Anatomy of ear and mastoid

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ANATOMY OF THE EAR AND MASTOID

• Dr. Krishna Koirala, MBBS, MS( ENT-HNS)
• Associate professor of ENT
• Manipal College of Medical Sciences
• Pokhara, Nepal.

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• 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

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• External Ear (Outer Ear)

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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

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• 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

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• 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

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External Auditory canal
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• 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

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• 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

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Middle Ear
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• Middle ear cleft
• Middle ear cavity
• Attic ,aditus, antrum
• Mastoid air cell system
• Eustachian tube

• Middle ear cavity
• Epitympanum
• Mesotympanum
• Hypotympanum
• Protympanum
• Post- tympanum

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• 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

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AD
Ant
ATTIC
ME
ET
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• 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)

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PF
PT
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• Landmarks of TM
• Short process of malleus
• Anterior and posterior malleolar folds
• Handle of malleus
• Umbo
• Cone of light
• Annulus tympanicus

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• Layers of tympanic membrane
• 1) Outer layer of squamous epithelium continuous
  with that of the meatus
• 2) Middle layer of fibrous tissue which 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

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Four Quadrants of pars Tensa
PS
AS
PI
AI
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• 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

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• 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

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• 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 Macewans triangle and Cymba
  conchae)

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Mac Ewans 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

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• 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)

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• 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

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Inner ear
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• Lies in the petrous temporal bone
• Divisions
• Bony labyrinth
• Membranous labyrinth

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• 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

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• 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

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• 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)

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• 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

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• Membranous labyrinth
• Membranous cochlea
• Triangular in cross section
• Bordered by Reisners membrane, Basilar membrane
  and stria vascularis
• Utricle and saccule
• Semicircular ducts
• Endolymphatic ducts and sac

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• 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

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Differences between inner and outer hair cells Differences between inner and outer hair cells
Inner Hair Cells Outer Hair Cells
3500 12,000
Single row Three or four rows
Flask-shaped Cylindrical
Primarily afferent Primarily efferent
Transmit auditory stimuli Modulate inner hair cell
Resistant to damage Vulnerable to damage