The normal ossicular chain consists of the malleus, incus, and stapes (Fig. 3.5). The stapes weighs only 2.5 g and as such is only about one tenth the weight of either of the Table 3.2 Appropriate Projections for Other Middle Ear Structures
Best CT Projection Either
Structure Axial Coronal (Both)
Prussak’s space X
Attic X
Scutum X
Antrum and central mastoid tract X
Aditus X
Tegmen tympani X
Tensor tympani muscle X
Sigmoid sinus groove X
Tensor tympani tendon X
Fossa incudis X
Round window X
Facial recess X
Pyramidal eminence X
Sinus tympani X
Lateral mallear ligament X
Superior mallear ligament X
Anterior mallear ligament X Pattern of pneumatization X
Tympanic membrane X
Anterior epitympanic recess (air cell) X
Ponticulus X
Subiculum X
Abbreviations:CT, computed tomography.
Source:From Swartz JD. High resolution computed tomography of the middle ear and mastoid. Part I: Normal anatomy including normal variations. Radiology 1983;148:449–454. Reprinted with permissison.
Fig. 3.2Tympanic membrane (solid arrow, pars flaccida; outlined arrow, pars tensa; long thin arrow, umbo [malleus handle]). (Adapted from Platzer W. Pernkopf Anatomy, 3rd ed. Munich: Urban & Schwarzenberg;
1989.) (See Color Plate Fig. 3.2.)
Fig. 3.3 (A)Coronal CT anatomy (sml, superior malleal ligament; mh, malleus head; mn, malleus neck; man malleus, manubrium; s, scutum;
p.flac, pars flaccida of tympanic membrane; lml, lateral malleal ligament;
tt, tensor tympani muscle and tendon; tac, tegmental air cells [above
external auditory canal]; the lml and the p.flac. subtend Prussak’s space).
(B)Sagittal CT image; long white arrow, facial nerve canal (labyrinthine segment); long black arrow, cochleariform process (tensor tympani muscle);
triple black arrows, inferior tympanic canaliculus (nerve of Jacobson).
A B
other two ossicles.1A review of paleontology reveals that the precursors of the ossicular chain were part of the jaw and that primitive vertebrates such as the bullfrog have only one middle ear bone.16The development of the ossic- ular chain was presumably a survival mechanism, as it amplifies sound pressure on the TM by 30%. A primitive stapes (solid, no crura) persists in various marsupials. The development of crura improved hearing, as the resultant ossicle is much lighter. The tympano-ossicular system is responsible for transmission of sound from the EAC to the cochlea in the normally functioning ear. This is referred to as ossicular coupling. Direct stimulation of the oval and round windows in those with a nonfunctioning ossicular chain is referred to as acoustic coupling.17
The malleus is described in terms of the head, neck, lateral (short) process, anterior process, and handle (manubrium) (Fig. 3.6, Fig. 3.7, Fig. 3.8). The lateral process and manubrium are embedded within the TM and are best seen utilizing coronal CT images.4,18There is a diarthrodial articulation between the malleus and incus in the attic, the malleoincudal articulation, easily and consistently seen on axial and sagittal CT images (Fig. 3.9 and Fig. 3.10).6,19There are medial and lateral incudomal- lear ligaments that are difficult to resolve even with the highest resolution CT equipment.19The bulk of the incus,
the largest ossicle, is made up of the body; however, short, long, and lenticular processes are also described (Figs. 3.6, 3.7, 3.9, 3.10). The short process lies posteriorly within the fossa incudus and acts as a fulcrum on which the rest of the incus rotates. The fossa incudus is located immediately below the aditus and can only be appreci- ated with axial and sagittal CT sections.4,10 Surgeons are aware of the close relationship between the short process and the second genu of the facial nerve, generally in the 3 mm range.20 The very fine long process and lenticular process represent the most vulnerable seg- ments of the ossicular chain and are commonly eroded in the context of inflammatory disease.7,21,22They meet at a variable angle, usually almost 90 degrees (Fig. 3.11and Fig. 3.12). The long process is visualized to best advantage on these coronal CT sections. The cup-shaped lenticular process articulates directly with the ball-shaped capitu- lum (head) of the stapes via a cartilaginous disk and is also a synovial, diarthrodial articulation (Fig. 3.13, Fig. 3.14, Fig. 3.15, Fig. 3.16,and Fig. 3.17).11,23–25The stapes super- structureis a term that is used to describe the portion of the stapes that is derived from the second branchial arch.
This includes the capitulum (head), anterior crus, poste- rior crus, and the tympanic portion of the footplate. The vestibular portion of the footplate and contiguous annular
Chapter 3 The Middle Ear and Mastoid
61
Fig. 3.4 (A)Short black arrow, facial nerve (labyrinthine segment);
dotted white arrow, facial nerve (tympanic segment); cross-hatched white arrow, tensor tympani muscle; thin white arrow, tensor tympani tendon; thick white arrowhead, malleus neck. (B)Artist’s rendering of Prussak’s space (arrowheads). This is the space subtended by the lat- eral mallear ligament, the malleus neck, and the pars flaccida of the tympanic membrane. Insert: Otoscopic view of normal tympanic membrane. (C)Drawing, coronal plane (lml, lateral mallear ligament;
sml, superior mallear ligament; mall, head of malleus). White arrow, pars flaccida of tympanic membrane (pf); double white arrows(pars tensa of tympanic membrane).
A
B
C
Fig. 3.5 Ossicular chain (SPM, malleus-short process; MH, malleus han- dle; MIA, malleoincudal articulation; IB, incus body; SPI, incus, short process; LONG, incus, long process; LEN, incus, lenticular process; ISJ, incudostapedial joint; SH, stapes head; ANT, stapes, anterior crus; POST, stapes, posterior crus; SF, stapes footplate [note tympanic and vestibu- lar segments]). (See Color Plate Fig. 3.5.)
Fig. 3.6 Axial anatomy (isp, incus short process; ib, incus body; mh, malleus head; cog, cog; aer, anterior epitympanic recess [single cell]; f.
tymp, facial nerve tympanic segment; mia, malleoincudal articulation).
A B
Fig. 3.7 (A) Axial anatomy (aml, anterior malleal ligament; mia, malleoincudal articulation; isp, incus, short process [in fossa incudus]; fi, fossa incudus). Level-trapped fluid in petrous apex cell. (B)Sagittal view.
White arrow, malleus head; outlined white arrow, incus body; short,thick white arrow, incus short process; thin white arrow, tympanic membrane (pars flaccida).
Chapter 3 The Middle Ear and Mastoid
63
Fig. 3.7 (Continued) (C)Drawing, sagittal plane; tympanic cavity (AML, anterior malleal ligament; CTN, chorda tympani nerve; M.HE, malleus head; MH, malleus handle; MIA, malleoincudal articulation;
IB, incus body; SPI, Incus, short process; PIL, posterior incudal liga- ment; LPI, Incus, long process; LEN, incus, lenticular process [stapes is removed]; PT, pars tensa, tympanic membrane). (See Color Plate Fig. 3.7C.)
C
Fig. 3.8 Coronal computed tomography image, well positioned. Both ears are appreciated in a symmetric fashion due to superb patient positioning (mild tilt; teg, tegmen tympani [roof of middle ear/attic];
mall, malleus head; tac, tegmental air cells [variable in number]; sc, scutum; ttt, tensor tympani tendon [5th nerve, 1st branchial arch deri- vation]; fnc, facial nerve canal; coch, apical/middle cochlear turns).
in the annular ligament that supports the syndesmotic (fibrous) stapediovestibular articulation (Fig. 3.13Dand Fig. 3.16B).
Contrary to popular belief, the vast bulk of the ossicu- lar chain is best appreciated on evaluation of axial CT sec- tions. The malleoincudal and incudostapedial articulations as well as the stapes superstructure are all appreciated to best advantage in this projection.4The oval window (stapes footplate/annular ligament) is of uniform thickness and has an anteroposterior orientation. In our opinion this structure is also best seen in this projection (Fig. 3.15).26,27 Coronal CT images allow for better appreciation of struc- tures oriented vertically, such as the malleus and incus long process. The right-angle junction of the incus long and lenticular processes is also well appreciated in this projection. Axial, coronal, and sagittal CT images are therefore highly complementary for ossicular evaluation (Fig. 3.11and Fig. 3.12).28,29
Sagittal CT imaging is more readily available with current techniques utilizing volumetric acquisitions (see Chapter 1).
In this projection, the malleoincudal articulation is well seen as the classic “molar tooth” configuration (Fig. 3.7C and Fig. 3.9B). This appearance was originally described with complex motion tomography when direct sagittal (lateral) imaging was routine. Other structures visualized in this projection include the recess for the stapedius mus- cle, the posterior semicircular canal, and the anterior tym- panic spine (Fig. 3.9B, Fig. 3.11B,C,and Fig. 3.17B–D). The latter forms the undersurface of the glaserian (anterior tympanic) fissure and is a common fracture site.
ligament are derived from the otic capsule (neuroecto- derm). The space between the crura is referred to as the obturator foramen.7The stapedial artery resides in this location during fetal life. Persistence of this vessel is rare (see Chapter 4). The footplate of the stapes is embedded
Fig. 3.9 (A)Axial anatomy (law, lateral attic wall; ma, mastoid antrum;
ssp, sigmoid sinus plate). (B)Sagittal computed tomography image.
Thin white arrow, long process of incus; outlined white arrow, handle of
malleus (note the classic “molar tooth” appearance); thick white arrow, anterior tympanic spine; black arrow, glaserian fissure (passage of chorda tympani nerve and anterior tympanic artery).
Fig. 3.10 Axial anatomy (aer, anterior epitympanic recess [multiple cells]; cog, cog; mh, malleus head; ib, incus body; adit, aditus ad antrum; ma, mastoid antrum; ssp, sigmoid sinus plate; f.tymp, facial nerve, tympanic segment).
A B
The malleus is supported by superior, anterior, and lateral mallear ligaments. These structures are well seen on a careful study of the CT scan.4The superior and lat- eral ligaments are seen best on coronal sections and the anterior ligament best on axial sections (Fig. 3.4, Fig. 3.7, Fig. 3.13). A posterior incudal ligament exists; however, it is thin and not visualized on CT (Fig. 3.7C). The incus is therefore quite poorly supported, particularly distally (see Chapter 6).11,30–32
Two muscles, the tensor tympani, which is a first (Meckel) branchial arch derivative innervated by the fifth cranial nerve (CN V), and the stapedius, which is a sec- ond (Reichert) branchial arch derivative innervated by the CN VII, also participate in ossicular support. The ten- sor tympani muscle lies within a narrow bony channel (semicanal) parallel and medial to the eustachian tube.6,7,33,34 The tendon of this muscle continues to course posterolaterally until it reaches a spoon-shaped depression adjacent to the cochlea referred to as the cochleariform process. The latter is an important surgical landmark indicating proximity to the facial nerve canal.
From here, the tendon courses laterally to reach the neck of the malleus. The tendon is easily visualized on both coronal and axial CT sections due to its mediolateral orientation
(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