Introduction

The rate of congenital severe hearing loss is approximately 1–3‰ among new-born babies in China. Most of these patients can acquire hearing and speech through cochlear implantation and postoperative speech rehabilitation training [6, 16]. As a result of continuous review and development, cochlear implantation has become an established surgical method. It is considered as a safe and effective otological surgery technique for severe sensorineural hearing loss [3]. However, the surgeon may encounter difficulties due to abnormal anatomical variations in the ear that can lead to operative complications [15]. Therefore, the surgeon needs effective methods for evaluating operative risk that can assure the accurate insertion of implant electrodes in the cochlear scala tympani and effectively prevent complications (especially facial paralysis) [2, 7]. Temporal bone high-resolution computed tomography (CT) can clearly display temporal structures, including the mastoid cavity, ossicles, facial nerve canal, cochlea, and internal auditory canal, and can clearly display disease conditions, such as cochlear malformation and internal auditory canal stenosis [4]. To predict the operative risk and decrease the risk of complications, we retrospectively analysed the preoperative CT scans of 176 cases of cochlear implantation (MED-EL) through the round window between 2013 and 2015. In this study, we measured the distance (d value, from the leading edge of the facial nerve to the posterior wall of the external auditory canal) and the angle (α value, between the line from the leading edge of the facial nerve to the midpoint of the round window membrane and the median sagittal line), and we observed the intraoperative exposure levels of the facial recess and round window niche by retrospectively watching videos of the operations. In addition, we analysed the relationship between these two values and the intraoperative degree of round window membrane exposure to investigate the difficulty of cochlear implantation through the round window membrane using preoperative temporal bone high-resolution CT.

Study methods

Clinical information

The cases observed in this study were patients with profound bilateral sensorineural hearing loss who underwent cochlear implantation (MED-EL Sonata) at our hospital between June 2013 and September 2015. All the operations were performed by two surgeons. There were 176 patients, including 165 right ear implantations, 9 left ear implantations, and 2 binaural implantations. The patients comprised 80 males and 96 females. The ages at surgery were 1.3–15.1 years, with a mean age of 4.4 years. The patients received relevant audiological examinations, including preoperative brainstem auditory evoked potentials and 40-Hz event-related potentials, and imaging evaluations, including temporal bone high-resolution CT, cranial magnetic resonance imaging (MRI), and inner ear hydrography. One month after the cochlear implantation, the cochlear implants were turned on, a temporal bone CT was performed again to assess the condition of the electrodes in the cochleae, and language rehabilitation training was initiated.

Intraoperative observation of the round window niche and round window membrane

After the facial recess was fully opened and the positions of the patient’s head and the microscope were adjusted, the round window membrane was observed through the facial recess. Based on the exposure conditions of the round window membrane, the patients were divided into a complete round window membrane exposure group, a partial exposure group, and an unexposed group. The complete exposure group (group A) was defined as having a round window membrane that was not covered by the facial nerve canal, resulting in the clear observation of the entire round window membrane. The partial exposure group (group B) was defined as having a round window membrane that was partially covered by the facial nerve canal, with only a portion of the round window membrane observable. The unexposed group (group C) was defined as having a round window membrane that was completely covered by the facial nerve canal or a round window niche marker that was not clear; thus, the round window membrane could not be localized.

CT observation content and methods

Scanning

The patient was placed in a supine position. Axial scanning was performed from the mastoidale to the leading edge of the petrous bone that paralleled the canthomeatal line. A LightSpeed VCT XT64-section spiral CT machine (GE, USA) was used. The scanning parameters were as follows: pitch 1.0; tube voltage 140 KV; tube current 140 mA; matrix 512 × 512; spiral time 1.0 s; slice thickness 0.625 mm; and scanning time 4.5–5.5 s.

The method for reviewing CT images and measurement indicators

The vertical distance between the leading edge of the vertical section of the facial nerve and the posterior wall of the external auditory canal (d value) was measured on the round window membrane plane of the preoperative temporal bone CT images [Fig. 1(1)]. The α value of the angle between two lines [the line from the leading edge of the facial nerve on the plane to the midpoint of the round window membrane and the line from the nasal septum or the perpendicular plate of the ethmoid bone to the occipital protuberance (the median sagittal line)] was measured [Fig. 1(2)].

Fig. 1
figure 1

The vertical distance d between the leading edge of the vertical section of the facial nerve and the posterior wall of the external auditory canal. a The linear line from the leading edge of the facial nerve to the midpoint of the round window membrane. b The angle α

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Statistical analysis

The number of cases effectively measured using the two methods was statistically analysed. The d value and α value for the preoperative temporal bone axial CT images are presented as the mean ± standard deviation. The cases in which both the d value and α value could both be measured were analysed using Pearson correlation with SPSS 20.0 statistical software. The intraoperative round window membrane exposure conditions were compared using one-way analysis of variance (ANOVA).

Results

Preoperative CT observation results

Measurement of the d value and α value of the angles

The d value could be measured for 95 of the 176 cases (53.98%). The d value of the vertical distance was 5.53 ± 1.00 mm. Data for the CT images in which the d value could not be measured are shown in Fig. 2. The α value of all 176 cases could be measured (100%). The α value was 62.60° ± 7.12°. A comparison of these two methods is shown in Table 1.

Fig. 2
figure 2

CT images for which the d value could not be measured

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Table 1 Comparison of the methods used to measure the α and d values (n = 176)
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The data from the 95 cases in which the d value and α value could both be measured were used for the statistical analysis. The Shapiro–Wilk normality test results showed that the d value and α value was all obtained from samples with a normal distribution (Pd = 0.270 (> 0.05) and Pα = 0.338 (> 0.05)). A scatterplot of the α value and d value was generated (Fig. 3). The Pearson correlation analysis showed that the d value and α value exhibited a significant negative correlation [r = − 0.395, P = 0.000 (< 0.05)]. The α value of the angles changed with the change in the d value of the distances. When the d value increased, the α value decreased; otherwise, the α value were larger.

Fig. 3
figure 3

The scatterplot of the correlation between the α value of the angles and the d value of the vertical distances

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Observation of the round window niche

The morphology of the round window niche was observed on the plane that had full exposure of the round window membrane on the temporal bone axial CT images. The round window niches were classified as funnel, triangle, semicircle, and closed shapes (Fig. 4) according to the method for classifying round window niche morphology on axial images proposed by Hou et al [5]. The round window membrane could be observed on the corresponding plane of the first three types of round window niches. In the closed type, the round window niche completely covered the round window membrane; therefore, the round window membrane could not be exposed.

Fig. 4
figure 4

Four types of round window niches: a tunnel type, b triangle type, c semicircular type, and d closed type

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Intraoperative round window membrane exposure conditions for all cases

According to the recorded intraoperative round window membrane exposure conditions, the round window membrane was completely exposed in 94 out of 176 cases (group A), which accounted for the majority (53.41%). In 48 cases, the round window membrane was partially exposed (group B) (27.27%). Only 34 cases (19.32%) had unexposed round window membranes (group C; Fig. 5; Table 2). There were two cases of facial nerve palsy after surgery in our study; these patients had angles of 65° and 66° and round windows that were partially exposed and unexposed, respectively.

Fig. 5
figure 5

a complete round window membrane exposure, b partial round window membrane exposure, and c unexposed round window membrane

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Table 2 Intraoperative exposure of the round window and the α values of the preoperative CT
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Correlation between intraoperative round window exposure conditions and α values of all cases

The angles of the three groups were measured and statistically analysed. The angles are presented as the mean ± standard deviation. Among the 176 cases, the angle of the complete round window membrane exposure group (group A) was 60.77 ± 5.63, the angle of the partial round window membrane exposure group (group B) was 63.42 ± 7.04, and the angle of the unexposed round window membrane group (group C) was 66.56 ± 9.05 (Table 2). Observations of the distribution of the α values of these three groups showed that the complete exposure and partial exposure groups accounted for the majority (80.68%). The α value of the complete exposure group was smaller and the α value of the unexposed group was larger than that of the partial exposure group; however, there was some overlap among the three groups (Fig. 6). The homogeneity test of variance results showed that the variance was homogenous. The one-way ANOVA results showed that the differences among the three groups were significant [F = 9.55 and P = 0.000 (< 0.05)]. The pairwise comparison analysis showed that the differences all had statistical significance [PA-B = 0.000 (< 0.05), PB-C = 0.001 (< 0.05), and PA-C = 0.000 (< 0.05)]. These results suggest that the α value sizes were correlated with the intraoperative degree of round window membrane exposure.

Fig. 6
figure 6

Distribution of the α value of the angles in all groups. The long line in the middle was m and the upper and lower short lines were ± s

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Discussion

Temporal bone CT scans can show the air cells and size of the mastoid process and the position of the vertical section of facial recess accurately and are useful preoperative imaging examinations for cochlear implantation [2, 11]. Currently, it is generally believed that the size of the mastoid cavity, the size of the facial recess, and the morphology of the round window niche are important factors that influence cochlear implantation surgery [13]. Our study showed that the α value of the angle between the line connecting the leading edge of the facial nerve to the midpoint of the round window and the median sagittal line measured in the preoperative CT scans was associated with the difficulty of intraoperatively exposing the round window membrane. When the α value was larger than a certain degree, the round window membrane was not easily exposed. The purpose of this study was to determine a simple and practical measurement method using preoperative CT scans to predict several potential risks that surgeons might encounter during cochlear implant surgery. If a patient’s abnormal anatomical variations can be determined before surgery, surgeons can fully prepare for the surgery or preoperatively formulate surgical strategies that could address these issues.

Abnormal anatomical variations, such as anterior position of the facial nerve, may be present [8]; in addition, other undiscovered anatomical variations of the inner ear, such as the presence of a small cochlea, osseous spiral lamina deformities, tympanomeningeal fissures, and hypotympanic air cells, might be present [12]. These conditions suggest that surgeons must carefully examine patients’ CT results, delineate the vertical segment of the facial nerve, and expose the chorda tympani nerve. The posterior wall of the bony external auditory canal can be ground to make it thinner and lower; the bone that wraps the chorda tympani nerve can be ground out and gently pushed forward to fully expose the facial recess, allowing the surgeon to carefully distinguish between the round window niche and the promontorium tympani structure and grind out the bone of the round window niche. If the round window membrane still cannot be exposed, the surgical approach should be changed; the surgeon should determine whether to open the window at the anterior inferior round window membrane or position the window opening at the scala tympani based on other anatomical markers and should insert the electrode through the scala tympani. If the patient has a small cochlea, a short electrode should be prepared. For patients with insertion difficulty, detection of impedance and NRT should be routinely performed. If any abnormalities are detected during the surgery, intraoperative cochlear plain film radiography can be used to ensure that the electrode is implanted into the scala tympani. If the location cannot subsequently be confirmed by intraoperative plain films, emergency temporal bone CT can be performed under anaesthesia to confirm implantation of the electrode into the cochlea.

Factors affecting the size of the facial recess include the location of the vertical section of the facial nerve, the chorda tympani nerve, and the angle of the facial nerve and chorda tympani nerve [10]. The location of the vertical section of the facial nerve is directly associated with the exposure of the round window niche and round window membrane [14]. For example, in patients with an anteriorly positioned facial nerve or posteriorly positioned chorda tympani nerve [1], the facial recess is decreased. Facial nerve damage may occur during surgery if the surgeon does not notice this abnormal anatomy. If the location of the vertical segment of the facial nerve is shallow, the facial recess shows a “deep well” shape, and the round window niche becomes difficult to access; this also affects the exposure of the round window membrane and may cause failure of electrode insertion or lead to facial paralysis. Some researchers measure the shortest distance between the vertical segment of the facial nerve and the posterior wall of the external auditory canal through the round window plane on axial CT to determine the size of the facial recess, which is a simple and practical method [9]. However, in some cases, CT scans cannot show the posterior wall of the external auditory canal, and this distance cannot be measured on the round window niche plane or even on any of the planes above or below the round window niche; therefore, this method cannot be used in all patients to determine the size of the facial recess. In addition, the position of the round window membrane exhibits large variations. The line connecting the leading edge of the facial nerve and the midpoint of the round window membrane can more intuitively determine the difficulty of exposure of the round window membrane through the facial recess approach.

The α value of all patients in this study could be easily measured. However, the d value (the distance between the vertical segment of the facial nerve and the posterior wall of the external auditory canal) could not be measured on CT scans in more than 40% of patients. Correlation analysis was performed on the patients, whose d and α values could both be measured, and the results suggest that these two values have a negative correlation. We considered the measurement of the α value applicable for all patients, while measurement of the d value was only practical for some patients. In addition, the measurement method for the α value could also be used to evaluate the size of the facial nerve access and to preoperatively evaluate the difficulty of exposing the facial recess during cochlear implantation.

Based on the surgical records of two surgeons with many years of cochlear implantation experience at our hospital and retrospective surgery videos, the researchers classified the patients based on round window membrane exposure conditions into a complete exposure group, a partial exposure group, and an unexposed group. The results showed that the size of the α value in these three groups was associated with the round window membrane exposure conditions. When the angle was smaller, exposure of the round window membrane was easier; otherwise, exposure was difficult. There were two cases of transient facial nerve palsy after surgery in our study in patients, whose angles were larger than 65°; these patients’ round windows were partially exposed and unexposed, respectively. Upon review, we believe a large angle is one that is greater than 65°; such angles predict a partially exposed or unexposed round window membrane, conditions associated with increased difficulty of operation and risk of complications of facial paralysis.

Conclusion

The angle (α value) between the line connecting the leading edge of the facial nerve to the midpoint of the round window and the median sagittal line measured in preoperative CT scans was associated with the intraoperative difficulty of exposing the round window membrane. When the α value was larger than a certain degree, the difficulty of exposing the round window membrane was increased. In such cases, the surgeon should fully expose the round window membrane, which could decrease the risk of complications.