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90 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
It is common for cholesteatoma of the epitympanic space to extend into
the anterior epitympanic recess. Surgeons must be aware that the facial
nerve and geniculate ganglion may be dehiscent in its medial wall.
The Oval Window Niche
The oval window niche is located in the medial wall of the posterior part of
the mesotympanum and harbors the stapes (Figs. 84 and 88). It is bounded
superiorly by the facial nerve and inferiorly by the promontory. Located
anteriorly is the cochleariform process and posteriorly the ponticulus, sinus
tympani, and pyramidal eminence.
With the advent of tympanoplasty surgery for chronic infections, stapes
operations for otosclerosis, labyrinth operations for Méni`ere’s disease, and
reconstructive surgery for congenital anomalies of the middle ear, the oval
window area has become an important anatomic site. The stapes is vulnerable to fracture or subluxation, and the facial nerve, which may be dehiscent
and bulging from its canal, can be injured. Some of the spatial relationships
concerning the middle ear and oval window are seen in Figure 89.
The Round Window Niche
The round window niche (fossula fenestra cochleae) is a depression of variable depth located in the posteroinferior aspect of the medial wall of the
tympanic cavity (Figs. 84 and 90). A bony ridge of bone, the subiculum, separates the round window niche from the sinus tympani posterosuperiorly.
The round window niche is delimited anterosuperiorly by the promontory
and inferiorly by the hypotympanum (Fig. 91).
Figure 88
This vertical section depicts the
anatomical relationships of inner and
middle ear structures (male, age 47yr).
Abbreviations: EAC, external auditory
canal; TM, tympanic membrane.
CHAPTER 3: THE MIDDLE EAR ■ 91
Figure 89
The measurements shown here are
average normal for the adult ear (4).
Figure 90
The subiculum is shown in relation to
the round window niche and sinus
tympani. The microfissure extending
from the round window niche to the
ampulla of the posterior canal is normally present in adult temporal bones.
The posterior part of the tympanic
membrane is pathologically thin and
retracted medially, probably as a consequence of previous otitis media (male,
age 83yr).
92 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 91
The tympanic ostium of the cochlear
aqueduct is located just medial to the
semilunar crest of the round window.
The inferior cochlear vein provides the
principal venous drainage for the inner
ear (male, age 68yr).
Ultrastructural studies of the round window membrane in guinea pigs
(32–34) reveal that it is composed of three layers. In the external layer, facing the
tympanic cavity, there are four distinct types of cells: osmiophilic, osmiophobic,
dark granulated, and goblet. Numerous microvilli stud the free epithelial surface, the cells of which are flat and mostly nonciliated. The internal layer, facing the scala tympani, consists of a single layer of thin cells having long, thin,
cytoplasmic extensions into the scala tympani, a cytoarchitectural pattern very
similar to that of the perilymphatic surface of Reissner’s membrane. An intermediate layer consists of a dense latticework of fibrocytes interspersed with
large intercellular spaces containing collagen and elastic fibers, blood
vessels, and myelinated as well as unmyelinated nerve fibers.
The round window membrane functions as a yielding area of the bony
labyrinth, permitting movement of the inner ear fluids associated with
movement of the stapedial footplate. Conductive hearing loss associated
with congenital absence of the round window has been successfully treated
by surgical fenestration (35). The round window membrane is believed to be
a route by which toxic substances (bacterial exotoxins, chemical solutions)
may enter the inner ear to cause sensorineural hearing loss.
The round window niche is angulated posteroinferiorly with respect to
the external auditory canal. The membrane lies mostly in the horizontal
plane, but assumes a more vertical orientation as it curves anteriorly toward
the scala tympani. It frequently lies partly hidden behind an incomplete
curtain of mucosa which bridges the mouth of the niche (Fig. 91).
Goodhill et al. (36) have implicated rupture (fistulization) of the round
window membrane as a cause of sudden sensorineural hearing loss occurring in association with barotrauma or head injury. When a surgical exploration is done for a suspected round window fistula, the mucous membrane
fold must not be mistaken for the round window membrane. The bony lip of
the round window niche will usually have to be removed if the round
window membrane is to be visualized. Patency of the niche is essential for
efficient acoustic transmission. In tympanoplasty surgery, the objective is to
achieve a pneumatized round window niche behind a protective tympanic
membrane.
Section of the posterior ampullary nerve has been advocated for relief of
benign paroxysmal positional vertigo (37). The singular canal that contains this
CHAPTER 3: THE MIDDLE EAR ■ 93
nerve lies immediately inferior to the posterior attachment of the round
window membrane. This membrane and the ampulla of the posterior canal are
vulnerable to injury in the surgical approach to the posterior ampullary nerve.
The Sinus Tympani
The sinus tympani is one of the three depressions in the medial wall of the
posterior part of the mesotympanum (Fig. 92), the other two are the round
and oval windows. Superiorly, it is bounded by the ponticulus and lateral
semicircular canal, posteriorly by the posterior semicircular canal, and
inferiorly by the subiculum, styloid eminence, and jugular wall (38). It is
bounded medially by the bony labyrinth and laterally by the pyramidal
eminence and facial nerve. It extends for variable distances in posterior
direction, medial to the facial nerve (Figs. 93–95).
Figure 92
Located lateral to the facial nerve is the
facial recess and medial to it is the
sinus tympani. The subiculum separates the round window niche from the
sinus tympani. The operculum is a
scale of bone that partly overlies the
endolymphatic sac (male, age 47yr).
Figure 93
The sinus tympani may extend for several millimeters medial and posterior to
the facial nerve as seen in this ear.
Invariably, there is a bony partition
between the sinus tympani and mastoid
air cells due to their different routes of
pneumatization (male, age 49yr).
94 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 94
The ponticulus (little bridge), which
forms the superior boundary of
the sinus tympani, stretches from
the pyramidal eminence to the
promontory. The facial recess lies
lateral to the facial nerve and is used by
otologic surgeons as a route from the
mastoid to the middle ear (the
posterior tympanotomy approach)
(male, age 9yr).
Figure 95
The sinus tympani and round window
niche are seen in relationship to the
subiculum. The posterior ampullary
nerve, located in the singular canal, is
surgically accessible by an approach
through the middle ear (male, age
40yr).
The sinus tympani may harbor diseased tissue such as cholesteatoma,
and is not directly visible by the usual surgical approaches to this area
(39–41). Visualization may be facilitated by the use of mirrors, and access to
its depths may be afforded by 3- or 4-mm right-angled picks and small,
curved, metal suction tubes. Partial removal of the bony tympanic annulus
is often necessary to gain access to the sinus tympani.
The Facial Recess
The facial recess is a depression of variable depth in the posterior wall of the
middle ear. It is bounded medially by the facial canal and styloid complex
and laterally by the tympanic bone (Fig. 96). The styloid complex (26) is the
CHAPTER 3: THE MIDDLE EAR ■ 95
Figure 96
This photomicrograph shows the anatomical relationships of several middle ear
structures (female, age 60yr).
term used to describe the derivatives of the superior portion of the second
branchial arch; once ossified, it gives rise to three projections present in all
adult temporal bones—the pyramidal, styloid, and chordal eminences. The
facial recess, like the sinus tympani, is a potential site for the sequestration
of disease, such as cholesteatoma.
The surgeon will find that the facial nerve is occasionally dehiscent in
the medial wall of the facial recess, making it vulnerable to surgical injury
(Fig. 97). In intact-canal-wall tympanoplasty, the route from the mastoid to
the middle ear is via the facial recess, a surgical maneuver commonly known
as posterior tympanotomy.
Figure 97
In this ear, the mastoid segment of the
facial nerve bulges into the facial recess
(male, age 61yr).
96 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
THE EUSTACHIAN TUBE
The eustachian tube, a mucosally lined pathway between the nasopharynx
and the middle ear, permits ventilation of the pneumatized spaces of the
temporal bone while safeguarding against bacterial contamination of these
spaces. The posterolateral one-third is bony while the anteromedial
two-thirds is fibrocartilaginous; these two sections are joined at the tubal
isthmus. The overall length of the eustachian tube in the adult varies from
31 to 38mm (42, 43).
The Bony Eustachian Tube
The bony part of the eustachian tube lies lateral to the internal carotid artery
(Fig. 98); the thin bone separating these structures has dehiscences permitting the passage of the caroticotympanic arteries. The tubal isthmus marks a
region of structural transition, with cartilage forming its anterolateral and
superior walls and bone forming its posteromedial and inferior limits
(Fig. 99). The lining mucosa is a low columnar ciliated epithelium with abundant goblet cells on a tunica propria of basement membrane and loose
connective tissue. The tympanic ostium of the eustachian tube is in the anterior wall of the middle ear cavity, about 4 to 6mm superior to the inferior
wall of the hypotympanum. Here, the lumen is triangular, measuring 3 to
5mm in diameter. As the osseous part of the tube heads anteriorly and
inferiorly to the isthmus, its vertical diameter shrinks to 2 to 3mm and its
horizontal diameter to 1 to 1.5mm.
The Fibrocartilaginous Eustachian Tube
This part of the tube in cross section resembles a shepherd’s crook. The
lumen is maintained by a larger medial cartilaginous lamella and a smaller
Figure 98
The next four photomicrographs are
from vertical sections in the coronal
plane of the eustachian tube of a onemonth-old infant presented serially
from posterior to anterior. This section
at the tympanic orifice shows the thin
bony shell separating the internal
carotid artery from the cartilage of the
eustachian tube. Source: Courtesy of
Drs. Doyle and Rood, University of
Pittsburgh.
CHAPTER 3: THE MIDDLE EAR ■ 97
Figure 99
This section shows the eustachian tube
at the junction of the cartilaginous and
bony portions (the isthmus tubae).
Source: Courtesy of Drs. Doyle and
Rood, University of Pittsburgh.
lateral one (Fig. 100). The salpingopharyngeal fascia stretches between the
inferior edge of the medial lamella and the free edge of the lateral lamella.
The tubal incisura is a groove in the middle one-third of the inferior margin
of the medial lamella which accommodates the levator veli palatini muscle.
The histology of the tubal cartilage varies with age; in the newborn it is
entirely hyaline, while in the adult an elastic component can be found concentrated at the inner aspect of the junction of the medial and lateral lamellae.
In the adult, the nasopharyngeal orifice lies some 15mm inferior to the
tympanic ostium (Fig. 101). At rest the orifice is a vertical slit measuring
about 8mm ⫻ 4 mm. The posterior lip of the pharyngeal orifice is the mobile
portion which forms the torus tubarius. The fossa of Rosenmüller is located
posterior to the torus.
A series of photomicrographs showing the anatomical features of the
eustachian tube of an adult is seen in Figures 102 to 105.
The Lining Membrane
The lining (mucosal) membrane is composed of a pseudostratified, ciliated
columnar epithelium interspersed with goblet cells that are most abundant
at the pharyngeal orifice (Figs. 106–108). The supporting lamina propria is of
variable thickness and can be divided into three layers: (1) a basement membrane directly beneath the epithelium, (2) a sheet of lymphoid tissue, the
thickness of which varies inversely with the age of the individual, and (3) a
layer of compound tubuloalveolar glands. The mucosa of the eustachian
tube consists of pseudostratified ciliated columnar cells mixed with a
cuboidal ciliated epithelium and goblet cells (44). Some authors (45, 46)
believe that lymphatic tissue of the nasopharynx never reaches up to, or into,
the pharyngeal orifice of the eustachian tube but occurs as a distinct extratubal lymphoid mass (the so-called Gerlach’s tubal tonsil).
98 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 100
This section shows the classical shepherd’s crook (Hirtenstabkrümmung)
configuration of a cross section of the
cartilage of the eustachian tube. The
anatomic relationships of the levator
veli palatini, tensor veli palatini, and
medial pterygoid muscles are shown.
Source: Courtesy of Drs. Doyle and
Rood, University of Pittsburgh.
Figure 101
The nasopharyngeal orifice of the
eustachian tube is seen in this section.
Source: Courtesy of Drs. Doyle and
Rood, University of Pittsburgh.
CHAPTER 3: THE MIDDLE EAR ■ 99
Figure 102
The next four photomicrographs are
from vertical sections in the coronal
plane of the eustachian tube of an adult
presented serially from posterior to
anterior. A thin bony partition separates the bony eustachian tube from the
internal carotid artery. Source: Courtesy
of Dr. Sando, University of Pittsburgh.
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