Anatomy of the middle ear, describing the various walls and its correlation

2. MIDDLE EAR CLEFT
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Eustachian Tube
Tympanic Cavity
Mastoid Air Cell System , Aditus ,
Antrum

3. TYMPANIC CAVITY
 Irregular air filled space ,within temporal bone.
 Bounded laterally by Tympanic Membrane.
 Medially by osseous labyrinth.
 Contains auditory ossicles, middle ear muscles.
 Tympanic segment of VII CN run along the medial wall
 Communications:
- Anteriorly - Eustachian tube
- Posteriorly - Antrum, mastoid air cells through aditus.
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6. 3 parts:
 Epitympanum:
 Situated above the malleolar folds of tympanic membrane.
 It contains the head of malleus, incudomalleolar joint and body
and short process of incus.
 Lined by the pavement epithelium.

7.  Mesotympanum:
 situated medial to the pars tensa of tympanic membrane.
 Middle ear proper is lined by cuboidal epithelium

8.  Hypotympanum:
 situated below the level of tympanic membrane.
 Lined by ciliated columnar epithelium

9. LATERAL WALL
Formed by
1. Bony lateral wall of Epitympanum superiorly.
2. Tympanic membrane centrally.
3. Bony lateral wall of hypotympanum inferiorly.
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10. TYMPANIC MEMBRANE
 Thin oval in shape
 9-10mm x 8-9mm
 550 with floor
 Circumference thickened to form fibrous annulus which anchors it
to tympanic sulcus
 Superiorly becomes a fibrous band Anterior Malleal Fold (AMF) &
Posterior Malleal Fold (PMF)
-triangular TM above the fold called as Pars Flaccida
(Sharpnell’s membrane)
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11. ROOF
 Formed by Tegmen tympani
 Separates tympanic cavity from MCF dura
 Formed by Petrous & Squamous part of temporal
bone
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12. FLOOR
 Thin plate of bone separating the tympanic cavity
from the dome of the Jugular bulb
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13. ANTERIOR WALL
 Separates middle ear cavity from ICA.
 Lower one third perforated by
 Superior and Inferior Carotico
tympanic Nerves to Tympanic plexus.
 Tympanic branches of ICA
 Middle One third
 Canal of Tensor Tympani muscle above
 Tympanic orifice of Eustachian Tube below
 Upper One Third
 Pneumatized
 May house anterior epitympanic sinus – can hide residual
cholesteatoma.
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14. MEDIAL WALL (Surgical floor of middle ear)
 Separates tympanic cavity from inner ear
 Promontory: rounded elevation
 Formed by part of basal turn of cochlea.
 Surface contains tympanic plexus
 Tympanic branch of glossopharyngeal nerve
 Carotico-tympanic nerve
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16.  Fenestra vestibuli (Oval Window)
 Behind & above promontory
 Oval shaped: 3.25 x 1.75 mm,
 Above it lies tympanic part of facial nerve
 Closed by foot plate of stapes & surrounded by
annular ligament

17.  Fenestra cochlea (Round Window)
 Below & behind Oval window
 The round window is posteroinferior to the promontory
 Separated by posterior extension of promontory - Subiculum
 Ponticulus : spicule of bone from promontory to pyramid
 RW membrane (2.3*1.9mm) is covered by bony overhang
from promontory.
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18. Tympanic segment of facial nerve
 Facial Nerve canal (Fallopian Canal)
 Above promontory & Oval Window it runs anteroposteriorly
 Marked anteriorly by processus cochleariformis: (constant
landmark)
 Geniculate ganglion- above processus cochleariformis
 Behind Oval Window it runs inferiorly
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19.  The posterior wall can be divided into two distinct parts:
 The upper third which corresponds to the aditus ad
antrum and represents the posterior limit of the
epitympanum
 The lower two thirds which correspond to the posterior
wall of the retrotympanum.
POSTERIOR WALL

20.  Aditus: aditus ad antrum connects middle ear space
with mastoid antrum. Dimension 4 × 4 × 4 mm
 Fossa incudis: lodges short process of incus and
posterior ligament.
 Pyramid: contains stapedius muscle.
 Recess:
 Facial recess
 Sinus tympani

21. Posterior wall eminences
 The pyramidal eminence
 The pyramidal eminence is situated at the centre of the
posterior wall immediately behind the oval window; it is
about 2 mm height.
 It lodges the body of the stapedial muscle and its apex
gives passage to the stapedial tendon. The pyramidal
eminence communicates with the facial bony canal by a
minute aperture which transmits the stapedial branch of
the facial nerve

22.  The styloid eminence
 The styloid eminence or Politzer eminence is a
recognized smoothed elevation at the inferior part of the
posterior wall; it represents the base of the styloid
process.

23.  Facial recess (suprapyramidal recess): lies lateral to facial
nerve.
 Boundaries: Fossa incudis superiorly
Chorda tympani laterally
Facial nerve medially
 Sinus tympani (infrapyramidal recess): the niche of two
labyrinthine windows communicate posteriorly with this
deep recess
 Boundaries :
-Laterally bounded by vertical segment of the facial nerve.
-Medially by the oval window.
-Superiorly by ponticulus and inferiorly by subiculum.
-Can be up to 9 mm deep from the tip of pyramid.

24. CONTENTS
 Three bones:
 Malleus
 Incus
 Stapes
 Two muscles:
 Tensor tympani
 Stapedius
 Two nerves:
 Tympanic plexus
 Chorda tympani

25. MALLEUS (HAMMER)
 Largest and lateral most
 9mm length
 Head , neck , handle/manubrium , anterior and lateral process.
Head
- Lies in epitympanum
- Has a saddle shaped facet articulating with body of incus – synovial
joint.
Handle
- Runs between mucosal and fibrous layers of TM.
- Upper part of medial surface gives insertion to tensor tympani muscle

27. INCUS (ANVIL)
 Body, short process, long process
 Body – In epitympanum , suspended by superior incudal
ligament attached to tegmen tympani.
 Short Process projects into attic ,lies in fossa incudis
 Long Process descends into mesotympanum.
- have lenticular process at its tip – articulates with stapes.
- Lenticular process sometimes called fourth ossicle because
of its incomplete fusion with tip.

29. STAPES (STIRRUP)
 Shaped like stirrup, smallest
 Head, neck, anterior and posterior crura, foot plate.
 Head – articulates with lenticular process
 Stapedial tendon attaches to neck and upper portion of
posterior crura.
 Foot Plate – 3mm long , 1.4 mm wide
- lies in oval window, attached to bony margin
by annular ligament.

32. MUSCLES OF MIDDLE EAR
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Stapedius
Arises from the walls of conical cavity within
pyramid.
Inserts into neck of stapes.
Nerve supply: small branch of facial nerve.
Tensor tympani
Arises from walls of the bony wall
lying above the Eustachian tube.
Inserts into upper end of malleus handle.
Nerve supply: Branch of mandibular nerve.

33. MASTOID ANTRUM
 Communicates with middle ear via the aditus.
 Antrum is well defined at birth.
 Measurements:
 Volume: 1ml
 Antero-posterior diameter: 14mm
 Vertical diameter: 9mm
 Transverse diameter: 7mm

34. Relations
 Medial wall : Lateral semicircular canal and more
deeply to posterior cranial fossa and endolymphatic
sac
 Roof : Middle cranial fossa
 Posterior wall : sigmoid sinus
 Lateral wall : Thickness at birth 2mm, adult life 12-
15mm, corresponds to suprameatal/Macewen’s
triangle.
 Floor: Digastric muscle laterally and sigmoid sinus
medially.

35. SUPRAMEATAL / MACEWEN’S TRIANGLE
 Region felt through the cymba conchae of the auricle.
 Bound by the suprameatal crest, posterosuperior margin of
the external meatus and a vertical tangent through the
posterior margin of the external meatus.
 In adults, antrum lies 1.5-2cm deep to Macewen’s triangle
 Landmark in cortical mastoidectomy.

36.  It is a shallow closed space that lies in between the pars
flaccida and the neck of the malleus
 Roof - lateral mallear fold
 Floor – lateral process of malleus along with its mucosal folds
lying in horizontal plane
 Through the gap present between lateral malleal and lateral
incudal folds, Prussaks space communicates with the attic
 The most common site for origin of cholesteatoma