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Figure 9–8

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186

Figure 9–9

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187

Temporal Bone Histology and Radiology Atlas188

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population in the infant is about 35,000 per ear, but this number decreases with aging. Reissner’s membrane extends from the spiral ligament to the spiral limbus. It separates scala media from scala vestibuli. The basilar membrane extends from the osseous spiral lamina to the spiral ligament and contains the organ of Corti. The corresponding CT image cannot show the intracochlear detail seen in the histology section. However, clearly visible are the following structures: part of the external auditory canal, part of the malleus and the incus, the entire stapes, the second genu of the facial nerve, the horizontal semicircular canal, a portion of the posterior semicircular canal, the vestibule, the IAC, the cochlea, the tensor tympani muscle, the protympanum, the bony Eustachian tube, the carotid artery, an aerated petrous apex, and pneumatized mastoid air cells.

FIGURE 9–11

This midmodiolar section shows all 5 “portions” of the 2¾ turns in the normal human cochlea. Note the density of the otic capsule bone of the cochlea.

FIGURE 9–12

FIGURE 9–13

At this level, the stapes footplate is evident. The stapes possesses a footplate, two legs (crus anterius and crus posterius), and a head. The site of the stapedotomy hole is preferably in the posterior third of the footplate since in this area the distance between the membranous labyrinth and the footplate is larger than the rest. Medially the vestibular aqueduct is seen. This is the internal opening of the aqueduct that passes from the area of the elliptical recess to the posterior surface of the temporal bone. The endolymphatic duct, a part of the endolymphatic sac, and their vein are contained within the vestibular aqueduct. The distal part of the endolymphatic sac lies on the posterior surface of the petrous portion of the temporal bone between the layers of the dura mater.

Looking at the tympanic membrane and ossicle relationship, it is obvious that the insertion of the ventilation tube is safer in the anterior quadrants of the tympanic membrane than in the posterior quadrants because of the risk of damaging the incus and stapes. The CT scan conﬁrms this idea. In the histologic section, a bit of the chorda tympani nerve can be seen as it ﬂies between the incus and malleus in the middle ear on its journey from iter chordae posterior to iter chordae anterior. The CT scan cannot show that small a nerve, but the facial recess air cell is clearly seen just lateral to the facial nerve posteriorly in the ear.

The tendon of the tensor tympani makes a right angle to its long axis, to attach to the neck of the malleus. Within the vestibule, the saccule and their maculae are clearly seen. The saccule, which is smaller than the utricle, is ovoid in shape. The saccule lies in the anterior and inferior part of the vestibule, bound to the spherical recess by ﬁbrous tissue. Note the distance between the stapes footplate and the saccular membrane, which is to be kept in mind during stapes surgery to avoid too-deep insertion of the prosthesis which can cause vertigo and sensorineural hearing loss. This section can be compared with the CT image in Figure 9–10b.

CLINICAL CAVEAT: The saccule is responsible for linear acceleration of the head. Recently, VEMP (vestibular evoked myogenic potential) is being used to test the saccular function.

FIGURE 9–14

Posteriorly, near the aditus, we see a recess known as fossa incudis, where the posterior ligament of the incus is attached. The stapedial footplate is medially located

CLINICAL CAVEAT: Although other ear pathologies can also be seen in this area, cystic structures and cholesterol granuloma are the most commonly seen pathologies in the petrous apex. They may occur due to Eustachian tube dysfunction and when there is bleeding into the air pockets in the petrous apex. The treatment is basically drainage—unlike cho lesteatomas, petrous apex cholesterol granuloma does not need full cyst wall removal. The petrous apex is also the most common site for secondary metastasis of malignant systemic cancers in the ear.

9. Complete Temporal Bone Study 189

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in the middle ear and is attached to the oval window with the annular ligament. In the internal auditory canal, the inferior vestibular nerve is seen. Anterior to the internal auditory canal, the edge of the petrous apex is seen.

FIGURE 9–15

Anteriorly, the Eustachian tube starts to appear in the middle ear. This tube connects the middle ear with the nasopharynx. In the course of this connection, it also inclines medially and anteriorly. Its medial two-thirds is cartilaginous and the lateral one-third is bony. Just posterior to the malleus the chorda tympani passes between the descending crus of the incus and the membrane. The bony lumen of the non-ampullated end of the lateral semicircular canal has now opened.

FIGURE 9–16

The mesotympanum level of the middle ear is in view. The apical turn of the cochlea has disappeared from view. The tensor tympani muscle has disappeared. The Eustachian tube and carotid artery are seen anterior to the cochlea. The nonampullated end of lateral semicircular canal as well as the posterior semicircular canal are seen.

CLINICAL CAVEAT: In recalcitrant benign paroxysmal positional vertigo (BPPV), as a last resort, a selective denervation of this particular nerve can be performed under local anesthesia. That kind of surgery requires a transcanal approach and drilling the inferior attachment of round window membrane to identify the nerve, and carries a high risk of permanent sensorineural hearing loss. Over 93% of BPPV cases respond favorably to canalith repositioning procedures, ally treated with posterior semicircular canal plugging via a mastoidectomy, with less hearing risk.

CLINICAL CAVEAT: The facial recess is bounded by the chorda tympani nerve and tympanic membrane annulus laterally and the facial nerve in the pyramidal process medially. Familiarity with the recess and its boundaries are important for the otologic surgeon.

and the few remaining cases are gener-

FIGURE 9–18

As we move further inferiorly down the temporal bone, at this level, the lateral semicircular canal and the middle turn of the cochlea gradually recede from view. Jacobson’s nerve is again seen here as it crosses the promontory on its way to the carotid plexus. In the posterior wall of the middle ear, the air cells of the facial recess are seen.

The two crura of the stapes are visible in this histologic section, as are the long processes of the malleus and the incus. The chorda tympani nerve is seen in crosssection as it traverses the middle ear between them. The facial nerve is seen in the pyramidal process at the posterior wall of the middle ear. On the promontory is visible Jacobson’s nerve, a branch of the glossopharyngeal nerve (CN IX), whose vascular structures are the ones involved in glomus tympanicum tumors. The singular canal and the singular nerve are in view, posterior to the posterior wall of the internal auditory canal. This nerve is a branch of the inferior vestibular nerve and supplies the posterior semicircular canal crista.

FIGURE 9–17

FIGURE 9–19

In this histological section, the incudostapedial joint is seen. This joint is one of the most vulnerable ossicular joints in the body. The lenticular process of incus (which comes off at a right angle from the long process of the incus) articulates with the head of the stapes via this joint. The facial recess, pyramidal eminence, and beginnings of the sinus tympani are seen making up the posterior wall of the middle ear cavity. Anteriorly, the close relationship of the cochlea, the carotid artery, and the bony Eustachian tube are seen. The corresponding CT image shows portions of the three ossicles, the incudo-stapedial joint clearly, the facial