Abstract
With increasing number of patients with residual hearing being implanted, there is a renewed interest in round window (RW) as the preferred route for electrode insertion to reduce intracochlear trauma. The degree of round window membrane (RWM) visibility and its orientation might hamper the accessibility of RW for electrode insertion. This study is an attempt to identify the various factors affecting the accessibility of RW for electrode insertion. 30 children fulfilling the CI candidacy criteria were recruited for the study. All the surgeries were performed by the standard posterior tympanotomy technique. Round window membrane (RWM) visibility was graded into four types from grade I to grade IV. The membrane visibility was assessed prior to niche drilling. Grade III RW was the most common type. RW insertion could be achieved in 66.7% cases, while extended RW was performed in 33.3% cases. Bony cochleostomy was not required in any of the cases. There was no significant association between grade of RWM visibility and the route of electrode insertion. RW insertion could be achieved in most cases with normal cochlear anatomy. We observed that the grades of RWM visibility did not affect the route of electrode insertion. RW insertion can be performed in a manner that is potentially less traumatic by following the surgical steps meticulously. Both RWM visualization and insertion angle can be improved with careful bone removal in RW niche region making the round window a viable option, when minimizing insertion trauma in patients.
Keywords: Cochlear implant surgery, Round window membrane visibility, Electrode insertion route
Introduction
The candidacy criteria for cochlear implantation have expanded significantly from bilateral severe to profound sensorineural hearing loss to include cases with residual hearing. Preservation of residual hearing is crucial when implanting these patients [1]. Soft surgery technique with round window insertion has played an important role in hearing preservation during cochlear implantation [2]. However, while considering the RW insertion route, it is important to have adequate RWM visibility following an optimum posterior tympanotomy. The present study was undertaken to identify the various anatomical and surgical factors affecting RWM visibility and to study the effect of round window visibility on the route of electrode insertion.
Materials and Methods
This was an observational study carried out at a tertiary care centre in Central India. Prior approval from the institutional ethics committee was obtained. 30 children with bilateral severe to profound hearing loss fulfilling the CI candidacy criteria were included in the study. A detailed clinical assessment including complete audiological and radiological workup was done. Children with cochlear anomalies detected on preoperative imaging were excluded from the study. All the surgeries were performed under the Govt. funded program and by a single surgeon. Implants from two different companies provided under the scheme were used. Cochlear implantation was performed by the standard posterior tympanotomy technique.
The following surgical steps were undertaken for optimum round window membrane visibility.
Cortical mastoidectomy with thinning of the outer posterior canal wall.
Identifying the facial and the chorda tympani nerve.
Creating a wide posterior tympanotomy.
Removing the lateral bony overhang of sinus tympani anterior to facial nerve and inferior to pyramid.
Meticulously drilling the bony overhang of the RW niche.
If still insufficient room, 1 mm of bone anteroinferior to RW annulus was removed.
Direction of electrode insertion was from posterosuperior to anteroinferior.
RWM visibility was graded into 4 types from grade I to grade IV (Fig. 1). In grade I > 50% of RWM is visible, grade II 50 − 25% RWM is visible, in grade III < 25% RWM is seen and in grade IV 0% RWM is visible [3]. Intraoperatively, various parameters like the thickness of posterior canal wall, facial recess dimensions– anteroposterior and craniocaudal, round window size, shape and orientation were measured. Facial recess dimensions were measured with sterile Schirmer’s strips. Posterior canal wall thickness was measured with vernier caliper (Fig. 2). Depending on the RWM visibility, its orientation and implant type the route of electrode insertion was selected.
Fig. 1.
Classification for RWM visibility (Grade I to Grade IV)
Fig. 2.
Intra-operative measurement of different parameters. a) posterior tympanotomy- anteroposterior dimension. b) posterior tympanotomy- craniocaudal dimension. c) & d) posterior canal wall thickness measured after drilling
Observations and Results
30 children under 5 years of age were included in the study. Out of the total children, 47% were male and 53% were female. The mean age at implantation was 4.2 years. Implants from two different companies provided under the Govt funded program were used. The CI 422 slim SRA electrode from Cochlear corporation was implanted in 57% patients while in 43% the MEDEL Concerto 2Mi 1050 STD implant was used.
Depending upon the RWM visibility to the surgeon through the posterior tympanotomy the grading was done into four types from grade I to grade IV. RWM visibility was assessed before niche drilling. Grade I RW with visibility > 50% was seen in 20% (6/30) cases, grade II RW (25–50% visibility) was seen in 10% (3/30) cases. Grade III RW (< 25% visibility) was present in 56.7% (17/30) cases, while grade IV RW (0% visibility) was noted in 13.3% (4/30) patients (Table 1). Grade III RW was the most common type encountered in our pediatric population.
Table 1.
Distribution of patients based on round window membrane visibility
| Grade of RWM visibility | RWM visibility | No of patients | Percentage |
|---|---|---|---|
| Grade I | > 50% | 6 | 20% |
| Grade II | 50 − 25% | 3 | 10% |
| Grade III | < 25% | 17 | 56.7% |
| Grade IV | 0% | 4 | 13.3% |
| TOTAL | 30 | 100% |
Surgical steps were undertaken to achieve maximum exposure of RW. Different parameters were measured. The outer posterior canal wall was straightened and thinned to a mean of 1.16 ± 0.4.mm (range 0.5–2.0 mm). An optimum posterior tympanotomy was performed after identification of facial and chorda tympani nerve. The facial recess dimensions were measured in both craniocaudal and anteroposterior directions. The craniocaudal dimension achieved was 4.58 ± 0.66 mm (range 3.5–6 mm) and the anteroposterior dimension was 2.66 ± 0.48 mm (range 2–3.5 mm). The characteristics of RW were also noted. The mean height of RW was 1.09 ± 0.24 mm (0.6–1.6 mm) while the median width was 1 mm (Interquartile range 0.6–1.3 mm) (Table 2). In 53% cases the shape of RW was circular, in 40% cases it was oval and in 7% patients kidney shaped RW was seen. In 57% patients the orientation of RW was favorable (angle < 45 degree) while 43% cases it was not favorable (> 45 degree). Depending upon the amount of RW niche overhang, drilling was performed to achieve complete exposure of RWM. Drilling of the superior lip of niche was required in all (100%) cases, while anterior and posterior niche drilling was done in 66% and 85% cases respectively.
Table 2.
Various parameters studied to achieve optimum round window membrane visibility
| Parameters | Mean + SD | Range |
|---|---|---|
| Thickness of outer posterior canal wall | 1.16 ± 0.40 mm | 0.5–2 mm |
| Facial recess craniocaudal dimension | 4.58 ± 0.66 mm | 3.5–6 mm |
| Facial recess anteroposterior dimension | 2.66 ± 0.48 mm | 2–3.5 mm |
| RW height | 1.09 ± 0.24 mm | 0.6–1.6 mm |
| RW width | Median = 1 mm | IQR = 0.6–1.3 mm |
In the cases with grade I and grade II RWM visibility, electrode array could be easily inserted through RW in 66.7% cases in each category, while in remaining 33.3% cases extended RW was performed. In both grade III and IV category, RW insertion could be achieved in > 50% cases (52.9% and 50% cases respectively) (Fig. 3). Cochleostomy was not required in any of the cases. There was no statistically significant difference in grades of RWM visibility and route of electrode insertion (p value = 0 0.942, not significant), implying that the route of electrode insertion was not influenced by the grade of RWM visibility (Table 3).
Fig. 3.
Graph showing the relation of the grades of RWM visibility with the routes of electrode insertion
Table 3.
Electrode insertion routes according to the grading
| GRADE OF ROUND WINDOW VISIBILITY | Round window insertion | Extended round window insertion | Total | |||
|---|---|---|---|---|---|---|
| No of pts | % | No of pts | % | No of pts | % | |
| I | 4 | 66.7% | 2 | 33.3% | 6 | 20% |
| II | 2 | 66.7% | 1 | 33.3% | 3 | 10% |
| III | 9 | 52.9% | 8 | 47.1% | 17 | 56% |
| IV | 2 | 50% | 2 | 50% | 4 | 13.3% |
| TOTAL | 17 | 56.7% | 13 | 43.3% | 30 | 100% |
Chi-Square test − 0.5350, P Value = 0.942, Not significant
We could identify factors which were important for decision making and for selection of the most appropriate route. These were the size of the round window after niche drilling, the orientation of RW and the implant design. The patients with smaller size, non-favorable orientation of RW and electrode array with a wider diameter at its proximal end required an extended RW approach to achieve smooth and complete electrode insertion. Intraoperative C arm was done in all cases to confirm the position of the electrode array in the cochlea. In difficult cases with grade III or grade IV RWM visibility we did not encounter any complications like tip fold over or scalar excursions of electrode array.
Discussion
In the early 1980’s, CI electrodes were inserted through the round window, in the mid 1980’s Prof Graeme Clark found that the crista fenestra frequently prevented smooth insertion of the cochlear multichannel electrode as its contacts were raised above the level of the silastic and could get easily caught on the crista. And the new bony cochleostomy was recommended to bypass the crista and to have the direct trajectory into the scala tympani of the cochlea. However, standard bony cochleostomy may be associated with the risk of perilymph loss, acoustic trauma, contamination with bone dust, osseus spiral lamina injury and so forth. With the advancement in the electrode design and expansion in the candidacy criteria the focus shifted back to RW insertion.
The benefits of RW insertion for cochlear implantation have been well studied [2, 4, 5]. The round window represents a natural door to scala tympani. It reduces the amount of drilling required, decreases the risk of acoustic trauma, minimizes the loss of perilymph, decreases the entry of bone dust into the scala tympani. In addition, the round window insertion has been found to increase the length of basal spiral lamina available for stimulation by approximately 2 mm relative to standard bony cochleostomy insertion [6]. RWM cochleostomy is a preferred technique and has been associated with improved hearing preservation, however unusual circumstances may require a bony cochleostomy. These are the small RW, obscure RW, cochlear ossification, cochlear rotation, high jugular bulb and anomalous facial nerve implying that the type of cochleostomy should be specific to the patient’s anatomy and the electrode type.
Different classifications have been proposed to anticipate the preferred route for electrode insertion based on the degree of RWM visibility through standard posterior tympanotomy. The St Thomas’ Hospital (STH) classification by Leong et al. graded the RWM visibility after niche drilling into type I, type IIa, type IIb, type III. 96% of type I and 63% of type IIa cases underwent the membranous cochleostomy, 71% of type IIb necessitated an extended membranous cochleostomy while all type III required a conventional bony cochleostomy [2]. Panda et al. classified the RWM visibility before niche drilling into 4 types, grade I to grade IV. The authors observed that RWM insertion could be achieved in all cases of grade I RW, in grade II minimal drilling of RWN was required followed by RWM insertion. In Grade III and IV either an extended round window or a standard cochleostomy was needed. They proposed that this classification prepares the surgeon to have a prior idea about the extent of drilling required to achieve RWM insertion [3]. We observed that grade III RW was the most common type of RW and the grade of RWM visibility does not affect the selection of the route for electrode insertion (P value = 0.942, not significant).
Various anatomical studies of the cadaveric temporal bone showed that the angulation of RWM was appropriate for the RWM cochleostomy in approximately 90% of the cases [7]. We observed favorable RW orientation in 57% cases and unfavorable in 43% patients.R W angulation of > 45 degree inferiorly resulted in electrode trajectories that caused insertion impact of the electrode tip on the modiolus. When the electrode tip impacts the modiolus, causing the tip to ride up the bone and too often translocate into scala vestibuli [8]. Studies done to see the effect of various trajectories on the tissue preservation in the cochlear implant surgery showed that cochleostomy approaches often traumatized the soft tissue of hook region and for optimal structural preservation of the RW approach is recommended [9].
In the present study surgical steps were undertaken to get an optimum exposure of the round window membrane through the facial recess. The management of the outer bony ear canal is crucial in difficult cases. The outer bony canal should be thinned until the origin of the chorda tympani and the facial nerve is identified. Opening the facial recess adequately at the chordo-facial angle and as wide as possible. RW niche can be identified after opening the inferior facial recess. The bone of fallopian canal anterior to the facial nerve should be removed to increase the posterior exposure of promontory. In cases where the RW is positioned more posteriorly in relation to the vertical segment of the facial nerve, adjusting the position of the operating table can help to provide a better view of the RWM. RWM is in a small niche called fossula fenestra rotunda and often hidden behind the bony ridges from superior, posterior and anterior which limits the visibility of RWM during surgery [10]. Meticulous drilling of the bony overhang of RWN without breaching the RWM should be done to achieve complete exposure of RWM. RW niche drilling is needed not only for better visibility, but also it allows insertion along the midscala axis of the scala tympani and thereby protecting important structures such as modiolus and basilar membrane. The axis of electrode insertion is important to facilitate safe insertion into scala tympani. During electrode placement, once the tip is inserted into the RW, the electrode should be carefully rotated in the posterosuperior direction and advanced with the insertion angle in the anteroinferior direction. This step is highly crucial for successful hearing preservation [6].
The view of the RWM through posterior tympanotomy is incomplete because of its orientation and conical shape. The RWM is placed at 90 degrees angle to the oval window, has a bony overhang and is conical in shape with the horizontal posterosuperior part which is closer to the osseous spiral lamina and the vertical membranous anteroinferior part through which the electrode array is inserted [11, 12]. The position of the RW niche opening and size and shape of the RWM are variable in each individual and have implications relative to the site and type of electrode insertion [13, 14]. The maximum height of the round window ranged from 0.6 to 1.6 mm (mean of 1.03 ± 0.24 mm). The interquartile range of the width of RW was 0.6–1.3 mm (median = 1 mm).
A good view of basal turn is obtained after the antero- inferior overhang of the RW niche and the crista fenestra have been removed [15]. Superior niche drilling was required in all 100% cases; posterior lip was drilled in 65% while anterior lip was removed in 66% cases to completely expose the round window membrane. A 360 degrees exposure of RWM would ensure good visualization of RWM and a correct electrode insertion trajectory into the scala tympani. Care should be taken while drilling the posterosuperior overhang of the RWN because of its proximity to the modiolus. The shape of the RWM was round in maximum number of patients (53%), oval in 40% and kidney shaped in 7% cases. Most of the studies reported in the literature, the temporal bone parameters have been measured in the cadaveric temporal bone. This is probably one of the first studies in which the parameters were measured intraoperatively.
The result of this study demonstrates that in children with normal cochlear anatomy, round window insertion could be achieved in maximum number of cases irrespective of the grade of round window membrane visibility. Extended round window was required in our cases implanted with Concerto 2 MI1050 STD implant to accommodate the electrode array with broader diameter (1.3 mm) at its base. Size of the RWM after niche drilling, its position in relation to the vertical portion of facial nerve and the electrode design selected would be the deciding factors for the selection of insertion portal.
Conclusion
RWM insertion is a step towards preserving residual hearing. Advances in electrode design and surgical techniques have considerably reduced the insertion trauma. A complete facial recess dissection with skeletonization of facial and chorda tympani nerve is required to gain better access to RW. Surgical preparation of the round window niche is crucial for optimum RWM visibility. When summing up the data of our study, all grades of RW could be managed with RW or extended RW insertion. We believe that extremely careful preservation and exposure of all anatomical structures mentioned by experienced surgeons can contribute to the high rate of RWM visibility and to achieve RW insertion.
Author Contributions
All authors contributed substantially to the paper in conception, design and writing.
Funding
No funding sources.
Declarations
Ethical Approval
The study was approved by the institutional review board (Local ethics committee).
Informed Consent
Informed consent was obtained from parents of all children.
Disclosure
This work has never been published and is not currently under evaluation in any other peer reviewed journal.
Conflict of Interest
The authors declare that they have no conflict of interest.
Footnotes
Publisher’s Note
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References
- 1.Konrad Stuermer,Tanja Winter, Nachtsheim L, Klussmann JP, Jans Christoffer Leurs (2021) Round window accessibility during cochlear implantation European archives of Oto-rhino-laryngology. 278(2):363–370 [DOI] [PubMed]
- 2.Annabelle CL Dan Jiang.Andress agger.Alec Fitzgerald-O,Connor (2013) evaluation of round window accessibility to cochlear implant insertion. Eur Arch Otorhinolaryngol; 270 (4):1237–1242 [DOI] [PubMed]
- 3.Naresh Panda M, Kameswaran SK, Patro, Saran S, Gyanranjan N (2017) Evaluation of round window accessibility for electrode insertion: validation study from two centres. J Otolaryngology- ENT Res: 8, Issue 5
- 4.Robert JS, Briggs,Michael Tykocinski J, Xu F, Risi M, Svehla R, Cowan T, Stover P, Erfurt T Lenarz (2006) Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode, vol 11. Audio Neurotol, pp 42–48 [DOI] [PubMed]
- 5.Seong-Chang Bae Y-R, Shin Y-M Chun (2019) Cochlear implant surgery through round window approach is it always possible; annals of Otology. Rhinology Laryngology 128(65):385–445 [DOI] [PubMed] [Google Scholar]
- 6.Paprocki A et al (2004) The topographical anatomy of the round window and related structures for the purpose of cochlear implant surgery. Folia Morphol (Warsz) 63(3):309–312 [PubMed] [Google Scholar]
- 7.Yisgav Shapira,Adrien A, Esraghi T, Balkany (2011) The perceived angle of the round window affects electrode insertion trauma in round window insertion– an anatomical study. Acta Otolaryngol 131:284–289 [DOI] [PubMed]
- 8.Nadine Schart-Moren et al et al (2019) Effects of various trajectories on tissue preservation in cochlear implant surgery: a micro- computed tomography and synchroton radiation phase contrast imaging study. Ear hear. 40(2):393–400 [DOI] [PubMed]
- 9.LeursJC HKB, Beutner D (2018) Surgical anatomy of the round window- implications for cochlear implantation. Clin Otolaryngol 43(3):417–424 [DOI] [PubMed] [Google Scholar]
- 10.Roland PS, Wright CG, Isaacson (2007) Cochlear implant electrode insertion: the round window revisited. Laryngoscope 117(8):1397–1402 [DOI] [PubMed]
- 11.Nomura Y (1984) Otological significance of the round window. Adv Oto-rhino- laryngology 33:1–162 [PubMed] [Google Scholar]
- 12.TakahashiH, Sando I (1990) Computer aided 3- D temporal bone 3 D temporal bone anatomy for cochlear implant surgery. Laryngoscope 100(4):417–421 [DOI] [PubMed] [Google Scholar]
- 13.Tóth M, Alpár A, Patonay L, Oláh I Development and surgical anatomy of the round window niche. Ann Anat. 188:93–101. [DOI] [PubMed]
- 14.Aslan A, Tekdemir I, Gunhan K, Eskiizmir G, Elhan A(2006) Anatomic observations on variations of the round window niche and its relationship to the tympanic membrane. Mediterr J Otol. 2:52–57.
- 15.B K, Franz GM, Clark DM, Bloom (2006) (1987) Surgical anatomy of round window with special reference to cochlear implant. J Laryngol, Otol101 (2):99–102 [DOI] [PubMed]