Clinical correlation research of 3D reconstruction of retrolabyrinthine space based on HRCT of temporal bone

Abstract Background: The retrolabyrinthine approach helps clinicians perform complex surgeries such as vestibular neurectomy, resection of petrous apex cholesteatoma, or use this space to complete endoscopic combined with microscope surgical operations in a relatively safe buffer space. Some of our current studies using 3D reconstruction in the clinic have also helped us perform some complex surgical procedures.Objective: This study aims to reveal the relationship between important structures in retrolabyrinthine space through objective parameters. These measurement data help clinicians locate intraoperatively and provide a reference for clinical surgery. Also, we are intended to help improve surgical techniques and expand the operating space to increase reachable anatomic structure.Material and Methods: The inner structures of the temporal bone from HRCT (High-resolution computed tomography) images which were taken at the Eye & ENT Hospital of Fudan University were reconstructed. Precise measurement of the structures was accomplished by using the software 3D-Slicer (3D Slicer, https://www.slicer.org/; version 4.8.0, Massachusetts, USA).Results: 3D model of temporal bone structures, including the cochlea, semicircular canals (SCCs), the internal auditory canal (IAC), facial nerve (FN), jugular bulb(JB), and carotid artery was reconstructed. The combination of HRCT and 3D models is utilized to analyze the Quantitative data of the retrolabyrinthine space and its adjacent structures.Conclusions and Significance: 3D reconstruction of CT images clearly displayed the detailed structures of the temporal bone. Surgical adaptability of the retrolabyrinthine approach can be assessed preoperatively by image and other methods, and anatomical parameters play an important role in the retrolabyrinthine space. Therefore, this study helps to skeleton the bone as much as possible to expand the surgical space, so that the surgeon can contact the anatomical structure more diversified to expand the surgical indications.


Introduction
Retrolabyrinthine approach was first defined by Pulec and Hitselberger in 1972 [1], and later elaborated by Silverstein and colleagues [2,3].The retrolabyrinthine approach helps clinicians complete complex clinical procedures in a relatively safe buffered space, such as the lesions in the region of the cerebellopontine angle (CPA), tumors of the internal auditory canal (e.g.auditory neuroma), and cholesteatoma of the petrous apex [4][5][6].In the preliminary work of this research, we accurately measured the spatial distance between the facial nerve and its surrounding anatomical structures using 3D-Doctor software [5,7].Previous anatomical studies of the retrolabyrinthine approach mainly focused on the jugular bulb, sigmoid sinus, and rarely anatomical measurements of retrolabyrinthine space.So this study used CT imaging to establish a 3D model and analyze relevant parameters to evaluate how much we can overcome the narrowness of the surgical corridor and guide the selection of the surgical approach.Measuring the anatomy of the retrolabyrinthine space helps clinicians performing surgery in this area to provide a safety margin assessment and to master the clinical application of this approach.

CT image data:
We randomly selected HRCT images (including sagittal, coronal, and transverse planes) of diseaseless temporal bones (excluding history of chronic otitis media, history of trauma and history of surgery, etc.) that were taken at the Eye & ENT Hospital of Fudan University, and selected 56 sides of healthy temporal bone HRCT images of 28 cases.High riding JB being a limiting anatomic variation to the retrolabyrinthine approach was previously shown by Ferreira Bento et al. [8].During the reconstruction process, 3 cases of high riding JB were found, one of which was unilateral (right).So, this study included 25 cases of bilateral temporal bone, 1 case of unilateral (left) temporal bone, 14 cases of male cases, and 12 cases of female cases, with a mean age of 44 ± 16 years.Using Siemens Samatoma Definition Edge128-slice spiral CT, the scan position was taken in a supine position in the Eye and ENT Hospital of Fudan University in 2018.Transverse scans were acquired in the helical mode with a tube voltage of 120.0 kV and a current of 250.0 mA.The images were 0.60-mm-thick sections and 512 × 512 matrices, and display field of view (DFOV) of 22.0 × 22.0 cm.All images were displayed at a window center of 700 HU and a window width of 4000 HU.This study was in accordance with protocols approved by the Institutional Research Review Board at EENT Hospital affiliated with Fudan University, Shanghai, China.

Reconstruction and measurement
The image data sets were loaded into 3D slicer freeware (3D Slicer, https://www.slicer.org/;version 4.8.0,Massachusetts, USA) followed by segmentation of the cochlea along with IAC, FN, SCCs, JB, vestibular organ and carotid artery.Segmentation of these structures was performed as precisely as possible in the axial plane in every slice.From this, we can create a 3D model that can be freely moved around the screen and viewed from any angle.The reconstructed image is given in Figure 1.
In this study, the posterior semicircular canal (PSC) and the lateral semicircular canal (LSC) were used as references.The intersection of two semicircular canal planes was used as the localization point to help locate the orientation intraoperatively.In the 3D model, the LSC plane and the PSC plane can be outlined, and the intersection point of the two planes is point A, which can be positioned to the transverse plane of the HRCT image.The outermost point of the posterior margin of the IAC is positioned in this plane at point B. The distance from the PSC to the IAC is connected to the two points AB (as in Figure 2).
The intersection of the line perpendicular to AB with the posterior margin of the petrous bone is made at point A, which is the distance of the PSC from the posterior margin of the petrous bone.The distance between the LSC and the posterior margin of the petrous bone can likewise be measured (Figure 3).Through the temporal bone MRP (Multiplanar Reconstruction) images, point A was localized at the coronal and sagittal planes, and a straight line perpendicular to the transverse plane to the inferior margin of the middle cranial fossa (MCF) brain plate distance was made upward according to the coordinates.The angle between the posterior semicircular canal plane and the internal auditory canal can be measured by making the PSC plane and the IAC posterior margin in the 3D model.
It is difficult to accurately locate the common bony crus (CBC) on CT images of the temporal bone.Therefore, the 3D model was reconstructed to determine the position of the CBC and to locate the C point on the transverse plane of the CT image (Figure 4).The closest distance between the posterior margin of the IAC and point C is the distance CD from the CBC to the IAC.
The intersection of the vertical line of CD with the posterior margin of the petrous bone is the distance from the CBC to the posterior margin of the petrous bone (Figure 4).The plane of the superior and inferior margins of the IAC were made on the 3D-model, and the vertical distance between the two planes was measured.The t-test was applied to the side and gender, and there was no statistical difference with p > .05.

Results
The mean distance from the PSC to the posterior margin of the petrous bone was 4.04 ± 1.05 mm, the mean distance from the PSC to the IAC was 12.41 ± 0.79 mm, and the mean distance from the PSC to the middle cranial fossa was 8.57 ± 1.28 mm.The mean distance from the CBC to the posterior margin of the petrous bone was 4.78 ± 1.67 mm, the mean distance from the CBC to the IAC was 5.05 ± 0.86 mm, the mean distance from the LSC to the posterior margin of the petrous bone was 9.27 ± 1.49 mm, and the mean vertical distance from the IAC was 5.81 ± 0.90 mm.The mean angle between the PSC plane and the posterior margin of the IAC was 125.26 ± 20.58°.The measured data were tested for normal distribution separately, and they all conformed to normal distribution.The t-test was applied for side and gender, and p > .05,there were not significant differences (Table 1, Table 2).

Discussion
The retrolabyrinthine approach ensures access to the higher surgical success with less damage to the surrounding anatomical structures is targeted in practices.The retrolabyrinthine space can be considered as a safe area in which there are no important blood vessels, nerves, and vital anatomical structures [9].The boundary of the space that can be identified in the surgery with retrolabyrinthine approach: the anterior border is the PSC plane, and the posterior border is the posterior margin of the petrous bone.The upper border is the middle cranial fossa meninges, the lower border is the JB, the shallow interface is the apex of the PSC, and the deep interface is the posterior margin of the IAC [10].
Previous studies about anatomical landmarks of the retrolabyrinthe approach mainly focused on the position of the sigmoid sinus (SS), the height of the JB, and the position of the facial nerve.The distance to the IAC was found to be 19.06 ± 3.19 mm for the retrolabyrinthine approach, which is a shorter surgical route compared to other surgical approaches.Fernando Alonso found that when the sigmoid sinus was completely contoured, the distance between the two semicircular canals and the SS was increased to expand the operating space [11].Some researchers found that ligating or temporary clamping of the sigmoid sinus to enlarge the view of the retrolabyrinthine space is permission with a venous pressure increase of less than 10 mmHg [12][13][14].The semicircular canals bone was first revealed after contouring Trautmann's triangle [15][16][17].The bony semicircular canal is a relatively stable structure [17,18], and with the help of bony landmarks can help the surgeon to locate it intraoperatively.
Two critical points of the posterior labyrinth have to be identified: the loop of the LSC and the common bony crus, defined as the junction between the posterior and superior SCCs [17].So based in the previous study, we think that the SSCs and common bony crus were used as localization points to measure relevant anatomical data.The measurements were performed in combination with HRCT and 3D models of the temporal bone to avoid measurement errors due to structural damage during dissection.
After the mastoidectomy, the part of the posterior and lateral SCCs bone were revealed to determine the anatomical locus A which as mentioned above.In this study, it was measured that the posterior margin of the internal auditory canal could be reached by continuing to a depth of approximately 12.41 ± 0.79 mm from this locus, and the SD was <1.This anatomical distance is a value with a small measure of dispersion, which can reflect the small variability of this structure.After approaching the posterior margin of the IAC, the angle between the plane of the PSC and the IAC was obtained by measurement as 125.26 ± 20.58°.We measured the mean value of the vertical height of the IAC as 5.81 mm, with a range of 4-7.5 mm.Operating within a certain distance when the surgeon reaches the posterior margin of the IAC can prevent damage to the nerve.The common bony crus is also a site prone to injury in the retrolabyrinthine space [19], and analysis revealed that the anatomical relationship between the CBC and the IAC is also more stable.Thomas et al. found the median length of the internal auditory canal to be 12 mm by anatomical measurements and analogized the retrolabyrinthine operational space to a cone [20].For the retrolabyrinthine approach, the anteriorposterior surgical freedom ranged 6-23 degrees.The superior-inferior surgical freedom ranged 33-46 degrees [20].In combination with this study, the mean distance between the LSC and the posterior margin of the petrous bone was 9.27 mm, the mean distance between the common crus and the posterior margin of the petrous bone was 4.78 mm, and the mean distance between the PSC and the middle cranial fossa was mm, thereby reducing the surgical risk of cerebrospinal fluid leakage that is predicted to occur when performing operations of retrolabyrinthine space.
There are many authors in the past have demonstrated through extensive clinical data that the retrolabyrinthine approach is successful in the complete resection of different types and sizes of lesions with corresponding functional preservation [8,21].Studies have shown that the retrolabyrinthine approach is better for hearing preservation surgery in the internal auditory canal, and Bento completed 22 cases of small <1.5 cm auditory neuromas confined to the internal auditory canal with successful preservation of hearing and facial nerve [8].Wang et al. found that the retrolabyrinthine approach procedure for small vestibular schwannoma is a safe and effective surgical method that can achieve almost 100% facial nerve function and 50% hearing persation [22].And it has been reported in the literature that the retrolabyrinthine approach able to accomplish tumor excision and cochlear implantation (CI)which was beneficial to hearing rehabilitation, in one stage without other skin incisions [23].Therefore measuring critical structures within a standard internal coordinate system better informs the clinician for surgical planning.More room to maneuver helps us to both remove the tumor and preserve the function at the same time.Using 3D reconstruction studies is now also helping us to perform some complex surgeries in the clinic.However, when complete exposure of the internal auditory canal is not always possible, the combination of endoscopy and microscopy can help the operator obtain a wider field of view.Muelleman et al. found that the retrolabyrinthine approach using microscopy alone can expose 69%-91% of the IAC, which can be completely exposed with the help of different angled endoscopes [24][25][26].The 3D model shows that the narrower space of the retrolabyrinthine approach is the distance between the PSC and the posterior margin of the petrous bone, which was found to be 5.98 mm at the maximum and 4.04 mm at the average.The diameter of the endoscope is 4, 3, and 2.7 mm, we think that a thinner endoscope can create more space for other instruments and help to obtain a more angular view.
The advantages of retrolabyrinthine approach include the ability to preserve important anatomical structures and their corresponding functions as much as possible.In particular, the avoidance of unnecessary retraction of the cerebellum, the extradural and direct access to the IAC and the tumor [27].Otherwise, this approach affords excellent visualization of CN III to XI which provides a trajectory for resecting lesions seated at the petrous apex or even lateral clivus [19].Besides these common advantages, the surgical corridor can be broadened by adding various modifcations and the accessible anatomic structures can be diversified.And the applicability of this surgical approach has become easier with the help of endoscopy.But retrolabyrinthine approach is time-consuming with the narrowness of the surgical area.The data measured in this study help the surgeon to more accurately orient himself intraoperatively and minimize unnecessary damage to the anatomical structures of this approach.The limitation of this study is that the current study only measured the corresponding structures in imaging and 3D models, without anatomical comparisons in the cadaver head.

Conclusion
The retrolabyrinthine space gives the surgeon a relatively safe buffer area.Furthermore, the procedure should be achieved with minimal disturbance of the anatomic structures and should be easily reproduced by other surgeons with minimal technical difficulty and limited operating time.The surgical suitability of retrolabyrinthine approach can be evaluated preoperatively by imaging and other ways, therefore, this study helps the surgeon to expand the surgical space by skeletoning the bone as much as possible and helps the surgeon to expand the surgical indications by accessing diversity anatomical structures.

Significance
The 3D reconstruction can help us to present the maximum possible realism of the internal structure of the temporal bone.The three-dimensional structure of the posterior labyrinthine space is reconstructed to the greatest extent possible so that the parameters between the important anatomical structures behind the labyrinth can be accurately measured.Compared to physical dissection, it can reduce the expenses and the errors caused by improper operation during dissection.

Figure 1 .
Figure1.Facial nerve and iAc nerve (yellow), cochlea and semicircular canals' bony labyrinth (green), jugular fossa (blue), and carotid artery (red).it is surrounded by a cranium that can be made transparent.

Figure 2 .
Figure 2. in the 3D model, the LSc plane and the PSc plane can be outlined (figure 2a), and the intersection point of the two planes is point A, which can be positioned to the transverse plane of the Hrct image.the outermost point of the posterior margin of the iAc is positioned in this plane at point B. the distance from the PSc to the iAc is connected to the two points AB (Figures 2b, 2c).

Figure 3 .
Figure 3. the intersection of the line perpendicular to AB with the posterior margin of the petrous bone is made at point A, which is the distance of the PSc from the posterior margin of the petrous bone.the distance between the LSc and the posterior margin of the petrous bone can likewise be measured.

Figure 4 .
Figure 4. 3D-model was reconstructed to determine the position of the cBc and to locate the c point on the transverse plane of the ct image.the intersection of the vertical line of cD with the posterior margin of the petrous bone is the distance from the cBc to the posterior margin of the petrous bone.

Table 1 .
Maximum, minimum and average values of measurment structure.

Table 2 .
the left side, right side and total values of measurement structure.