2     Surgical Anatomy in Auditory Implantation

2.1 Anatomy of the Middle Ear and Mastoid

2.1.1 The Mastoid (Fig. 2.1)

The usually well-aerated cells of mastoid and antrum are a perfect route for access to the middle ear and epitympanum. The landmarks for opening up the mastoid cavity are, posteriorly, the sigmoid sinus and, posterior fossa dura; superiorly, the middle cranial fossa dura; and, anteriorly, the posterior wall of the external ear canal. For easiest access to the facial recess, the posterior wall of the external ear canal needs to be thinned well. The digastric ridge is the most caudal landmark in the mastoid cavity, defining the facial nerve leaving the mastoid via the stylomastoid foramen, situated just anterior-medially to this ridge.

2.1.2 The Antrum (Fig. 2.1)

The antrum is the largest mastoidal cell and connects the mastoid air cells with the tympanic cavity via the aditus ad antrum (“the entrance of the antrum”). It is located just posteriorly to the epitympanum, inferiorly to the middle fossa plate, and posterior-lateral to the labyrinth. Since the antrum is very consistent and there is no important structure lateral to it, it serves as one of the most important landmarks in the initial stage of mastoidectomy. On the medial wall of the antrum is found the prominence of the lateral semicircular canal: one of the most important landmarks for identification of the facial nerve and the incus. The prominence of the lateral canal, together with the digastric ridge, defines the vertical segment of the facial nerve. Both structures can be used to identify the facial nerve before starting to drill the posterior tympanotomy.

2.1.3 The Tympanic Cavity

The mesotympanum is the middle portion of the tympanic cavity located just medial to the tympanic membrane (Fig. 2.2). It is bordered superiorly by the epitympanum (attic), which hosts the corpora of malleus and incus, and inferiorly by the hypotympanum, which starts inferior-laterally from the inferior border of the round window. The mesotympanum and epitympanum are separated by the tympanic segment of the facial nerve. The protympanum, located in the anterior part of the tympanic cavity, harbors the tympanic orifice of the eustachian tube, just inferior to the semicanal of the tensor tympani muscle and lateral to the genu of the carotid artery. The anterior epitympanum can be found anterior from James’s cog, a bony eminence at the roof of the epitympanum. The cog is sometimes also regarded as a landmark for the facial nerve; it points at the facial nerve.

2.1.4 The Ossicles

The malleus is the most lateral of the ossicles and is only a minor landmark in auditory implantation (Fig. 2.3). The tendon of the tensor tympani muscle is attached to the neck of the malleus and originates from the cochleariform process, which is an important landmark in the middle ear for identification of the horizontal part of the facial nerve. The tensor tympani muscle is located in the semicanal anterior to the cochleariform process, on the medial wall of the anterior epitympanum.

The Incus

The short process of the incus projects posteriorly and is lodged in the fossa incudis located just anterior-laterally to the eminence of the lateral semicircular canal. This is an important landmark for drilling the posterior tympanotomy at the facial recess. The long process projects into the tympanic cavity, and forms an articulation with the stapes at its lenticular process (Fig. 2.3). The incus is also supported by the malleus anteriorly at the malleoincudal articulation and by the posterior incudal ligament posteriorly.

The Stapes and Stapedius Muscle

The smallest bone in the human body is located in the oval window. It has two crura, the anterior crus and posterior crus, on top of the stapes footplate. The footplate is accommodated in the oval window. The connective tissue lying between the footplate and the edge of the oval window is called the annular ligament. The stapedius muscle inserts onto the head and the posterior crus of the stapes and originates from the eminentia pyramidalis on the posterior mesotympanic wall, where this muscle is hidden in the bone, anterior-medial to the facial nerve. Its length can vary. To improve the access to the round window during the drilling of the posterior tympanotomy, this muscle must sometimes be skeletonized or even be uncovered.

A contraction of the stapedius muscle tilts the stapes and its footplate, and the resulting tension in the annular ligament limits sound transmission into the inner ear, to some extent protecting the inner ear from noise trauma. This contraction is seen when the cochlea and cochlear nerve endings are stimulated at a signal that mimics 80–100 dB. This is the physiologic function of the stapedius reflex; moreover, the stapedius reflex can also be used to test the correct position of the electrode in the cochlea during CI surgery: the electrical stapedius reflex (see Chapter 4).

2.1.5 The Oval Window

The oval window is located at the bottom of a deep depression surrounded by eminences: the promontory inferiorly, the fallopian canal superiorly with the tympanic segment of the facial nerve, the cochleariform process anterior-superiorly, and the pyramidal eminence (eminentia pyramidalis) with stapedius tendon posteriorly (Fig. 2.4). The tympanic segment of the facial nerve runs just superior to the oval window and near the posterior edge of it, the nerve turns inferiorly toward its vertical segment. The oval window and the stapes (if still present) are landmarks for the round window. The stapes and oval window can be seen through the facial recess, whereas the round window is not always as easily visible. The round window is more medial and caudal to the oval window and mostly perpendicular in orientation. Especially in ossification of the round window, the oval window, stapes, and promontory become the most important landmarks to perform a drill-out procedure of the basal turn of the cochlea (see Chapter 11).

2.1.6 The Facial Nerve

The tympanic segment of the facial nerve courses obliquely in the medial wall of the tympanic cavity separating the epitympanum from the mesotympanum (Fig. 2.5). The tympanic segment of the facial nerve starts at the first genu at the geniculate ganglion just superior to the cochleariform process and the tensor tympani tendon and then runs superior-laterally to the oval window. Around the posterior edge of the oval window, the nerve follows a gentle curve, the second genu, coursing inferior-laterally, changing its location from the medial wall of the tympanic cavity to the posterior wall of the mesotympanum. At this curved section, the nerve is positioned inferior-medially to the lateral semicircular canal, and courses almost parallel to it. The short process of the incus lies laterally to the nerve and to the lateral semicircular canal. This position should be kept in mind at the start of the posterior tympanotomy. Note that the vertical segment of the facial nerve, immediately after the second genu of the facial nerve, can vary in anatomy, not only in malformations or syndromic patients. A more aberrant position of the nerve located inferiorly to the oval window or divided into branches can also be found. Coronal CT images can provide valuable preoperative information on the individual anatomy and course of the facial nerve (see Chapter 3).

2.1.7 The Chorda Tympani (Fig. 2.6)

The chorda tympani, a branch of the facial nerve, courses lateral to the long process of the incus and medially to the manubrium of the malleus after emerging from the posterior wall of the middle ear at the posterior spine. This nerve contains sensory fibers for taste and secretory fibers innervating the submandibular and sublingual glands. The chorda tympani can branch off the mastoid segment of the facial nerve at a variable level, either just above the stylomastoid foramen or at a more superior or even more inferior level.

The size of the posterior tympanotomy (a surgical entity) is defined by the angle between the chorda tympani and facial nerve as explained later in the text.

2.1.8 The Cochleariform Process and Tensor Tympani Muscle (Fig. 2.7 and Fig. 2.8)

The tensor tympani muscle lies in its semicanal, which runs superior and parallel to the eustachian tube toward the cochleariform process. At this bony process, the tendon of the tensor tympani muscle makes a right angle and directs laterally to attach to the neck of the malleus. The cochleariform process is the posterior end of the semicanal of the tensor tympani muscle. It is located just anterior-superior to the oval window, and just inferior-lateral to the tympanic segment and first genu of the facial nerve. It remains an important landmark in the temporal bone, especially when the ossicles are not present, because it defines the position of the tympanic portion of the facial nerve. When performing a surgical drill-out in an ossified cochlea, the cochleariform process is the main landmark to find the apical turn of the cochlea, which is located directly medially. Also James’s cog can be used as a landmark when the cochleariform process is eroded: this is a bony process attached to the roof of the epitympanum and is positioned in the same plane as the cochleariform process.1–3

2.1.9 The Round Window (Fig. 2.9 and Fig. 2.10)

The round window is located in the round window niche, inferiorly to the oval window. The round window is covered superiorly by a bony overhang, especially superior-posteriorly, creating the round window niche. It lies in a mostly horizontal plane, slightly tilted anteriorly. A bony projection (“lip”) can also be found at the anterior-inferior margin of the round window niche. Both overhangs make it usually difficult to see the round window membrane directly and more extensive exposure to appreciate the complete round window is regularly needed.⁴ A pseudomembrane consisting of a mucosal layer closing off the round window niche is frequently present, recognizable by a whitish color (Fig. 2.9) instead of the normal dark gray color of the true membrane (Fig. 2.10). The round window is situated in the inferior-posterior wall of the basal turn, parallel to the direction of the basal turn, more or less like a “side entrance” (Fig. 2.16).

To appreciate the angle of the round window, one has to imagine that a 0° view would be a perfect perpendicular view at the round window, equivalent to the inferior and superior border of the window both at equal distances from the vertical segment of the facial nerve.⁵ The round window angle (in adults) is tilted, with the inferior border ~48° (±9°) turned more anterior.⁵ The size of the round window is ~1.35 mm², but varies from 0.8 to 1.75 mm².^4,6

2.1.10 The Facial Recess and the Posterior Tympanotomy (Fig. 2.11 and Fig. 2.12)

The posterior mesotympanum contains deep recesses. The facial nerve running in the middle divides them into the tympanic sinus medially and the facial recess laterally. The facial recess is bordered by the bony tympanic annulus laterally and the vertical segment of the facial canal medially. It is subdivided into two segments by a bony bridge called the chordal crest that connects the pyramidal eminence and the emergence of the chorda tympani called the chordal eminence (Fig. 2.11). During surgery one could encounter this crest as a last lateral-medially oriented layer within the posterior tympanotomy before entering the tympanic cavity. The inferior extension of the facial recess is to the hypotympanum and is not related to the position where the chorda tympani branches off the facial nerve.

By performing a posterior tympanotomy, the facial recess is opened, giving access from mastoid cavity to the tympanic cavity. Care must be taken not to damage the tympanic annulus in this procedure, which lies just few millimeters lateral from the chorda tympani (Fig. 2.6 and Fig. 2.12). An inferior tympanotomy reaches the hypotympanum, opening up the facial recess usually inferior from the chorda tympani.

Proceeding, the chorda tympani is found running from inferior-posteriorly toward superior-anteriorly, creating an angulation, a V-shape, with the facial nerve. After removal of the chordal crest generally the tympanic cavity can be seen and the stapes and the promontory can be visualized. The angle between the facial nerve and the round window is very variable.10 To create a clear view of the round window and to obtain the best angle of electrode insertion during cochlear implantation, the facial nerve usually has to be skeletonized.¹¹

2.1.11 The Labyrinth

Semicircular Canals (Fig. 2.13)

The prominence of the lateral semicircular canal in the medial wall of the antrum slopes ~30° from a true horizontal line/surgical view, running from anterior-superiorly to posterior-inferiorly. At the anterior end of the lateral semicircular canal it widens to form the lateral ampulla that accommodates sensory cells and opens to the utricle. It is located in the medial wall of the posterior attic.

The Vestibule (Fig. 2.14)

The vestibule is the hollow space within the petrous bone medial to the second genu of the facial nerve, containing the utricle and saccule. It lies anterior-medially to the semicircular canals, medial to the oval window, lateral to the fundus of the internal auditory canal, and posterior to the cochlea. Its posterior-lateral surface receives five openings from the semicircular canals. There is no connection between the vestibule and the scala tympani, while the vestibule is anteriorly in continuation with the scala vestibuli.

The Cochlea and Promontory (Fig. 2.15)

The promontory is the prominent eminence located anterior-inferiorly to the oval window, and anteriorly to the round window. It corresponds to the basal turn of the cochlea. The Jacobson nerve, a branch of the glossopharyngeal nerve (CN IX), can be found crossing the promontory.

The cochlea is located on the medial surface of the middle ear and can be found by the promontory, the oval window niche, and the round window niche as the most important landmarks. The cochlea is situated tilted forward and superiorly with its apex (Fig. 2.16 and Fig. 2.17). The 2.5 turns of the cochlea curve around a central cone of bone, the modiolus, arising from the fundus of the internal auditory canal and containing the fibers of the cochlear nerve and the spiral ganglion cells. From the modiolus, a shelflike bony projection, the osseous spiral lamina extends halfway into the cavity of the cochlear turns. The basilar membrane and subsequently the membranous spiral ligament connect the edge of the osseous spiral lamina to the lateral surface of the cochlear turns, dividing into the scala vestibuli and the scala tympani. The membranous structure, in the middle between the two scalae, is called the scala media and is located laterally to the osseous spiral lamina. This is the scala where the organ of Corti is located. In addition, the round window corresponds to the scala tympani, and the oval window corresponds to the scala vestibuli (Fig. 2.18).

The cochlea is anteriorly in close proximity to the carotid artery, so performing a cochlear drill-out procedure of the basal turn could lead to damage of the carotid artery. The middle turn of the cochlea can be found 3 to 4 mm anteriorly from the stapes footplate, while the apical turn is situated medial to the cochleariform process. Removal of the tensor tympani muscle gives better access to the apical turn (Fig. 2.8). The intracochlear anatomy is visible in Fig. 2.16 and subsequent figures.

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