Surgical anatomy of the ear

Diksha CheetooRoll number 10

Table of content

• Anatomy of the External auditory meatus vs the eustachian tube

• The anatomy of the middle Ear and the tympanic membrane

• The anatomy of the inner ear especially the scala media

• The middle Ear Transformer mechanism• The important triangles and their significance

Anatomy of the middle Ear

1. The middle ear cleftMiddle ear + ET+ aditus+ antrum+ mastoid air

cells2. Middle Ear The hypotympanum

Lies below pars tensa

Epitympanum

Lies above pars tensa

Mesotympanum

Lies opposite the pars tensa

The middle ear can be linked to a six- sided box.

The roof

• The tegmen tympani

The floor

• A thin plate of bone which separates tympanic cavity from the jugular bulb

The medial wall• Promontory- bulge due

to coil of cochlea• Round window/fenestra

cochlea- lies post-inf to scala tympani

• Oval window-opens in scala vestibuli, foot plate of stapes is fixed to it

• Prominence of facial nerve canal-lies above oval window

• The processus cochleariformis- ant to oval window, hook like projection- for the tendon of tensor tympani

The lateral wall

• The tympanic membrane

• It is semitransparent• Pearly grey• Thickness is 0.1-

0.15mm• Placed at 55 degrees

from the floor• It separates the EAM

from ME

• Peripheral part of the TM is thickened to form a fibrocartilaginous ring attached to the tympanic sulcus

• Membrane-• 1. tense part is called pars

tensa • 2. flaccid part is called pars

flacida/shapnell’s membrane

• Outer TM- 3 layers• 1. outer cuticular• 2. middle fibrous of inner

circular and outer radiating fibres

• 3. inner mucosal• Blood supply-1. deep

auricular branch of maxillary artey 2.post auricular artery 3. tympanic branch of maxillary artery

• Veins- ext jug vein and transverse sinus

The posterior wall• Lies close to mastoid air

cells• Presents a bony

projection called as the pyramid/pyramidal recess- contains the stapedius muscle

• Aditus- opening through the attic, communicates with the antrum- lies above the pyramid

• Facial nerve runs behind the pyramid

• Facial recess/ suprapyramidal recess

• Collection of air cells lying lateral to facial nerve

• Medially- ext genu of facial nerve

• Lat-chorda tympani• Sup- fossa incudis• Anterolaterally- tympanic

membrane.

• The importance of this recess is that one can approach the ME without disturbing the post meatal wall

MCQ

• True about the facial recess is :

A. Bony landmark on the lateral wall of middle ear

B. Separates facial from vestibular nerve

C. Lateral boundary by fossa incudis

D. One can enter middle ear without removing posterior bony meatal wall

• May 2013

• Facial recess is bounded laterally by :

A. Facial nerveB. Chorda tympani nerveC. Short process of incus D. ponticulus

MCQ

• Facial recess is bounded by all except :

A. Facial nerveB. Chorda tympani

nerveC. Short process of

incusD. ponticulus

Anatomy of the EAM vs ET

EAM• LENGTH

24mmOuter 1/3rd is

cartilaginous and inner 1/3rd is bony

ETLength 36mmAlso called pharyngotympanic

tubeOsseus part is 12mm – from

ant tympanic wall, narrows to the end to attach to the cartilaginous part -24 mm

EAM• COURSE• Forms a mild s –shape• Curve is directed medially

upwards and forwards and then medially backwards downwards

ET• Descends at an angle of 45

degrees with sagital plane and 30 degrees with horizontal plane

• In infants the auditory tube is wide and short and is placed horizontally

EAM• RELATIONS1. Floor and ant part of the

meatus are longer than the roof and post part

2. ant, inf and post bony part is formed by the tympanic part of the temporal bone and roof is formed by the squamous part of the petrosal bone

3. The meatal recess is present in relation to the inf wall of the meatus

4. Ceruminous glands and hair are present mostly in sbc tissue of cartilaginous part.

ET

1. The muscles of ET are • Tensor palati• Levator palati• Tensor palati separates the ET

from the ottic ganglion, mandibular nerve , chorda tympani nerve and middle meningeal artery

EAM

• SUPPLY• Nerve supply• The ant and sup wall are

supplied through the auriculotemporal branch of mandibular nerve and inf wall is supplied by the auricular branch of vagus

ET

• Tympanic plexus and pharungeal branches of pterygopalatine ganglion.

Anatomy of the inner Ear

internal ear

Bony labyrinth Membranous labyrinth

bony labyrinth

Vestibule Semicircular canals

cochlea

Conical snail shell

At its base – 3 openings-oval window, round

window and cochlear

canaliculus

3 scc

Ant postlater

al

Forms central part of bony

labyrinth

Membranous labyrinth

utricle saccule Semicircular ducts

Scd open in it through 5

ampullary endsThe sensory part is

called maculaConcerned with

linear acc and dec

Connected to utricle through the utricosacular duct

3 in numberCorrespond to 3

bony canals. They open in utricle. The ampullated end of each duct

contain neuro epithelium called crista ampullaris

Cochear duct/ Scala Media

• Lies within the bony cochlea

• It is a blind coiled tube• Occupies the middle

part of the cochlear canal between the scala vestibuli and scala tympani

• It is

Floor- basilar membraneRoof – vestibular

membraneOuter wall- stria

vestibularis- contains vascular epithelium and secretes endolypm

The basilar membrane supports the organ of corti

The OOC is innervated by peripheral process of bipolar cells located in the spiral ganglion

• The ganglion is present within the spiral canal present within the modiolus at thebase of spiral lamina

• The central process of the ganglion form the cochlear nerve

• Post the scala media is connected to saccule by the ductus reuniens

• OOC consists of2 rows of cells

• External rod cells ( 4000)

• Internal rod cells (6000)• Supporting cells of

deiters are found between the outer hair cells

• Cells of hesner’s lie outside the deiter cells

Middle ear transformer mechanism/ impedance matching• Impedance matching is

the process in which the TM and auditory ossicles convert the sound energy to mechanical vibrations in the fluid of inner ear with min loss of energy by matching the impedance offered by fluid

• Hydaulic action of the TM

• Effective vibratory area of the TM is nearly 45- 55 mmsq as compared to footplate of stapes (3.2 sqmm). Hence it is 14 X greater.

• Hence when sound waves fall on the TM part of the sound is transmitted through the ossicles

• Lever action of the ossicles

• Handle of malleus is longer than the incus by 1.3 X so the pressure exerted at the oval window works out to be 14x 1.3= 18.3 times

Important triangles and their significance

• Macewan’s triangle• Sup- suprameatal recess• Antero-inf by the post

sup margin of the EAM• Post- tangent drawn

from zygomatic arch• Significance-1. Spine of henle lies in

the triangle2. Mastoid antrum lies

15mm deep

• Trautmann’s triangle• Post- sigmoid sinus• Ant- bony labyrinth• Sup-sup petrosal sinus• Significance• Inf into the post cranial

fossa can spread through this triangle and be can be approached by removing bone in between the triangle.

t