1 Anatomy and Radiology of the Normal Temporal Bone
Basic anatomical knowledge of important structures that may be encountered during middle ear surgery is described here. Since three-dimensional anatomy of the middle ear is so complicated, it is impossible to figure out its entity only through these flat pictures. Intensive work in temporal bone dissection laboratory is mandatory. More detailed anatomy of deep structures in the temporal bone is described in Chapter 14.
1.1 The External Auditory Canal
The osseous portion accounts for medial one-third of the external auditory canal. The skin lying on the bony canal is extremely thin, only 0.2 mm in thickness, and requires meticulous care during dissection. Two sutures between elemental structures forming the temporal bone appear in the canal. The tympanosquamous suture is located anterosuperiorly and the tympanomastoid suture posteroinferiorly. Connective tissue enters into these suture lines and sharp dissection may be required during meatal skin elevation. The glenoid fossa, which receives mandibular condyle to form the temporomandibular joint, is located just anterior to the canal, and separated from the canal only by thin bony shell.
1.2 The Tympanic Membrane
The conically shaped tympanic membrane is tilted anteroinferiorly. Because of this dimension, the anteroinferior bony wall is longer than the posterosuperior one, and the anterior tympanomeatal angle is more acute than the posterior. The anterior angle is often hindered by a bony protrusion of the anterior wall. Sufficient visualization of this angle is the key for successful tympanic membrane reconstruction. The tympanic membrane is composed of three layers. Laterally, it is covered with an epidermal layer, and medially with a mucosal layer. Between these two layers, a fibrous layer, the lamina propria exists. The lamina propria may be lost in an atrophic tympanic membrane, and may be thickened by a tympanosclerotic plaque. The tympanic membrane is divided into two parts. The pars tensa, located inferiorly to the lateral process of the malleus and the anterior and the posterior malleal folds, occupies majority of the tympanic membrane. The lamina propria thickens in the periphery of the pars tensa to form tympanic annulus. The tympanic annulus is attached to a groove on the bony canal called the tympanic sulcus. The pars flaccida is located superiorly to the lateral process of the malleus, and delineated superiorly by a bony notch in the superior canal wall called the Rivinus notch. Medial to the pars flaccida and lateral to the neck of the malleus is the Prussak’s space, where epitympanic cholesteatoma starts to invaginate medially from the pars flaccida.
1.3 The Ossicular Chain
1.3.1 The Malleus
The manubrium of the malleus is firmly attached to the tympanic membrane. Its tip corresponds to the umbo of the tympanic membrane which is the bottom of its conical shape. The lateral process is located at the superolateral end of the manubrium. Because of its proximity to the superolateral canal wall, meticulous care should be taken not to touch this process with burrs during canalplasty. The head of the malleus is located in the attic, and its neck connects the head and the manubrium. The tendon of the tensor tympani muscle attaches to the medial surface of the neck. Contraction of the muscle pulls the ossicle medially, and resultant tension to the tympanic membrane limits sound transmission to the inner ear to some extent. The head of the malleus is supported by the superior suspensory ligament and the anterior suspensory ligament.
1.3.2 The Incus
The body of the incus forms an articulation anteriorly with the head of the malleus. The short process of the incus projects posteriorly. The short process is lodged in the fossa incudis located just anterior to the eminence of the lateral semicircular canal (LSC). It is important to know. The long process projects into the tympanic cavity, and forms an articulation with the stapes at its lenticular process. The incus is supported by the malleus anteriorly and the posterior incudal ligament posteriorly.
1.3.3 The Stapes
The smallest bone in the human body is located in the oval window. The stapes sits in a deep depression called oval window niche as described in subsequent text. The head of the stapes forms an articulation with the incus. The stapedius muscle inserts onto the head and the posterior crus. The footplate is accommodated in the oval window that is an opening of the vestibule and the scala vestibuli of the cochlea. The connective tissue lying between the footplate and the edge of the oval window is called the annular ligament. A contraction of the stapedius muscle tilts the stapes and its footplate, and resulting tension in the annular ligament limits sound transmission into the inner ear to some extent.
1.4 The Tympanic Cavity
Mesotympanum is a portion located just medial to the tympanic membrane. It is bordered superiorly from the epitympanum (attic) by the tympanic segment of the facial nerve. A recess inferior to the tympanic membrane is the hypotympanum. The protympanum, located anteriorly to the tympanic membrane, has the tympanic orifice of the eustachian tube, just inferior to the semicanal of the tensor tympani muscle. A branch of the facial nerve, the chorda tympani, courses lateral to the long process of the incus and medially to the manubrium of the malleus after emerging from the posterior wall. The nerve contains sensory fibers for taste and secretory fibers innervating the submandibular and sublingual glands.
1.4.1 Medial Wall
The Facial Nerve
The tympanic segment of the facial nerve courses obliquely in the medial wall of the tympanic cavity from the area just superior to the eustachian tube, toward the area just superior to the oval window. Since the nerve is dehiscent in this segment in about one-third of the cases, meticulous care should be taken during operation. Above the oval window, the nerve forms a prominence that may be large enough to cover the area of the footplate. Around the posterior edge of the oval window, the nerve follows gentle curve directing inferolaterally, changing its location from the medial wall to the posterior wall. At that turning portion, the nerve is positioned inferomedially to the LSC, and courses almost parallel to it. The short process of the incus lies just laterally to the nerve (see also Chapter 1.8).
The Cochleariform Process
The cochleariform process lodges posterior end of the semicanal of tensor tympani. It is located just medial to the neck of the malleus, anterosuperior to the oval window, and just inferolateral to the tympanic segment of the facial nerve. At this bony process, tendon of the tensor tympani muscle makes a right angle and directs laterally to attach to the neck of the malleus.
The promontory is a prominent eminence located anteroinferiorly to the oval window, and anteriorly to the round window. It corresponds to the basal turn of the cochlea. The axis of the cochlea directs anteriorly and laterally.
The Oval Window
The stapes footplate is lodged in this window to transmit the mechanical energy to the scala vestibuli of the cochlea. The window is located at the bottom of a deep depression surrounded by eminences, the promontory inferiorly, the fallopian canal superiorly, the cochleariform process anteriorly, and the pyramidal eminence posteriorly. The window edge and the stapes footplate are connected by the connective tissue called the annular ligament. The tympanic segment of the facial nerve runs just superior to the window, and near the posterior edge of it, the nerve turns inferiorly toward the stylomastoid foramen.
The Round Window
The round window is located in the round window niche, inferiorly to the oval window. The round window is the other opening of the labyrinth to the middle ear. With this window, the cochlear fluid contained in bony structure is vulnerable to mechanical vibration. The round window membrane lodges superior aspect of the round window niche, and lies mostly in the horizontal plane. Therefore, it is difficult to see directly the membrane without removing superior overhang of the niche.
1.4.2 Posterior Wall
The posterior tympanum contains deep recesses. The facial nerve running in the middle divides them into the tympanic sinus medially and the facial recess laterally. These recesses are subdivided by two bony eminences that join the pyramidal eminence on the facial nerve. The eminence contains the stapedius muscle that attaches to the head of the stapes.
The Tympanic Sinus
The sinus is located medially to the facial nerve. The posterior extension of the sinus tympani is variable, and it may extend far medially to the facial nerve. Since direct visualization of its bottom is impossible in the majority of cases, eradication of the disease from this sinus requires considerable experience. The tympanic sinus is subdivided into two segments located superiorly and inferiorly, by a bony bridge named the ponticulus that connects pyramidal eminence and the promontory. The tympanic sinus is bordered inferiorly by another bony bridge lying between the posterior wall and the round window niche, called the subiculum.
The Facial Recess
The facial recess is bordered by the bony annulus laterally and the facial canal medially. This is the portion to be drilled for posterior tympanotomy in canal wall up tympanoplasty. The facial recess is also subdivided into two segments by a bony bridge called the chordal crest that connects the pyramidal eminence and an emergence of the chorda tympani called the chordal eminence. The inferior extension of the facial recess is variable, depending on the point of exit of the chorda tympani from the facial nerve.
1.4.3 The Attic
A bony spur called the cog extends vertically from the tegmen to a point just anterior to the head of the malleus. With this structure, the attic is divided into a posterior division and an anterior division known as the supratubal recess. Cholesteatoma often advances into the recess, and the recess often becomes a site of residual if it is not opened fully during surgery. Since the cog is located superiorly to the facial nerve with its tip pointing it, the structure serves as one of the landmarks of the nerve. The floor of the anterior attic recess contains the pregeniculate portion of the facial nerve. Posterior to the attic, an opening of the antrum called aditus ad antrum is located.
1.4.4 The Antrum
The antrum connects the mastoid air cells with the tympanic cavity. It is located just posteriorly to the epitympanum, inferiorly to the middle fossa plate, and laterally to the labyrinth. Since the antrum is very consistent and there is no important structure lateral to it, the antrum serves as one of the most important landmarks in the initial stage of mastoidectomy. The prominence of the LSC is one of the most important landmark for the facial nerve.
1.5 The Labyrinth
The prominence of the LSC in the medial wall of the antrum is sloping about 30 degrees, running from anterosuperior to posteroinferior. The bony capsule of the labyrinth is compact and hard, and more resistant to bone erosion. However, due to its proximity to the antrum, the LSC is the most vulnerable labyrinth to pathologies that erodes the medial wall of the antrum such as cholesteatoma. At the anterior end of the LSC is the ampulla that accommodates sensory cells and opens to the utricles. The ampulla is located in the medial wall of the posterior attic.
Other two semicircular canals are sited nearly perpendicularly to the LSC. The posterior semicircular canal (PSC) lies just posterior to the LSC, and the posterior edge of the LSC points almost the center of the PSC. The PSC courses nearly in parallel with the posterior fossa dura. Its ampulla is located at its inferior end, just medial to the facial nerve. The superior end of the PSC joins the superior semicircular canal (SSC), forming the common crus.
The SSC is located just beneath the middle cranial fossa plate. Its ampulla is at the anterior end, just superomedial to the ampulla of the LSC. It courses nearly perpendicular to the long axis of the pyramis, which places the canal deeper and farther from the antrum posteriorly. Therefore, the canal is not seen in the majority of cases, and the entire canal may be exposed in limited cases in which intensive removal of perilabyrinthine cells are required such as petrous bone cholesteatoma. In rare cases, the canal is dehiscent and directly contacts with the middle fossa dura.
The ampullae of the superior and the LSCs are located in the medial wall of the posterior attic. The anterior walls of the two ampullae become parallel to the light axis of the microscope medially when seen from the posterior cavity. Therefore, if there is any necessity to drill deeply the medial wall of the attic, great care should be taken not to open these two ampullae. The labyrinth is less resistant to an insult of the ampulla than that of semicircular canals.
1.6 The Jugular Bulb
The structure is located in the foramen jugulare, connecting the lateral sinus and the internal jugular vein. The jugular bulb is located medial to the mastoid segment of the facial nerve, and inferior to the semicircular canals. Distance from the facial nerve and the labyrinth varies, and the bulb is variably positioned in the hypotympanum. In some cases, the bulb in the hypotympanum is dehiscent. It should be remembered that the lower cranial nerves from 9th to 11th pass the skull base with this venous system.
1.7 The Carotid Artery
The carotid artery enters the temporal bone through the carotid foramen. It ascends vertically and emerges in the medial wall of the hypotympanum at the area just beneath the cochlea. Then, it turns anteromedially with nearly a right angle toward the petrous apex, forming horizontal segment just posteroinferior to the eustachian tube, and anteroinferior to the cochlea. In 2% of cases, a bony shell separating the carotid artery and the eustachian tube is absent. Distance between the cochlea and the carotid artery ranges from 1 to 5 mm.
1.8 The Facial Nerve
Total length of the facial nerve in the temporal bone is around 30 mm. Within the temporal bone, the nerve is divided into three segments. The labyrinthine segment runs laterally from the fundus of the internal auditory canal until it reaches to the geniculate ganglion that lodges cell bodies of sensory somatic nerves serving for taste. The ganglion is located immediately superior to the cochlea, beneath the middle fossa plate. As described, the ganglion may have direct contact with the middle fossa dura in the anterosuperior aspect without any bony shell in between. At the geniculate ganglion, the nerve turns abruptly posteriorly. The tympanic segment starts from the geniculate ganglion, coursing posteroinferiorly toward the area just superior to the oval window. The segment may be dehiscent, and the facial nerve may be exposed to the tympanic cavity. Around the posterior edge of the oval window and medial to the short process of the incus, the nerve follows gentle curve directing inferiorly. The curve is called the second genu, located inferomedially to the LSC. The mastoid segment of the nerve runs nearly vertically toward the stylomastoid foramen that is located anteriorly to the digastric ridge. Therefore, the mastoid segment of the nerve courses near the line between the short process of the incus and the digastric ridge, and in the posterior wall of the tympanic cavity. On the anterior surface of the mastoid segment, the stapedius muscle receives its innervation from the facial nerve.
The chorda tympani branches from the nerve at last, usually before the nerve exits from the stylomastoid foramen. It ascends in the posterior wall of the tympanic cavity at the canaliculus chordae tympani, and emerges into the tympanic cavity near the chordal crest. It courses laterally to the long process of the incus and medially to the neck of the malleus, and exits from the cavity through the petrotympanic fissure.
One of the most serious complications in the middle ear surgery is injury to the facial nerve. The surgeon must have complete knowledge about anatomical relationships of the nerve and surrounding structures. Positive identification of the nerve in the surgical field considerably reduces risks to injure it. Therefore, surgeon should always be prepared for identifying the nerve at any segments using available landmarks. Anomalies of the nerve should always be kept in mind. The nerve may be located inferiorly to the oval window, or may be divided into branches.
1.8.1 Landmarks and Signs for Positive Identification
The digastric ridge points directly to the nerve at the stylomastoid foramen. The facial nerve courses perpendicular to the ridge.
The lateral semicircular canal. The facial nerve runs anteriorly to the canal and almost always medially to the level of it. The short process of the incus lies laterally to the nerve.
The oval window niche, with the nerve lying superior to it. Care should be taken not to mistake the canal of the tensor tympani for the tympanic segment of the nerve. The canal of the tensor tympani muscle is horizontal, ending at the cochleariform process, while the nerve courses obliquely in the tympanic cavity.
The cochleariform process. The nerve runs directly superior to it. Since it is resistant to cholesteatoma and often identifiable in revision surgery, the cochleariform process is a very important and consistent landmark.
The cog points to the facial nerve in the floor of the anterior epitympanic recess.
Bleeding usually occurs when the nerve sheath is exposed. Any tubular bleeding structure should be suspected to be the facial nerve until proved otherwise. A useful method is to gently probe the suspicious structure with a blunt instrument. The facial nerve will return to its place when the instrument is removed.
Patients usually complain of some discomfort when the chorda tympani nerve is insulted under local anesthesia. This is informative when the facial ridge is lowered.
The facial nerve courses inferolaterally in the third portion, which makes the nerve run more laterally, closer to the tympanic annulus inferiorly. The distance between the nerve and the annulus is only 2 to 3 mm, and the nerve may run laterally to the tympanic annulus inferiorly. The relationship is of tremendous importance especially in meatoplasty.
Refer to ▶Fig. 1.1, ▶Fig. 1.2, ▶Fig. 1.3, ▶Fig. 1.4, ▶Fig. 1.5, ▶Fig. 1.6, ▶Fig. 1.7, ▶Fig. 1.8, ▶Fig. 1.9, ▶Fig. 1.10, ▶Fig. 1.11, ▶Fig. 1.12, ▶Fig. 1.13, ▶Fig. 1.14, ▶Fig. 1.15, ▶Fig. 1.16, ▶Fig. 1.17, ▶Fig. 1.18, ▶Fig. 1.19, ▶Fig. 1.20, ▶Fig. 1.21.