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Cummings Ch 127: T-Bone and Ear
Anatomy
Sameer Ahmed
9/25/2013
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The temporal bone consists of four embryologically distinct
components:
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Squamous, mastoid, petrous, and tympanic parts
A horizontal ridge known as the temporal line is formed
along the most inferior insertion by the temporalis muscle
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Aligned with the zygomatic process,
Surface landmark that estimates the location of the middle
fossa floor (Tegmen)
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“The middle fossa dural plate (MFD) is located on average 5 mm
above the LTI (linea tempralis inferior)”
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“This study confirms that the mastoid antrum is located 15 mm
deep to the lateral surface of the mastoid bone”
Facial Nerve Course
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Cisternal segment/intracranial segment
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Meatal segment → 8mm
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Porus acoustics: Medial IAC
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Fundus: Lateral IAC
Labyrinthine segment → 5mm
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Brainstem to IAC --> 16 -24 mm
Fundus to geniculate ganglion(1st genu)
Tympanic segment → 8-11mm
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Geniculate to 2nd genu
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Courses above oval window; dehisc 50% of time
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2nd genu is just anteroinferior to the horizontal SCC
Vertical segment → 10-14mm
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2nd genu to stylomastoid foramen
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At the fundus (lateral IAC)
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Bill's Bar separates CN 7 anteriorly from SVN
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“Jail Bar”
Falciform cr. separates CN 7 from cochlear n.; SVN from IVN
Anatomic relationship changes closer to the brainstem
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Jugular foramen:
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Pars nervosa: CN X, CN XI, Arnold's nerve, jugular
bulb, and posterior meningeal branch of ascending
pharyngeal artery
Pars venosa: CN IX, Jacobson nerve, and venous
return from inferior petrosal sinus
Inferior limit for a translab approach to IAC
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Cochlear aqueduct
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The cochlear aqueduct eventually opens into the
scala tympani at the cochlear base
Keel: Ridge of bone between Jugular bulb and ICA
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EAC:
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Lateral cartilaginous: 1/3rd
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Medial bony: 2/3rd
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Think skin; continuous with TM epithelium
Bony-cartilaginous junction in EAC
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Lateral canal skin thicker, more sebaceous units
Site of granulation tissue in malignant OE
Routes of tumor/infxn spread
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Foramen of Hushke
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Incomplete ossification of bony anterior EAC; medial
Fissures of Santorini
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Defects in cartilaginous EAC
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1st and 2nd branchial arches → external ear
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6 Hillocks of His
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1st branchial arch: 1-3 hillocks (tragus, superior helix)
2nd branchial arch: 4-6 hillocks (antihelix, antitragus,
lobule, and inferior helix)
External ear blood supply
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Posterior auricular & Superficial temporal art.
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Tympanic Membrane
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Outer layer: epidermal/squamous (ectoderm)
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Middle layer: fibrous (mesoderm)
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Can be subdivided into radial outer and circular inner
Inner layer: mucosal (endoderm)
Fibrous annulus: thickened pars tensa forming a
fibrous outler ring for the attachement to the TBone; lies within tympanic sulcus except superiorly
where it is deficient at the Notch of Rivinus
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Pars flaccida = Shrapnell's Membrane
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Eustachain Tube
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Medial 1/3rd → Bony
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Lateral 2/3rd → Cartilaginous
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Collapsed at rest
Tensor veli palatini intermittenly enlarges ET during
yawning or swallowing
Bony cartilaginous jnxn of ET is the narrowest
portion of the ET
Carotid artery is just medial to ET
Middle Ear
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Relative to tympanic annulus
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Epitympanum: above annulus
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Mesotympanum: confined by the annulus
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Hypotympanum: below annulus
Mestoympanum
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Anterior limit: ET
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Posterior limit: Facial nerve
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Medial wall: Cochlear promontory
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Postero-superior: Oval window
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Postero-inferior: Round window
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Sinus tympani: posterior to both windows and medial to vertical
division of FN
Pyramidal eminence: anterior to 2nd genu
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Hypotympanum
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Limited inferiorly by the jugular bulb
Epitymapnum
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Superior-medial wall of bony EAC (scutum) forms
the lateral wall of the epitympanum
Epitympanum divided into 3 spaces:
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Prussak's space, just medial to pars flaccida and lateral
to the head and neck of the malleus;
The compartment anterior to the malleus
The posterior compartment, which communicates with
the antrum
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Attic Cholesteatomas can spread postero-superiorly into the
antrum or postero-inferiorly into posterior mesotympanum
Ossicles
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Malleus
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Manubrium (handle; tip is the umbo)
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Lateral/short process
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Anterior process
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Head
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Neck
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Tensor tympani attaches to malleus neck and manubrium
by a tendon originating from the cochleariform process
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Incus
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Body
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Short process
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Long process
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Lenticular process
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Diarthrodial joints (for malleus-incus and incus-stapes
connections)
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Stapes
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Head (capitulum)
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Anterior and posterior crus
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Footplate (base)
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Stapes footplate attaches to bony margin of oval window
via an annular ligament (syndesmosis joint)