Introduction
Over the last decades, there has been a tremendous increase in the number of cochlear
implant recipients, and, consequently, there is a recent increase of interest in the
proper understanding of the anatomy of the round window (RW), which is the most important
anatomical landmark, not only in cochlear implant surgery but also in other otologic
procedures. A thorough and proper understanding of the anatomy of the RW is now considered
essential and mandatory for the practicing otolaryngologist rather than for an experimental
scientist.[1 ]
The round window membrane (RWM) is normally hidden under a boney overhang, termed
the RW niche (RWN), which is formed by a posterior pillar (postis posterior), the
tegmen, and the anterior pillar (postis anterior). The RWM with its niche is termed
the RW prechamber. The membrane itself is located at the end of the scala tympani
anterolateral to the hook region.[2 ]
Insertion of the electrode array either through the RWM or by means of creation of
a cochleostomy has been debated for a long time a; however, both approaches depend
on the meticulous identification of the RW. The early reports of cochlear implantation
emphasized insertion through the RWM; this approach was replaced by drilling a cochleostomy
anteroinferior to the RWM. The crista fenestra, which is preserved in the first approach,
usually obscures the vision of the surgeon during RW insertion. Recent researches
emphasized the residual hearing preservation and a soft atraumatic surgery. Round window membrane insertion with preservation of the crista fenestra is now the
insertion site of choice for many surgeons, as it is claimed that the electrode will
be kept in place by this bone; therefore, if the electrode is inserted through the
cochleostomy, the point of first contact to the basilar membrane is 5 mm far anterior,
so the contact length toward the modiolus is 5 mm shorter.[3 ]
[4 ]
[5 ]
The position of the RWN, its shape, and the direction that its opening faces are variable
among individuals. The awareness of possible anatomical variations of the RW, as it relates to the different
anatomical structures in the tympanum, can help with the decision-making process before
surgery regarding the type, the length, and the site of insertion of the cochlear
implant electrode array and it, subsequently, minimizes the rate of complications
and misplacement.[6 ]
[7 ]
Round Window Measurements
The mean maximum height of the round window niche was 1.53 ± 0.41 mm, ranging from
0.9 to 2.3 mm; 75% of the specimens measured between 1 and 2 mm, while 15% measured > 2 mm,
and 10% measured < 1 mm. We found that the RW height gives an idea about the depth of the RW prechamber.
The mean width of the RW was 1.18 ± 0.25 mm, ranging from 0.8 to1.65 mm. A total of
75% of the specimens measured > 1 mm, and 25% measured < 1 mm.
The surface area of the RWN was determined by measuring the circumference of the niche
after the excision of the RWM in square millimeters; this surface area was 1.52 ± 0.28 mm2 , ranging from 1.24 to 1.52mm2 .
In our study, we assessed the minimal distance between the RW niche and the surrounding
anatomical structures. The results were as follows: the mean minimal RW-OW distance
was 2.44 ± 0.42 mm, ranging from 1.7 to 3.05 mm; the mean minimal RW-CC distance was
7.9 ± 1.44 mm, ranging from 5.3 to 11.05 mm; a total of 5% measured < 6 mm, 75% measured
between 6 and 8 mm, and 5% measured > 8 mm. The mean minimal RW-JF distance was 2.77 ± 0.42 mm,
ranging from 1.95 to 3.9 mm; a total of 5% measured < 2 mm, 70% measured between 2 and 3 mm, and 25% measured > 3 mm. Lastly, we measured the minimal RW-FC distance, which measured 5.55 ± 1.18 mm, ranging
from 3.25 to 7.75 mm. A total of 15% measured < 4 mm, 50% measured between 4 and 6 mm,
and 35% measured > 6 mm. ([Fig. 1 ])
Fig. 1 Microscopic view illustrating the measurement of the minimum distance between (A ) round window and oval window, (B ) round window and internal carotid, (C ) round window and jugular fossa, (D ) round window and the vertical segment of the facial nerve.
The crista fenestra was clearly identified in 18 specimens out of the 20 dissected
temporal bones; its shape was rectangular, causing significant narrowing of the RW
region and subsequently to cochlear implant electrode insertion among 6 temporal bones,
and showed semilunar shape in 7 cadavers, triangular in shape among 3 cases, and,
lastly, it was rudimentary in 2 temporal bones. ([Fig. 2 ])
Fig. 2 Different shapes of the crista fenestra. (A ) Absent, (B ) Rudimentary, (C ) Rectangular obstructing most of the round window region, (D ) Semilunar in shape.
Radiological Assessment
The mean maximum transverse diameter of the round window niche was measured in the
axial and coronal plane of the high-resolution computed tomography (HRCT) scan; it
measured 1.27 ± 0.4 mm with a range between 1.2 and 2.3 mm. The height of the RW prechamber
(the depth of the prechamber) was measured in the sagittal plane, and it measured
1.52 ± 0.38 mm with a range between 1.1 and 2.4 mm.
Serial sections in the sagittal view using the OTOPLAN otological tablet based planning
platform was used in cases prior to cochlear implantation. It can generate patient-specific
3D reconstruction from ordinary medical images and visualizes the unique anatomy of
each patient, including the optimal electrode array for each candidate. This software
enables the surgeon to achieve proper visualization of the cochlear anatomy, including
the RW and its relations with the surrounding anatomical structures prior to the surgery.
In the current study, imaging data was transformed to this software for assessment
of the infracochlear air cell track pneumatization. [Fig. 3 ]
Fig. 3 Serial sections in the sagittal view using the MED-EL otological tablet based planning
platform (OTOPLAN) showing the degree of pneumatization of the infracochlear tunnel.
(A, B ) the pneumatization is limited just inferior to the cochlea, (C, D ) the pneumatization reaches far anterior until the petrous apex.
Four different types of pneumatization of the infracochlear air cell track were found,
and, subsequently, we classified the subcochlear canaliculus in our study into:
Type 1: Poorly pneumatized infracochlear air cell tract ([Fig. 3A ]). This was noticed in only 3 cases.
Type 2: Well-pneumatized but not reaching the petrous apex ([Fig. 3B ]), and this type was found in 6 cases.
Type 3: Well-pneumatized reaching the petrous apex, no connection with the petrous apex air
cells ([Fig. 3C ]); this type was the most common type in our study, and was found in 9 cases.
Type 4: Well-pneumatized and connected to the petrous apex air cells ([Fig. 3D ]); only 2 cases had this type of pneumatization.
In two cases, the cochlear implant electrode was misplaced through this tunnel to
an extracochlear site, one into the petrous apex (Type 4 infracochlear tunnel), while
in the other case the electrode was found in the hypotympanum inferior to the cochlea
(Type3 infracochlear tunnel). Both cases have been excluded from the study. ([Fig. 4 ])
Fig. 4 Two cases of misplaced cochlear implant electrode not included in our study, in which
the cochlear implant electrode passed through the subcochlear canaliculus into an
extracochlear site.
Surgical Assessment
The surgical anatomy of the RWN was evaluated in 20 patients subjected to cochlear
implantation; the endoscopic assisted suprameatal approach was used in 9 cases. The
Advanced Image and Data Acquisition (AIDA) system with a high definition Storz endoscopic
camera (Karl Storz, Tuttlingen, Germany) (Image1) was attached to either the surgical
microscope or directly to the endoscope using 0° and 30° rigid Hopkins rod telescopes
with a 3 mm outside diameter, 15 cm length, (Karl Storz, Tuttlingen, Germany).
A) The direction of the RWN; the RW was found facing posteroinferior in 11 cases (55%),
in 7 cases the RW direction was inferior, and, lastly, in only 2 cases (10%), the
direction was posterior. ([Fig. 5 ])
Fig. 5 The round window prechamber during endoscopic assisted suprameatal approach for cochlear
implantation showing the direction at which the RW faces according to the relation
between the postis posterior and the postis anterior. (A ) It faces posteroinferior, both pillars are equal in length, (B ) It faces posterior, the anterior pillar is much longer than the posterior, C, it
faces inferior, the posterior pillar is much longer than the anterior.
B) The length of the anterior and posterior pillars were measured ([Table 4 ]), the photographs were imported to a computer, and by using a special software (Image
J1.46r software [National Institute of Mental Health, Bethesda, Maryland, USA]) after
proper calibration and at 5x magnification, they were used to assess:
The postis anterior length (the anterior pillar), from the dome (the most superior
point) of the RW tegmen until its end where it meets the fustis of the RW prechamber
(a smooth bony structure, which forms the floor of the RW prechamber indicating the
entrance to the round window membrane). ([Fig. 5 ])
The postis posterior length (the posterior pillar), from the dome (the most superior
point) of the RW tegmen until its beginning (the junction with the fustis). ([Fig. 5 ])
Statistical analysis of the relation between the length of both pillars and the direction
at which the RW faces was done using IBM SPSS Statistics for Windows, Version 20.0
(IBM Corp., Armonk, NY, USA), the difference in length between the anterior and posterior
pillars (anterior pillar length - posterior pillar length) was calculated in mm.
[Tables 2 ], [3 ], [4 ] show a significant difference among the three directions of the RW (posteroinferior,
inferior, and posterior) in relation to the difference in length between the anterior
and posterior pillar. If the mean difference between both pillars was (0.02 ± 0.03); both pillars are almost
equal, the RW was facing postero-inferior, and this was found in 11 cases. The direction
of the RW was inferior in 7 cases when the mean difference was (-0.41 ± 0.06); the
posterior pillar was significantly longer than the anterior one. Lastly posteriorly
facing RW niche was found only in two cases and the difference between both pillars
was (0.61 ± 0.01); Significantly longer anterior pillar. ([Fig. 4 ])
Table 1
Summary of the round window visibility, shape, direction, and the crista fenestra
among the 20 cadaveric specimens
Visibility through FR
Visibility through EAC
RW shape
Direction of opening
Crista fenestra
Infracochlear tunnel
1
Visible
Not Visible
Pear shape
Posteroinferior
Rectangular
Clearly visible
2
Visible
Visible
Rounded
Posteroinferior
Rectangular
Clearly visible
3
Visible
Visible
Rounded
Inferior
Rudimentary
Clearly visible
4
Visible
Visible
Pear shape
Posteroinferior
Semilunar
Clearly visible
5
Visible
Visible
Rounded
Inferior
Triangular
Barely seen
6
Not Visible
Not Visible
Triangular
Posterior
Semilunar
Barely seen
7
Visible
Visible
Oval
Inferior
Rectangular
Barely seen
8
Visible
Visible
Oval
Inferior
Triangular
Clearly visible
9
Not Visible
Visible
Oval
Posterior
Semilunar
Barely seen
10
Visible
Not Visible
Rounded
Inferior
Rectangular
Barely seen
11
Visible
Visible
Oval
Posteroinferior
Semilunar
Barely seen
12
Not visible
Visible
Oval
Posteroinferior
Absent
Clearly visible
13
Visible
Visible
Oval
Inferior
Absent
Clearly visible
14
Visible
Not Visible
Triangular
Posteroinferior
Semilunar
Invisible
15
Visible
Visible
Oval
Posteroinferior
Triangular
Clearly visible
16
Not Visible
Visible
Oval
Posteroinferior
Rectangular
Clearly visible
17
Visible
Not Visible
Rounded
Inferior
Semilunar
Invisible
18
Visible
Visible
Oval
Posteroinferior
Rudimentary
Clearly visible
19
Visible
Visible
Oval
Inferior
Semilunar
Clearly visible
20
Visible
Not Visible
Quadrangular
Posteroinferior
Rectangular
Clearly visible
Abbreviations: EAC, external auditory canal; FR, facial recess; RW, round window.
Table 2
Summarizes the statistical data for the measurements of both the postis anterior and
posterior of the round window prechamber in millimeters
(n = 20)
Anterior pillar length
Posterior pillar length
Mean ± SD
2.22 ± 0.18
2.09 ± 0.20
Median
2.3
2.17
Minimum–Maximum
1.97–2.64
1.70–2.37
Range
0.63
0.67
Abbreviation: SD, standard deviation.
Table 3
Distribution of the studied round window directions according to difference in length
between anterior and posterior pillars
Direction of cord
Posteroinferior
(n = 11)
Inferior
(n = 7)
Posterior
(n = 2)
Kruskal-Wallis test
Mean ± SD
0.02 ± 0.03
−0.41 ± 0.06
0.61 ± 0.01
X2
= 15.58
p < 0.001*
Median
0.0
−0.42
0.61
Minimum–Maximum
0.00–0.10
−0.49–- 0.30
0.60–0.62
Range
0.10
0.19
0.02
Abbreviation: SD, standard deviation.
Table 4
Distribution of studied round window directions according to the length of the anterior
and of the posterior pillar
Direction of cord
Posteroinferior
(n = 11)
Inferior
(n = 7)
Posterior
(n = 2)
Anterior pillar length
Mean ± SD
2.07 ± 0.15
1.85 ± 0.07
2.35 ± 0.07
Median
2.11
1.85
2.35
Minimum–Maximum
1.80–2.35
1.77–1.95
2.30–2.40
Range
0.55
0.18
0.10
Posterior pillar length
Mean ± SD
2.05 ± 0.16
2.27 ± 0.06
1.74 ± 0.06
Median
2.12
2.25
1.74
Minimum–Maximum
1.80–2.30
2.19–2.37
1.70–1.74
Range
0.50
0.18
0.08
Discussion
The RW has been a subject of anatomical interest ever since it was discovered and
named by Gabriel Fallopius in the 16th century. The RW prechamber is a three-dimensional space lying between the RWN and
the RW membrane. It is funnel-shaped or conical, becoming narrower toward its fundus,
laterally toward the middle ear is the RWN, and it ends medially by the round membrane.[8 ]
In the current work, a prospective study on 20 cadaveric specimens and on 20 patients
subjected to cochlear implantation at the main university hospital was conducted to
assess the detailed surgical and radiological anatomy of the RW prechamber relevant
for cochlear implantation.
In our collection, among the 20 cadaveric specimens, different shapes were identified;
the oval RW was the most commonly encountered. Our results were similar to that of
a previous study conducted by Singla et al on 50 gross cadaveric temporal bones. In
another study done by Tóth et al, they reported other RW shapes, such as comma shape
and pinpoint ones, and they also emphasized that age does not influence the shape
of the RW.[9 ]
[10 ]
[11 ]
The RWN is often directly seen within the tympanic cavity after tympanomeatal flap
elevation; however, this is not valid in every case. In the current study, it was visible through
the external auditory canal in 14 specimens, and was visible in 16 out of the 20 specimens
through the FR. These results were consistent with the results concluded by Hamamoto
et al using 22 temporal bones; in 77% of the temporal bones, the RWN was visible through
posterior tympanostomy; on the other hand, the results were slightly different from
those of Goravalingappa, who reported 40% of invisible RWNs through posterior tympanostomy.
The slight variability between our results and those in literature might be attributed
to the number of temporal bones used in each study.[12 ]
[13 ]
Among the 20 cadaveric specimens, the direction at which the niche faced was posteroinferior
in 50% of the specimens, and in 40% it faced inferiorly, while in only 10% the direction
was posterior. These results were consistent with the results documented by Aslan
on 11 dry temporal bones and in 9 temporal bone specimens preserved in formalin; they
reported that the direction of the niche opening was posterior in 3 (15%), inferior
in 9 (45%), and posteroinferior in 8 (40%).[14 ]
Regarding the RW dimensions, measurement parameters were very variable in the literature,
Takahashi et al measured the diameter or half diameter of the RW, other authors, as
Su et al, and Cohen et al measured its length or width. In our study, owing to the
great variability of the RW shape, we preferred to measure the maximum height and
width of the RWN. An extremely narrow RW makes the insertion of the electrode array
difficult and usually necessitates drilling of the anteroinferior margin of the niche.
This area is termed the crista fenestra, which is an obstacle to the insertion of
the electrode array to the scala tympani of the basal turn of the cochlea. This drilling
allows adjustment of the vector of insertion of the electrode array into the scala
tympani; however, it might be not only potentially hazardous because of its close
proximity to the hook region, but it might also be traumatic to the cochlea, leading
to loss of the residual hearing in hearing‐preservation protocols.[15 ]
[16 ]
[17 ]
Previous authors, such as Stewart et al, Su et al, Takahashi et al, and Singla et
al have measured the RWw, which were 1.5 mm, 1.66 mm, 2.98 ± 0.23 mm, and 1.15 ± 0.39 mm,
respectively; in the present work, the mean RWw was 1.18 ± 0.25 mm. The present results are consistent with Stewart et al, Su et al, and Singla et al.
results, but were different from those of Takahashi et al. This can be attributed to the fewer number of cases in their study work, which were
only 5 specimens. Regarding RWh, many authors have measured the RWh, as Stewart et
al, Cohen et al, and Singla et al; their results were 1.2 mm, 1.665 ± 0.258 mm and
1.62 ± 0.77 mm, respectively; in the present study, the mean RWh was 1.53 ± 0.41 mm.[9 ]
[15 ]
[16 ]
[18 ]
The anatomical location of the vertical segment of the FN is at risk of damage during
cochlear implantation, especially during posterior tympanotomy. Wide variability in
the distance between the RW and the vertical FC is usually present. In the present
work, again owing to the wide variability of the shape of the RW, the minimal distance
between both of them was measured. The mean minimal distance was 5.55 ± 1.18 mm. Our
results were different from those of Singla et al, who reported the mean distance
between the FC and the RW to be 4.28 ± 0.67 mm, and this might be due to the larger
number of cases in their study (50 cases) and wider age range (2.5–70 years old).[9 ]
[19 ]
[20 ]
[21 ]
Jugular bulb injury during cochlear implantation is a reported complication, especially
in high dehiscent bulb. In the present work, the mean minimal RW-JF distance was 2.77 ± 0.42 mm.
Our results were not too far from those of many other authors in the literature; Duan
et al, in a study on the computed tomography (CT) scans of 15 normal head specimens,
reported the mean RW-JF distance to be 2.10 mm on the right side and 2.39 mm on the
left side; and our results are also in line with another study conducted by Singla
et al, who reported the RW-JF distance as measuring 2.98 ± 0.68 mm. The study by Stewart
et al conducted on 12 vertically sectioned temporal bones, reported the mean distance
between the inferior margin of the RWN to the jugular bulb to be 4.4 mm, ranging from
1.4 to 5.7 mm; these measurements were different from those in the current study,
the wide variability was attributed to the parameters of measurement and the number
of specimens.[9 ]
[15 ]
[22 ]
During cochleostomy, anteroinferior to the RW, the knowledge of the precise safe distance
of drilling is essential to avoid injury to the internal carotid artery (ICA), as
this injury could be potentially fatal. The mean minimal RW-CC distance in the present
work was 7.9 ± 1.44 mm; our results are in agreement with the results of the study
by Wysocki et al conducted on 100 temporal bones; they found that the mean RW-CC distance
was 8.08 ± 1.55 mm, and are also in agreement with the results of Singla et al, who
stated that the mean RW-CC distance was 8.03 ± 1.55 mm.[9 ]
[10 ]
According to the present work results, the mean minimal RW-OW distance was 2.44 ± 0.42 mm;
this result was consistent with the results found by Stewart et al; they found that
the mean distance between the superior margin of the RWN to the OW was 2.7 mm; and
our results are also consistent with the results of Singla et al, who reported a mean
of 2.19 ± 0.43 mm, ranging from 1.39 to 3.57 mm, but were different from the measurements
found by Paprocki et al on 10 cadaveric temporal bones; they reported the mean RW–OW
distance to be 1.43 ± 0.279 mm; the possible reasons for this difference were attributed
to the number of specimens analyzed.[9 ]
[15 ]
[23 ]
In the present study, the crista fenestra was evaluated only for its presence and
its shape. The crista fenestra is a sharp bony crest, which occupies a considerable
area of the circumference of the RWN; during cochlear implant surgery, it can be considered
an obstacle to electrode insertion to the scala tympani.
A HRCT scan was performed for the 20 cases subjected for cochlear implantation. The
mean transverse diameter of the RWN was measured in the axial and coronal planes,
and measured 1.27 ± 0.4 mm, while the height (the depth of the prechamber) was measured
in the sagittal plane, and was 1.52 ± 1.38 mm. these measurements were consistent
with the measurement taken from the cadaveric specimens (1.18 ± 0.25 mm) for the maximum
diameter of the niche, and which were 1.53 ± 0.41 mm for the maximum height.
There was no significant difference between the average width and height measured
radiologically and those measured on the temporal bone study, which ensures that the
new imaging modalities are reliable methods that give an accurate data about anatomical
measurements.
Our results were consistent with the results found by Veillon et al, who performed
a radiological study using 75 temporal bones and found a RW transverse diameter ranging
between 1.3 and 1.9 mm; on the other hand it was completely different from the results
of Takahashi et al, who measured the diameter and the height of the RW niche in only
5 specimens.[18 ]
[24 ]
The radiological identification of the width and the height of the RW prechamber prior
to surgery is essential in surgical planning regarding the type of implant, and the
amount of niche drilling, if needed, as a very tight RW niche usually requires the
use of slim electrodes, and a short RW prechamber (procident tegmen), will require
minimal tegmen drilling to avoid going through a wrong scala or injuring the osseus
spiral lamina.
The Subcochlear Canaliculus (The Infracochlear Tunnel)
Interestingly, to date, there are no published reports highlighting the surgical anatomy
of the subcochlear canaliculus relevant for cochlear implantation, and to our knowledge
this is the first work in the literature to do so.
Anatomically, the subcochlear canaliculus is a tunnel located between the fustis and
the finiculus, and connects the RW prechamber with the petrous apex and, subsequently,
the petrous carotid via a series of pneumatized air cells. Marchioni et al evaluated
the endoscopic anatomy of this canaliculus in cases with cholesteatoma, and as a corridor
for petrous apex lesions, they found it in only 84% of the 42 specimens in their study;
endoscopically, they reported three different types of subcochlear canaliculus: Type A: represents a large tunnel to the petrous apex, detectable endoscopically; Type B: a small tunnel, with a connection to the petrous apex, not detectable endoscopically;
Type C : the RW chamber is not connected to the petrous apex.[25 ]
In the current study, the lateral end of the infracochlear canaliculus was invisible
in 2 out of the 20 (10%) microscopically dissected cadaveric specimens; the finiculus
bone merged medially with the fustis, and these tunnels were classified as type C
according to Marchioni. On the other hand, among the 20 cases subjected for cochlear
implantation, the infracochlear tunnel was evaluated radiologically in the coronal
and the sagittal views of the HRCT using the new OTOPLAN software. The tunnel was
present in all cases in the study with variable degrees of pneumatization, and subsequently,
we classified the subcochlear canaliculus radiologically into 4 types ([Fig. 2 ]):
Type 1: Poorly pneumatized infracochlear air cell tract; limited subcochlear pneumatization.
Noticed in only 3 cases.
Type 2: Well-pneumatized but not reaching the petrous apex in 6 cases.
Type 3: Well-pneumatized reaching the petrous apex, no connection with the petrous apex air
cells in 9 cases.
Type 4: Well-pneumatized and connected to the petrous apex air cells was found only in 2
cases.
In our study, we consider the infracochlear air cell track a very common site and
cause of misplacement of the electrode array not only in the hypotympanic air cells
inferior to the cochlea, but also in the adjacent extracochlear sites, such as the
petrous apex and the internal carotid canal. The configuration and the orientation
of this track during a microscopic approach through posterior tympanotomy can be easily
misinterpreted during surgery as a RW prechamber. The final position and the encountered
resistance to insertion of the electrode, if inserted into this track, depend on the
degree of its pneumatization. If the track is nonpneumatized or poorly pneumatized,
the final position of the electrode array will be in the hypotympanic air cells and
resistance to the insertion will be encountered; on the other hand, in well-pneumatized
tracks, the electrode can reach the petrous apex and even the petrous carotid with
minimal resistance. ([Fig. 6 ])
Fig. 6 The round window was not visible both thorough the facial recess and the external
auditory canal.
The Direction of the Round Window Niche
The boney walls of the RW prechamber start to develop by the 16th week of the intrauterine life. The tegmen, the postis anterior (anterior pillar)
and the postis posterior (posterior pillar) are the first to appear, while the fustis
(inferior wall) is completely absent at this time. One week later, the fustis starts
to develop, only reaching the anterior pillar by the 18th week. By the 20th week, the anterior and posterior pillar and the tegmen show a rapid increase in growth;
the most intensive growth can be found in the anterior wall, where the inferior tympanic
artery and the tympanic nerve run, which can be found as a boney canal parallel to
the anterior pillar, followed by the tegmen (the superior wall).[11 ]
The anterior pillar and tegmen of the round window prechamber begin to ossify as chondral
bone, but the main parts of both will be finally formed by membranous bone. On the
other hand, the posterior pillar ossification remains until the end of the development
in the chondral bone ossification pattern. Membranous bones are rich in blood vessels
and shows rapid growth, more than chondral bones, which results in a wide range of
variability of the RW prechamber walls.
Depending on the rate of growth of the anterior pillar and the tegmen, the plane of
the RWN or the direction at which it faces varies. In cases in which the anterior
wall lengthens more than the tegmen, the RW will face posteriorly. When the tegmen
becomes longer than the anterior wall, the direction of the RW will be inferior. The
most common variant, when both of them grow in a proportionate way to each other,
the RW prechamber will face the posteroinferior direction.
Owing to the impossibility to measure the lengthening of the tegmen, in the current
study we measured the length of both the anterior and posterior pillar. The anterior
pillar was measured from the most superior point or the dome of the tegmen until the
junction with the fustis, the mean length of the postis anterior was 2.22 ± 0.18 mm,
which was larger than the postis posterior, which was measured from the same point
until it meets the fustis inferiorly, with a mean length of 2.09 ± 0.20mm. The only
limitation was the tremendous variations in the tegmen shape, and so the most superior
point or its dome was taken as a reference for measurement.
There was a statistically significant difference between the mean difference between
both pillars and the direction at which the niche faced. When both pillars were almost
equal; the difference in the anterior pillar-posterior pillar length was 0.02 ± 0.03.
In this occasion, there was no difficulty in the identification of the RW or in electrode
insertion through the posterior tympanostomy, as the direction vector of insertion
was directly to the scala tympani. This was found in 11 out of the 20 cases subjected
to cochlear implantation in the current study.
The direction of the RW was inferior in 7 cases; among those patients, the posterior
pillar was significantly larger than the anterior, and the mean difference was - 0.41 ± 0.06 mm.
During the surgery, full visualization of the RW through the posterior tympanostomy
was difficult despite progressive thinning of the posterior wall of the external auditory
canal, and in two cases it was not visualized except through the external auditory
canal after elevation of the tympanomeatal flap; however, we were able to insert the
electrode through the RW after its visualization. In the situation of an inferiorly
facing RW prechamber, misplacement of the electrode array can easily happen in an
extracochlear site, mostly in the hypotympanic air cells, so a wider exposure and
the identification of other anatomical landmarks, such as the OW or the stapedius
tendon might be needed. ([Fig. 3 ], [4 ])
Lastly, a posteriorly-facing RWN was encountered only in two cases, with significant
longer anterior pillar; the difference between both pillars was 0.61 ± 0.01mm. During surgery, the RW was not completely visualized despite posterior tilting of
the patient head and progressive thinning of the posterior wall of the FR until the
mastoid segment of the facial nerve was clearly visible through a very thin covering
bone. Among those two patients, we used the 30° otoendoscope after meticulous elevation
of the tympanomeatal flap for proper localization of the RW, and successful insertion
of the electrode array was performed through a cochleostomy anteroinferior to the
RW and not through the RW itself. In the situation of a posterior facing RW prechamber,
the difficulty in visualization may not only lead to misplaced cochlear implant electrode,
but also may end up in injury to the surrounding extracochlear structures during blind
drilling along the cochlear curvature to the modiolus or to more catastrophic anterior
drilling toward the internal carotid canal. ([Fig. 4 ])
Conclusion
Proper understanding of the topographic anatomy of the RW, including its direction
of opening, and the distances from different adjacent structures in the tympanum,
is essential for successful cochlear implantation surgery; it can help decision-making
before surgery and is very useful to avoid many complications, such as misplacement
into extracochlear sites and iatrogenic injury to the surrounding structures.
In our anatomical study, we were able to assess the parameters of relation of the
round window to the surrounding structures in the tympanum; these parameters are more
of cadaveric dissection limits than of real cases, because of the limited number of
temporal bone specimens, which lacks age, racial and gender variations.
In the current surgical study, the anatomy of the subcochlear canaliculus was evaluated
and was classified radiologically into four subtypes according to the pattern of its
pneumatization. Lastly, the orientation of the RW, its direction, or at which direction
it faces, was evaluated according to the relation between the different components
comprising the RW prechamber (anterior, posterior pillars, and the tegmen).
The limitations in the present study were that the crista fenestra was evaluated only
for its presence and its shape. This boney crest occupies a considerable area of the
circumference of the RWN. Future studies have to be directed to the assessment of
the relation between RWh, RWw, and the surface area and between the surface area and
the shape of this boney crest among a bigger number of temporal bones, and in the
presence of an accurate way for measurement. Also, the limited number of temporal
bones was another limitation.
