Article Type : Research Article
Authors : Hussain Albannai
Keywords : Oral mucosa; Pigmentary lesions; Diastema; Ankyloglossia; Maxillary sinus; Infratemporal fossa; Pterygoid venous plexus; Pterygopalatine fossa; Maxillary nerve; Maxillary artery; Lingual nerve; Mylohyoid nerve
The oral cavity forms the first portion of the
digestive tract and hence the entry to the whole body. It extends anteriorly
from the oral fissure (which is formed by the upper and lower lips meeting at
the commissures) to the fauces marking the boundary with the pharynx
posteriorly. The oral cavity has important communications with the nasal
cavity, maxillary sinus, orbits and even the cranium. This connection can be
either direct via bony canals or indirect through vascular anastomosis. Various
functions associated with the oral cavity include mastication, deglutition,
phonation, respiration and facial expression. These functions are affected in
case of structural or pathological abnormalities. Therefore, the oral cavity is
a common anatomical location where various procedures are performed by
dentists, otolaryngologist and oral and maxillofacial surgeons. Other
specialties like anesthesiologists, pneumologist and gastroenterologists need
the oral cavity as a port during their different procedures. In addition, the
oral cavity is often viewed as a reflection of general health as many
manifestations of systemic diseases manifest in the oral region before systemic
ones. Knowledge of the normal anatomical structures in the oral cavity is
therefore required by many health care professionals. The oral cavity is
divided into the vestibule and the oral cavity proper. The aim of this article
is present clinically relevant anatomy of the oral cavity along with common
conditions involved.
Mucous membranes consist of epithelium covering
connective tissue which in turn is made of lamina properia and reticular layer.
The epithelium is keratinized stratified squamous epithelium which loses
keratinization at the areas of lining mucosa. Attached or masticatory mucosas
is keratinized and have lamina properia that have numerous papillary
projections. The submucosa is the loose layer of CT under the mentioned layers
which contains: blood vessels, muscles, fat, glandular tissue, and loose CT.
The submucosa contains deep vascular plexus that send branches to the papillary
layer forming the secondary or superficial plexus close to the basement
membrane of the epithelial layer [1]. The general arrangement of blood supply
is such that blood vessels travel along the buccal and lingual vestibules
towards the gingiva and run from posterior to anterior direction [2]. Due to
the posterior anterior orientation of blood vessels, it probably is better to
avoid distal releasing incisions during flap surgery to avoid healing complications.
The described vascular distribution would leave the mid crest of the alveolus
relatively avascular. Therefore, crestal incisions may heal slowly and may be
involved in wound dehiscence.
The normal mucosal color varies between pale pink to
dark red depending on the type and thickness of the overlying epithelium and
the number and size of the vascular content. In general, discoloration is
related to vascularity, hemoglobin concentration and the presence of endogenous
pigments like hemosiderin and melanin or foreign bodies [3]. Anemic patients
are generally pale due to reduced concentration of hemoglobin in blood. Other
features include dyspnea and fatigue. Anemia also causes epithelial atrophy due
to changes in the basic metabolic demands by keratinocytes which leads to
glossitis, angular cheilitis, oral ulceration and mucosal pallor [4]. Diagnosis
of anemia is made by measuring hemoglobin concentration and serum ferritin.
Anemia may be associated with congestive heart failure, dizziness, apathy and
cognitive impairment altogether increasing the risk of falls, morbidity and
mortality.
Hemosiderin is a product of hemoglobin breakdown and
causes bluish-gray pigmentation. In hemochromatosis which is autosomal
recessive disease of increased iron absorption, oral pigmentations are commonly
found at the hard palate and gingiva. Hemochromatosis may be associated with
liver cirrhosis, bronze diabetes and heart failure [5]. Melanocytes are present
in the basal layer of oral epithelium and many pigmentary conditions are
attributed to either their increase function or number (Table 1). Melanocytes
produce and store melanin in intracellular vesicles called melanosomes. Melanin
is produced to protect cellular DNA from ionization radiation. Other sources of
color change of oral mucosa include ingestion of heavy metals, drug side
effects and foreign bodies [6,7].
The vestibule is the U-shaped space between teeth and
cheeks. It is where the mucous membrane reflects from cheeks or lips to the
alveolar mucosa and gingiva. Important contents of the vestibule include freni,
Stenson’s duct orifice, lina alba and Fordyce granules.
Freni are folds of mucous membrane seen across the
vestibule attaching the lips and cheeks to the alveolar mucosa, gingiva and periosteum
[8,9]. Variable contents may be observed histologically including loos
connective tissue, elastic fibers and in about third of the times, portion of
adjacent skeletal muscle fibers [10]. Fibers of orbicularis oris are present in
median freni while buccal freni may contain fibers of levator anguli oris and
depressor anguli oris in the maxilla and mandible, respectively. Freni have
prominent position which allows easy inspection and examination. Abnormally
positioned freni are those that extend their attachment to the level of the
interdental papilla or even to the other side of the alveolar ridge [8]. A
simple clinical test includes pulling the lip and observing the blanch or
gingival retraction that occur. Abnormal size or attachment level have clinical
significance in gingival recession and mucogingival defect [11]. In addition,
tight freni pull the gingival sulcus during mastication causing trauma and
enhancing plaque accumulation. Muscle attachment may also contribute to flap
retraction after suturing causing wound dehiscence. Prominent upper midline
frenum prevents midline space closure causing persistent diastema and
interrupting the eruption of the upper lateral incisors [12]. Frenal
abnormality may also be one of the manifestations of syndromes, for example:
Ehler’s Danlos and Ellis-van Creveld syndromes [13].
The parotid papilla or the orifice of parotid duct,
after it pierces the buccinator, is located next to upper 6 or 7 and located
about 3.56 mm above the buccal cusps of upper molars [14]. Stenson’s duct stone
may associate with pain and swelling in the parotid gland. Clinical examination
may reveal pus discharged from the duct or frankly show the associated stone at
the orifice. Aberrant location of Stenson’s duct may confuse the clinician
between buccal space infection and parotitis [unpublished case report].
Lina alba is a whitish raised line on the inner aspect
of cheeks and at the level of the occlusal plane. Its extension is from the
commissures to the molar area. The whitish discoloration along this line is
attributed to keratosis which may be a response to parafunction. Lina alba may
be present in as much as 5-7% of the population and other than correlating to
parafunction or TMD, it may be considered as a normal finding [15,16].
Fordyce granules are sebaceous glands with no hair
follicles and commonly found in the lips, cheeks and genitalia [17,18]. They
appear as separated or coalesced yellowish granules and become obvious at
puberty possibly in response to androgenic hormones. These granules are
considered normal, but unilateral distribution has been attributed to skin
hypopigmentation [18]. Cosmetic concerns indicate their removal [19].
The area in the oral cavity from the vestibules to the
oropharynx constitutes the oral cavity proper and contains the teeth, hard and
soft palate, tongue and the floor of the mouth.
The hard palate is formed by fusion of the horizontal
plates of the maxilla with the premaxilla in a Y-shaped suture at the midline.
It is covered by tight keratinized epithelium which is thinnest at the midline
and becomes thicker at the posterolateral aspect. The incisive papilla is a
soft tissue bulge covering the incisive foramen retro-incisal in position and
contains nasopalatine nerves and blood vessels. Another surface anatomical
landmark includes the greater and lesser palatine foramina. Normal palate is
required to make tongue contact during oral phase of swallowing and
pronunciation of palate-lingual sounds [20]. Abnormal palatal height is generally
considered when it exceeds 2 cm at the intermolar area. Average linear width is
about 3.4 cm. High narrow palate is associated with increased nasal resistance
due to reduced nasal volume. This leads to increased air turbulence and nasal
obstruction, contributing to obstructive sleep apnea [21]. Constricted palate
also predisposes to skeletal and occlusal disharmonies including crowding of
anterior teeth, cross bite and open bite deformities. It appears that tongue
forces are required for transverse palatal and maxillary growth. Torus
palatinus is a form of bony exostosis at the center of hard palate and is
composed of dense cortical with or without spongy bone [22]. Incidence varies
from 0.5%-74% based on racial differences. Strong association between masticatory
stress and tori formation. It is hypothesized that masticatory stress generates
functional hyperplastic response. Genetic factors may contribute 30% in the
formation of tori. Tori may be associated with TMD, bruxism, sleep disturbance,
irritation and ulceration. Denture fabrication may be difficult in the presence
of tori and mandates its removal. Tori may be similar to osteomas which may be
multiple in Gardener’s syndrome. Other clinical applications of anatomy of the
hard palate includes choosing the thickest areas for connective tissue
harvesting. Areas lateral to the median raphae at next to the first premolar
appears to have the thickest tissue [23].
The tongue is a muscular organ covered by specialized
mucosa containing filiform and fungiform papilla. Other specialized taste
papillae are present in the laterally oriented foliate and the V-shaped
circumvallate papilla. Bald tongue or atrophic glossitis denotes the absence of
tongue papilla and may be related either to factors accelerating their damage
and removal like chemical, physical and chemical factors or to those causing
reduced rate of their production as in metabolic, hematologic, immunologic and
nutritional disorders [24]. The pathophysiology of atrophic glossitis is
related to abnormal cellular oxygenation in relation to nutritional deficiency
or indirectly by the presence of a disease that causes nutritional deficiency
either by interference with absorption or metabolism. Lingual freni attach
between the tongue and the mandibular alveolar ridge. It contains fibrous
tissue and parts of the genioglossus muscle. Abnormality in lingual freni is
called ankyloglossia or tongue tie. In ankyloglossia, the tongue tip cannot be
protruded beyond the lower incisors [25]. Ankyloglossia is also implicated when
the free tongue (defined as the distance from the frenal attachment in the
ventral surface of the tongue to the tip) is shorter than 16 mm.
Clinical findings associated with ankyloglossia
include breastfeeding problems, interference with speech, gingival recession
and diastema. Its hypothesized that ankyloglossia may predispose to dentofacial
deformity sequala like narrow maxillary and mandibular prognathism [26]. The
floor of the mouth is present underneath the tongue and contains the mylohyoid
muscle, the sublingual glands, the lingual nerves, Wharton’s ducts and branches
of the lingual and facial arteries. These arterial branches pass through and
supply the mandible through multiple lingual foramina.
The sublingual caruncle is a mucosal elevation at the
base of the lingual frenum which contains duct opening of the sublingual and
submandibular glands. The sublingual gland opens via multiple ducts the largest
of which is the Bartholin’s duct which joins the Wharton’s duct of the
submandibular gland beneath the sublingual caruncle. The remaining openings
form the ducts of Rivinus and open at the sublingual fold which extends from
the sublingual caruncles posteriorly. Due to the presence of dense vascular
structures and the potential to develop hematoma that may displace the tongue
placing the airway in danger, vertical releasing incisions in the lingual
aspect of the mandible are better avoided [27]. Sialolithiasis of submandibular
or sublingual glands may be associated with swelling of the involved gland or
its duct with history of prandial pain. The stone may be detected clinically by
bimanual palpation. Radiographic evaluation is needed to determine the location
and number of duct or gland stones [28]. Another clinically important swelling
in the floor of the mouth includes ranula or mucocele of the sublingual gland.
Plunging or cervical ranulas are not confined to the floor of the mouth and by
dissecting through the mylohyoid muscle and reach the submental or
submandibular space. Removal of the mucocele and the gland is needed to avoid
recurrences.
The paired maxillary sinuses are the largest paranasal
air sinuses. The maxillary sinus development starts at 5th IU as an extension
from the nasal capsule. Its rate of growth is slow and at birth its dimensions
are about 3mm, 6mm, 8 mm. At about the age of 7 years, the maxillary sinus
growth is accelerated and stops with the eruption of maxillary third molar. The
maxillary sinus has a role in facial growth and modeling as it provides
surfaces for bone resorption and deposition [29]. Secondary pneumatization
occurs throughout life and is especially accelerated in association with tooth
loss [30]. The average adult sinus dimensions are about 26mm, 28mm, 40 mm and
volume of about 30 cm³. The maxillary sinus is pyramidal in shape and occupies
the posterior maxilla, with its base towards the lateral nasal cavity and its
tip extending into the zygomatic body. The canine fossa and infratemporal fossa
are located in the anterior and posterior aspect, respectively. The roof formed
by the orbit, while the floor is just above the alveolar process of maxillary
teeth. During development of the face, the sinus floor is located above the
nasal floor by about 4mm, while in adults, however, the sinus floor is about
4-5 mm lower than that of the nasal floor. This is possible as a result of
facial growth and sinus pneumatization. In about 50% of the population, the
floor of the sinus is in the confines of the maxillary alveolar process.
Regarding which tooth is closes to the sinus, different studies [29,31]
reported first molar while still others [32] found that the second molar are
closest to the maxillary sinus. Dental proximity to the maxillary sinus bears
significant clinical correlation as will be described. Sinus septa are bony
elevations of more than 3mm and are found in the sinus floor about 27-33% of
the time [33,34]. Three locations have been defined and these are: premolar,
molar and third molar area with the molar areas as most common location. This
may correspond to three different periods of dental development of premolars,
molars and third molars. In edentulism, subsequent pneumatization may increase
rate of septal formation. Buccolingual orientation is the most common.
The maxillary sinus has a role in humidification and
conditioning of inspired air as well as trapping dust and foreign bodies. This
is made possible as the sinus is lined with a ciliated pseudostratified
columnar epithelium layer that have abundant mucus secreting goblet cells.
Mucus acts as a barrier that reduces water loss and traps foreign bodies. Cilia
are hair-like extension from the apical part of the columnar cell, the core of
which contains 9+1 pairs of microtubules allowing it to beat back and forth.
Ciliary beats move the mucus blanket spirally and toward the sinus ostium and
into the nasal cavity. The maxillary sinus drains into the middle meatus which
is located at the posterior aspect of the hiatus semilunaris corresponding to
2/3rd the way up along the medial wall. The presence of rich vascularity allows
for thermoregulation of the inspired air. Other important functions include
regulation of intranasal pressure, imparting resonance to voice and shock
absorption.
Arterial supply to the maxillary sinus is derived from
superior alveolar (which gives anterior, middle and posterior branches), the
infraorbital and the palatine arteries. These branches extensively divide and
anastomose forming intraosseous and extraosseous plexus. Venous drainage is
through facial, sphenopalatine veins and pterygoid venous plexus. It is
important to point that through the pterygoid venous plexus there exist
communication to the cavernous sinus intracranially making infection spread
possible.
Nerve supply to the maxillary sinus is derived the
same nerves that supply the maxillary dentoalveolar structures: superior
alveolar nerve (with its anterior, middle, posterior branches), infraorbital
and anterior palatine nerves. This common source of nerve supply and proximity
to the sinus makes referred pain a possibility between both the sinus and
maxillary teeth.
Sinus health is dependent on patent ostium and
continuous drainage. If mucus is not cleared, bacterial accumulation causes
sinusitis. Therefore, ostium obstruction, ciliary dysfunction and increased
nasal secretions are important etiological factors in the development of
sinusitis. The clinical features of sinusitis include nasal obstruction,
offensive purulent discharge with pain in malar area. Examination reveals
mucosal redness, turbinate edema and tenderness on palpating the anterior and
posterior wall of the maxilla.
Referred sinus pain may produce diagnostic problems
regarding the source of pain. Sinusitis may irritate branches of the superior
alveolar nerve during their course through the wall of maxillary sinus.
Conversely, dental pain may project over the sinus area. However, pain may be
the result of both dental and sinus problems. Dental source has to be explored
and teeth should be examined thoroughly with inspection, palpation, percussion
and radiographs. A suggested differentiating test is to keep a piece of cotton
saturated with 5% lidocaine in the nostril of the affected side for 20-30
seconds. Pain relieve occur in case of sinusitis [29].
Sinusitis of odontogenic origin was noted as
periodontal lesions were long associated with sinus membrane thickening
[31,32]. It seems that microorganisms and their toxins can permeate through
tissue barriers and affect the sinus. The rate of this group of sinusitis
ranges between 5-10% and in some studies up to 40% [35,36].
Oroantral communication developed after tooth
extraction allows oral flora and other contaminants to lodge within the sinus
causing sinusitis. Oroantral communication after extraction occurs when the
roots are inside the sinus [31]. Punwutikorn reported sinus perforation rate of
about 0.3 % (a study on 27,984 extractions at the posterior maxilla). OAC needs
identification and primary closure if has a diameter more than 5 mm [37].
Foreign bodies like surgical burrs, root tips or even
a teeth, dental implants and bone graft particles all initiate foreign body
reaction and cause sinus inflammation. A special challenge is presented to the
endodontist when treating teeth in close relation the maxillary sinus as even
if instrumentation is kept within the confines of the root canal, extrusion of
infected tooth debris or irritating filling material still possible. Sodium
hypochlorite, calcium hydroxide, gutta percha and silver points have all been
reported to irritate the sinus [38].
Sinus augmentation procedures are done in dental
implant practice to avoid violation of the maxillary sinus. Special attention
should be directed to the type and location of septa [33,34]. Implant
displacement or migration has been reported. Possible explanation to this
phenomenon includes negative pressure exerted by the sinus during inspiration
causing suction effect on the implant. This can be helped by loss of
osseointegration and traumatic occlusal forces. Another clinically useful note
regarding the maxillary sinus is that orthodontic movement of roots across the
sinus results in tipping movement and root resorption [39].
Due to its location and contents, the THE
infratemporal fossa (ITF) is part of the skull base and the masticatory
compartment. The ITF is an irregularly shaped space located deep to the ramus,
posterior to the maxilla and anterior to the styloid process [40]. Major
contents include: the pterygoid muscles, the maxillary artery, the pterygoid
venous plexus, the mandibular and chorda tympani nerves. Deep lobe of parotid
gland and carotid sheath area are located posterior to the ITF and medially, it
is bounded by the lateral pterygoid plate and the fibers of the superior
pharyngeal constrictor muscle. The roof is formed by the infratemporal surface
of the greater wing of the sphenoid bone which contains many of the foramina
leading to the skull base. The ITF communicates superiorly with the temporal
fossa by a space deep to the zygomatic arch where the fibers of temporalis
muscle course to insert to the coronoid process. More importantly, it is
connected to the cranium by the foramen ovale, foramen spinosum and sphenoidal
emissary foramen (of Vesalius). Valveless emissary veins connect the pterygoid
venous plexus to the cavernous sinus intracranially. The pterygopalatine
fissure is the cleft between the maxillary tuberosity and the pterygoid plates
provides route for communication with the pterygopalatine fossa anteriorly. The
ITF has no floor and it is continuous with neck spaces.
The maxillary artery (MA) is one of the terminal
branches of the external carotid artery, the other being the superficial
temporal artery. The originates at the level of the condyle within the parotid
gland. As it enters the ITF it is located between the condyle and
sphenomandibular ligament and runs in close relation to the inferior border of
the lateral pterygoid muscle [41]. The MA is located 15 mm from the
pterygopalatine fissure. The maxillary artery is divided into three parts according
to its anatomical relations. The first or mandibular part is related to the
mandibular condylar neck. In this location, the MA is inferior to the
auriculotemporal nerve and superior to the maxillary vein.
Branches of the first part include the deep auricular
artery to the tympanic membrane, skin of the external auditory meatus and TMJ.
The anterior tympanic artery leaves the ITF through petrotympanic fissure to
the middle ear. The middle meningeal artery supplies the dura mater and enters
the cranial fossa through the foramen spinosum. The accessory meningeal artery
runs deep to the mandibular nerve in relation to the tensor veli palatini and
levator veli palatini and enters the cranium through the foramen ovale. The
inferior alveolar artery (IAA) follows the inferior alveolar nerve (IAN) and
together, enter the mandibular foramen. Before entering the canal, it gives a
branch to the mylohyoid muscle. Inside the mandibular canal, the IAA is found
lateral to the IAN and provides dental branches to supply teeth. More
anteriorly, it divides into the incisive artery that continues within the canal
to the incisor area and to a mental branch that exits through the mental
foramen with the mental nerve. The second or pterygoid part of the MA is
related to the pterygoid muscle and gives five muscular branches. These are the
deep temporal, masseteric, pterygoid, buccal and lingual artery. The third part
of the MA enters and terminates within the pterygopalatine fossa. It gives
branches that follow those of the maxillary nerve.
The pterygoid venous plexus (PVP) is a dense
collection of interconnected venous channels located around the lateral
pterygoid muscle and maxillary artery. Essentially, all structures within the
ITF drain into the PVP. Emissary veins pass through of the foramen ovale,
foramen lacerum and emissary sphenoidal foramina and connect the PVP with the
cavernous sinus. A branch from the PVP also pass through the inferior orbital
fissure and joins the inferior ophthalmic vein. The PVP drains into the
maxillary vein which runs with the first portion of the MA deep to the neck of
mandibular condyle. Within the parotid gland, the maxillary vein joints the
superficial temporal vein forming the retromandibular vein.
During extraction of upper third molar, its
displacement into the ITF is possible especially in cases where only thin bone
is present. Improper force application without distal support may direct the
tooth toward the ITF. Inadequate vision and poor surgical access also play
important roles [42]. Clinically, the patient may be asymptomatic or complain
of limited mouth opening due to pain or mechanical obstruction. If infection
develops, swelling with collection of pus may be seen by radiographic imaging. Infection
of the ITF may be difficult to diagnose and low threshold to obtain adequate
radiographic evaluation should be practiced when a patient complains of
trismus. Dimitrakopoulos reported subcutaneous emphysema because of Valsalva
maneuver was tried [43]. This was attributed to the created communication
between the maxillary sinus and the ITF by the displacement. Radiographic
evaluation also allows localization of the displaced tooth. Usually the tooth
is lateral to lateral pterygoid plate and inferior to lateral pterygoid muscle.
Position of the tooth may change by the movement of the muscles and recent CT
or MRI is recommended before surgical intervention [44,46]. Different surgical
approaches have been described to remove displaced teeth into the ITF.
Borgonovo used high maxillary vestibular incision along with bone window in the
posterolateral wall of the maxilla [45]. Other reported surgical approaches
include Gillies approach and hemicoronal incision [44]. Delaying removal of the
tooth to 2 weeks after displacement may be advantageous as fibrous tissue may
form. However, some would argue that fibrosis may not occur at all and more
urgent decision regarding its removal should be taken to avoid complications
like infection in this vital space [42,43].
Nerve block to both IAN and PSAN require deep needle
insertion hence, vascular injury may occur. During the PSAN block, the
maxillary artery, PSAA, or the PVP may be injured and hematoma formation may be
the result. Gupta et al reported immediate hematoma formation during PSAN block
despite proper injection technique [47]. Swelling may develop rapidly and could
be seen in the buccal, temporal area and eyelid areas. Hrishi explained that
bleeding could reach such areas given the central position of the ITF [51]. LA
anesthetic solutions diffusion in proximity to the orbit have the potential to
cause ophthalmic manifestations like dipoplia, amaurosis and loss of power of
accommodation. Intraarterial injection can reverse the blood circulation and
allow backflow of the anesthetic agent to reach the maxillary artery if
forcefully injected into the PSAA or IAA [48]. The anesthetic agent then may
reach the ophthalmic artery through the meningeal and lacrimal arteries. In
addition, injecting PVP can also affect the orbit by way of the emissary veins
that connect it to the cavernous sinus which has close association with CN III,
IV, VI, and ophthalmic artery. Another possible route is that during injection,
the backflow reaches the internal carotid artery (ICA) which transmits the
agent to the brain [47].
Infection may reach the ITF from adjacent structures
like maxillary molars, maxillary sinus, the parotid gland and as complication
of invasive procedures like exodontia and TMJ arthroscopic procedures [49].
Needle injury to the major vasculature of the ITF may produce hematoma that
with seeded microbes can produce infection [50].
The pterygopalatine fossa (PPF) is inverted
cone-shaped concavity located between the posterior surface of the maxilla and
the pterygoid process of the sphenoid bone [52]. The PPF contains the maxillary
nerve and the pterygopalatine ganglion in addition to important foramina that
provide communication to the oral cavity, nasal cavity, ITF, orbit and the
cranial cavity. The PPF is about 18 mm long and 2.3 mm wide. It communicates
superiorly with the orbit via the inferior orbital fissure which contains the
infraorbital and zygomatic nerves. Inferiorly, the PPF converge into a canal
measuring approximately 17 mm in length [52]. This canal opens in the oral side
through the greater and lesser palatine foramina carrying the greater and
lesser palatine nerves, respectively. The anterior wall of the PPF is formed by
the infratemporal surface of the maxilla. Laterally, the ptrygopalatine
fissure, extends longitudinally between the pterygoid plate and the posterior
wall of maxilla ending at the posterior aspect of the inferior orbital fissure
superiorly. Through the pterygopalatine fissure, the third part of the MA
enters and the posterior superior alveolar nerve leaves the PPF. At the
superomedial aspect of the PPF, the sphenopalatine artery and nerve and the
posterior superior nasal nerve enter the lateral aspect of the nasal cavity
through the sphenopalatine foramen. The posterior border of the PPF is formed
by the pterygoid process of sphenoid bone and greater wing of sphenoid bone.
The foramen rotundum and pterygoid canal connect the PPF with the middle crania
fossa. The former conveys the maxillary nerve and the later contains the nerve
of pterygoid canal.
Branches of the third part of the maxillary artery
include the posterior superior alveolar artery (PSAA) which arises inside the
pterygopalatine fossa and leaves through the pterygopalatine fissure. Over the
tuberosity it becomes imbedded into the maxillary bone. The PSAA supplies the
posterior maxillary teeth, maxillary sinus, and the buccal gingiva. The
infraorbital artery leaves the pterygopalatine fossa and enters the orbit
through in the inferior orbital fissure. It runs along the floor of the orbit
through infraorbital canal and gives the anterior superior alveolar artery that
supply the anterior teeth and anterior wall of the maxilla. The infraorbital
artery exits the canal along with infraorbital nerve via the infraorbital
foramen. The third part of MA also gives a branch that passes through the
pterygoid canal to supply the auditory tube, tympanic cavity and upper part of
the pharynx. The descending palatine artery leaves the PPF through the palatine
canal where it divides into greater and lesser palatine arteries and these
leave the canal through the greater and lesser palatine foramina to supply the
hard palate and soft palate, respectively. The sphenopalatine artery leaves the
PPF through the sphenopalatine fossa to enter the nasal cavity and supply the
posterior lateral wall of the nose. It has branches that supply the posterior
aspect of the nasal septum as well.
One of the important contents of the PPF is the
maxillary nerve (MN). It is the second division of the trigeminal nerve (V2)
arising from the trigeminal ganglion at the middle cranial fossa and reaches
the PPF via the foramen rotundum. MN provides sensory innervation to the
maxillary dental and sinus structures, hard and soft palate and portion of the
nasal cavity. It also contributes to the formation of the PPG. Direct branches
of the MN include the meningeal, zygomatic, ganglionic, posterior superior
alveolar and infraorbital nerves. The zygomatic nerve leaves the MN in the PPF
to enter the lateral aspect of the orbit through the inferior orbital fissure.
The zygomatic nerve then divides to the zygomaticotemporal and zygomaticofacial
branches which exit the orbit through corresponding foramina to supply the
overlying skin. The zygomaticotemporal nerve gives off a lacrimal branch that
carries autonomic fiber to the lacrimal gland. The posterior superior alveolar
nerve leaves the PPF through the pterygopalatine fissure and runs in area above
the tuberosity. It passes through the maxilla posteriorly to supply the
maxillary sinus and posterior teeth. The infraorbital nerve (ION) is the
terminal branch of the MN and leaves the PPF through the inferior orbital
fissure. It then runs through the inferior orbital groove and canal and leaves
the orbit through infraorbital foramen located 4-10 mm below the infraorbital
rim. In 15% of the times, there exist an accessory infraorbital foramen [48].
Within the infraorbital canal, the ION gives off the middle and anterior
superior alveolar nerves that supply the maxillary premolars and incisors and
the anterior maxillary sinus wall by contributing to the formation of the
maxillary dental plexus. In some situation, the middle superior alveolar nerve
is absent and the nerve supply to the premolars is derived from the PSAN or the
dental plexus. Terminal branches of the ION includes inferior palpebral,
external nasal, internal nasal and superior labial branches.
The pterygopalatine ganglion (PPG) is small
heart-shaped neural mass located close to the sphenopalatine foramen [53]. It
is connected to the MN from its upper pole by two ganglionic nerve fibers. The
PPG supplies sensory and autonomic fibers to the nose, palate, tonsils and
lacrimal glands. It contains nerve fibers from the facial, trigeminal and
autonomic nervous system. The parasympathetic component of the PPG originates
at the superior salivatory nucleus of the brain stem with fibers carried by the
nervus intermedius (NI) branch of the facial nerve (CN VII). The NI gives
greater petrosal nerve that passes through the facial canal of the temporal
bone to enter the middle cranial fossa. It then courses towards the PPF by
passing through the foramen lacerum to enter the pterygoid canal. The
sympathetic fibers, pass through the PPG without synapse, originate from the
superior cervical ganglion which sends branches surrounding the ICA. A branch
from this plexus emerge as the deep petrosal nerve and enters the pterygoid
canal where it fuses with the greater petrosal nerve to form the pterygoid
nerve. Sensory fibers from the MN pass without synapsing through the PPG.
Branches from the PPG include orbital, nasal, nasopalatine, greater palatine,
lesser palatine and pharyngeal nerves. Orbital branches of the PPG carry
sensory fibers to the orbital periosteum and the maxillary sinus in addition to
sympathetic fibers to the orbitalis smooth muscle overlying the infraorbital
groove. The nasopalatine nerve enters the nasal cavity through the nasopalatine
foramen and supplies the posterior inferior portion of the nasal septum. It
then courses towards the nasopalatine canal and reach the oral cavity to supply
mucosa of the hard palate through the incisive foramen. It provides nerve
supply to the anterior hard palate, incisor teeth and palatal gingiva. The
greater and lesser palatine nerves leave through the greater and lesser
palatine foramina to supply the hard and soft palate, respectively. The greater
palatine foramen is located next to the third molar, between the third and
second molar or distal to the third molar in about 54%, 6.19%, 38.9%,
respectively [48]. Pharyngeal branches to the nasopharynx via the
palatalovaginal canal.
With a single injection and small amount of local
anesthetic solution, the whole maxillary nerve and the PPG can be anesthetized.
Blocking the PPG has also important roles in diagnosis and management of
various pain disorders like headache, neuralgias and atypical facial pain [53].
The PPF can be approached either intraorally through the greater palatine
foramen or extraorally through the pterygopalatine fissure. Piagkou et al [52]
calculated the mean distance from the overlying surface mucosa till the PPF and
found it to be around 23 mm. Therefore, to avoid overpenetration during the
maxillary nerve technique, it is recommended to bend the needle at a distance
of about 23 mm from its tip. The greater palatine foramen is located next to
the third molar midway between the median palatine suture and the CEJ of the
wisdom tooth in 54% of the time and in 38.9% it may be located more distally.
Palpation helps confirm the foramen location. However, it should be remembered
that using this technique may endanger adjacent important vital structures like
the orbit and the brain by several routes. Diffusion has been suggested to be
through the inferior orbital fissure. In addition, intravascular injection may
involve a branch from the PVP that is connected to the inferior ophthalmic vein
that is also connected to the cavernous sinus [54,55]. Bleeding and hematoma
formation may also affect orbit. Orbital hematoma has been reported despite
proper technique. Injection at recommended depth, avoiding rapid injection and
performing aspiration before injection in at least two planes is recommended to
avoid such complications.
As the mandibular branch of the trigeminal nerve (V3)
descends to the ITF through the foramen ovale, it divides to anterior and
posterior trunks. The lingual nerve is a branch from the posterior trunk
arising above the mandibular notch about 13.5-14.3 mm below the foramen ovale
[56]. The lingual nerve then courses between the tensor veli palatini and the
lateral pharyngeal muscle [56,57]. At the level of inferior border of the
lateral pterygoid muscle about 15 mm inferior to the foramen ovale, the lingual
nerve is joined by the chorda tympani which provides secretomotor supply to the
submandibular and sublingual glands and taste sensations to the anterior 2/3rds
of the tongue. The LN crosses the medial pterygoid muscle and passes below the
pterygomandibular raphe reaching the mandibular third molar area. It is
documented that the LN is only covered by mucosa and periosteum at the area
just distal to the distal root of the mandibular third molar [56,57]. In
addition, the incidence of LN location above the alveolar crest is 17.6% being
covered by gingiva only, while in 62% of the times, the LN is actually making
contact with the lingual crest of the mandible. The mean vertical and
horizontal distance from the lingual plate is 0.58-3.45 mm and 2.2-8.3 mm,
respectively [48,58]. As the LN approaches the superficial lobe of the
submandibular gland, it gives two to three glandular branches and continues
over the mylohyoid muscle. The lingual nerve makes medial turn and becomes
crossed by the submandibular gland duct at area corresponding to the first or
second mandibular molars. The LN is located inferior and lateral to the duct
[59]. The LN provides general, gustatory and secretomotor innervation to the
presulcus tongue, floor of the mouth, mandibular gingiva and sublingual and
submandibular glands.
The LN may be at risk at various surgical and
non-surgical procedures. LN dysfunction is manifested by sensory (e.g.:
anesthesia, hypoesthesia, paresthesia), gustatory (dysgeusia) and secretomotor
deficits (xerostomia). Other reported symptoms include affected speech and
drooling [56,59]. Most common procedure that risks the LN is third molar
surgeries [48]. Incidence of temporary and permanent LN injury in relation to
third molar surgery is 1% and 0.3%, respectively. The mean distance between
distal releasing incision and the LN is only 4.4 mm. Factors that increase the
risk of LN injury during 3rd molar surgery include lingual flap retraction,
vertical sectioning of the tooth, perforation of the lingual plate, insertion
of lingual flap retractor, follicle attached to the lingual gingiva and
procedure performed under general anesthesia [48,56]. Other than 3rd molar
surgeries, inferior alveolar nerve block (IANB) using the Halsted technique
also places the LN at risk of injuries. The incidence of associated temporary and
permanent LN injury is 0.15% - 0.54% and 0.001-0.01%, respectively [56]. During
the injection procedure the needle may actually penetrate the nerve during
insertion and after hitting bone it becomes barbed and causes more injury
during withdrawal as well. Such injury is capable of producing direct nerve
damage. Compression as a result of extraneural or intraneural hematoma may be
of greater concern. Tan et al have found that the LN is surrounded by larger
number of perineural layers when compared to the IAN which makes ischemia a
possible sequela of hematoma formation. In addition, the LN becomes
unifascicular or oligofascicular at the level of the mandibular foramen which
reduces the chances of nerve recovery if injury occurs [59].
Entrapment neuropathy is another possible mechanism by
which LN injury can occur [4,7]. Various structures within the ITF may
mechanically impinge on the LN. Passage through or close to hyperactive muscle
places sufficient compression on the nerve to cause neuropathy [6]. Other
possible sources of compression neuropathy include calcified pterygospinous
ligament and enlarged lateral pterygoid plates [57,60].
Another branch of the posterior division of V3 is the
mylohyoid nerve which branches from the IAN at area 14 mm superior to the
mandibular foramen [48]. In some cases, the mylohyoid nerve was given off
directly from the main trunk of V3 or even from the LN [61]. The mylohyoid
nerve provides sensory innervation to mandibular teeth and submental skin and motor
fibers to the anterior belly of digastric and mylohyoid muscles. Along with the
IAN which passes through the mandibular foramen, the mylohyoid nerve descends
more posteriorly and around the sphenomandibular ligament to lodge in the
mylohyoid groove at the lingual aspect of the ramus. The presence of
retromandibular foramina suggests sensory supply to mandibular molars. The
mylohyoid nerve continues anteriorly beneath the mylohyoid muscle till it
enters the mandible through the midline retromental foramen and joins the
ipsilateral or contralateral incisive nerve or directly supplies the lower
incisors [62]. The mylohyoid nerve also sends sensory branches through the
lateral lingual foramen to the premolar teeth.
The mylohyoid nerve provides accessory innervation to
the mandibular ipsilateral or contralateral incisors in about 60% of the time
[62]. With inferior alveolar nerve block alone, the patient feels numb lip but
still painful manipulation at the incisors. Anesthesia to the posterior
mandibular teeth may need supplemental mylohyoid nerve block. The mylohoid
nerve may be spared from anesththsia during the IANB due to several reasons.
The mylohyoid nerve is positioned higher than the IAN. In addition, the
mylohyoid may pass in a position shielded by the sphenomandibular ligament or
the pterygomandibular fascia [57,60]. Therefore, supplemental injection at the
pterygomandibular fossa for posterior teeth and at the submental fossa for
anterior teeth may be needed. The mylohyoid nerve communication with the lingual
nerve has been reported [63]. It is possible that the mylohyoid neve has role
in tongue innervation which may serve to explain incomplete anesthesia to the
tongue. This also may provide a means of LN recovery after injury.