2 Anatomy of the Temporal Bone

The temporal bone actually consists of four fused parts—the squamous, tympanic, mastoid, and petrous bones.

Squamous Bone

The squamous part of the bone represents the major part of the lateral surface of the bone. Above the level of the zygomatic process, the vertical portion of the squamous bone extends upward to cover part of the temporal lobe of the brain. The zygomatic process is actually part of the squamous portion of the bone. It originates anterior to the external auditory canal at the level of the junction of the vertical and horizontal parts of the squamous bone. The root of the zygomatic process shows an initial swelling known as the posterior zygomatic tubercle. Traced anteriorly, the root thins out to form the glenoid fossa for the articulation of the head of the mandible, and then thickens again to form the anterior zygomatic tubercle. The zygomatic process then thins out and flattens as it separates from the squamous bone and ends by articulation with the zygomatic bone. Posterior to the external auditory canal, the zygomatic process can be traced as a somewhat faint line, the supramastoid crest, indicating the level of the middle cranial fossa. The squamous bone then extends inferiorly in its retromeatal portion, forming the flattened lateral part of the mastoid process. The squamous part of the temporal bone also forms the superior parts of both the anterior and posterior walls of the bony external auditory canal. On the posterosuperior border of the canal, the spine of Henle can be seen. The squamous part of the temporal bone also forms the superior parts of both the anterior and posterior walls of the bony external auditory canal. On the posterosuperior border of the canal, the spine of Henle can be seen.

Tympanic Bone

The gutter-shaped tympanic bone forms the inferior wall and major parts of the anterior and posterior walls of the bony external auditory canal. Two sutures between the elementary structures that form 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 temporomandibular joint is located just anterior to the canal and is separated from the canal only by a thin bony shell. The lateral border of the tympanic bone is roughened for the attachment of the cartilaginous part of the external auditory canal, which forms the outer two-thirds of the canal. The inferior edge of the tympanic bone expands to form the vaginal process where the styloid process lies.

Mastoid Process

The mastoid process can be seen at the posterior and inferior border of the temporal bone, protruding anteroinferiorly to variable levels, depending on the pneumatization of the mastoid. The process serves as the anterior part of the attachment of the sternocleidomastoid muscle. On its medial surface lies the digastric groove, from which the posterior belly of the digastric muscle originates. On the posteromedial end of the groove, an impression of the occipital artery can be seen. The stylomastoid foramen, from which the main trunk of the facial nerve exits the temporal bone, can be seen at the anterior border of the digastric ridge posterior to the styloid process.

The temporal component of the jugular foramen can be seen anteromedial to the stylomastoid foramen and medial to both the tympanic bone and the styloid process. From the lateral border of the foramen, the jugular spine of the temporal bone can be seen extending into the foramen toward its occipital counterpart and separating the foramen into the what are known as the vascular and nervous compartments. Through the fossa and at a more superior level, the dome of the jugular bulb can be seen. Posteriorly lies the small canal for the passage of Arnold's nerve (the auricular branch of the vagus nerve), while anteriorly the end of the groove of the in ferior petrosal sinus can be seen lateral and anterior to the opening of the cochlear aqueduct. The foramen of the internal carotid artery is separated from the anterior border of the jugular foramen by a thin wedge of bone called the jugulocarotid spine, through which a canal for the passage of Jacobson's nerve (the tympanic nerve) to the tympanic cavity lies.

Petrous Bone

The most prominent feature of the medial aspect of the temporal bone is the petrous part. Shaped like a pyramid, this part protrudes in an anteromedial direction, with the base located laterally and formed by the semicircular canals, vestibule, cochlea, and carotid artery. The apex of this bone forms part of the anterior foramen lacerum. Through the apex, the internal carotid artery exits the petrous bone to the anterior foramen lacerum, where it curves superiorly on its way to the cavernous sinus. The end of the bony part of the eustachian tube, the isthmus, is also located in the apex anterior to the carotid opening and just medial to the spine of the sphenoid. The superior surface of the petrous bone forms part of the middle cranial fossa. It begins from the arcuate eminence and ends at the foramen lacerum. The groove of the greater petrosal nerve can be seen coursing close to the bone near the anterior border of this surface; in 10% of cases, the nerve can be traced posteriorly into a dehiscent geniculate ganglion. The bisection of the angle formed by this groove and the arcuate eminence marks the position of the internal auditory canal. Near the foramen lacerum, the impression of Meckel's cavity can be seen. The posterior border of this surface is marked by the groove for the superior petrosal sinus, which separates the superior and posterior surfaces.

The posterior surface of the petrous bone forms part of the posterior cranial fossa. The opening for endolymphatic duct and sac can be seen at the lateral end of this surface. This opening represents an important landmark for the posterior semicircular canal in procedures using the retrosigmoid approach. The most important feature of the posterior surface is the internal auditory meatus.

The Middle Ear

The Tympanic Membrane

The conically shaped tympanic membrane is tilted anteroinferiorly. As a result of this, 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 obstructed by a bony protrusion of the anterior wall. Adequate visualization of this angle is the key to 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, there is a fibrous layer, the laminapropria. The tympanic membrane is divided into two parts. The pars tensa, located inferior to the lateral process of the malleus and the anterior and posterior malleal folds, represents the majority of the tympanic membrane. The lamina propria thickens in the periphery of the pars tensa to form the tympanic annulus. The tympanic annulus is attached to a groove on the bony canal, called the tympanic sulcus. The pars flaccida is located superior to the lateral process of the malleus and is 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 Prussak's space, in which epitympanic cholesteatomas start to invaginate medially from the pars flaccida.

The Ossicular Chain

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. Due to 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 the resulting tension on the tympanic membrane limits sound transmission to the inner ear to some extent. The head of the malleus is supported by the superior and anterior suspensory ligaments.

The incus. The anterior surface of the body of the incus forms an articulation with the head of the malleus. The short process of the incus projects posteriorly. The short process is lodged in the fossa incudis. 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.

The stapes. The smallest bone in the human body, the stapes, is located in the oval window. 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, which opens into the vestibule. The connective tissue lying between the footplate and the edge of the oval window is called the annular ligament. Contraction of the stapedius muscle tilts the stapes and its footplate, and the resulting tension on the annular ligament limits sound transmission into the inner ear to some extent.

The Tympanic Cavity

The mesotympanum is a portion located just medial to the tympanic membrane. Superior to the epitympanum (attic), it is bordered by the tympanic segment of the facial nerve. A recess inferior to the tympanic membrane is the hypotympanum. The protympanum, located...