Prediction of Difficult Round Window Visibility during Cochlear Implantation via a Reformatted CT Facial Recess View: A Retrospective Study with Surgical Correlation ===================================================================================================================================================================== * Si Wei Kheok * Jia Hui Ng * Lishya Liauw * Vanessa Yee Jueen Tan * Jiun Fong Thong ## Graphical Abstract ![Figure1](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F1.medium.gif) [Figure1](http://www.ajnr.org/content/46/3/572/F1) ## Abstract **BACKGROUND AND PURPOSE:** Cochlear implant surgery is performed commonly through the facial recess via the round window (RW) approach. This study aims to evaluate the utility of reformatting the preoperative CT temporal bone scan into a CT facial recess view in alerting surgeons to a potentially difficult surgery with poorly visualized RW. **MATERIALS AND METHODS:** This is a retrospective study of 41 patients (43 ears), who had undergone cochlear implant surgery. Intraoperative findings of RW position relative to second genu-mastoid portion of facial nerve, and RW membrane orientation were recorded by the surgeons. Preoperative CTs were analysed by 2 radiologists in both axial and reformatted planes, with the later simulating the surgeon’s view via the facial recess. Radiologic assessment markers include the facial nerve-chorda tympani nerve width measured 1.2 mm inferior to the exit point of the chorda tympani nerve into the tympanic cavity, RW position relative to second genu-mastoid segment of the facial nerve, and RW membrane’s angle from the vertical axis. **RESULTS:** The best predictor for difficult RW intraoperative visibility is the RW position relative to the second genu-mastoid segment of the facial nerve lying lateral to it on CT facial recess reformatted images. A RW that lies partially to completely posterior to the posterior border of the second genu-mastoid segment of the facial nerve had up to 55.6% risk of encountering difficult access, while those positioned anterior to or partially anterior to the anterior edge of the second genu-mastoid segment of the facial nerve had 0% risk of difficult access (*P* < .05). There are substantial agreements in the intrarater (κ = 0.751, *P* < .001) and interrater reliability (κ = 0.698, *P* < .001). There is no significant association between surgical difficulty and facial nerve to chorda tympani distance or RW angle (*P* > .05). **CONCLUSIONS:** Identification of RW positions in the reformatted CT facial recess view is a useful tool in predicting potentially difficult RW access in cochlear implant surgery. ## ABBREVIATIONS: FN-CTN : facial nerve to chorda tympani nerve width ICC : intraclass correlation coefficient IQR : interquartile range RW : round window SUMMARY #### PREVIOUS LITERATURE: The preferable method of cochlear implantation for better surgical outcomes is via the facial recess and the electrode is inserted through the RW. However, difficulty in visualizing the RW is associated with a higher risk of complications and requires additional surgical steps to circumvent this. Preoperative CT temporal bone methods to predict a difficult surgery may not be easy to apply due to individual differences in temporal bone morphology. #### KEY FINDINGS: The RW position determined on the reformatted CT facial recess view from the preoperative temporal bone CT shows 0% risk of encountering a difficult surgery when it falls into positions 1–2. CT RW position 4 has a 55.6% risk of encountering a difficult surgery. #### KNOWLEDGE ADVANCEMENT: The CT facial recess view can simulate the surgical facial recess view and is useful in identifying the RW position in relation to the facial nerve. The method is applicable as long as the chorda tympani, facial nerve, and RW are identifiable on CT. Cochlear implantation is a treatment option for patients with severe to profound sensorineural hearing loss. As of 2019, it is estimated that 736,900 registered devices have been implanted worldwide.1 The standard surgical approach is via the facial recess in posterior tympanotomy (the mastoid bone is drilled to reach the middle ear cavity), and the electrode is preferably inserted through the round window (RW)2,3 rather than through a cochleostomy created adjacent to the RW. The RW approach has been found to be superior to cochleostomy approach in terms of causing less intracochlear trauma,4 increased likelihood of perimodiolar placement5 and better outcomes in terms of ease of scala tympani insertion, reduced scalar shift, superior speech perception, language acquisition, and speech production.6,7 However, difficulty in visualizing the RW could necessitate maneuvers such as removal of a portion of the posterior wall of the external auditory canal, sacrifice of the chorda tympani, performing a cochleostomy or a canal wall down mastoidectomy, and blind sac closure to successfully insert the electrode.8,9 Difficulty in visualizing the RW is primarily due to obscuration of RW from the surgeon’s view by the second genu-mastoid segment of the facial nerve that lies lateral to the RW. To a lesser extent, the orientation of the RW membrane could also contribute to difficult RW visualization. As such, this study aims to discover if a preoperative temporal bone CT could be reformatted to mimic the surgical facial recess view and identify similar CT surrogate features that could alert the surgeon of a potentially difficult surgery. Some of the structures or measurements on temporal bone CT that had been assessed include aeration of the bone at the facial recess, the posterior tympanotomy depth, chorda-facial angle, chorda-facial angle to stylomastoid length, angle of the facial nerve second genu,10 facial recess width measured perpendicularly from the external auditory canal to the facial nerve,11,12 thickness of overhanging RW bone,13 anatomic variations of the round window,14 angles between lines connecting the RW to the facial nerve and external auditory canal,11 and position of the RW relative to the facial nerve based on an intersecting line.3,12 Imaging methods used to predict difficult cochlear implantation in the posterior tympanotomy approach rely on temporal bone landmarks. Yet, the orientation and course of the temporal bone contents vary even among individuals with normal temporal bone anatomy. For instance, the mastoid is known to vary significantly in size and configuration,1 and the facial and chorda tympani nerves range in length and morphology.2,3 The authors found it difficult to adopt some of the existing methods where the landmarks were not clearly defined. For instance, the continuous, smooth bony margin of the external auditory canal to the curved mastoid makes it difficult to reliably reproduce the exact connecting points for assessment.11,12 Unlike most prior studies that evaluate the RW based on angles and connecting lines created on axial views, this study includes the analysis of the RW through a reformatted facial recess view similar to what the surgeon sees intraoperatively. ## MATERIALS AND METHODS ### Ethics This study was approved by the institutional review board, CIRB ref: 2020/2685, and consent for research participation was obtained from all patients. ### Patients In this retrospective study, 41 patients (24 right ears and 19 left ears), who underwent cochlear implantation through the facial recess were consecutively recruited. The eligibility criteria are as follows: 1) age > 1 year old, 2) no inner ear deformity precluding surgery, 3) no prior surgery, 4) availability of preoperative CT temporal bone scans that could be reformatted, 5) RW niche is clear on CT that allows analysis of RW membrane, and 6) visible chorda tympani on CT. ### Preoperative CT Temporal Bone Assessment and Measurements High-resolution temporal bone images were acquired without intravenous contrast administration on Aquilion (Toshiba), Somatom Definition Flash, or Somatom Force (Siemens). These were performed at 120 kV and 240 mAs on the Toshiba CT machine and at 120 kV and 140 mAs, on the Siemens CT machines with a slice thickness of 0.6 mm. Images were reformatted on a PACS. The facial nerve and chorda tympani were traced along their entire course to ensure accurate identification. The chorda tympani branches from the mastoid segment of the facial nerve. In the rare event that the chorda tympani arose from the extratemporal segment of the facial nerve, we took care to ensure that the identified structure indeed represented the chorda tympani by following its course toward the chordae posterius of the tympanic canaliculus and that there were no other contentious branches. Three radiologic markers were assessed. The first measurement was the facial nerve to chorda tympani nerve width (FN-CTN) as described by Jwair et al.3 In the axial plane that is parallel to the horizontal semicircular canal, the entry of the chorda tympani into the middle ear is identified, and 2 slices (1.2 mm) inferior to this level, the FN-CTN distance was measured. The FN-CTN is defined as the shortest distance between the inner margins of the facial nerve and chorda tympani on the chosen image slice (Fig 1). ![FIG 1.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F2.medium.gif) [FIG 1.](http://www.ajnr.org/content/46/3/572/F2) FIG 1. Magnified view of the axial CT temporal scan. The FN-CTN is defined as the shortest distance between the margin of the chorda tympani (*white arrow*) and facial nerve (*black arrow*) 2 CT slices caudal to where the chorda tympani enters the middle ear, as per Jwair et al.3 The FN-CTN measures 2.5 mm in this example. The other 2 radiologic markers analyzed were the RW position relative to second genu-mastoid segment of the facial nerve (RW position) and the RW membrane’s angle from the vertical axis (RW angle). These were assessed in a reformatted view to mimic the surgical facial recess view. This reformatted oblique sagittal plane is obtained parallel to the FN-CTN measurement line15 (Fig 2, Supplemental Data). ![FIG 2.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F3.medium.gif) [FIG 2.](http://www.ajnr.org/content/46/3/572/F3) FIG 2. Axial CT temporal bone image on the right shows the slice where the FN-CTN is measured. The reformatted view on the left is in the oblique sagittal plane, and is oriented parallel to the plane of the FN-CTN (*red line*). The *black arrow* points to the mastoid segment of the facial nerve, and the *white arrow* points to the smaller chorda tympani. In the reformatted plane, RW angle is measured between the best visualized RW membrane and the vertical axis, and this is often seen at the center of the RW membrane (Fig 3). ![FIG 3.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F4.medium.gif) [FIG 3.](http://www.ajnr.org/content/46/3/572/F4) FIG 3. The best view of the RW membrane is usually at the center of the RW as identified on the left-most axial CT temporal bone image. The angle of the RW membrane is measured in the CT temporal bone reformatted sagittal oblique view. The middle image shows the slice of the reformatted view where the RW angle is measured, and the RW angle measured is superimposed on the right-most image. By scrolling through the reformatted plane, the RW’s position relative to the second genu-mastoid segment of the facial nerve can be determined. RW positions are classified as such: position 1 refers to RWs that are completely anterior to the facial nerve; position 2 are RWs that are partially obscured posteriorly by the facial nerve; position 3 are RWs completely obscured by the facial nerve; position 4 are RWs partially obscured anteriorly by the facial nerve or that lie beyond the posterior border of the facial nerve (Figs 4 and 5). ![FIG 4.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F5.medium.gif) [FIG 4.](http://www.ajnr.org/content/46/3/572/F5) FIG 4. Illustration depicting the criteria used to assess RW positions on reformatted temporal bone CT relative to the second genu-mastoid segment of the facial nerve: position 1 is completely anterior to the facial nerve; position 2 is partially obscured posteriorly by the facial nerve; position 3 is completely obscured by the facial nerve; position 4 is partially obscured anteriorly by the facial nerve or lies beyond the posterior border of the facial nerve. ![FIG 5.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F6.medium.gif) [FIG 5.](http://www.ajnr.org/content/46/3/572/F6) FIG 5. Temporal bone CT in the oblique sagittal views. The RW position identified medially (*dashed line*, *left image*) is superimposed on a more lateral image (*right image*). As it overlaps at its anterior-inferior aspect with the second genu-mastoid segment of the facial nerve (*black arrow*, *right image*), the RW is in position 4. The *white arrows* points to the chorda tympani. The scans were independently reformatted and the 3 radiologic markers were analyzed independently by 2 head and neck radiologists, R1 (with 6 years, specialist experience) and R2 (with 20 years, specialist experience), who were blinded to the surgical findings, and reader 1 repeated the measurements 3 months later (R1 repeat). To determine reproducibility of methods described in the literature that require good visualization of the RW membrane at the level of the external auditory canal in axial view,11⇓–13,16 R1 recorded the number of cases in which the RW was visible at or near the central level of the external auditory canal. ### Surgical Details and Findings The patients were operated on by any of the 3 surgeons, who were blinded to the CT findings analyzed in this study. Surgical findings of RW position relative to second genu-mastoid portion of facial nerve, and the RW membrane orientation were recorded. The intraoperative RW position relative to the second genu-mastoid segment of the facial nerve was recorded as “on” (if the RW was directly medial to the facial nerve from the surgeon’s view), “anterior,” “posterior,” or “unable to visualize.” “Anterior” RW positions were favorable for easy surgical access to the RW (Figs 6 and 7). On the contrary, “on,” “posterior,” and “unable to visualize” denoted difficult surgical access. ![FIG 6.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F7.medium.gif) [FIG 6.](http://www.ajnr.org/content/46/3/572/F7) FIG 6. Intraoperative microscope view of the right ear, showing an easily accessible RW (white arrow) anterior to the second genu-mastoid segment of the facial nerve (*black arrow*). Its membrane is posteroinferiorly oriented. With regards to the RW membrane orientation, the surgeons recorded if it was posteroinferior in orientation, which was favorable (Figs 6 and 8), inferior (Figs 7 and 8) or unable to visualize that naturally indicated a more challenging surgery.14 ![FIG 7.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F8.medium.gif) [FIG 7.](http://www.ajnr.org/content/46/3/572/F8) FIG 7. Intraoperative microscope view of the right ear shows an almost inferiorly oriented RW membrane (*white arrow*) and the RW lies anterior to the facial nerve. The *black arrow* points to the second genu-mastoid segment of the facial nerve. ![FIG 8.](http://www.ajnr.org/https://ajnr-sso.highwirestaging.com/content/ajnr/46/3/572/F9.medium.gif) [FIG 8.](http://www.ajnr.org/content/46/3/572/F9) FIG 8. Graphic demonstration of the RW membrane orientation from the surgeon’s view. From the facial recess view, which is also the oblique sagittal view of the ear, the black arrow points to the second genu-mastoid segment of the facial nerve. The inferiorly oriented RW appears slitlike to the surgeon (unfavorable) (*white arrow*), while the posteroinferiorly oriented RW tilts toward the surgeon’s view (*gray arrow*), resulting in a larger exposed surface area that makes the RW membrane more surgically visible. ### Statistical Analysis Intraoperative evaluation of the RW was taken as the reference standard, for which the CT assessment markers (RW position, FN-CTN, and RW angle) were compared against. CT RW position and FN-CTN were compared against surgical RW position. RW angle on CT was compared against surgical RW membrane orientation. Categoric variables were compared using the χ2 test or the Fisher exact test. Continuous variables were presented as median and interquartile range (IQR) and compared using the Mann-Whitney *U* test. Nonparametric tests were used due to the non-normal distribution of values. Binomial logistic regression was performed to ascertain the effects of CT RW position, FN-CTN, and RW angle on likelihood of an unfavorable RW position intraoperatively. Analysis for inter- and intrarater reliability was performed using the weighted κ statistic for categoric variables and the intraclass correlation coefficient (ICC) for continuous variables. ## RESULTS ### Population Characteristics A total of 41 patients (44 ears) were recruited for the study. In 1 ear, both radiologists were unable to visualize the chorda tympani well in a pediatric patient on the preoperative CT. After the exclusion of this case, a total of 41 patients (43 ears) were included in this study. Median age was 58 (IQR 43.5–70). Fifty-eight percent were men and 42% were women. Operative RW position was anterior in 37 ears (86%) and on, posterior, or unable to visualize in 6 ears (14%). RW position 4 was found in 20.9% of patients on preoperative CT. The median FN-CTN on preoperative CT was 2.0 mm (IQR 1.7–2.5). The median RW angle on preoperative CT was 32.4° (IQR 26.1–37.8°). Nineteen of 43 ears had RW at or near the central level of the external auditory canal. This is summarized in Table 1. One patient had incomplete partition type 2 of the cochlea. View this table: [Table 1:](http://www.ajnr.org/content/46/3/572/T1) Table 1: Population details ### CT RW Position Intraoperative finding of anterior RW position was taken as the reference standard for a straightforward surgery, while the other RW positions denoted challenging intraoperative RW access. Of the 6 surgically difficult cases, 5 had CT RW position 4, and 1 had CT RW position 3. The 5 patients with CT RW position 4 and had surgically difficult RW access were found to have operative RW positions “posterior,” “superior,” or “unable to visualize.” In only 1 of these patients did reader 2 differ from reader 1 by grading the CT RW position as 3 rather than 4. The patient with surgically difficult RW access and graded CT RW position 3 by both readers was found intraoperatively to have a RW directly medial to the second genu-mastoid segment of the facial nerve from the surgeon’s view (ie, surgical RW position classified as “on”). With a CT RW position of 4, there is up to 55.6% risk of encountering difficult RW access intraoperatively. Comparatively, CT RW prediction of positions 1–3 has only a 2.9% risk of encountering difficult RW access intraoperatively (*P* < .001). The method has a sensitivity of 0.833, specificity of 0.892, negative predictive value of 0.971, and positive predictive value of 0.556. Furthermore, CT RW positions of 1 to 2 have 0% risk of encountering difficult RW access intraoperatively. In comparison, CT RW positions 3 to 4 face a 26.1% chance of encountering difficult RW access intraoperatively (*P* = .017). This has a sensitivity of 1.000, specificity of 0.541, negative predictive value of 1.00, and a positive predictive value of 0.261. ### FN-CTN Width Cases with difficult RW access intraoperatively had a smaller FN-CTN median, although this did not reach statistical significance (FN-CTN median 1.85 mm [IQR 1.7–2.13 mm] versus 2.10 mm [IQR 1.7–2.5 mm], U = 88.5, *P* = .440). ### RW Membrane Angle Of 43 ears, 3 of the RW niches were opacified with soft tissue on CT and the RW angle could not be measured. Of the remaining 40 ears, there were 6 ears with difficult surgical access as they were seen to have an inferiorly or slightly inferiorly directed RW. The RW angles measured on CT were compared against the intraoperative RW membrane orientation, with posteroinferior orientation considered favorable, and inferior orientation deemed unfavorable. The median RW angle on CT is 33.4° (IQR 26.9°–38.3°) for favorable intraoperative RW membrane orientation and 34.4° (IQR 28.8°–39.9°) for unfavorable intraoperative RW membrane orientation (U = 89.5, *P* = .644). They overlapped in IQR and did not reach statistical significance. ### Analysis of CT RW Position, FN-CTN, and RW Angle on Likelihood of Unfavorable RW Position Intraoperatively Binomial logistic regression was performed to ascertain the effects of CT RW classification (CT RW position 4 versus others), FN-CTN, and RW angle on the likelihood of an unfavorable RW position intraoperatively. The model was statistically significant X2 = 17.1, *P* < .001. The model explained 61.0% of the variance in intraoperative RW position (Nagelkerke R square) and correctly classified 90% of cases. Again however, only CT RW classification (CT RW 4 versus others) was found to predict for intraoperative RW position (*P* = .004), while FN-CTN (*P* = .78), and CT RW angle (*P* = .188) did not. ### Incomplete Partition Type 2 The patient with cochlear incomplete partition type 2 was surgically found to have an anteriorly positioned RW and posteroinferiorly oriented RW membrane. The surgery was straightforward and no complication was encountered. The CT RW position was consistently determined to be of position 2 in all 3 readings by the 2 radiologists. The FN-CTN was small at 1.2 mm. The RW angle was 34.9°. ### RW Visibility at or Near the Central Level of the External Auditory Canal Only 24 of 43 ears in our study had visible RWs at or near the central level of the external auditory canal on axial view. ### Intra- and Interrater Reliability A second independent radiologist, and the first radiologist 3 months later, reanalyzed the CT RW position, FN-CTN, and RW angle on the preoperative CT. Substantial intrarater and interrater reliability were observed in obtaining the RW positions (κ = 0.751, *P* < .001 and κ = 0.698, *P* < .001, respectively) and measuring the FN-CTN distance (ICC = 0.949 and 0.922, respectively; *P* ≤ .001). Moderate intrarater and fair interrater reliability was revealed for CT assessment of RW angle (ICC = 0.6921, *P* < .001, and 0.679, *P* < .001, respectively) (Table 2). View this table: [Table 2:](http://www.ajnr.org/content/46/3/572/T2) Table 2: Intrarater and interrater reliabilities in the assessment of the CT findings ### Surgical Outcomes of Operatively Unfavorable RW Positions Of the 6 cases that had unfavorable surgical RW positions, additional surgical steps were taken. One patient had part of the posterior canal wall drilled to angle the microscope further posteriorly, the RW had to be extended, and the drilled wall was subsequently reconstructed with cartilage. One resulted in facial nerve dehiscence after facial recess drilling and still required cochleostomy. The last 4 cases had cochleostomy performed instead, without facing any complication. ## DISCUSSION In our attempt to find suitable and reliable anatomic landmarks on temporal bone CT to predict difficult RW access in cochlear implantation, we discovered the wide anatomic variation in the appearance and contours of the temporal bone structures among individuals that rendered it difficult to reproduce some of the methods highlighted in the introduction. Analyzing the relationship of the external auditory canal to the RW by means of creating an angle or extrapolated lines on axial view is a popular method.11,12,16 The chief problem we faced with this is that the external auditory canal wall is not a straight line, which obviates accurate creation of a straight line parallel to it.11,12 Second, the center of the external auditory canal may not be in the same plane as the best view of the RW,16 which was what we encountered in 24 of 43 ears in our study. The facial recess width deemed the distance between the facial nerve and chorda tympani had mixed results for difficult surgical access.3,16 While the study by Jwair et al3 reported positive finding, our finding was similar to that of Kang et al16 and we did not find the smaller FN-CTN in surgically difficult cases to be statistically significant and there was an overlap of the interquartile ranges between the 2 groups, that suggests it is not a sole reliable method. One of the features that could make surgery challenging is an inferiorly oriented RW, because the RW prechamber may not be immediately identifiable during microscopic surgery through the posterior tympanotomy, and some effort is required to locate it.9 This can be especially tricky for the surgeon when there is also a well-pneumatized infracochlear air cell tract that has an opening with the configuration and orientation mimicking that of the RW prechamber.14 The well-pneumatized air cell tract could be wrongly entered as there is minimal resistance encountered during electrode insertion, which would otherwise alert the surgeon to the mistake. Hence, 1 of the features we analyzed is the RW angle from the CT reformatted facial recess view. Unfortunately, this method had not been found to be reliable. To provide some background, the RW width ranges from 1.15–2.98 mm,14 while CT resolution reaches 0.6 mm in slice thickness. We found that with the current CT resolution, the RW membrane was visible in only 2 to 3 slices in the reformatted view, which made it difficult to ascertain the best slice to use. Furthermore, as one scrolls medially the RW membrane becomes more inferiorly oriented, which creates volume averaging artifact. We postulate that the use of scanners with higher resolution, such as the conebeam CT, may be able to provide more reliable results. Another possible reason is that the RW membrane orientation may be affected by slight differences in the way the patient’s head is positioned during surgery, and it is not possible to replicate this routinely on CT. In this study, 14% of cases were found to have RWs hidden from the facial recess view, which is comparable to the finding in a cadaveric study.14 This suggests a balanced study sample for analysis. To standardize the axial slice where the CT facial recess view was reformatted from, we adopted the same axial level that Jwair et al3 described for the assessment of the facial recess width due to its replicability. This study showed it has excellent intrarater and interrater reliability. With the CT facial recess view, the best, statistically significant predictor of a simple or difficult surgery was CT RW classification. CT RW positions 1–2, where the RW was found anterior to or only posteriorly obscured by the second genu-mastoid segment of the facial nerve, had 0% chance of a difficult surgery. CT RW position 4, where the RW is obscured anteriorly or lies beyond the second genu-mastoid segment of the facial nerve portends up to 55.6% chance of a difficult surgery. Comparing CT RW position 4 to CT RW positions 1–3 showed high specificity of 0.833 and sensitivity of 0.892 in predicting a difficult surgery. Of the 6 difficult surgical cases, 1 had an RW immediately medial or “on” the mastoid segment of the facial nerve rather than partially or completely beyond the posterior border of the nerve and this was accurately identified as CT RW position 3. Substantial intra- and interrater reliability imply that this is a dependable method that could be used by specialists familiar with CT temporal bone anatomy. A limitation of this technique is that although it mimics the surgical view, it does not fully replicate it as there are surgical factors such as patient operative position, and microscope position that can be manipulated in various angles to optimize the field.8 The chorda tympani and RW membrane need to be visible, so the method may not be suitable when the chorda tympani and RW niche are obscured or destroyed. Due to the tiny size of the structures analyzed, the reader should be meticulous. Last, the true extent to which the CT reformatted view can be used to analyze other potentially useful findings, such as the presence of a prechamber cell and structural abnormalities, have not been explored. That said, the reformatted CT facial recess view described in this study is a novel method for the viewer to mimic the surgical view preoperatively. Not only are the facial nerve and chorda tympani visualized in the same way as the surgeon, the horizontal and vertical coordinates of the RW relative to the facial recess can be assessed in a manner that the traditional axial view is not able to replicate.3,11,12,16 This could enhance communication with surgeons and patients. Another advantage is by relying only on the facial nerve and chorda tympani anatomy that makes up the facial recess view, as well as the RW membrane, the method should be applicable in the presence of other anatomic deformities, such as external auditory canal atresia and cochlear deformities. In fact, 1 of our patients had type 2 incomplete partition of the cochlea and CT RW position 2 accurately predicted an easily accessible surgical RW. Given the ubiquity of the reformatting tool in most basic imaging packages, the reformatted CT facial recess technique can be readily applied in most imaging viewer software. It took approximately 30 seconds to create the reformatted view, and 30 seconds for confirmation of the CT RW position (Supplemental Data). ## CONCLUSIONS Posterior tympanotomy coupled with cochlear implant insertion through the RW stands as the preferred surgical approach. However, when the RW is hidden from view or the membrane is unfavorably oriented, there is heightened risk of complications and alterations in surgical techniques may be necessary. Identifying unfavorable RW positions on preoperative CT scans serves as a crucial warning for surgeons, alerting them to the potential challenges ahead. Our study introduces a novel CT reformatted facial recess view, enabling comprehensive evaluation of the RW’s position relation to the second genu-mastoid segment of the facial nerve. Notably, identifying a RW position anteriorly obscured or positioned beyond the posterior aspect of the facial nerve on preoperative CT scans emerges as a valuable predictor for challenging RW access during cochlear implant surgeries. Meanwhile, CT RW position anterior to or only posteriorly obscured by the second genu-mastoid segment of the facial nerve predicts a very low likelihood of a difficult surgery. This predictive tool offers assistance in preoperative counseling and surgical planning, enhancing overall patient care. ## Footnotes * [Disclosure forms](https://www.ajnr.org/sites/default/files/additional-assets/Disclosures/March%202025/0316.pdf) provided by the authors are available with the full text and PDF of this article at [www.ajnr.org](http://www.ajnr.org). ## References 1. 1.Jung H, Woo EJ. Evaluation of mastoid process as sex indicator in modern white Americans using geometric morphometrics. J Forensic Sci 2016;61:1029–33 doi:10.1111/1556-4029.13079 pmid:27364284 [CrossRef](http://www.ajnr.org/lookup/external-ref?access_num=10.1111/1556-4029.13079&link_type=DOI) [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=27364284&link_type=MED&atom=%2Fajnr%2F46%2F3%2F572.atom) 2. 2.Liu L, Arnold R, Robinson M. Dissection and exposure of the whole course of deep nerves in human head specimens after decalcification. 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