Microsurgical Management of Middle Ear and Petrous Bone Cholesteatoma (eBook)

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

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...