EXERCISE 11

ENDOSCOPIC ENDOMEATAL DISSECTION OF COCHLEA
COCHLEA AND COCHLEAR IMPLANT SURGEON


Bony cochlea resemles like a snail, the base lying up on the floor of IAC and its apex or copula is situated anteriorly, laterally.
cochlear cone is a bone between the horizontal FN and vertical ICA .
The base of the cochlea is towards posterior side with oval and round windows and its apex is under the genu of FN.
Most of the cochlea consists of spiral cochlear canal of 30 mm in length spirals round the modiolus. The height of the cochlea is 5mm

STEP A COMPLETE EXPOSURE OF MEDIAL WALL OF MIDDLE EAR
Using 0 degree endoscope the medial wall of the middle ear is exposed.
The promontory is visualised including the cochlear cone .
The major part of the promontory is formed by the basal turn of the cochlea.
The inter oval and round window bony part is mainly occupied by the subvestibular part of cochlea ( origin of osseus spiral lamina and blind end of cochlea).
Surgical importance:
In over hanging of facial nerve with stapedopromontopexy the site of hole is important in these cases

STEP B – VISUALISATION OF SUPPORTING STRUCTURES.
The finicululus and sibiculum supports the coclear capsule inferiorly and posteriorly
The average distance between vertical ICS to anterior commissure is 5.5 to 7mm is observed.

STEP C – DRILLING OF LATERAL COCHLEAR WALL WITH PIEZO BLADE.
It is important step Piezo blade removes removes the lateral wall of cochlea carefully leaving the membranous structures of cochlea intact (like removal of egg shell leaving yolk intact)
The thickness of lateral cochlear wall is 1.5 mm, that is why the monopolar cautery is not used over promontory to seal the vessels

STEP D – DRILLING OF FINICUS
The finiculus is drilled to isolate cochlea

STEP D1 – SKELETONIZATION OF COCHLEAR CAPSULE
Using 0 degree endoscope and piezo electric blade the cochlear capsule is carefully skeletonized with out injuring the membranous labyrinth.
The cochlear capsule is very hard bone

Labelling:
1 – Subvestibular portion of cochlea in inter oval window round window bony bar.
2 – Basal turn of cochlea
3 – Horizontal FN
F – Finiculus
S – Sibiculum

ENDOSCOPIC ENDOMEATAL DISSECTION OF COCHLEA

SKELETONIZATION OF COCHLEA

STEP D2 – COCHLEAR CAPSULE
The location of the cochlear capsule is noted.
The apex of the cochlea is towards the genu of facial nerve
Cone is bounded superiorly by horizontal facial nerve, anteriorly by internal carotid artery and the base towards posteriorly

STEP E – SKELETONIZATION OF BASAL TURN OF COCHLEA.
The first 3mm of basal turn starts under the vestibule below the oval window
This part is called sub vestibular portion of cochlea and it contains blind end of cochlear duct (cochlear caecum).
The direction of the basal turn is observed.
The relation of basal turn to oval window is noted
Surgical importance:
The direction of basal turn of cochlea is important for cochlear implant surgeon for inserting electrode.
(Note – we dissected this bone in spain under pro carlo cenjor)

STEP F – SUBVESTIBULAR PORTION OF COCHLEA AND BONY SPIRAL LAMINA.
The bony spiral lamina arises under the oval window passes along the inferior surface of the oval window to join the spiral portion of basal turn of cochlea
The relation between the cochlea and ICA is noted

STEP G – COCHLEARIFORM PROCESS AND COCHLEAR TURNS
Basal turn of cochlea ends below the CP and second turn starts infront of basal turn.

Labelling:
CC – Cochlear capsule
BT – Basal turn of cochlea
OSL – Osseus spiral lamina imaginary line
SVP – Sub vestibular portion of cochlea

ENDOSCOPIC DISSECTION OF SUBVESTIBULAR PORTION OF COCHLEA

STEP H – DISSECTION OF SUBVESTIBULAR PORTION OF COCHLEA..
Careful dissection of bony bridge between round and oval windows visualises the subvestibular portion of cochlea.
The first 3 to 4 mm of cochlear duct arises inferior to the oval window, runs horizontally from posterior to anterior and forms at the base of cochlea as scala media
This caecum of cochlear duct is communicated with saccule with ductus reuinions.

STEP I – VISUALIZATION OF OSSEUS SPIRAL LAMINA
once the cochlear duct is removed the bony ledge arising from inferior wall of vestibular cavity around sibiculum is seen and this gives attachment to cochlear duct.

SURGICAL IMPORTANCE:
I) In stapedectomy with complete overhanging facial nerve over stapes, stapediopromotopexy is done.
This type of cases over drilling of inferior wall of stapes injures cochlear duct.
2) In cochleosacculotomy procedure surgeon has to perforate and rupture cochlear caecum to enter the round window to oval window with pick.

Labelling:
CC – Cochlear caecum.
OSL and SL – Osseus spiral lamina.


EXERCISE 11 (Cont..)

ENDOSCOPIC DISSECTION OF SUBVESTIBULAR PORTION OF COCHLEA

STEP H – DISSECTION OF SUBVESTIBULAR PORTION OF COCHLEA..
Careful dissection of bony bridge between round and oval windows visualises the subvestibular portion of cochlea.
The first 3 to 4 mm of cochlear duct arises inferior to the oval window, runs horizontally from posterior to anterior and forms at the base of cochlea as scala media
This caecum of cochlear duct is communicated with saccule with ductus reuinions.

STEP I – VISUALIZATION OF OSSEUS SPIRAL LAMINA
once the cochlear duct is removed the bony ledge arising from inferior wall of vestibular cavity around sibiculum is seen and this gives attachment to cochlear duct.

SURGICAL IMPORTANCE:
I) In stapedectomy with complete overhanging facial nerve over stapes, stapediopromotopexy is done.
This type of cases over drilling of inferior wall of stapes injures cochlear duct.
2) In cochleosacculotomy procedure surgeon has to perforate and rupture cochlear caecum to enter the round window to oval window with pick.

Labelling:
CC – Cochlear caecum.
OSL and SL – Osseus spiral lamina.