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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4399_Библиотеки_им_академика_М_И_Перельмана
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Temporal Bone Histology and Radiology Atlas126
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VERTICAL SECTION 1
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CLINICAL CAVEAT: When drilling in the mastoid
near the facial nerve canal and/or skeletonizing the
mastoid portion of the facial nerve, it is imperative
that the direction of drilling be from superior to
inferior. This is to avoid inadvertent injury to the
inferior vertical section of the facial nerve, as the
fallopian canal has a medial-to-lateral swing from
second genu to stylomastoid foramen. Therefore,
the inferior portion of the vertical (mastoid) facial
nerve is more lateral than the superior portion just
distal to the second genu.
VERTICAL SECTION 2
The relationship of the dome of the horizontal (or lateral) semicircular canal, the short process of the incus,
the vertical facial nerve, and the facial recess is one that
the otologic surgeon must be intimately familiar with.
The second surgical genu of the facial nerve is where the
nerve turns from its horizontal (tympanic) lie to its
vertical (mastoid) lie. This is the most frequent site for
iatrogenic facial nerve injury.2 After drilling through
Koerner’s septum (the petrosquamous fusion plane, or
the petrosquamous septum) and entering the mastoid
antrum, the deep structure to be identified is the dense
otic capsule bone of the curvature or dome of the lateral semicircular canal. Immediately inferior to that is
the genu prior to the vertical facial nerve. The short process of the incus just anterior and superior to the LSCC
“points” to the facial recess air cells which are between
the descending facial nerve and the ascending chorda
tympani nerve which lies close to the posterior external
CLINICAL CAVEAT: The facial recess is demarcated by the vertical facial nerve medially, the
chorda tympani nerve/posterior tympanic ring
annulus laterally, and the incus bar superiorly. The
short process of the incus “points” to the FR. The
vertical facial nerve lies immediately inferior to
the LSCC. Opening of the facial recess is important
in chronic ear surgery as well as in auditory implant surgery.
auditory canal. A surgical picture is shown in Figure 8–9.
Through the facial recess can be seen the round window niche, the promontory, the long and lenticular
processes of the incus, and the head of the stapes. A
draw ing of the important relationships is shown in Fig ure 8–10 on page 137. The facial nerve moves from medial to lateral at up to a 30-degree angle as it descends
from the second genu to the stylomastoid foramen.
Although abnormalities of the Fallopian canal are
frequent in congenital aural atresia cases, the otologic
surgeon must be aware that the facial nerve’s course
may be abnormal in a certain percentage of “normal”
ears. There is significant variability in the angle between
the horizontal and vertical facial nerves. In one study1
of 25 cadaver specimens, that angle was 110° in 64%,
95° in 32%, and 125° in 4% of bones. In the same study,
the second genu was observed to have a normal course
in 68% of the specimens, a lateral hump or posterolateral bulge below the horizontal semicircular canal
in 20%, and in 12% it was lateral to the prominence
of the horizontal semicircular canal. Vigilance during
mastoidectomy is of paramount importance.
VERTICAL SECTION 3
CLINICAL CAVEAT: After entering the mastoid
antrum, identification of the dome of the LSCC and
short process of the incus is best accomplished by
staying anterosuperior, skeletonizing the tegmen,
and following the zygomatic root. The otic capsule
bone over the LSCC is denser and much whiter
than the surrounding mastoid bone. Care is taken
to avoid drilling on the incus itself, in order to
avoid communication of drill trauma to the stapes
and inner ear.
Moving deeper past the facial recess enables us to
see the following structures. The bipennate stapedius
muscle, innervated by the stapedius branch of the facial nerve, lies medial to the vertical facial nerve in the
same plane. At the pyramidal process, it gives off its
tendon to the head (capitulum) of the stapes. Past the
dome of the LSCC, we now see the anterior ampullated
and posterior non-ampullated ends of the LSCC, the
posterior semicircular canal (PSCC), the otic capsule
bone of the superior SCC (SSCC), and the subarcuate
fossa. A valuable surgical tool—Donaldson’s line—is
an imaginary line through the plane of the LSCC, bi-
8. Vertical Temporal Bone Sections with Corresponding Radiographic Images 131
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secting an imaginary line through the plane of the
PSCC. In the anteroinferior quadrant of the “X” that is
thus created will lie the endolymphatic sac in the posterior fossa dura. The relationship of the second genu
of the facial nerve just inferior to the LSCC is clearly
demonstrated on both histologic and radiographic images here.
VERTICAL SECTION 4
The relationship of the muscles and nerves in the ear
are constant. The stapedius muscle, which is innervated by the stapedius branch of the facial nerve, lies
medial to the vertical lie of the facial nerve. At the pyramidal process it gives off its tendon to the capitulum
( head) of the stapes. The tympanic ( horizontal) facial
nerve runs between the cochleariform process, travels
posteriorly just superior to the oval window, and ends
at the second surgical genu inferior to the horizontal
semicircular canal. It is grossly dehiscent at least once in
56% of temporal bones, most commonly over the oval
window, but there are additional microdehiscences
in one-third of all temporal bones.3 These may be the
pathogenesis of the facial palsy that is seen in acute otitis media in 0.04% to 0.16%, even in the post-antibiotic
era.4 The tensor tympani muscle, innervated by the trigeminal nerve (CN V), runs in its semicanal superior to
the Eustachian tube and in the middle ear posteriorly
until the cochleariform process. There, it gives off its
tendon which goes to the neck of the malleus.
CLINICAL CAVEAT: Depending on the size of the
bony overhang of the RWN, direct round window
insertion of a cochlear implant electrode array may
require some bony removal in order to expose the
window directly. This is accomplished with microdrills under direct visualization, being careful
not to drill on or into the actual round window
membrane.
The ampullated end of the LSCC is seen on histologic section, which is visible only as a larger bore of
the LSCC on CT imaging.
VERTICAL SECTION 5
This histological section shows very clearly the relationship between the oval window and round window: they are at 90 degrees to each other. The bone of
the cochlear promontory lies between them. There is
a variable depth to the round window niche, with a
mean bony overhang of 2.1 mm (range 1.9 to 2.4 mm),
a mean height of 2.0 mm (1.8 to 2.15 mm), and a mean
anterior-posterior length of 4.0 mm (3.3 to 4.95 mm).5
As noted in Chapter 6, an adhesion across the mouth
of the RW niche should not be mistaken for the actual
RW membrane.
CLINICAL CAVEAT: Acute otitis media complicated by facial nerve palsy or paralysis requires
emergent myringotomy, aspiration of middle ear
(purulent) fluid, and placement of a pressureequalizing tube in the tympanic membrane. By releasing the pressure of the pus on dehiscences of the
facial nerve in the fallopian canal, the palsy can be
reversed completely.
CLINICAL CAVEAT: The cochleariform process is
a valuable landmark in middle ear surgery, particularly in infected or inflammatory cases. Anterior to
this process, the tubular horizontal structure is the
tensor tympani muscle; posterior to it, it is the facial
nerve.
VERTICAL SECTION 6
In this section, we have come to the anterior tympanomeatal angle. The histologic image shows just a corner
of the EAC and TM. Internally, we are at the protympanum, which will lead into the Eustachian tube orifice.
The facial nerve is at its first surgical genu, the geniculate ganglion, lying superiorly and medially to the tensor tympani muscle. The anterior-most portion of the
inner ear is the cochlea. This section shows a portion of
the cochlea as well as a portion of the utricle inside the
bony vestibule. The stain used highlights the density of
the otic capsule bone compared with the mastoid bone.
The corresponding CT image shows the entire cochlea and a portion of the superior semicircular canal,
as well as the tensor tympani muscle lying superiorly
in the aperture of the Eustachian tube.
Temporal Bone Histology and Radiology Atlas132
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VERTICAL SECTION 2
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