EXERCISE 9

ENDOSCOPIC ENDOMEATAL DISSECTION OF ROUND WINDOW

ROUND WINDOW AND COCHLEAR IMPLANT SURGEON

Round window is outlet of cochlear sound pathway.
When we observe with 0 degree endoscope the round window is situated postero inferior end of the promontory.
The round window is basically having 3 parts
A) Round window niche
B) Round window window chamber
C) Round window membrane

STEP A – EXPOSURE OF ROUND WINDOW
By using 0 degree endoscope the round window is clearly visualised at the posterior inferior part of the promontory.
Round window niche consists of

  • The TEGMEN (roof),
  • The anterior pillar (POSTES ANTICUS) which connects the tegmen to finiculus and
  • The posterior pillar (POSTES POSTICUS) which connects tegmen to the sibiculum.
    The round window chamber is mainly occupied by THE FUSTES which connects the retrotympanum to round window niche.
    The fustis has two arms the pyramidal arm and the styloid arm which attaches to the retrotympanum.

STEP A1 – VISUALISATION OF THE FALSE ROUND WINDOW MEMBRANE
In majority of cases the false round window membrane attaches to the both pillars of the round window niche.
After removal of the false membrane the round window chamber is seen clearly below the fustis the middle ear opening of the subcochlear tunnel.

STEP B – EXPOSURE OF POSTES POSTICUS
It forms a bridge between oval and round windows
The SUBICULUM starts from the posterior end of posterior pillar and ends at retrotympanum.

STEP C – EXPOSURE OF ROUND WINDOW CHAMBER
After removal of false round window membrane and mucosal folds the fustis is visualised.
The fustis a bony bar divides the fundus of round window into concomereta medialis and lateralis.
Following the fustis leads to round window membrane.

Labelling:
T – Tegmen
1 – Postes posticus
2 – Postes anticus
3 / FRM – False round window membrane
4 / F – Fustis
5 – Subcochlear tunnel
6 – Finiculus
7 – Concomereta lateralis
8 – Concomereta medialis
T – Tegmen
PA – Postis anticus
PS – Posterior pillar
SCT – Subcochlear tunnel

ENDOSCOPIC ENDOMEATAL DISSECTION OF ROUND WINDOW

ROUND WINDOW AND COCHLEAR IMPLANT SURGEON

STEP D – EXPOSURE OF ROUND WINDOW MEMBRANE
By removing overhanging of tegmen, anterior and posterior pillars of round window the round window membrane (RWM) is exposed.
The upper border of fustis is directed towards RWM
The position of RWM is horizontal to the ceiling of the niche
There is a recess between the fustis and inferior bony margin of RWM called inferior recess.

SURGICAL IMPORTANCE:
The recess allows the fluids or drug liquids to stay there and causes slow absorption through RWM
STEP D1 AND F1 –
PARTS OF ROUND WINDOW MEMBRANE
The RWM is divided in to

  • Antero superior (PARS ANTERIOR)
  • Postero inferior (PARS INFERIOR)
    The RWM is not flat anteriorly.
    The pars anterior attaches to CRISTA SEMILUNARIS and bulges slightly towards middle ear side
    The pars anterior is major part of the membrane, serves as a sound vibrating part during sound transmission.
    The pars posterio is flat and is attached to the root of osseus spiral lamina and it serves as diffusion of drugs (it contains more melanocytes).
    The posterior lip of round window (bridge between round and oval windows) is mainly occupied by subvestibular portion (cochlear duct caecum ororigin of cochlear
    duct) along osseus lip.

SURGICAL IMPORTANCE:
If the cochlear implant is inserted in posterior direction towards oval window it hinges bony spiral lamina.
If the electrode is directed too anteriorly it hits the crista semilunaris .
So the correct placement of electrode is initially upwards to wards tegmen and then gentle of electrode anteriorly towards anterior lip of round window in to scala tympani.

STEP E – VISUALIZATION OF SUB COCHLEAR TUNNEL
In majorty of cases this tunnel is present and opens below the fustis and passes under the basal turn of cochlea and medial to ICA to reach petrous apex

SURGICAL IMPORTANCE:
This is misleading pathway for false insertion of electrode in the tunnel.
So the electrode should not be inserted any opening below fustis.

Labelling:
F – Fustis
RW – Round window membrane
PA – Pars anterior
PP – Pars posterior
C / CS – Crista semilunaris
SVC – Subvestibular portion of cochlea
S – Stapes
IC – Inferior chamber
ICT – Infra cochlear tunnel

EXERCISE 9 (Cont..)

ENDOSCOPIC ENDOMEATAL DISSECTION OF ROUND WINDOW

ROUND WINDOW AND COCHLEAR IMPLANT SURGEON

STEP F2 – RIGHT INSERTION OF ELECTRODE, THE ANATOMICAL BASIS
If the electrode is inserted in anterior direction it impenges on crista semilunaris .
If the electrode is directed towards oval window side it hits the osseus spiral lamina
So the right direction of electrode is first towards tegmen of round window and then towards anterior lip of round window to pass into scala tympani

STEP F – REMOVAL OF RWM
The round window membrane is first perforated with pick and it is removed

STEP G – VISUALISATION OF SCALA TYMPANI
Once round window is completely removed the semilunar shaped bony crest is seen anteriorly, it is crista semilunaris.
Towards oval window side the sub vestibular portion of cochlea with osseus spiral lamina with basement membrane of cochlear duct is seen.

STEP G1 – EXPOSURE OF OSSEUS SPIRAL LAMINA
The osseus spiral lamina (bony spirale) a bony lamina is seen separating oval and round windows to which ceacum of cochlear duct is attached is seen clearly.

STEP H – EXPOSURE OF SCALAE
Both scala tympani and scala vestubuli are exposed after complte removal of round window

STEP I – VISUALISATION OF COCHLEAR AQUEDUCT OPENING
After removal of RWM and lowering of crista, the inner opening of cochlear aqueduct is seen just inside of Rwm Medial to cochlear aqueduct vertical ridge is visualised to protect inferior cochlear vein .
Outside of RWM the singular nerve is seen in concomereta medialis.

Labelling:
P – RWM perforation
CS – Crista semilunaris
X – Incorrect insertion of electrode
C – correct insertion
OSL – Osseus spiral lamina
BS – Basilar membrane
1 – Position of RWM
2 – Osseus spiral lamina
3 – Crista
10 – Inner opening of cochlear opening
11 – Vertical ridge
12 – Singular nerve
ST – Scala tympani