APPROACHES TO TEMPORAL BONE DISSECTION

General considerations

Pre-requisites and equipment

• Dissection laboratory

• Dissection bench

• Microscope

• Drill machine

• Hand piecess

Burr Points• The largest possible size of burr should be used

for the given area of dissection.• In early part of drilling shorter length of burr

gives better control.0• The hand piece should be gripped like a pen.• The burr should be moved parallel to important

structures.• The burr should never be kept static at one point.• Rotating burr should never be introduced or

removed from the dissection field.• The least possible pressure should be used while

drilling.• Side of the burr should be used rather than the

tip.• Movement of drilling (direction) should be away

from the important structures, to avoid damage in case of accidental slipping.

Suction and irrigation

• Profuse irrigation with continuous suction keeps the field clean and improves the visibility.

• It prevents the clogging of the burr and suction tips.

• It cools the drilling area.

Temporal bone holder (House urban temporal bone holder)

• Bone is placed with glenoid fossa anteriorly and zygoma facing perpendicular and anteriorly (away from the surgeon).

Instruments

• Tooth forceps

• Handle with 15 number blade

• Picks

• Canal elevator

• Sickle knife

• Cup forceps

• Scissors

• Alligator forceps

• Periosteal elevator

Surface anatomy of the temporal bone

The following landmarks are identified on the lateral surface:

• Linea temporalis (Inferior Temporal Line)

• Spine of Henle

• External Auditory Canal

• Base of Zygoma

• Clenoid Fossa

• Mastoid Tip

Planning of dissectionThe temporal bone dissection should be planned in a manner to utilize the bone maximally. Following steps are recommended:

• Cortical Mastoidectomy• Transmastoid Tympanotomies (Epi-, Posterior

and Hypotympanotomy)• Identification of semicircular canals• Identification of Endolymphatic Sac• Canaloplasty• Atticotomy (permeatal) • Facial Nerve Decompression (Horizontal &

vertical portion)• Canal wall down mastoidectomy• Labyrinthectomy• Identification of Internal auditory canal and its

contents.

• The third cut joins the posterior limit of the first cut to the mastoid tip, thus forming a bony triangle. This has been described as "Triangle of Attack".

Cortical mastoidectomy

•The dissection is started with cortical mastoidectomy.

•Using the largest cutting burr the first horizontal cut is made along the Linea temporalis starting at the base of the Zygoma.

•Another cut is made posterior to the posterior canal wall vertically downwards towards the mastoid tip.

The boundaries of this triangle are saucerized preserving the following structures :-Superiorly - Middle Fossa Plate- Posteriorly - Sigmoid Sinus Plate- Anteriorly - Posterior canal wall- Inferior angle- Mastoid tip

Antrotomy • Once the antrum is opened the dome of the

lateral semicircular canal is visualized.• The drilling is continued widening the antrum

the aditus is visualized.• 'Incus shadow' - In the process of widening the

aditus, the shadow of incus is visualized in the irrigation fluid. This shadow appears before the incus is seen. This helps in avoiding blind probing and possibility of dislodging the incus.

• The sino-dural angle is opened in the whole length till the dural plate meets the sinus plate.

• The posterior canal wall is thinned till a shadow of the instrument can be seen through it.

• Then the mastoid air cells are removed systematically.

• The superficial and deep mastoid tip cells are opened to identify the digastric ridge.

Landmarks to identify the mastoid segment of the facial nerve

• The facial nerve at the second genu is always medial to the short process of incus.

• The facial nerve is always medial & anterior to the lateral semicircular canal.

• It is 1-2 mm anterior to ampullary end of the posterior semicircular canal.

• The digastric ridge points to the facial nerve at its exit from the stylomastoid foramen.

End points of the cortical mastoidectomy

• Thin dural and sinus through which durra & sigmoid sinus can be visualized.

• Sino-dural angle widely opened showing the junction of dural and sinus plates.

• Mastoid up with complete exenteration of air cells.

• Well-delineated digastric ridge along with its relation to the facial canal.

• Thin posterior canal wall.

• The origin of chorda tympani (chorda-facial angle) should be visible inferiorly.

• Skeletonized vertical segment of the facial canal.

• Widely opened mastoid antrum and aditus showing the dome of the lateral semicircular canal and short process of incus.

Epitympanotomy• Aditus is widened removing the posterior attic cells.• Care should be taken not to damage the dural plate

and canal wall.• This exposes the Incudo-malleolar joint, body and

short process of incus with its ligaments. • The COg - It is a vertical spur of bone arising from

middle fossa going just anterior to the malleus head. It separates the posterior attic from the anterior attic.

• After removing the cog, anterior attic cells are removed to expose the anterior attic recess.

• Processus cochleariformis with the tendon of tensor tympani can be visualized.

• Antero-inferior to the processus cochleariformis opening of the Eustachian tube can be visualized.

• Anterosuperior to it lies the geniculate ganglion, which may be visible in a well pneumatized bone.

Posterior tympanotomy

Boundaries

- Superior - Fossa Incudis

- Inferior - Chorda-Facial Angle

- Lateral - Tympanic Annulus

- Medial - Facial Canal

Structures visualized

• Chorda Tympani• Incudo-Stapedial joint and

Stapes • Pyramid with Stapedius

Muscle• Horizontal Segment of the

Facial Nerve/Canal.• Processus Cochleariformis• Eustachian Tube opening • Round window • Facial Recess• Sinus Tympani• Round window• Posterior Hypotympanic

cells• Promontory with

jacobson's Nerve• Medial surface of the

Handle of Malleus and Umbo.

Extended facial recess

•The facial recess in extended inferiorly.

•Care is taken not to damage the tympanic annulus, as it is closer in this area because of angulation of the tympanic membrane.

•Inferior mesotympanum is better visualized.

•Care is taken, as the jugular bulb may be dehiscent and high.

Skeletonization of semicircular canals •The dome of the lateral semicircular canal is

visualized when the mastoid antrum is opened in the early part of the dissection.

•The posterior semicircular canal lies posterior and perpendicular to the lateral canal.

•The ampullary end of the posterior canal is just 1 to 2 mm from the vertical segment of the facial canal.

•Care is taken to prevent damage to the facial nerve while removing the perilabyrinthine air cells in this area.

•The ampullary end of the superior semicircular canal is identified medial to the body of incus, where it is adjacent to the ampulla of lateral semicircular canal.

•Non-ampullary ends of the superior and posterior canals join to from the crus commune, which is identified by drilling the air cells/bone between the canals and sino-dural angle (Trautman's triangle)

Facial nerve decompression • At this stage the facial canal is visualized again in the

vertical segment (in the mastoid) and horizontal segment (through the posterior tympanotomy).

• The vertical segment of facial nerve is decompressed with the biggest possible diamond burr.

• Direction of the drill should be parallel to the nerve with profuse irrigation.

• The facial canal bone is thinned till the blood vessels of the nerve can be visualized through it.

• The bone over the anterior aspect of the second genu is thinned keeping the direction of the burr parallel to the nerve. This prevents damage to the lateral semicircular canal.

• Care is taken not to touch the ossicles while drilling.• Using a small diamond burr the lateral and inferior bone

of the horizontal segment of the facial canal is thinned.• The dissection is continued anteriorly towards the

Geniculate ganglion (First Genu), which lies above and anterior (1-2mm) to the processus cochleariformis.

• With the help of blunt dissector, the thin bone over the dissected length of the facial nerve is removed.

Canal wall down mastoidectomy •With the help of a rongeur or drill the posterior canal wall is removed.

•The anterior buttress is removed to make the anterior attic and anterior canal wall continuous and smooth.

•Inferiorly the floor of the external canal is flushed along with the cavity.

•If necessary the mastoid tip is excised.End points •Well saucerized cavity (bevelled edges)•Round in appearance•The posterior canal wall is lowered to the level of the facial canal.

•The floor of the external canal and the mastoid cavity should be at the same level.

•No obvious air cells left in the mastoid cavity.

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