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| ORIGINAL ARTICLE |
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| Year : 2017 | Volume
: 23
| Issue : 3 | Page : 141-145 |
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Significance of round window niche drilling for cochlear implant surgery
Deepak Dalmia, Sanjaya Kumar Behera
Department of ENT, Dr. Babasaheb Ambedkar Memorial Central Railway Hospital, Mumbai, Maharashtra, India
| Date of Web Publication | 31-Aug-2017 |
Correspondence Address:
Deepak Dalmia
Department of ENT, Dr. Babasaheb Ambedkar Memorial Central Railway Hospital, Ranibaugh, Byculla East, Mumbai - 400 027, Maharashtra
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/indianjotol.INDIANJOTOL_43_17
Aim: The aim of this study is to determine the extent of round window niche (RWN) drilling for optimal visualization of the round window membrane (RWM) during cochlear implant surgery. Materials and Methods: This is a prospective study of 38 candidates between 1 and 8 years for cochlear implant surgery from March 2014 to March 2016. We classified the visibility of RWM and its cover by RWN on the surgical view through the posterior tympanotomy during surgery into four types (1) Type 1 - fully visible, (2) Type 2-2/3rd (75%–99%) of RWM visible, (3) Type 3 - ½–2/3rd (50%–74%) of RWM visible, and (4) Type IV - ¼th–½ (25%–49%) of RWM visible. Extent of drilling of RWN into (1) drilling of anterosuperior and superior lip, (2) drilling of anterosuperior, superior, and posterosuperior lip, (3) drilling of the anterior lip with anterosuperior, superior, and posterosuperior lip, and (4) drilling of the posterior lip with Type III drilling. Results: Type-4 RWN is the most common and Type-1 least common. The extent of drilling most commonly performed was Type IV drilling of anterosuperior, superior, and posterosuperior and anterior and posterior lip of RWN. Conclusion: Sufficient drilling of the bony overhang is essential for complete exposure of RWM for insertion of electrodes without any hindrance.
Keywords: Cochlear implant, electrode insertion, round window approach, round window niche
How to cite this article:
Dalmia D, Behera SK. Significance of round window niche drilling for cochlear implant surgery. Indian J Otol 2017;23:141-5 |
| Introduction | |  |
Cochlear implantation is performed in patients with severe to profound hearing loss.[1],[2] It is done both in pre- and post-lingual patients with hearing impairment.[2] The surgical approach is posterior tympanotomy. There are several surgical steps for a cochlear implant with the intention of a round window insertion. They include cortical mastoidectomy, opening the facial recess, round window membrane (RWM) identification, and opening.
The facial recess known as a posterior tympanotomy is a well-established otologic surgical pathway that gains to assess the middle ear without violating the tympanic membrane. Facial recess bounded by vertical segment of the facial nerve medially, the chorda tympani nerve laterally, and the fossa incudis superiorly.
For cochlear implant surgery, proper visualization of round window niche (RWN) and RWM is prerequisite.[3],[4],[5] The round window is usually partially hidden by the bony RWN, and this anatomical landmark must be identified before the bony niche can be drilled away to fully expose the RWM. After complete exposure of RWM, it can be opened to enter the perilymphatic space of the scala tympani by the electrode which can be carefully and slowly inserted.[6],[7]
The ossification of the RWN starts in the 16th fetal week and is complete at birth. A process of the otic capsule, called the cartilage bar, forms the inferior wall of the RWN. The anterior and superior walls of the niche form by intramembranous ossification, whereas the posterior and inferior walls predominantly form by endochondral ossification.[8]
Due to the variation of in the position of round window, most of the time, it is difficult to visualize RWN. Our aim is to determine the extent of RWN drilling for optimal visualization of the RWM before electrode insertion in cochlear implant surgery, to classify the RWN according to its intraoperative visibility, to classify the type of RWN drilling for proper visualization of RWM, and access the accuracy of high-resolution computer tomography (HRCT) temporal bone for predicting type of drilling of RWN.
| Materials and Methods | | |
The study was undertaken in our hospital Dr. Babasaheb Ambedkar Memorial Central Railway Hospital, Mumbai, from March 2014 to March 2016. The Institutional Ethical Committee clearance taken for this study and written informed consent was taken from the patients' parents regarding inclusion in this study. This is a prospective study of 38 candidates aged between 1 and 8 years who were undergone cochlear implant surgery. Written informed consents were taken from parents of all the children, and all the patients had severe to profound bilateral hearing loss. The patients were referred for HRCT of temporal bone and magnetic resonance imaging of brain for cochleovestibular nerve complex.
The cochlear implant surgery was performed by a single surgeon (main author) who has 10 years of experience in cochlear implant surgery. HRCT measurement was done by single reader in the institution who has experienced 10 years of HRCT of temporal bone along with the main author. The computed tomography (CT) scan contained high-resolution images that were 0.7 mm thick with reconstructed coronal and sagittal and axial images. The images were viewed in the standard bone window setting.
The CT scan was analyzed mainly for the degree of round window bony overhang, height of jugular bulb and facial nerve with inner ear anomaly, and vestibular aqueduct. The degree of round window bony overhang relates to difficulties associated with round window assess. This was correlated to the intraoperative scores of difficulties with assess to facial recess.
The degree of round window bony overhang was measured by assessing four consecutive axial cuts, beginning with the most inferior cut showing the RWN, and proceeding superiorly for RWM.
We classified the visibility of RWM and its cover by RWN on the surgical view through the posterior tympanotomy (facial recess) during surgery into four types:
- Type 1 - fully visible
- Type 2-2/3rd (75%–99%) of RWM visible
- Type 3 - ½–2/3rd (50%–74%) of RWM visible
- Type 4 - ¼th–½(25%–49%) of RWM visible.
Extent of drilling of RWN into:
- Drilling of anterosuperior and superior lip
- Drilling of anterosuperior, superior, and posterosuperior lip
- Drilling of anterior lip with anterosuperior, superior, and posterosuperior lip
- Drilling of posterior lip with Type III drilling.
| Results and Statistical Analysis | | |
Our study included the cases of age ranging from 1.50 to 8.0 years with average age being 4.50 years. Male were 57.9% of case, and 42.1% were female [Table 1]. Preoperative HRCT of temporal bone was done in all the cases to assess the extent of RWN covering the RWM obscuring its visibility. The degree of round window bony overhang was compared between the four groups by assessing four consecutive axial cuts beginning with the most inferior cuts showing the RWN then proceeding superiorly for cuts showing the RWM. The number of cuts showing full thickness bony overhang around the round window was counted out of four. Suppose if there are three slices showing bony overhang it was measured as – or 0.75 [Figure 1],[Figure 2],[Figure 3]. This variable was then compared to the intraoperative drilling of RWN through posterior tympanotomy. Enlarged vestibular aqueduct was seen in two patients, and Mondini (incomplete partition II) was seen in two patients, and the rest were having normal cochlea [Table 2]. | Figure 1: Four consecutive axial cuts of high-resolution computer tomography of temporal bone left side for round window niche assessment. Preoperative score 0.75
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 | Figure 2: Four consecutive axial cuts of high-resolution computer tomography of temporal bone left side for round window niche assessment. Preoperative score 1
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 | Figure 3: Four consecutive axial cuts of high resolution computer tomography of temporal Bone right side for round window niche assessment. Preoperative score 0.5
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Correlation between HRCT of temporal bone with type of drilling by Chi-square test showed that with increasing in preoperative score of HRCT, type of drilling also increases (P < 0.05).
- Type 1 RWN - fully visible RWM was seen in 2 (5.26%) patients [Figure 4]a
- Type 2 RWN - 2/3rd (75%–99%) of RWM visible in 4 (10.53%) patients [Figure 5]a
- Type 3 RWN - ½–2/3rd (50%–74%) of RWM visible were seen in 9 (23.68%) cases [Figure 6]a
- Type IV RWN - ¼th–½ (25%–49%) of RWM visible were seen in 23 (60.53%) cases [Figure 7]a and [Table 3].
 | Figure 4: (a) Type I round window niche. (b) Drilling of anterosuperior an superior lip
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 | Figure 5: (a) Type II round window niche. (b) Drilling of anterosuperior, superior, and posterosuperior lip
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 | Figure 6: (a) Type III round window niche. (b) Drilling of anterosuperior, superior, posterosuperior, and complete anterior lip
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 | Figure 7: (a) Type IV round window niche. (b) Drilling of posterior lip with anterosuperior, superior, posterosuperior, and complete anterior lip
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The extent of drilling of RWN was found type:
- Drilling of anterosuperior and superior lip was done in 2 (5.26%) cases [Figure 4]b
- Drilling of anterosuperior, superior, and posterosuperior lip was done in 3 (7.89%) cases [Figure 5]b
- Drilling of anterior lip with anterosuperior, superior, and posterosuperior lip done 10 (26.32%) cases [Figure 6]b
- Drilling of posterior lip with Type III drilling was done in 23 (60.53%) cases [Figure 7]b and [Table 4].
Chi-square test showed (P = 0.007) there was a significant correlation between type of RWN and type of drilling as type of RWN increases drilling also increases [Table 5].
We used Nucleus-24 cochlear implant device in 18 (47.37%) cases advanced bionic in 7 (18.42%) cases and Digisonic-SP in 13 (34.21%) cases [Table 6].
| Discussion | | |
Preoperative HRCT of temporal bone is must for patients undergoing cochlear implantation.[1],[8],[9] These images are routinely reviewed by the ear, nose, and throat surgeon before surgery to visualize if there are no anatomical abnormalities to implant insertion. By this, the surgeon is better prepared intraoperatively.
There can be a high degree of variability in how the preoperative CT scan is analyzed by the individual surgeon.[1],[9],[10],[11],[12],[13],[14],[15] However, round window assessment intraoperatively is paramount importance during RWM exposure.[6],[9],[11]
A thick bony overhang (RWN) often precludes the true location and orientation of the round window. This overhang must be drilled away to expose the round window and to allow smooth insertion of the electrode into the scala tympani of the cochlea. RWN overhang was assessed radiologically by assessing four consecutive axial cuts of the preoperative CT scan as well as intraoperatively.
RWM cannot be fully exposed without drilling of the bony overhangs because its size and orientation of the opening vary.[2],[6],[9] We classified the visibility of the RWM through posterior tympanotomy approach into four types with Type IV - ¼th–½ (25%–49%) of RWM visible seen in 23 patients (60.53%) was the most common type of RWN overhang and Type 1 - RWM fully visible was least common seen in two patients (5.26%).
The extent of drilling most commonly performed was Type IV drilling of anterosuperior, superior and posterosuperior, and anterior and posterior lip was in 23 patients for complete visualization of RWM (60.53%) and Type I RWN drilling was done in 2 (5.26%) patients. Chi-square test showed that there was a significant correlation between type of RWN and type of drilling, as type of RWN increases type of RWN drilling for proper visualization of RWM also increases.
| Conclusion | | |
There are several principles for correct electrode insertion through the RWM. This approach requires a comparatively large posterior tympanotomy for sufficient exposure of the posterior aspect of round window through the facial recess. For atraumatic electrode insertion, proper exposure of RWM by drilling of RWN is necessary for anterior lip with anterosuperior, superior, and posterosuperior lip with posterior lip depending on the bony overhang.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
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