(Fig. 3.8, Fig. 3.13,and Fig. 3.16). The stapedius muscle travels in a bony sulcus just medial to the second genu of the facial canal. It emerges from the pyramidal eminence and courses anteriorly to attach to the stapes anywhere from the incudostapedial region to the junc- tion of the posterior crus with the footplate. It can only be appreciated on axial CT sections (Fig. 3.16). The tensor tendon tightens the TM, and the stapedius tendon stretches the annular ligament. As such, these muscles both play a role in damping the response of the ossicular chain, thus protecting the cochlea from intense acoustic stimulation.24,35
A B
C–E
F
Fig. 3.13 (A)Axial anatomy, right ear (mn, malleus neck; t, tensor tympani tendon; ilp, incus, long process; isj, incudostapedial joint; ss, stapes superstructure). (B)Axial anatomy, stapes superstructure. (C)Line drawing. (D)Anatomy of ossicular chain. Coronal drawing (slightly oblique). (E)Axial drawing (isj, incudostapedial joint; ttt, tensor tympani tendon; st, stapedius tendon; lpi, long process of incus; len, lenticular process of incus). (F)Artist’s rendering of nor- mal ossicular chain and tendinous attachments. Derivations are indicated. Cross-hatched; first branchial arch; stippled: second branchial arch; filled: otic capsule (lamina stapedialis). (From Swartz JD, Glazer AU, Faerber EN, et al. Congenital middle ear deafness: CT study. Radiology 1986;159:187–190. Reprinted with permission.)
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Fig. 3.15 Axial anatomy (mn, malleus neck; asc, anterior stapes crus;
psc, posterior stapes crus; ilp, incus, lenticular process; isj, incudostape- dial joint; ow, oval window [stapes footplate/annular ligament]).
Fig. 3.14 Axial anatomy (p, pyramidal eminence; f, facial recess; s, sinus tympani; ps, posterior semicircular canal; ss, stapes superstructure).
A B
Fig. 3.16 (A)Axial anatomy (ttm, tensor tympani muscle; ttt, tensor tympani tendon; mn, malleus neck; ow, oval window [small unlabeled arrows, anterior and posterior margins]; asc, anterior stapes crus; psc, posterior stapes crus; isj, incudostapedial joint). (B)Tympanic cavity, corresponding axial illustration (CTN, chorda tympani nerve; SH, stapes
head; ASC, stapes, anterior crus; PSC, stapes, posterior crus; SF, stapes footplate [note tympanic and vestibular segments]; ST, stapedius ten- don; FN, facial nerve, second genu in pyramidal eminence; CN, cochlear nerve; IVN, inferior vestibular nerve). (See Color Plate Fig. 3.16B.)
A
B D
E C
outlined white arrow, pyramidal eminence; black arrow, sinus tympani;
white arrow, incudostapedial articulation; dotted white arrow, chordal eminence. (B)Sagittal computed tomography (CT) image, more lateral.
Black arrow, lateral semicircular canal; outlined black arrow, facial nerve canal, second genu; thin white arrow, sinus tympani; double black arrows, facial nerve canal (mastoid segment); outlined white arrow, stylomastoid foramen. (C)Sagittal CT image, more medial. Black arrow, pyramidal eminence; double black arrowheads, stapedius muscle canal; white arrow, incus lenticular process; white arrowhead, malleus neck. (D)Sagittal CT image, most medial. Outlined white arrow, subiculum; outlined black arrow, sinus tympani; dotted black arrow, lateral semicircular canal; long white arrow, facial nerve (tympanic segment); short white arrow, stape- dial head; double white arrows, anterior tympanic spine; short thick black arrow, glaserian fissure (exit of chorda tympani nerve). (E)Posterior tym- panum, coronal CT image. Outlined white arrow, facial recess; thick black arrow, pyramidal eminence; thick white arrow, sinus tympani.
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lateral mallear ligament, and medially by the neck of the malleus (Fig. 3.4, Fig. 3.8). 4,9,36,37The lateral mallear ligament courses from the scutum (junction of the lateral attic wall and EAC) to the neck of the malleus. Prussak’s space opens posteriorly into the epitympanum and repre- sents the most common site of origin for acquired attic cholesteatoma (CH).
The anterior epitympanic recess (AER), also known as the supratubal recess (STR), is located superior to the bony eustachian tube and anterior to the attic and consists of a variably sized single air cell (61%) or multiple small cells.
There is symmetry in 78% of patients.38This region is visu- alized on axial section anteromedial to the head of the malleus. It is bounded posteriorly by a thin transverse bony septum (“cog”) and anteriorly by the anterior petrosal tegmen (Fig. 3.6, Fig. 3.10, and Fig. 3.18). The middle cranial fossa forms a portion of the superior and anterior boundary. The chorda tympani nerve and the tympanic bone form the lateral boundary.39The “cog” may be bony or fibrous and extends from the cochleariform process to the tegmen.40The shape of this recess is deter- mined by the embryologic development of the saccus an- ticus and anterior saccule of the saccus medius41; however, development of the AER/STR is independent of the middle ear/mastoid air cells system, instead relating directly to eustachian tube formation.42Growth of the AER/STR may continue into early childhood in contradistinction to the remainder of the attic, which is generally believed to be
complete by the end of gestation. The proximal tympanic segment of the facial nerve canal lies immediately adja- cent to the recess on its medial side (Fig. 3.10and Fig. 3.18).
The mucosal fold investing the tensor tympani is also in close apposition.4,43,44When this fold is embryologically absent, cholesteatomatous masses have direct access to this segment of the facial nerve.45Koerner’s septum (see below) is the posterior continuation of the “cog” and therefore has similar embryologic significance (Fig. 3.19).46 A complex set of recesses and ridges lies posterior to the bony tympanic annulus along the posterior and medial borders of the tympanic cavity. This region is referred to in the literature as the posterior tympanumor retrotympanum (Fig. 3.14, Fig. 3.17, Fig. 3.20, Fig. 3.21, Fig. 3.22, Fig. 3.23,and Fig. 3.24).36,47The pyramidal eminence (PE), from which the stapedius tendon emerges, represents the most promi- nent ridge in the posterior wall. Immediately medial to the PE lies the sinus tympani, a recess of variable depth that is bordered medially by the cortical bone overlying the posterior semicircular canal.12,36,47 The ponticulus (an extension of the oval window niche) is its superior border;
the subiculum forms the inferior border (Fig. 3.20A). The subiculum separates the sinus tympani from the round window niche (Fig. 3.23).4,48Directly lateral to the PE lies the facial recess, which is often much shallower than its more medial counterpart. Lateral to the facial recess is the chordal eminence, which forms the medial border of the canaliculus chordae tympani through which the chorda tympani branch of the facial nerve enters the middle ear cavity. A chordal ridge is described that links the pyramidal eminence to the chordal eminence. The facial recess is limited further laterally by the bony tympanic annulus (origin of the TM).
Fig. 3.18 Anterior epitympanic recess. Outlined white arrow, anterior epitympanic recess (supratubal recess); thin white arrow, cog; outlined black arrow, facial nerve/canal (tympanic segment).
Fig. 3.19 Axial anatomy (aml, anterior malleal ligament; ks, Koerner septum; st, sinus tympani; f.tymp, facial nerve canal, tympanic segment).
A
B Fig. 3.20 (A)Axial anatomy (p, ponticulus; rw, round window niche).
(B)Artist’s rendering of posterior tympanum [C.T.N., chorda tympani nerve (arrow); F.R., facial recess (arrow); S.T., sinus tympani; SU, subicu- lum; R.W.N., round window niche (arrowhead); PON, ponticulus; P.E., pyramidal eminence (open arrowhead); stapedius tendon (arrow); SS, stapes superstructure].
The medial wall of the posterior tympanum is described as having two ridges and three depressions (Fig. 3.14, Fig. 3.20, Fig. 3.23,and Fig. 3.24). The ridges are the more inferior subiculum (posterior prolongation of the cepha- lad border of the round window) and the more superior ponticulus, which extends from the pyramidal eminence to the promontory. Between these ridges lies the sinus
tympani.12 Beneath the subiculum is the round window niche, and superior to the ponticulus is the oval window.
These ridges are variable in size. They are identified with difficulty on coronal section; however, their importance is limited to both the surgeon and the radiologist. Two
Fig. 3.21 Axial anatomy (pt, pars tensa, tympanic membrane; s, subicu- lum; f.mast, facial nerve canal, mastoid segment; hypo, hypotympa- num; fo, foramen ovale; fs, foramen spinosum; cc, carotid canal).
Fig. 3.22 Axial anatomy (ttm, tensor tympani muscle; cc, carotid canal, horizontal portion; f.mast, facial nerve canal, mastoid segment; jf, jugular foramen; eac, external auditory canal; gs, glossopharyngeal sulcus).
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additional eminences, the styloid and the chordal, and two other depressions, the posterior and lateral tympanic sinuses, are also probably identifiable on CT; however, their importance is limited as well. The styloid and chordal eminences are inferior and posterior to the pyramidal eminence, respectively. The posterior tympanum is derived virtually in its entirety from the second branchial arch.7,12 These recesses may be hidden from view during surgery and are often the site of residual collections of granula- tion tissue or CH. They are consistently well seen on axial CT section.4,49,50CT visualization of the sinus tympani is especially important preoperatively, as extensive involve- ment in this location may require a retrofacial (nerve) surgical approach (Fig. 3.14and Fig. 3.24). A highly posi- tioned jugular bulb and a contracted space between the facial nerve and the posterior semicircular canal preclude this type of exploration.47,48
These structures form the posterior portion of the tym- panic cavity proper. The lateral border is the TM, and the medial border is the labyrinth, particularly the promon- tory. The roof of the epitympanum, which is referred to as the tegmen tympani, separates the epitympanum from the middle cranial fossa. The inferior wall of the tympanic cavity (hypotympanum) is separated by plates of bone anteriorly from the carotid canal and posteriorly from the jugular bulb (Fig. 3.22).