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ANATOMY & DEVELOPMENT OF THE MIDDLE EAR
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ANATOMY & DEVELOPMENT OF THE MIDDLE EAR SAYAN BANERJEE
M/C site of labyrinthine fistula- dome of LSCC MEDIAL WALL • Projection of bone from cochlea • Tensor tympani muscle takes a turn, moves laterally and attaches to the handle of malleus • Tensor tympani reduces loud excessive noises like during chewing • Tensor tympani, tensor veli palatini and mastication muscles- all are supplied by the mandibular branch of the trigeminal nerve • Acts as a landmark for 1st genu of facial nerve Connects to scala tympani
PROCESSUS COCHLEARIFORMIS PYRAMID PROMONTORY ROUND WINDOW
The prominence of the facial nerve canal indicates the position of the upper part of the bony facial canal (Fallopian canal), which contains the facial nerve. • The canal crosses the medial tympanic wall from the cochleariform process anteriorly, runs just above the fenestra vestibuli (oval window), and then curves down into the posterior wall of the cavity. • Its lateral wall may be partly deficient. • Length- 27mm
THE PONTICULUS • It is a bony ridge extending from the pyramidal process to the promontory. • The ponticulus is a central structure in the retrotympanum. • THE SUBICULUM • The subiculum is a smooth bony projection that is situated posterior to the promontory and extends inferiorly from the posterior lip of the round window niche toward the styloid eminence. • Therefore, it intervenes between the sinus tympani superiorly and the RW niche inferiorly.
Below and behind the promontory, is the ROUND WINDOW NICHE, which is directed downward and posteriorly. Because of such direction of the round window niche the round window cannot be easily seen from external or lateral side even through a large perforation of tympanic membrane. • The round window is covered by SECONDARY TYMPANIC MEMBRANE (ROUND WINDOW MEMBRANE), which is usually out of sight, obscured by the overhanging edge of the promontory forming the niche and mucosal folds within it. The membrane is roughly oval in shape, about 2.3 x 1.9 mm in dimension and lies in a plane at right angles to the plane of the stapes footplate. • It tends to curve towards the scala tympani of the basal coil of the cochlea, so that it is concave when viewed from the middle ear. • It appears to be divided into an anterior and posterior portion by a transverse thickening. • The round window niche is of an average of 2 mm from the inferior margin of the oval window and is separated from the promontory by the subiculum. • The mean width of the RW niche is around 2 mm. Its height varies up to 1 mm.
Above and behind the promontory is OVAL WINDOW. • The Oval window is a kidney-shaped opening that connects the tympanic cavity with the vestibule, which is closed by the footplate of the stapes and its surrounding annular ligament. • The size of oval window varies with the size of the footplate, but on average it is 3.25 mm long and 1.75 mm wide. • The oval window niche is limited anteriorly and superiorly by the cochleariform process and posteriorly by the ponticulus and pyramidal eminence. It is situated in a depression called the FOSSULA VESTIBULI where its depth depends on the position of the facial nerve superiorly, and the prominence of the promontory inferiorly. • The bony semicircular cannel lie above and posterosuperior to oval window, on the medal aspect of aditus ad antrum.
BONY LATERAL WALL OF ATTIC/ EPITYMPANIC RECESS The lateral epitympanic wall is wedge-shaped in section and its sharp inferior portion is also called the outer attic wall or SCUTUM (Latin: ‘shield’). It is thin and easily eroded by cholesteatoma, leaving a telltale sign on a high-resolution coronal CT scan. BONY LATERAL WALL OF HYPOTYMPANUM MEMBRANOUS LATERAL WALL aka TYMPANIC MEMBRANE
EMBRYOLOGY OF LATERAL WALL This explains the difficulty of a good exposure of the tympanic membrane in newborns during otoscopic examination or paracentesis.
THE PETROTYMPANIC GLASERIAN FISSURE opens anteriorly just above the attachment of the tympanic membrane. It is a slit about 2 mm long, which receives the anterior malleal ligament and transmits the anterior tympanic artery, a branch of the internal maxillary artery to the tympanic cavity . THE CANAL OF HUGUIER/ ITER CHORDAE ANTERIUSis a separate canaliculus placed in the medial end of the petrotympanic fissure; through this canal of Huguier, the chorda tympani nerve leaves the tympanic cavity toward the infratemporal fossa. ITER CHORDAE POSTERIUS Situated medial to the posterior tympanic spine. It leads into a minute canal through which the chorda tympani nerve exits to enter the tympanic cavity. It lies immediately medial to the tympanic membrane at the level of the upper limit of the malleus handle. The changes in the depth of the tympanic sulcus reflect the stability of the annulus insertion; in the posterosuperior quadrant, the annulus is not totally inserted into the sulcus and is merely supported. This weak insertion of the posterosuperior quadrant of the tympanic membrane in the tympanic ring makes it lax and predisposed to retraction and skin canal migration.
The TEGMEN is a thin bony plate that forms the roof of the middle ear cavity and separates it from the overlying temporal lobe. The part of the Tegmen overlying the tympanic cavity is called TEGMEN TYMPANI The part of the Tegmen overlying the Eustachian tube is called TEGMEN TUBARI The part of the Tegmen overlying the mastoid antrumis called TEGMEN ANTRI The superior surface of the tegmen forms part of the middle cranial fossa floor and is covered by the dura; the inferior surface of the tegmen is lined by middle ear mucosa. The tegmen separates the CSF superiorly from the air space of the middle ear inferiorly.
The floor of the middle ear cavity is narrow and consists of a thin plate of bone that separates the middle ear from the jugular bulb posteriorly and the internal carotid artery anteriorly. In the bone between the carotid artery canal and the jugular bulb fossa, near the medial wall, opens a small canal, THE INFERIOR TYMPANIC CANALICULUS, localized near the fossula petrosa which houses the inferior ganglion of the glossopharyngeal nerve (CN IX). It is often a faintly marked depression on the inferior surface of the petrous portion of the temporal bone. • The surface of this wall may show irregularities due to the overlying pneumatized cells. • In the posterior part of the floor is the root of the styloid process which gives rise to a bony eminence, the STYLOID EMINENCE. The floor of the middle ear cavity is narrow and consists of a thin plate of bone that separates the middle ear from the jugular bulb posteriorly and the internal carotid artery anteriorly. THE INFERIOR TYMPANIC CANALICULUS (*) in the inferior wall gives passage for the Jacobson’s nerve originating from the inferior ganglion of IX which is located in the jugular fossa bulb and the inferior tympanic artery (branch of the ascending pharyngeal artery).
