthetic techniques can be classified
into three main categories: local infiltration, field block, and nerve
block.
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The local infiltration technique anesthetizes the terminal nerve endings
of the dental plexus. This is indicated when an individual tooth or a
specific, isolated area requires anesthesia. The procedure is performed in
the direct vicinity of the site of infiltration.
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The field block anesthetizes the terminal nerve branches in the area of
treatment. Treatment can then be performed in an area slightly distal to the
site of injection. The deposition of local anesthetic at the apex of a tooth
for the purposes of achieving pulpal and soft tissue anesthesia is often
used by many dental and maxillofacial professionals. Although this is
commonly termed “local infiltration,” it is important to note that this is
a misnomer. Terminal nerve branches are anesthetized in this technique, and
it is therefore correctly termed a field block.
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A nerve block anesthetizes the main branch of a specific nerve, allowing
treatment to be performed in the region innervated by the nerve.1
This chapter reviews the essential anatomy of orofacial nerves and
details the practical approach to performing nerve blocks and infiltrational
anesthesia for a variety of surgical procedures in this region.
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Anatomy of the Trigeminal Nerve
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General Considerations
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Anesthesia of the teeth and soft and hard tissues of the oral cavity
cannot be achieved without knowledge of the trigeminal nerve (fifth cranial
nerve) and its branches. Regional, field, and local anesthesia of the
maxilla and mandible depend on the deposition of anesthetic solution near
terminal nerve branches or a main nerve trunk of the trigeminal nerve.
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The largest of all the cranial nerves, the trigeminal nerve gives rise
to a small motor root originating in the motor nucleus within the pons and
medulla oblongata, and a larger sensory root which finds its origin in the
anterior aspect of the pons. The nerve travels forward from the posterior
cranial fossa to the petrous portion of the temporal bone within the middle
cranial fossa. Here, the sensory root forms the trigeminal (semilunar or
gasserian) ganglion situated within Meckel's cavity on the anterior surface
of the petrous portion of the temporal bone. The ganglia are paired, one
innervating each side of the face. The sensory root of the trigeminal nerve
gives rise to the ophthalmic division (V1), the maxillary
division (V2), and the mandibular division
(V3) from the trigeminal ganglion (Figure 20–1).
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The motor root travels from the brainstem along with but separate from the
sensory root. It then leaves the middle cranial fossa through the foramen
ovale after passing underneath the trigeminal ganglion in a lateral and
inferior direction. The motor root exits the middle cranial fossa along with
the third division of the sensory root; the mandibular nerve. It then unties
with the mandibular nerve to form a single nerve trunk after exiting the
skull. The motor fibers supply the muscles of mastication (masseter,
temporalis, medial pterygoid, and lateral pterygoid), mylohyoid, anterior
belly of the digastric, tensor veli palatini and tensor tympani muscles.
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The smallest of the three divisions, the ophthalmic division
(V1) is purely sensory and travels anteriorly in the
lateral wall of the cavernous sinus in the middle cranial fossa to the
medial part of the superior orbital fissure. Before its entrance into the
orbit through the superior orbital fissure, the ophthalmic nerve divides
into three branches: frontal, nasociliary, and lacrimal.
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The frontal nerve is the largest branch of
the ophthalmic division and travels anteriorly in the orbit, terminating as
the supratrochlear and supraorbital nerves. The supratrochlear nerve lies
medial to the supraorbital nerve and supplies the skin and conjunctiva of
the medial portion of the upper eyelid and skin over the lower forehead
close to the midline. The supraorbital nerve supplies the skin and
conjunctiva of the central portion of the upper eyelid, the skin of the
forehead, and the scalp as far back as the parietal bone and lambdoid
suture.
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The nasociliary branch travels along the medial
aspect of the orbital roof, giving off various branches. The nasal cavity
and the skin at the apex and ala of the nose are innervated by the anterior
ethmoid and external nasal nerves. The mucous membrane of the anterior
portion of the nasal septum and lateral wall of the nasal cavity are
innervated by the internal nasal nerve. The skin of the lacrimal sac,
lacrimal caruncle, and adjoining portion of the side of the nose are
innervated by the infratrochlear branch. The ethmoid and sphenoid sinuses
are supplied by the posterior ethmoidal nerve. The eyeball is innervated by the short
and long ciliary nerves.
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The lacrimal nerve supplies the skin and
conjunctiva of the lateral portion of the upper eyelid and is the smallest
branch of the ophthalmic division.
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The maxillary division (V2) of the trigeminal nerve
is also a purely sensory division. Arising from the trigeminal ganglion in
the middle cranial fossa, the maxillary nerve travels forward along the
lateral wall of the cavernous sinus. Shortly after stemming from the
trigeminal ganglion, the maxillary nerve gives off the only branch within
the cranium; the middle meningeal nerve. It then leaves the cranium through
the foramen rotundum, located in the greater wing of the sphenoid bone.
After exiting the foramen rotundum, the nerve enters a space located behind
and below the orbital cavity known as the pterygopalatine fossa. After
giving off several branches within the fossa, the nerve enters the orbit
through the inferior orbital fissure, at which point it becomes the
infraorbital nerve. Coursing along the floor of the orbit in the
infraorbital groove, the nerve enters the infraorbital canal and emerges
onto the face through the infraorbital foramen.
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The middle meningeal nerve is the only branch
of the maxillary division within the cranium and provides sensory
innervation to the dura mater in the middle cranial fossa.
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Within the pterygopalatine fossa, several branches are given off, including
the pterygopalatine, zygomatic, and posterior superior alveolar nerves. The
pterygopalatine nerves are two short nerves that
merge within the pterygopalatine ganglion and then give rise to several
branches. They contain postganglionic parasympathetic fibers, which pass
along the zygomatic nerve to the lacrimal nerve innervating the lacrimal
gland, as well as sensory fibers to the orbit, nose, palate, and pharynx.
The sensory fibers to the orbit innervate the orbital periosteum.
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The posterior aspect of the nasal septum, mucous membrane of the superior
and middle conchae, and the posterior ethmoid sinus are innervated by the
nasal branches. The anterior nasal septum, floor of the nose, and premaxilla
from canine to canine is innervated by a branch known as the nasopalatine
nerve. The nasopalatine nerve courses downward
and forward from the roof of the nasal cavity to the floor to enter the
incisive canal. It then enters the oral cavity through the incisive foramen
to supply the palatal mucosa of the premaxilla.
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The hard and soft palate is innervated by the palatine branches: the greater
(anterior) and lesser (middle and posterior) palatine nerves. After
descending through the pterygopalatine canal, the greater palatine nerve
exits the greater palatine foramen onto the hard palate. The nerve provides
sensory innervation to the palatal mucosa and bone of the hard and soft
palate. The lesser palatine nerves emerge from the lesser palatine foramen
to innervate the soft palate and tonsillar region.
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The pharyngeal branch leaves the pterygopalatine ganglion from its posterior
aspect to innervate the nasopharynx.
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The zygomatic nerve gives rise to two branches
after passing anteriorly from the pterygopalatine fossa to the orbit. The
nerve passes through the inferior orbital fissure and divides into the
zygomaticofacial and zygomaticotemporal nerves supplying the skin over the
malar prominence and skin over the side of the forehead, respectively. The
zygomatic nerve also communicates with the ophthalmic division via the
lacrimal nerve sending fibers to the lacrimal gland.
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The posterior superior alveolar (PSA) nerve
branches off within the pterygopalatine fossa before the maxillary nerve
enters the orbit. The PSA travels downward along the posterior aspect
of the maxilla to supply the maxillary molar dentition, including the
periodontal ligament and pulpal tissues as well as the adjacent gingiva and
alveolar process. The mucous membrane of the maxillary sinus is also
innervated by the PSA. It is of clinical significance to note that the PSA
does not always innervate the mesiobuccal root of the first
molar.1,2 Several dissection studies have been performed
tracing the innervation of the first molar back to the parent trunk. These
studies have demonstrated the variations in innervation patterns of the
first molar and this is of clinical significance when anesthesia of this tooth
is desired.
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In a study by Loetscher and Walton,3 29 human maxillae
were dissected to observe innervation patterns of the first molar. The study
evaluated the innervation patterns by the posterior, middle, and anterior
superior alveolar nerves on the first molar. The posterior and anterior
superior alveolar nerves were found to be present in 100% (29/29) of
specimens. The middle superior alveolar (MSA) nerve was found to be present
72% of the time (21/29 specimens). Nerves were traced from the first
molar to the parent branches in 18 of the specimens. The PSA nerve was found
to provide innervation in 72% (13/18) of specimens. The MSA nerve
provided innervation in 28% (5/18) specimens, whereas the anterior
superior alveolar nerve did not provide innervation to the first molar in
any of the specimens. In the absence of the MSA nerve, the PSA nerve may
provide innervation to the premolar region. In a study by
McDaniel,4 50 maxillae were decalcified and dissected to
demonstrate the innervation patterns of maxillary teeth. The PSA was found
to innervate the premolar region in 26% of dissections, when the MSA
was not present. Table 20–1 lists the branches of the ophthalmic,
maxillary, and mandibular divisions.
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Within the infraorbital canal, the maxillary division is known as the
infraorbital nerve and gives off the middle and
anterior superior alveolar nerves. When present, the MSA nerve descends
along the lateral wall of the maxillary sinus to innervate the first and
second premolar teeth. It provides sensation to the periodontal ligament,
pulpal tissues, gingival, and alveolar process of the premolar region as
well as the mesiobuccal root of the first molar in some
cases.1,2 In a study by Heasman,5
dissections of 19 human cadaver heads were performed and the MSA was found
to be present in seven of the specimens. Loetscher and
Walton3 found that the mesial or distal position at which
the MSA nerve joins the dental plexus (an anastomosis of the posterior,
middle, and anterior superior alveolar nerves described below) determines
its contribution to the innervation of the first molar. Specimens in which
the MSA joined the plexus mesial to the first molar were found to have
innervation of the first molar by the PSA and the premolars by the MSA.
Specimens in which the MSA joined the plexus distal to the first molar
demonstrated innervation of the first molar by the MSA. In its absence, the
premolar region derives its innervation from the PSA and anterior superior
alveolar nerves.4 The anterior superior alveolar nerve
descends within the anterior wall of the maxillary sinus. A small terminal
branch of the anterior superior alveolar communicates with the MSA to supply
a small area of the lateral wall and floor of the nose. It also provides
sensory innervation to the periodontal ligament, pulpal tissue, gingiva, and
alveolar process of the central and lateral incisor and canine teeth. In the
absence of the MSA, the anterior superior alveolar has been shown to provide
innervation to the premolar teeth. In the previously mentioned study by
McDaniel, the anterior superior alveolar was shown to provide innervation to
the premolar region in 36% of specimens in which no MSA nerve was
found.4
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The three superior alveolar nerves anastomose to form a network known as the
dental plexus, which comprises terminal branches coming off the larger nerve
trunks. These terminal branches are known as the dental, interdental, and
interradicular nerves. The dental nerves innervate each root of each
individual tooth in the maxilla by entering the root through the apical
foramen and supplying sensation to the pulp. Interdental and interradicular
branches provide sensation to the periodontal ligaments, interdental
papillae, and buccal gingiva of adjacent teeth.
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The infraorbital nerve divides into three terminal branches after emerging
through the infraorbital foramen onto the face. The inferior palpebral,
external nasal, and superior labial nerves supply sensory innervation to the
skin of the lower eyelid, lateral aspect of the nose, and skin and mucous
membranes of the upper lip, respectively.
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The largest branch of the trigeminal nerve, the mandibular branch
(V3), is both sensory and motor (Figure 20–2).
The sensory root arises from the trigeminal ganglion, whereas the motor root
arises from the motor nucleus of the pons and medulla oblongata. The sensory
root passes through the foramen ovale almost immediately after coming off
the trigeminal ganglion. The motor root passes underneath the ganglion and
through the foramen ovale to unite with the sensory root just outside the
cranium, forming the main trunk of the mandibular nerve. The nerve then
divides into anterior and posterior divisions. The mandibular nerve gives
off branches from its main trunk as well as from the anterior and posterior
divisions.
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The main trunk gives off two branches known as the nervus
spinosus (meningeal branch) and the nerve to the
medial pterygoid. After branching off the main trunk, the nervus
spinosus reenters the cranium along with the middle meningeal artery through
the foramen spinosum. The nervus spinosus supplies the meninges of the
middle cranial fossa as well as the mastoid air cells. The nerve to the
medial pterygoid is a small motor branch that supplies the medial (internal)
pterygoid muscle. It gives off two branches that supply the tensor tympani
and tensor veli palatini muscles.
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Three motor and one sensory branch are given off by the anterior division of
the mandibular nerve. The masseteric, deep temporal, and
lateral pterygoid nerves supply the masseter, temporalis, and
lateral (external) pterygoid muscles, respectively. The sensory division
known as the buccal (buccinator or long buccal)
nerve, runs forward between the two heads of the lateral
pterygoid muscle, along the inferior aspect of the temporalis muscle to the
anterior border of the masseter muscle. Here, it passes anterolaterally to
enter the buccinator muscle; however, it does not innervate this muscle. The buccinator muscle is innervated by
the buccal branch of the facial nerve. The buccal nerve provides sensory
innervation to the skin of the cheek, buccal mucosa, and buccal gingiva in
the mandibular molar region.
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The posterior division of the mandibular branch gives off two sensory
branches (the auriculotemporal and lingual nerves) and one branch made up of
both sensory and motor fibers (the inferior alveolar nerve).
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The auriculotemporal nerve crosses the superior
portion of the parotid gland, ascending behind the temporomandibular joint
and giving off several sensory branches to the skin of the auricle, external
auditory meatus, tympanic membrane, temporal region, temporomandibular
joint, and parotid gland via postganglionic parasympathetic secretomotor
fibers from the otic ganglion.
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The lingual nerve travels inferiorly in the
pterygomandibular space between the medial aspect of the ramus of the
mandible and the lateral aspect of the medial pterygoid muscle. It then
travels anteromedially below the inferior border of the superior pharyngeal
constrictor muscle deep to the pterygomandibular raphae. The lingual nerve
then continues anteriorly in the submandibular region along the hyoglossus
muscle, crossing the submandibular duct inferiorly and medially to terminate
deep to the sublingual gland. The lingual nerve provides sensory innervation
to the anterior two thirds of the tongue, mucosa of the floor of the mouth,
and lingual gingiva.
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The inferior alveolar branch of the mandibular
nerve descends in the region between the lateral aspect of the
sphenomandibular ligament and the medial aspect of the ramus of the
mandible. It travels along with, but lateral and posterior to, the lingual
nerve. While the lingual nerve continues to descend within the
pterygomandibular space, the inferior alveolar nerve enters the mandibular
canal through the mandibular foramen. Just before entering the mandibular
canal, the inferior alveolar nerve gives off a motor branch known as the
mylohyoid nerve (discussed below). The nerve accompanies the inferior alveolar artery and vein within the mandibular canal and
divides into the mental and incisive nerve branches at the mental foramen.
The inferior alveolar nerve provides sensation to the mandibular posterior
teeth.
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The incisive nerve is a branch of the inferior
alveolar nerve that continues within the mandibular canal to provide sensory
innervation to the mandibular anterior teeth.
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The mental nerve emerges from the mental foramen
to provide sensory innervation to the mucosa in the premolar/canine region
as well as to the skin of the chin and lower lip.
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The mylohyoid nerve branches off the inferior
alveolar nerve before its entry into the mandibular canal. It travels within
the mylohyoid groove and along the medial aspect of the body of the mandible
to supply the mylohyoid muscle as well as the anterior belly of the
digastric.1,2