Imaging Handbook on Anatomy of Cochlea (eBook)

2Cochlear Implant Related Anatomy: Temporal Bone

Introduction

A cochlear implant surgeon should have a thorough understanding of the intricate anatomy of the temporal bone. Possessing knowledge of the three-dimensional orientation of various structures and their inter-relationships helps in identifying intraoperative surgical landmarks while performing cochlear implant surgeries. It also helps to plan a rational surgical approach, especially in cases with middle ear as well as inner ear anomalies or abnormalities.

External Ear

The external ear has two parts, namely, the pinna and external auditory canal.

Tympanic Membrane

•It is 10 mm in diameter and attaches to the tympanic annulus (Fig. 2.1).

•It is faintly discerned on computed tomography (CT) images.

•Lateral one-third is fibrocartilaginous.

•Medial two-thirds is surrounded by the tympanic part of temporal bone.

Middle Ear

•It is an air-filled cavity within the petrous portion of the temporal bone.

•It contains an ossicular channel which is bounded by the following:

–Laterally by tympanic membrane.

–Medially by inner ear structure.

–Superiorly by tegmen.

–Inferiorly by jugular bulb.

•Scutum: A sharp bony projection to which the tympanic membrane is attached superiorly (Fig. 2.2).

•Tegmen: It is a thin plate of bone separating middle cranial fossa from the mastoid cavity (Fig. 2.3).

•Roof of the middle ear cavity is formed by the tegmen tympani.

•Roof of the mastoid cavity is formed by the tegmen mastoideum.

•Posterior wall is formed by the facial recess also known as facial nerve recess. Pyramidal eminence overlies the stapedius muscle.

•Subiculum separates the sinus tympani from the round window niche.

•Lateral to the pyramidal eminence is the facial recess where lies the second genu of the facial nerve.

•The facial recess is used by the surgeon to place cochlear implant electrode via round window (Fig. 2.4).

•Prussak’s space also known as superior recess is bounded by the following:

–Laterally by pars flaccida, scutum.

–Superiorly by lateral malleal ligament.

–Medially by neck of malleus.

•Middle ear is subdivided into:

–Epitympanum.

–Mesotympanum.

–Hypotympanum (opening of eustachian tube, internal carotid artery along its medial margin) (Fig. 2.5).

•Mesotympanum consists of ossicular chain.

–Malleus—head, neck, anterior process, lateral process, and manubrium.

–Incus—body, short process, long process, lenticular process.

–Stapes—head, anterior crus, posterior crus, foot plate.

•The manubrium of the malleus is attached to the tympanic membrane, and the head of the malleus articulates with the body of the incus in the epitympanum forming incudomalleal joint (ice-cream cone appearance which is seen in axial section of high-resolution computed tomography [HRCT] of temporal bone).

•The neck of the malleus anteriorly and the long process of the incus posteriorly give “two dots” appearance.

•The manubrium of the malleus anteriorly and the long process of the incus posteriorly are seen as two parallel lines.

•The lenticular process of the incus extends approximately at right angle from the long process of the incus.

•The head of the stapes and the long process of the incus articulate together to form the incudostapedial joint.

•The foot plate of the stapes attaches to the oval window of the vestibule.

•There are four suspensory ossicular ligaments:

–Superior malleal.

–Lateral malleal.

–Posterior malleal.

–Posterior incudal.

•The suspensory ossicular ligaments are seen on CT image as thin linear structures.

•The lateral malleal ligament is the most commonly identified ligament among all suspensory ligaments.

•The tensor tympani muscle arises from the cartilaginous part of the eustachian tube, and then turns sharply at cochleariformis process and attaches to the neck of the malleus (Fig. 2.6).

•Epitympanum communicates with the mastoid via the aditus and antrum.

•Mastoid is an air-filled cavity:

–It is divided into numerous compartments by mastoid septations (Fig. 2.7).

–Air cell sizes are variable.

•Korner’s septum: It is a bony structure (petrosquamous suture) separating mastoid air cells into two compartments. When thick may be confused with medial wall of the antrum by the surgeon.

•Petromastoid canal is a channel passing between superior and lateral semicircular canal.

–Commuting cranial cavity to mastoid antrum.

–Measures 0.5 to 1 mm.

–Contains subarcuate artery.

–Potential channel for spread of infection to and from the mastoid antrum.

–Not to be mistaken for fracture line.

Inner Ear

•It is situated in petrous part of temporal bone.

•It comprises bony and membranous labyrinth.

Cochlea

•It is a snail-like structure.

•It consists of basal turn, middle turn, and apical turn, which are separated by interscalar septae (Fig. 2.8a–c).

•The osseous spiral lamina is well appreciated on T2 magnetic resonance imaging (MRI). It parallels the interscalar septae.

Cochlear Promontory

•The bulge of cochlear promontory is prominently seen in the middle ear cavity as a dome (Fig. 2.9).

•Nerve of Jacobson courses above the promontory.

•Vestibule is ovoid shape, located superior and posterior to the cochlea.

Semicircular Canals

•They are three in number:

–Superior.

–Posterior.

–Lateral semicircular canal.

•They are placed at right angle to each other.

•Each has ampulla at one end.

•Posterior and superior semicircular canals have a common crus.

•The lateral semicircular canal has two separate openings into the vestibule (Fig. 2.10a, b).

•The endolymphatic duct runs from the vestibule and ends in a blind pouch in the posterior cranial fossa.

•Vestibular aqueduct:

–Envelopes endolymphatic duct.

–1 mm at mid-point and 2 mm at operculum.

•Cochlear aqueduct is a narrow bony channel that surrounds perilymphatic duct and extends from the basal turn of the cochlea anterior (round window) to the subarachnoid space.

–It measures 0.1 to 0.2 mm in the midportion.

–It roughly runs parallel and immediately inferior to the internal auditory canal (approximately 6–7 mm).

•Internal auditory...