
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12366-3
10.1016/j.heliyon.2024.e36335
e36335
Research Article
Innovative computed tomography based mapping of the surgical posterior tympanotomy: An exploratory study
Vranken Brecht a1
Schoovaerts Maarten b1
Geerardyn Alexander b
Kerkhofs Lore b
Devos Johannes c
Hermans Robert c
Putzeys Tristan b2
Verhaert Nicolas Nicolas.verhaert@kuleuven.be
bd⁎2
a Faculty of Medicine, KU Leuven, Herestraat 49, 3000 Leuven Belgium
b ExpORL, Department of Neurosciences, KU Leuven, Herestraat 49, 3000 Leuven, Belgium
c Department of Radiology, University Hospitals Leuven, Herestraat 49, 3000 Leuven, Belgium
d Department of Otorhinolaryngology – Head & Neck Surgery, University Hospitals Leuven, Herestraat 49, 3000 Leuven, Belgium
⁎ Corresponding author. University Hospitals Leuven Department of oto-rhino-laryngology, head and neck surgery Prof. Dr. Nicolas Verhaert Herestraat 49 3000 Leuven Belgium. Nicolas.verhaert@kuleuven.be
1 joint first author.

2 joint last author.

14 8 2024
30 8 2024
14 8 2024
10 16 e3633519 3 2024
12 8 2024
13 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Robotic devices have recently enhanced cochlear implantation by improving precision resulting in reduced intracochlear damage during electrode insertion. This study aimed to gain first insights into the expected dimensions of the cone-like workspace from the posterior tympanotomy towards the round window membrane.

This retrospective chart review analyzed ten postoperative CT scans of adult patients who were implanted with a CI in the past ten years. The dimensions of the cone-like workspace were determined using four landmarks (P1–P4). In the anteroposterior range, P1 and P2 were defined on the edge of the bony layer over the facial nerve and chorda tympani nerve, respectively. In the inferosuperior range, P3 was defined on the bony edge of the incus buttress and P4 was obtained at a distance of 0.45 mm between the facial nerve and the chorda tympani nerve. After selecting the landmarks, the calculations of the dimensions of the surgical access space were done in a standardized coordinate system and presented using descriptive statistics.

The cone-like space is limited by two maximal angles, α and β. The average angle α of 19.84 (±3.55) degrees defines the angle towards the round window membrane between P1 and P2. The second average angle β of 53.56 (±10.29) degrees defines the angle towards the round window membrane between P3 and P4. Based on the angles the mean anteroposterior range of 2.25 (±0.42) mm and mean inferosuperior range of 6.73 (±2.42) mm. The distance from the posterior tympanotomy to the round window membrane was estimated at 6.05 (±0.71) mm.

These findings present data on the hypothetical maximum workspace in which a future robotically steered insertion tool can be positioned for an optimal automated electrode insertion. A larger sample size is necessary before generalizing these dimensions to a population. Further research including preoperative CT scans is needed for planning robotic-steered cochlear implantation.

Highlights

• Dimensions of the posterior tympanotomy for future robotic insertion tools.

• This study maps the posterior tympanotomy in a standardized 2D and 3D way.

• The surgical access space for cochlear implantation is cone-shaped.

• Dimensions of posterior tympanotomy based on 10 postoperative CT scans.

Keywords

Posterior tympanotomy
Cochlear implant surgery
Dimensions
Three-dimensional
Robotics
==== Body
pmc1 Introduction

A cochlear implant (CI) is a surgically implantable medical device that directly stimulates the cochlear nerve by an electrode array in the cochlea. During CI surgery, the primary goal is the insertion of the electrode array into the scala tympani without damaging the intracochlear structures such as the basilar membrane, the osseous spiral lamina or the spiral ligament. Traditionally, the surgeon inserts the electrode array using their mental representation of the insertion axis based on their experience, visual assessment and pre-operative computed tomography (CT) scans. The major limitation is that the cochlea is fully surrounded by bone which makes surgeons insert the electrode array blindly since the pre-operative CT scans are not detailed enough to identify intracochlear microstructures. Recently, new imaging techniques have been explored in a research manner to visualize the tiny intracochlear structures [1,2]. Also, mechanical models have shed light on the specific insertion angle of electrode arrays [3].

Mechanical damage can lead to an inflammation reaction and the formation of fibrotic tissue [[3], [4], [5]]. This induces necrosis and/or apoptosis and causes a loss of the preserved auditory hair cells responsible for residual hearing [6]. It has been demonstrated that patients with preserved postoperative residual hearing have improved speech perception with electro-acoustic or electro-vibrational stimulation [7,8]. Besides, patients without preserved preoperative residual hearing can also benefit from atraumatic electrode array insertion because direct mechanical damage to the intracochlear structures can induce loss of ganglion cells [[9], [10], [11], [12]]. Furthermore, electrode array insertion or translocation into the scala vestibuli can result in a potential stimulation of ganglion cells in more than one cochlear turn. This cross-turn stimulation leads to pitch confusion and reduced speech perception [9,13].

Over the past few years, there have been many developments in the use of robotic devices in CI surgery with the aim of minimal invasive cochlear implantation and reduced intracochlear trauma. Robotic devices can improve CI surgery by assisting the surgeon during several steps of surgery [14]. First, robotics can reduce the invasiveness of the standard surgical procedure by approaching the round window through minimally invasive direct cochlear access and drilling automatically along a pre-operative planned path [15,16]. Second, the robot can insert the electrode array with a programmed optimal insertion vector [16,17]. Finally, electrode array insertion devices can be attached to the robot [18]. At present, five robotic systems are in use for clinical applications. Three systems (Microtable® Vanderbilt system, HEARO® system, and ROSA® system) use minimally invasive direct cochlear access to approach the round window [14]. The fourth system (iotaSOFT™Insertion System) does not use an automated drilling system to approach the round window. It is a pedal-controlled automated insertion device temporally screwed to the squamous cortex of the temporal bone contributing to a slow and atraumatic electrode array insertion [19,20]. The fifth system (RobOtol®) is also not an automated drilling system. The RobOtol® is a teleoperated robotic arm with a motorized insertion tool attached [21,22]. This assists the surgeon in inserting the electrode array along an optimal insertion axis based on calculations on preimplantation cone-beam CT images resulting in reduced intracochlear insertion trauma [14,17]. Gauging the surgical space towards the cochlea in which these robots can work is important for the optimization of their functionality.

Robotic systems overcome human limitations of the surgeon such as tremor and technique variability [23]. Furthermore, robotic insertion has a higher accuracy in comparison to manual insertion and contributes to the optimization of the insertion axis [24,25]. Ideally, the electrode array is inserted into the central axis of the scala tympani to prevent early contact with the cochlea's lateral wall, which could result in intracochlear trauma [17]. However, this ideal insertion axis is rarely achievable in a straight line through the posterior tympanotomy (PT) because the facial nerve (FN) frequently intersects with this ideal insertion vector. The optimal insertion axis approaches the ideal insertion vector as closely as possible [26]. A larger difference between the ideal and optimal insertion axis is associated with more damage to the intracochlear structures [17]. Finally, motorized insertion tools have the potential to maintain a constant insertion speed below human limits [18]. This results in a reduction of both the occurrence of force peaks and their amplitude. Each force peak has the potential to damage intracochlear structures, with the risk proportional to the force magnitude [18].

Previous studies have shown that the posterior access toward the round window is crucial to attain proper access to the cochlea [27]. In the development of atraumatic insertion and diagnostic or therapeutic features to the cochlea, the dimensions of the surgical PT are very important. It defines the maximal space in which an automated insertion tool can be manipulated. The PT is bordered anteriorly by the chorda tympani nerve (CTN), posteriorly by the mastoid segment of FN, and superiorly by the incus buttress [28]. The size of this surgical PT and the visibility of the round window are patient-specific. The anteroposterior angle and the inferosuperior angle define the area of the surgical PT. This area together with the distance from the PT to the round window membrane (RWM) forms a cone-like volume which is the anatomical dimension that defines the size of the surgical access space to the RWM. In this cone-like volume, the different insertion angles and ranges can be measured. In literature, the real size of the surgical PT measured on postoperative CT scans is very scarce. Only the radiological PT is in previous works described as the direct distance between the FN and CTN measured on a preoperative CT scan or a maximal drilled-out PT on cadaver studies without considering the bony layer over these two nerves during surgery [28,29].

This study aimed to map the surgical access space from the PT towards the cochlea. It is crucial to determine these dimensions in a standardized way in every patient. Therefore, we analyzed CT scans of patients with a CI in a retrospective format. On CT scans we segmented the important landmarks in three-dimensional space. To quantify the relative position between structures, we defined anatomical planes according to the coordinate system of Verbist et al. and selected important landmarks such as the basal turn of the cochlea, the RWM, the center of the modiolus and the top of the horizontal semicircular canal [30]. The result is a cone-like maximum workspace with an angle in both the longitudinal and the transverse direction and a distance from the surgical PT plane to the RWM.

2 Material and methods

2.1 Patient selection

Ten adult patients who were implanted with a CI between March 31st, 2012 and April 1st, 2022 at the oto-rhino-laryngology, head and neck surgery department of the university hospitals Leuven were included in the study. All the implanted patients had a normal cochlea shape and there were no congenital anomalies. All the included patients had a postoperative CT scan as part of the clinical routine. We chose to only include RWM and extended RWM insertions as a surgical procedure because the geometry of the cochlea would not or only be slightly affected by drilling out the bony overhangs of the round window niche. Cochleostomy approaches were excluded. No structural damage to the FN or the CTN was reported. None of the patients had a medical history of invasive ear surgery involving mastoid drilling procedures prior to cochlear implantation surgery nor a history of other non-otological medical implants/stents/drains in the head and neck region which could result in CT artifacts.

2.2 Clinical characteristics

The age of the ten included patients at the time of surgery varies between 35 and 79 years old, with a mean (±SD) age of 58.9 ± 15.2 years old. The study maintained a gender balance important for the anatomical variation between genders [31]. Four of them were female patients and six were male patients. All patients were of European origin. All patients had a unilateral CI, seven on the left side and the other three on the right side. The specifications of their CI vary from one another. Three patients had the HiRes™ Ultra 3D CI of Advanced Bionics with a Slim J straight electrode array. The other seven patients had Nucleus® implants of Cochlear™. The different types are CI522 (n = 1), CI532(n = 1), CI622(n = 3) and CI632(n = 2). Three out of seven Nucleus® implants had a modiolar type of electrode array. The other four had a straight electrode array.

2.3 Data origin

The ten pseudonymized CT scans of the mastoid were stored as DICOM files. The axial CT images of the temporal bone were obtained with SOMATOM Force, Siemens Healthineer, parallel to the nasion-biauricular plane and were acquired at 110 kV and 150 mA in UHR-mode with a matrix size of 512 x 512. The CT images had a slice thickness of 0.2 mm and the in-plane pixel size varied between 0.16 and 0.24 mm, matching the used Ur73-kernel with a maximum resolution of 23 lp/cm (0.217 mm).

2.4 Segmentations

DICOM files were imported into Avizo Software version 2019.1 (Thermo Fisher Scientific Inc.). The following structures were segmented manually in a fixed order (Fig. 1) by one author (BV) and afterward confirmed by two others (AG, LK). First, we started every segmentation with the electrode array to demarcate it from the segmentation of the cochlea. The metallic electrode array is higher in density compared to the cranial bones. Next, we segmented the relatively hypodense cochlea, vestibulum, and semicircular canals. We located subsequently the FN and the CTN. These nerves are also hypodense structures but in their passage through the temporal bone. The contours are not always visible through the air-filled cavities and the small size of CTN means its location often needs to be inferred. We approximated the location of the nerves based on adjacent slices where the nerves are visible. After defining the hypodense structures in the temporal bone, we segmented the temporal bone with attention to the bony boundaries of the mastoidectomy and the PT. Finally, between the bony boundaries of the surgical PT, the volume of the PT was segmented with a thickness according to the size of the FN.Fig. 1 Flowchart of the segmentation process. Facial nerve (FN). Chorda tympani nerve (CTN). Posterior tympanotomy (PT).

Fig. 1

2.5 Transformation of the planes

After the segmentation of the anatomical structures in the temporal bone, we made a multiplanar reconstruction according to three-dimensional cylindrical the coordinate system of Verbist et al. starting from a radiological standardized transverse, coronal and sagittal plane [30]. The xy-plane is aligned with the basal turn of the cochlea and the z-axis is defined through the center of the modiolus. The zero-degree reference angle is chosen at the top of the horizontal semicircular canal and defines the axis x. This landmark is outside the level of the round window because the normal anatomy can be distorted by surgery or metallic artifacts from the CI on postoperative CT scans. Each point can be defined by the θ (degrees), ρ (radius) and z (height). The reconstruction in a cartesian coordinate system defines a xy-plane through the basal turn of the cochlea and perpendicular to the axis through the center of the modiolus. The second yz-plane is reconstructed through the center of the modiolus and the top of the horizontal semicircular canal. The third xz-plane will be perpendicular to planes xy and yz.

2.6 Calculations of the dimensions

The dimensions of the PT define the surgical access space towards the cochlea based on bony landmarks. We segmented six additional boxes with a size of 2 × 2 × 2 voxels that define the boundaries of the surgical access space and the point in the center of these boxes, P1 to P6, which will be used in the calculation of the dimensions. First, we selected point one (P1) on the edge of the bony layer over the FN and point two (P2) on the edge of the bony layer over the CTN (Fig. 2). Second, we selected a point where we presume the center of the RWM would be based on the position of the electrode array at the beginning of the basal turn of the cochlea. These three points are located in the same plane parallel with plane z and through the round window. Third, we selected point three (P3) on the bony edge towards P4 and in the middle of the incus buttress performed by the surgeon. P3 is obtained at the level between the CTN and FN in plane x (Fig. 2). On the other side, we selected point four (P4) between the FN and the CTN with a distance of 0.45 mm to these two nerves also in plane x (Fig. 2). This point is chosen to maintain a safety distance from these nerves and is the only point that is not on the border of the surgeon's posterior tympanotomy [32,33]. It is very difficult to get closer than P4 to the branch point where the CTN branches off from the FN. Finally, we selected a point on the electrode array in the cochlea right before the electrode array makes its first turn. This point and the point of the RWM define the clinical insertion vector of the electrode array's position in the basal turn of the cochlea.Fig. 2 A visual representation of the surgical posterior tympanotomy in a left ear with the facial nerve (blue), the chorda tympani nerve (yellow), the cochlea (red) and bone (transparent grey) on the left. The bone is removed on the right. Point 1 (P1) is located at the bony edge of the bone layer over the facial nerve and point 2 (P2) is located at the bony edge of the bone layer over the chorda tympani nerve. The angle α measured between P1 and P2 with the vertex at the round window membrane (RWM). Point 3 (P3) is located at the bony edge of the incudal buttress and point 4 (P4) is located at a distance of 0.45 mm from the facial nerve and the chorda tympani nerve. The angle β is measured between P3 and P4 with the vertex at the round window membrane (RWM). The mediolateral range (ML range) is the distance from the round window membrane (RWM) to the point where the inferosuperior (IS range) and the anteroposterior range (AP range) intersect. Through the round window membrane (black dot) runs the clinical insertion vector

(red line) into the basal turn of the cochlea (red).

Fig. 2

Geometrical formulas were used to calculate the angles and the distances between the points. These dimensions define the working space of a robotic insertion tool. First, we calculated angle α formed with P1 and P2 and with the vertex at the point of the RWM (Fig. 2). Angle α is located in a plane parallel to plane z according to Verbist et al. and through the point at the RWM [30]. Also, the width of the PT or the anteroposterior range, defined as the distance between P1 and P2, was calculated. Second, the angle β, formed between P3 and P4, and also the vertex at the RWM (Fig. 2) was calculated. P3 and P4 are located in planes parallel to plane x according to Verbist et al. [30]. We calculated the length of the PT or the inferosuperior range, defined as the distance between P3 and P4. Third, the distance from the RWM to the PT is calculated by measuring the distance from the RWM to the point where the inferosuperior range intersects with the anteroposterior range. Finally, we verified if the orientation of the clinical insertion vector is inside the determined angles and if the electrode array could be inserted in a straight line through the PT.

With these dimensions, we approximated the shape of the PT fitting the standard Hügelschäffer model to the anteroposterior and inferosuperior range [34]. The standard Hügelschäffer model gives a more accurate representation of the actual shape of the PT, rather than fitting, for example, an ellipse. In this model, the inferosuperior range is the length while the anteroposterior range is the wide. A 2D graphical representation of the shape of the PT with this model and a 3D graphical representation of the PT volume from the PT towards the RWM was created.

2.7 Statistical analysis

The data were analyzed with the IBM SPSS version 29.0 (SPSS Inc., Chicago, IL). Quantitative data were summarized using mean, standard deviation (SD), minimum and maximum range.

3 Results

3.1 Segmentations Fig. 3 shows an example of the segmentations of the cochlea, the semicircular canals, the FN, the CTN, the electrode array and the volume of the PT.Fig. 3 Viewing an example segmentation of one patient along anterior (a), lateral (b) and inferior (c). The segmented structures are the cochlea (red), the facial nerve (blue), the chorda tympani nerve (yellow), the semicircular canals (orange), the electrode array (grey) and the volume of the posterior tympanotomy (green). (d) The multiplanar reconstruction of the planes through the basal turn of the cochlea according to the coordinate system of Verbist et al. The 0-degree angle is chosen at the top of the horizontal semicircular canal and defines axis x (red). The z axis is defined through the center of the modiolus and defines the axis y (blue). Each point can be defined by the θ (degrees), ρ (radius) and z (height).

Fig. 3

3.1 Dimensions of the posterior tympanotomy

The mean (±SD) angle α is 19.84 (±3.55) degrees with a minimal angle of 14.20° and a maximal angle of 28.10°. The mean (±SD) angle β is 53.56 (±10.29) degrees with a minimum angle of 40.20° and a maximum angle of 74.0°. This maximum for angle β is an outlier in patient 4. The mean (±SD) distance from the RWM to the PT or mediolateral range is 6.05 (±0.71) mm with a minimum distance of 4.65 mm and a maximum distance of 6.91 mm. This minimum mediolateral range is an outlier in patient 9. The width of the PT or the anteroposterior range has a mean (±SD) distance of 2.25 (±0.42) mm with a minimum of 1.38 mm and a maximum of 2.75 mm. The length of the PT or the inferosuperior range has a mean (±SD) distance of 6.73 (±2.42) mm with a minimum of 3.82 mm and a maximum of 10.89 mm. Only three out of ten patients have a clinical insertion vector within the determined angles. In the majority of patients, the clinical insertion vector that determines the orientation of the basal turn of the cochlea lies outside these dimensions and the scala tympani cannot be reached in a straight line through the PT at the ideal insertion angle (complete overview in Table 1). A 2D graphical representation of this model, and thus the shape of the PT, is depicted in Fig. 4 (bottom). The 3D graphical representation of the PT volume is illustrated in Fig. 4 (top). Note that the exact position of the RWM, which has an offset in the anteroposterior-inferosuperior plane, is taken into account and is considered as the origin in Fig. 4. The volume is for each patient rendered with the inferosuperior range, anteroposterior range, and the mediolateral range at the axes Fig. 4.Table 1 For each patient the calculated angles, α and β, the distance from the posterior tympanotomy toward the round window membrane (RWM) or the mediolateral range (ML), the anteroposterior range (AP range), the inferosuperior range (IS range) and the verification of the clinical insertion vector is oriented inside the boundaries of the surgical posterior tympanotomy.

Table 1	Angle α (°)	Angle β (°)	Distance to RWM (ML Range) (mm)	AP Range (mm)	IS Range (mm)	Insertion vector inside?	
Patient 1	17.50	59.30	5.77	1.87	6.72	NO	
Patient 2	20.00	65.30	6.17	2.22	10.89	NO	
Patient 3	20.90	56.20	6.33	2.38	7.21	YES	
Patient 4	19.40	74.00	6.91	2.45	10.89	NO	
Patient 5	19.60	46.30	6.55	2.37	6.11	NO	
Patient 6	14.20	52.50	5.35	1.38	5.37	YES	
Patient 7	19.20	47.50	6.83	2.39	6.43	YES	
Patient 8	18.00	40.20	5.54	1.94	3.82	NO	
Patient 9	28.10	49.00	4.65	2.75	4.48	NO	
Patient 10	21.50	45.30	6.39	2.75	5.35	NO	
Mean ± SD	19.84 ± 3.55	53.56 ± 10.29	6.05 ± 0.71	2.25 ± 0.42	6.73 ± 2.42		
Minimum	14.20	40.20	4.65	1.38	3.82		
Maximum	28.10	74.00	6.91	2.75	10.89		

Fig. 4 TOP: 3D graphical representation of the posterior tympanotomy volume for all ten patients based on the segmentations and data in Table 1, with corresponding standard Hügelschäffer curve; BOTTOM: 2D graphical representation of the PT shape with corresponding Hügelschäffer curve, as seen from the maximal mediolateral (ML) position; The proportions of these volumes are according to their anteroposterior range (APS range) as the distance between point 1 (P1) and point 2 (P2), inferosuperior range (IS range) as the distance between point 3 (P3) and point 4 (P4) and their mediolateral range (ML range). The red dot represents the round window membrane (RWM), and is considered as the origin.

Fig. 4

4 Discussion

This retrospective chart review describes the surgical access space in a unique three-dimensional way in contrast to previous studies that reported the facial recess in a two-dimensional way [23,28,29,[35], [36], [37], [38], [39], [40]]. To our knowledge, a similar description of the surgical dimensions of the PT has not been reported before. Additionally, the measurements are based on postoperative CT scans of patients who underwent CI surgery. This provides a more realistic estimate of the expected size of the surgical access space instead of a potential maximum facial recess measured on preoperative CT scans [23,37,39]. These two characteristics contribute to the unique estimation of the size of the surgical access space during cochlear implantation.

The shape of the surgical PT depends on the anatomic variability between patients and the individual surgeon. The consensus paper of Verbist et al. provides a frame of reference to calculate this in a fixed coordinate system [30]. The motivation to characterize using two different angles with their vertex at the RWM stems from a robotic point of view. When the RWM is reached, it is valuable to know which angles can be achieved with the tip fixed at the round window. In this way, an insertion tool can be optimally positioned for an atraumatic insertion. The measurements presume maximal maneuverability during robotic insertion and can be further applied to calculate the different insertion angles at which an automated insertion tool can be positioned. The two angles, α and β, and the average distance to the RWM of 6.05 (±0.71) mm form a cone-like space that describes the surgical access space from the PT towards the RWM (illustrated in Fig. 2, Fig. 4). The width of the PT or the anteroposterior range has a mean distance of 2.25 (±0.42) mm. This can also be described by an average angle α of 19.84 (±3.55) degrees. This range and angle give an estimation of the anteroposterior range that can be achieved which determines the insertion angle in a plane parallel to plane z, through the point at the RWM and basal turn of the cochlea. In the transverse direction has the length of the PT or the inferosuperior range a mean distance of 6.73 (±2.42) mm or described by an average angle β of 53.56 (±10.29) degrees. This range and angle define the maximal inferosuperior range that can be achieved in planes parallel to plane x. These dimensions give insights into the anatomical variation among patients and the intersurgeon variability of the performed PT.

To verify if an automated insertion tool can be positioned in such a way that the electrode array can be optimally inserted into the scala tympani, the clinical insertion vector was determined. Only in three out of ten patients the clinical insertions vector is oriented inside the surgical PT. This implies that the size of the surgical PT is a limiting factor for an ideal, automated electrode array insertion with a straight insertion tool. The clinical insertion vector is not always the ideal insertion vector through the center of the scala tympani [17]. However, it is the clinical insertion vector at a correct position inside the cochlea and a possible insertion axis without tip fold-over. Microscopic intracochlear trauma such as rupture of the basal membrane or the osseous spiral lamina cannot be excluded with current postoperative CT scans. Important to note is that the workspace is the maximal theoretical space. For each specific tool, the effective maneuver space will be smaller depending on its diameter.

The angles and distances of the anteroposterior range reported included a safe intra-operative procedure as all the surgeries were performed without nerve damage and neurological deficits, verified by the surgical reports. The inferosuperior angles and distances are the theoretical dimensions because P4 is chosen more inferior to the surgeon's PT. Additionally, these PTs are performed by experts in CI surgery which makes these results representative of the conditions during CI surgery. The size of an ideal PT is as small as possible to expose the round window for an adequate and easy electrode array insertion at a safe distance to the FN and the CTN. If surgeons try to increase the size of the PT for more freedom of movement for the robot, they need to be careful to prevent neurological damage. Currently, a larger PT is performed by the surgeon to create more visibility. When the entire procedure is robotized a larger field of view may become less important, resulting in a smaller PT size [41]. Another important factor to consider is that all scans were acquired postoperatively. The round window niche had been altered by the surgery in some cases to visualize correctly the RWM and the beginning of the scala tympani. The overhanging oblique ridge of the promontory can cover the RWM. The removal of the canonus, the superior part fo the overhanging ridge, (canonectomy) on the limit of human sensory capabilities may cause trauma to the hook region and consequently to the basilar membrane, osseous spiral lamina and spiral ligament [42].

The dimensions describe a cone-like workspace for an adult European patient population. Consequently, these findings cannot be extrapolated to a pediatric population or other etnic groups. Despite this, the pediatric population has been an important part of the patient population who were implanted with a CI in the last decades. The difference in the size of the facial recess between children and adults goes beyond the scope of this research paper [43]. No statement about the difference between the sexes can be made due to the preliminary nature of the results and the small sample size.

In previous studies, mostly the two-dimensional mean width of the PT at the level of the round window is used to describe the size of the PT. This varies between 2.40 mm and 4.30 mm with a median of 2.83 mm (overview in Fig. 5) [23,28,29,[35], [36], [37], [38], [39],44]. Some research groups calculated their dimensions on preoperative CT images [23,29,37,39]. The other groups prepared cadaveric temporal bones with maximal skeletonization of the FN and CTN and determined their dimensions on microscopic photographs of these temporal bones [28,35,36,38]. The limitation of this method is that the camera angle defines the size of the facial recess which makes it less standardized. The strength of this study is that the planes are translated according to the coordinate system of Verbist et al. to standardize the measurements [30]. Another important difference between previous studies and this study is that the direct distance between the two nerves is measured. This is theoretically the maximal size of the facial recess without taking the safety margins to the FN and the CTN into account. Typically a bony covering is left on the FN to avoid trauma during surgery or electrical stimulation later on. So these are not surgical PTs defined as a partial opening in the facial recess. In this study, we calculated the angles of the PT based on points located at the edge of the bony layers over the FN and the CTN. We can assume the safety margins were preserved in the patients included in this study because no postoperative neurological deficits are reported. This explains the slightly smaller mean of our anteroposterior range, although our measurements are in line with the previous studies (Fig. 5).Fig. 5 Overview of the width of the posterior tympanotomy at the level of the round window in the previous studies (blue). For each study is a Whisker plot created with the mean (the middle of the boxes), ± the standard deviation (SD) and the range with their minimum and maximum (if reported). This study is rendered in green.

Fig. 5

Another way to describe the size of the PT is to use the Chorda-Facial angle. This angle is defined as the angle of the bifurcation where the CTN branches of the FN [28,35,36,40]. Also, this is a two-dimensional description instead of a three-dimensional one. Only one study measured the distance from the round window to the FN [44]. On preoperative CT scans of the temporal bone (oblique sagittal view), a mean distance of 5.50 mm with a minimum distance of 4.00 mm and a maximum distance of 6.50 mm is measured. Thereafter the distance is verified peroperative and a similar mean distance of 5.90 mm is measured. These results are consistent with the mean distance of 6.05 (±0.71) mm, and a similar minimum and maximum distance of respectively 4.65 mm and 6.91 mm. The measurements in this study were not made from the RWM to the FN, but from the round window to the point in the middle between the FN and the CTN as described in the method section.

In later stages, the method described in this study could be extended to preoperative CT scans for planning the CI surgery. The dimensions of the PT should be combined with the dimensions of the cochlear anatomy to calculate the optimal insertion vector into the scala tympani [45]. The important factors in the interindividual variability of the cochlear anatomy are the cochlear lengths, angles between turns, the hook region and the crista fenestra [[46], [47], [48]]. This can be complemented with a novel optically-guided sheath with an integrated forward-facing optical coherence tomography probe for real-time feedback during the electrode array insertion [2]. The visualization of the intracochlear anatomy will contribute to an individualized and atraumatic cochlear implantation.

A limitation of this study is that the resolution of the CT scan (0.217 mm) determines the delineation of the structures like the bone margins and the CTN. The voxel sizes of the CT images were nicely matched to the used reconstruction kernel which will only have influenced our measurements to a minor extent. Also, metallic artifacts of the CI electrode array on these CT images may reduce the segmentation's accuracy. These artifacts make it difficult to differentiate the thickness of the bone layer on the FN and CTN because the electrode array passes closely between these two nerves. The FN was independently segmented twice and its smallest diameter was subsequently measured. The intraclass correlation coefficient from these two measurements was calculated to quantify the error of the metallic artifact. Intraclass correlation coefficient estimates and their 95 % confidence intervals based on single measurements, absolute-agreement, two-way mixed-effects model. The intraclass correlation coefficient of 0.77 with a 95 % confidence interval of 0.32–0.94 suggests moderate to good reliability [49]. The mean (±SD) difference between the two measurements is 0.11 ± .0.09 mm. Additionally, the electrode array was not near the selected points. It can be concluded that the metallic artifacts have a minor influence on the measurements. Finally, the power analysis described no significant difference (significance level of 0.05) between the anteroposterior range of this study (2.25 ± 0.42 mm) and the expected size based on the literature in Fig. 5 with a power of 0.98. Nevertheless, the dimensions cannot be generalized for the adult, European population with the small sample size of this study, and no inclusion criteria were specified to make a statement on the general population. Despite the sample size, the dimensions give a first impression of the expected conditions during CI surgery.

5 Conclusion

This retrospective chart review of postoperative CT scans studies the surgical access space from the PT toward the RWM. Through manual segmentation of these CT scans we described a cone-like space with its tip at the RWM and the surgical PT at its base. After the transformations into the standardized coordinate system of Verbist et al., the hypothetical maximum space is defined through two angles, α and β, with their corresponding ranges and the distance to the RWM [30]. Also, the clinical insertion vector into the scala tympani was calculated to verify if the electrode array could be inserted with a straight robotic insertion tool within the boundaries of the surgical PT according to the insertion vector of the electrode array's position. In only three out of ten patients it would be possible. All the calculations were done in a standard reference system and contain data with important first impressions in the surgical manipulation space of a future robotically steered insertion tool. This method and data could be extended to measuring the size of the posterior tympanotomy on preoperative CT scans for planning future robotic CI surgery.

Ethics approval and consent to participate

This research was approved on May 16th, 2022 by the Research Ethics Committee of the university hospitals/catholic university of Leuven (MP018629) before data collection and performed in compliance with the ICH-GCP principles (International Conference on Harmonisation Guidelines on Good Clinical Practice), with the most recent version of the Helsinki Declaration, and with applicable laws and regulations. Medical data was recovered and anonymized from the medical records of patients at the university hospitals Leuven. The collection, processing and disclosure of personal data are subject to compliance with the General Data Protection Regulation. This study was waived from informed consent due to its retrospective nature and lack of intervention.

Consent for publication

Not applicable.

Data availability statement

Data will be made available upon reasonable request.

Funding

This work was financially supported by Research Foundation 10.13039/501100011878 Flanders (FWO: 1SD3322N/1SD3324N (AG), 1804816N (NV), 1SG1623N (MS)) and the CHARIOT project IDN Internal fund of 10.13039/501100004040 KU Leuven .

CRediT authorship contribution statement

Brecht Vranken: Writing – review & editing, Writing – original draft, Visualization, Resources, Project administration, Methodology, Investigation, Data curation, Conceptualization. Maarten Schoovaerts: Writing – review & editing, Visualization, Software, Methodology, Investigation, Formal analysis, Conceptualization. Alexander Geerardyn: Writing – review & editing, Resources, Methodology, Data curation, Conceptualization. Lore Kerkhofs: Writing – review & editing, Software. Johannes Devos: Writing – review & editing. Robert Hermans: Writing – review & editing. Tristan Putzeys: Writing – review & editing, Visualization, Supervision, Resources, Project administration, Methodology, Formal analysis, Conceptualization. Nicolas Verhaert: Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Data curation, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Maarten Schoovaerts reports financial support was provided by Research Foundation 10.13039/501100011878 Flanders . Nicolas Verhaert reports financial support was provided by Research Foundation 10.13039/501100011878 Flanders . Alexander Geerardyn reports financial support was provided by Research Foundation 10.13039/501100011878 Flanders . If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Not applicable.
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