Anatomy of ear and mastoid

Dr. Krishna Koirala

• Paired sensory organs comprising of • Auditory system involved in the detection

of sound• Vestibular system involved in maintaining

body balance and equilibrium• Divided anatomically and functionally into• External ear• Middle ear• Inner ear

• All three regions are involved in hearing • Inner ear is involved in body balance and

equilibrium

External Ear (Outer Ear)

Pinna Framework formed by yellow elastic

cartilage except in the lobule and incissura terminalis

Functions

Collect and direct sound waves through the ear canal to the tympanic membrane

Protect the tympanic membrane Importance : Graft material for middle ear &

other reconstructive surgeries

Helix: Slightly curved rim of the auricle Antihelix: Broader curved eminence anterior

to helix Concha : Deep cavity in front of the helix Cymba conchae : Depression between the

antitragus and ascending crus of the helix (surface landmark of mastoid antrum)

Tragus Lobule : Structure made up of areolar tissue

& fat without cartilage

Sensory Nerve supply of pinna Lateral surface

Upper 2/3 : Auriculotemporal nerve (cranial nerve V)

Lower 1/3 : Greater auricular nerve (C2,3) Medial Surface

Upper 1/3: Lesser occipital nerve (C2) Lower 2/3 : Greater auricular nerve (C2, 3)

Posterior concha and antihelix : Auricular b/o Vagus

Facial : Small region at the root of concha

External Auditory canal

Extends from bottom of concha to the tympanic Membrane

24 mm long in adults Lateral 1/3 (8 mm) Cartilaginous : Directed

upwards, backward and medially Medial 2/3 (16 mm) Bony : Directed

downwards, forward and medially Pinna to be pulled upwards, backwards and

laterally to straighten the external auditory canal in adults

Only cartilaginous skin has hair

follicles, ceruminous and

pilosebaceous glands (wax)

Cartilaginous fissure of Santorini and

bony foramen of Huschke present in

anterior wall infection / metastasis

to and from the parotid gland

Middle EarMiddle Ear

Middle ear cleft Middle ear cavity

Attic ,aditus, antrum

Mastoid air cell system

Eustachian tube

Middle ear cavity

Epitympanum Mesotympanum Hypotympanum Protympanum Post- tympanum

Contents of middle ear cleft 3 Ossicles : malleus, incus, stapes

2 Nerves : Chorda tympani, Tympanic plexus

2 Muscles :Tensor tympani, stapedius

Air Mucosal folds & ligaments Blood vessels

ET

MEME

ATTICADAnt

Tympanic Membrane Partition between the external and

middle ear

Obliquely set with 550 to floor

Dimension: 10 mm x 8 mm x 0.1 mm

Parts Pars Tensa Pars Flaccida (Shrapnel's membrane)

PF

PT

Landmarks of TM1. Short process

of malleus2. Anterior and

posterior malleolar folds

3. Handle of malleus

4. Umbo5. Cone of light 6. Annulus

tympanicus

Layers of tympanic membrane

1) Outer layer of squamous epithelium continuous with that of the meatus

2) Middle layer of fibrous tissue that has radial and circular fibres

3) Inner layer of mucous membrane continuous with the lining of the tympanic cavity

Fibrous layer disorganized in pars flaccida Annulus deficient superiorly as notch of

Rivinus

Four Quadrants of pars Tensa

AS

AI

PS

PI

Borders of middle ear cavity Roof : Tegmen tympani

Floor : Separates tympanic cavity from jugular bulb

Medial wall Promontory : Bulge formed by basal turn of

cochlea Oval window : Communicates between middle

ear and the vestibule of the inner ear, closed by footplate of stapes

Round window :Communicates between scala tympani and tympanic cavity, covered by secondary tympanic membrane

Lateral wall Largely by TM Scutum (outer attic wall) Bone inferior to TM

Anterior wall Thin plate of bone Openings of canal for tensor tympani and

Eustachian tube Posterior wall

Separates middle ear cavity from mastoid bone

Contains aditus ,pyramid

The mastoid antrum and air cell system Mastoid antrum : Largest and most consistent air cell

of mastoid air cell system, well developed at birth

Relations

Roof : Part of floor of MCF

Floor : Digastric muscle, sigmoid sinus

Posterior : Bony covering of sigmoid sinus

Lateral : Squamous temporal bone (corresponds to suprameatal or Macewan’s triangle and Cymba conchae)

Mac Ewan’s Triangle ( Suprameatal triangle)

• Boundaries• Superior : Posterior prolongation of

upper border of root of zygoma • Anterioroinferior : Posterosuperior

margin of bony external meatus • Posteroinferior : Vertical tangent drawn

through the posterior margin of bony external meatus touching the first line

Mastoid air cell system Extensive system of interconnecting air filled

cavities arising from walls of mastoid antrum that extend throughout the mastoid

Lined with flattened non ciliated squamous epithelium Types

Cellular ( pneumatized) : Honeycomb appearance on plain X-Ray mastoid

Diploic : Air cells interspersed with marrow containing spaces

Acellular (sclerotic)

Five Recognized regions of mastoid pneumatisation (Allam -1969)

Middle ear: Epitympanum, Mesotympanum, Hypotympanum, Protympanum, posterior tympanum

Mastoid : Antrum, central mastoid, peripheral mastoid

Perilabyrinthine: Supralabyrinthine, infralabyrinthine

Petrous apex : Apical, peritubal

Accessory : Zygomatic, squamous, occipital, styloid

Inner ear

Lies in the petrous temporal bone

Divisions

Bony labyrinth

Membranous labyrinth

Bony labyrinth ( Vestibule, Semicircular canals , Bony cochlea)

Vestibule Central portion of bony labyrinth, ovoid in

shape

Oval window at the lateral wall, utricle and saccule in the medial

Openings of SCC (5) - lie on posterior, superior and inferior walls of bony vestibule

Semicircular canals (3)

Lie in planes at right angles to each other

Ampullated and non ampullated ends

Ampullated ends contain vestibular sensory epithelium and independently open into the vestibule

Bony cochlea Coiled tube like the shell of a snail,

contains 2 ½ to 2 ¾ turns

Height around 5mm,base around 9 mm in diameter

Coils turn around the modiolus - extends along the entire length of cochlea except for helicotrema ( small channel at the apex)

Three compartments

Scala vestibuli

Scala tympani

Scala media (membranous cochlea)

Within the modiolus lie spiral ganglion

Cochlear nerve lies within the bony modiolus throughout the entire length

Membranous labyrinth Membranous cochlea

Triangular in cross section

Bordered by Reisner’s membrane, Basilar membrane and stria vascularis

Utricle and saccule

Semicircular ducts

Endolymphatic ducts and sac

Organ of Corti Sense organ of hearing Situated on the basilar membrane Components

Tunnel of Corti Hair cells ( outer and inner) Supporting cells (Deiter's,

Hansen's) Tectorial membrane

Differences between inner and outer hair cells

Inner Hair Cells Outer Hair Cells

3500 12,000Single row Three or four rows

Flask-shaped CylindricalPrimarily afferent Primarily efferentTransmit auditory

stimuliModulate inner hair

cellResistant to

damageVulnerable to

damage