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Department of Otolaryngology, Head and Neck Surgery, Beijing Friendship Hospital, Capital Medical University, 95 Yong An Road, Western District, Beijing 100050, China
The relationship between the stapedius muscle and the vertical part of the facial nerve is important for surgery. The study aims to understand the spatial relationship between the stapedius muscle and the vertical part of the facial nerve in ultra-high-resolution computed tomography (U-HRCT) images.
Methods
A total of 105 ears from the heads of 54 human cadavers were analyzed using U-HRCT. The location and direction of the stapedius muscle were evaluated with the facial nerve as the reference. The integrity of the bony septum between the two structures and the distance between the transverse sections were examined. Paired Student's t-test and the nonparametric Wilcoxon test were applied.
Results
The lower end of the stapedius muscle emerged at the upper (45 ears), middle (40 ears), or lower (20 ears) level of the facial nerve and was positioned medial (32 ears), medial posterior (61 ears), posterior (11 ears), or lateral posterior (1 ear) to the facial nerve. The bony septum was not continuous in 99 ears. The distance between the midpoints of the two structures was 1.75 mm (IQR=1.55-2.16 mm).
Conclusion
The spatial relationship between the stapedius muscle and the facial nerve was varied. They were close to each other and in most cases the bony septum was not intact. Preoperative familiarity with the relationship between the two structures is helpful for avoiding unwanted injury to the facial nerve in surgery.
. The localization and protection of the facial nerve is particularly important during surgery, as facial nerve injury leads to nerve palsy, which is one of the most serious complications [
]. Preoperative understanding of the relationship between the facial nerve and the stapedius muscle can help boost the surgeon's confidence, especially when there are lesions in the area or if the facial nerve is abnormally positioned. Temporal bone imaging plays an important role in clarifying the mastoid anatomy accurately. However, the close relationship between the stapedius muscle and the facial nerve may confuse radiologists and otologists, especially residents with little practice interpreting images of this area [
]. Although there are several imaging studies on the facial nerve in the temporal bone, few have investigated the imaging of the stapedius muscle and its relationship with the facial nerve [
Today, multi-slice computed tomography (MSCT) is the best option for imaging the temporal bone. However, MSCT requires high radiation doses, making it unsuitable for young patients or patients suffering from chronic ear diseases who require regular imaging sessions [
Dyna-CT of the temporal bone for case-specific three-dimensional rendering of the stapedial muscle for planning of electrically evoked stapedius reflex threshold determination during cochlear implantation directly from the stapedius muscle via a retrofaci.
A comparative study for image quality and radiation dose of a cone beam computed tomography scanner and a multislice computed tomography scanner for paranasal sinus imaging.
. Cone-beam computed tomography (CBCT) uses a lower radiation dose and provides better spatial resolution than conventional MSCT. In the last 10 years, CBCT has been used in an increasing number of studies to obtain images of the head and neck region, especially the temporal bone [
A comparative study for image quality and radiation dose of a cone beam computed tomography scanner and a multislice computed tomography scanner for paranasal sinus imaging.
Imaging re-evaluation of the tympanic segment of the facial nerve canal using cone-beam computed tomography compared with multi-slice computed tomography.
Imaging re-evaluation of the tympanic segment of the facial nerve canal using cone-beam computed tomography compared with multi-slice computed tomography.
]. To resolve this issue, we designed a new device specialized for temporal bone imaging based on CBCT, named ultra-high-resolution computed tomography (U-HRCT). Compared with the current generation of MSCT, U-HRCT offers higher spatial resolution and lower radiation dose, which is well-suited to studying the tiny structures in the temporal bone [
Thus, we aimed to evaluate the relationship between the stapedius muscle and the facial nerve via U-HRCT to help provide a reference for accurate localization of the facial nerve during surgery.
2. Materials and methods
2.1 Study specimens
Between December 2019 and January 2020, temporal bones in 54 formalin-fixed heads of human adult cadavers were examined using the U-HRCT scanner (Ultra3D-SE, LargeV, Beijing, China). None of the specimens were deformed. If the target area contained abnormalities that affected the image, or if the field of view did not fully capture the region of interest, the specimen was excluded. This study was approved by the Institutional Review Board of Beijing Friendship Hospital (IRB: 2018-P2-210-02).
2.2 Image acquisition
All images were acquired on the same U-HRCT scanner with the following parameters: 100 kV; 3.5 mA; 6.5 cm × 6.5 cm field of view; 0.1 × 0.1 × 0.1 mm3 voxel size; 40 s exposure time; 370 images. Images of the unilateral temporal bone were acquired after each scan, exported in DICOM file format, and transferred to a workstation for three-dimensional reconstruction (RadiAnt DICOM Viewer; RadiAnt, Poland).
2.3 Image analysis
Multiplanar reformations were generated using the RadiAnt workstation. Standard axial and coronal image reconstructions were performed based on the vertical part of the facial nerve and the stapedius muscle. The standard axial plane was perpendicular to the vertical part of the facial nerve. The standard coronal plane was perpendicular to the standard axial plane and parallel to the middle line between the center of vertical part of the facial nerve and the stapedius muscle (Fig. 1). The range of the observation was from the lower end of the stapedius muscle to the base of the pyramidal eminence.
Fig. 1Reconstruction of standard axial and coronal U-HRCT images. On the (a) sagittal and (b) coronal views, the axial location line (yellow line) was adjusted until it was perpendicular to the vertical part of facial nerve (thick arrow) to obtain the standard axial image (c). (d) On the standard axial view, the coronal location line (pink line) was drawn to connect the middle point of the facial nerve (thick arrow) and the stapedius muscle (fine arrow). (e) On the sagittal view, the coronal location line (pink line) was adjusted until it was parallel to the vertical part of the facial nerve to obtain the standard coronal image (f)
With the vertical part of the facial nerve as the reference, the evaluation metrics were as follows:
(1)
Location of the stapedius muscle
As the location of the upper part of the stapedius muscle is fixed in the pyramidal eminence, the view of the stapedius muscle was determined by the location of the lower end. On standard coronal images, the vertical part of the facial nerve was divided into three equal parts. The position of the lower end of the stapedius muscle relative to the different parts of the facial nerve was observed (Fig. 2).
(2)
Direction of the stapedius muscle when extending upward
Fig. 2The location of the lower end of the stapedius muscle on standard coronal U-HRCT images. All the images denote the left side. Taking the vertical part of the facial nerve (thick arrow) as the reference, the lower end of the stapedius muscle (fine arrow) emerges at the upper (a), middle (b), and lower (c) levels
On standard axial images, the direction of the stapedius muscle as it ran from the inferior to the superior relative to the facial nerve was evaluated with the posterior wall of the external auditory canal as the reference line (Fig. 3).
(3)
Connection of the stapedius muscle canal and the facial nerve canal
Fig. 3The location of the lower end of the stapedius muscle on standard axial U-HRCT images. All the images denote the right side. The inset images are (a) coronal or (b-d) oblique sagittal views used to indicate location. The long dotted line is parallel to the posterior wall of the external auditory canal. The lower end of the stapedius muscle (fine arrow) is located medial (a), medial posterior (b), posterior (c), and lateral posterior (d) to the facial nerve (thick arrow)
On standard axial and coronal images, the integrity of the bony septum between the stapedius muscle canal and the facial nerve canal was observed. The positions of bone defects were described as two types: involving the lower end of the stapedius muscle canal; and sparing the lower end of the stapedius muscle canal (Fig. 4).
(4)
Distance between the stapedius muscle and the facial nerve
Fig. 4The bony septum between the stapedius muscle canal and the facial nerve canal on standard coronal U-HRCT images. All the images denote the right side. Bone defects are found in (a) and (b), and the facial nerve canal (thick arrow) and the stapedius muscle canal (fine arrow) seem to be connected. In (a), the lower end of the stapedius muscle canal is affected, and in (b), the lower end is spared (arrow head). The bony septum is intact in (c), and the two structures are independent
On standard axial images, the distance between the middle points of the stapedius muscle and the facial nerve was measured at the level of the thickest part of the stapedius muscle (Fig. 5).
Fig. 5Distance measurement. The distance between the stapedius muscle (fine arrow) and the facial nerve (thick arrow) on standard axial U-HRCT images
Images were reviewed by one radiologist (with 20 years of experience) and one otologist (with 12 years of experience). Differences in qualitative assessments were resolved after discussions. The mean of the measurements was calculated for quantitative variables.
2.4 Statistical analysis
Statistical analyses were performed using SPSS (version 22.0; IBM). The Kolmogorov-Smirnov test was used to determine the normality of quantitative variables. Paired Student's t-test and the nonparametric Wilcoxon test were applied to evaluate interobserver consistency and to assess differences between bilateral sides in normally and nonnormally distributed data, respectively. Paired Wilcoxon tests were used to examine the difference between the distance of the stapedius muscle and the facial nerve. Normally distributed quantitative data was described by the mean and standard deviation, and non-normally distributed data were described by the median and interquartile range (IQR). A P value <0.05 was considered statistically significant.
3. Results
In total, 108 ears were obtained from 54 specimens. Images of 105 ears were included, and 3 ears were excluded because of the structural destruction of the mastoid region, which affected the observation. Out of these 105 ears, 104 ears (52 specimens) were pairs, and one ear was unilateral.
The stapedius muscle and the facial nerve were demonstrated clearly in all images of the 105 ears.
There were no statistically significant differences between the conclusions of the two reviewers in location (P=0.564), direction (P=0.131), and distance (P=0.535). The observation of bone defects was consistent between the two reviewers.
(1)
Location of the stapedius muscle
In the mastoid process of the temporal bone, the lower point of the stapedius muscle emerged at the level of the lower (20/105 ears), middle (40/105 ears), or upper (45/105 ears) part of the facial nerve (Fig. 6a). The stapedius muscle could be observed from the location of appearance upward to the pyramidal eminence.
(2)
Direction of the stapedius muscle when extending upward
Fig. 6Scatterplot of the location and direction of the lower end of the stapedius muscle. (a) shows the numbers of lower ends of the stapedius muscle that emerged at different levels of the facial nerve. (b) shows the number of lower ends of the stapedius muscle located at different directions relative to the facial nerve.
The lower point of the stapedius muscle was situated medial (32/105 ears), medial-posterior (61/105 ears), posterior (11/105 ears), or lateral-posterior (1/105 ears) to the vertical part of the facial nerve (Fig. 6b). Then, the stapedius muscle ran inward and upward into the pyramidal eminence.
(3)
Connection of the stapedius muscle canal and the facial nerve canal
Of the 105 ears, bony defects between the stapedius muscle canal and the facial nerve canal were found in 99 ears, among which the lower end of the stapedius muscle canal was involved in 60 ears, and the upper or middle segment was involved in 39 ears sparing the lower end of the stapedius muscle canal. With these bony septum defects, the stapedius muscle appeared to be connected with the facial nerve, resembling branches from the nerve. In the other 6 ears, the bony septum between the stapedius muscle canal and the facial nerve canal was not damaged, and the stapedius muscle appeared to run independently relative to the facial nerve.
(4)
Distance between the stapedius muscle and the facial nerve
At the level of the thickest part of the stapedius muscle, the distance between the midpoint of the stapedius muscle and the facial nerve was 1.75 mm (IQR=1.55-2.16 mm).
There were no statistically significant differences between the left and right sides for any evaluation metric (P>0.05) (Table 1).
Table 1Observations of the stapedius muscle and the facial nerve on U-HRCT images.
Observations
Total
Left
Right
P
Emergence
Lower
20 (19.0%)
11
9
0.86
(ears)
Middle
40 (38.1%)
18
22
(n=105)
Upper
45 (42.9%)
24
21
Direction
Medial
32 (30.5%)
17
15
0.41
(ears)
Medial-posterior
61 (58.1%)
31
30
(n=105)
Posterior
11 (10.5%)
4
7
Lateral-posterior
1 (1.0%)
1
0
Connected
Yes
99 (94.3%)
50
49
0.16
(ears)
Lower end
60
27
33
(n=105)
Upper/middle part
39
23
16
No
6 (5.7%)
3
3
Distance (mm)
1.75 (1.55, 2.16)
1.74 (1.57, 2.15)
1.75 (1.54, 2.15)
0.85
Abbreviations: U-HRCT, ultra-high-resolution computed tomography. Median and IQR are reported for distance.
Not many radiologists are familiar with the imaging of the stapedius muscle, a structure that may inform facial nerve localization during surgery, thereby making it an important target in preoperative imaging. This study evaluated the relationship between the stapedius muscle and the facial nerve using U-HRCT, providing a basis for accurate localization of the latter during surgery.
The vertical part of the facial nerve is known to be the most variable, and can be vulnerable during mastoidectomy and transcanal surgical approaches [
]. Familiarity with the imaging of the facial nerve and its neighbors can help clinicians to acquire correct image information and avoid misdiagnosis. The stapedius muscle and the facial nerve both run longitudinally through the mastoid process of the temporal bone. They are situated close to each other, with a middle point distance of about 2 mm. The relative spatial relationship between the stapedius muscle and the facial nerve is variable. The stapedius muscle was reported to originate from the posterior belly of the digastric muscle and pass through the stylomastoid foramen [
]. In scans, the lower end of the stapedius muscle can emerge at the level of the lower, middle, or upper part of the mastoid segment of the facial nerve, with more than half of the cases emerging in the upper part. In only a few cases, the lower end of the stapedius muscle was observed to be connected to the stylomastoid foramen, potentially because the scan was unable to distinguish the lower part of the stapedius muscle running through the facial canal. As a result, further histological investigation is needed. Therefore, as a close neighbor of the vertical part of the facial nerve, the stapedius muscle can emerge at any height near the facial nerve, though it is most commonly observed at the upper part. In addition, the direction of the lower end of the stapedius muscle relative to the facial nerve was also varied. Taking the posterior wall of the bony external auditory canal as the reference, the end of the stapedius muscle was observed to emerge on the medial, medial-posterior, posterior, or lateral-posterior side of the facial nerve. The stapedius muscle then went upwards into the pyramidal eminence, which lies on the medial side of the facial nerve. Therefore, although the stapedius muscle and the facial nerve both ran vertically through the mastoid, they were not exactly parallel. The stapedius muscle with a medially located lower end was approximately parallel to the facial nerve, while the other stapedius muscles ran diagonally at different angles relative to the facial nerve. In a similar study, Rubini A et al. found that the inferior part of the stapedius muscle was medial and parallel to the facial nerve, which is partly consistent with the results of this study, but the variability was not mentioned [
]. The spatial variability of the stapedius muscle suggested that its level and location should be duly noted before mastoid surgery to minimize unwanted damage.
Several authors have described bifurcations of the facial nerve in the mastoid [
. In the current study, we found that the bony septum between the two structures was not intact in most ears (99/105 ears), which made them seem connected. In particular, when the defect was located at the lower end of the stapedius muscle, the stapedius muscle looked very much like a branch from the facial nerve. Radiologists and otologists should be aware that discontinuous bony septum between the stapedius and the facial nerve is common to avoid mistaking the stapedius muscle for facial nerve bifurcation. Careful evaluation is needed before surgery to avoid misjudging the ‘bifurcation’ as a facial nerve and causing damage to the real facial nerve. Bony septum defects between the stapedius muscle and the facial nerve were also reported in previous studies. In Rubini A et al.’s anatomical study, bony separation between the inferior part of the stapedius muscle and the facial nerve was not observed in 11 cadaver ears [
]. During the first period of embryonic development, Cisneros observed that the stapedius muscle and the facial nerve were housed in a common compartment. From week 35, a bone partition began to develop and separate the two structures. However, this partition had not developed in all the cases observed in this previous study, thus, the partition might be absent in some cases.
In the present study, the relationship between the stapedius muscle and the facial nerve was clearly displayed in images acquired by U-HRCT. Compared with MSCT, U-HRCT requires a significantly lower radiation dose while ensuring higher image quality of the fine bone structures in the temporal bone due to its higher spatial resolution. According to a previous study, the radiation dose of U-HRCT is about one third of that of MSCT during a unilateral temporal bone scan and the U-HRCT images are clearer [
]. This is the first time that the relationship between the stapedius muscle and the facial nerve was evaluated using U-HRCT. In this study, the bony canal of the two structures were presented clearly, providing the basis for image evaluation.
There were a few limitations to our study. First, because this was a pilot study, we imaged the heads of human cadavers instead of live patients. Thus, there were some differences between this in vitro study and true clinical examinations. For example, the scanning conditions would not be the same, and optimal conditions for clinical observations need to be determined. In addition, focus should be placed on imaging artifacts, especially motion artifacts, which were not present in this in vitro study. Second, a comparative anatomical study is required to verify the results of the present study. However, the number of specimens used here was limited, and it was difficult for us to dissect the specimens. Further research will be carried out in the future.
This study revealed that the spatial relationship between the stapedius muscle and the vertical part of the facial nerve was variable, and the bony septum between the two structures was not intact in most cases in U-HRCT images, which made them appear to be connected, resembling branches. These findings suggested that physicians should perform a full radiological evaluation before mastoid surgery to avoid injury to the facial nerve.
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
Funding
This work was supported by the Beijing Scholar [(2015)160]; and the Capital's Funds for Health Improvement and Research [2022-1-1111].
References
Cisneros A
Orozco JRW
Nogues JAO
Gotor CY
Orozco AW
de la Torre MAC
et al.
Development of the stapedius muscle canal and its possible clinical consequences.
Dyna-CT of the temporal bone for case-specific three-dimensional rendering of the stapedial muscle for planning of electrically evoked stapedius reflex threshold determination during cochlear implantation directly from the stapedius muscle via a retrofaci.
A comparative study for image quality and radiation dose of a cone beam computed tomography scanner and a multislice computed tomography scanner for paranasal sinus imaging.
Imaging re-evaluation of the tympanic segment of the facial nerve canal using cone-beam computed tomography compared with multi-slice computed tomography.
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