Introduction

Apex petrosae is located at the top of the pyramid-like pars petrosa section of the os temporale. It is located near structures such as meatus acusticus internus, auris interna and labyrinthus. Since the apex petrosae constitutes the thickest and densest part of the os temporale, surgical access to this area is quite difficult [1, 2]. The area between the apex petrosae and clivus is called the petroclival region. This region is truncus cerebri, a. carotid interna, a. It is located near many vital structures such as the basilaris, VI and VII cranial nerves. Lesions in the petroclival region are usually petroclival meningiomas and a. It includes rare and complex conditions such as basilaris aneurysms [3]. Surgical interventions to the apex petrosae and petroclival region are performed specifically for the purpose of removing tumors and aneurysms in these regions. Such interventions require preservation of surrounding neurovascular structures using minimally invasive techniques. In terms of surgical approaches, although there are many options such as anterior transpetrosal approach, subtemporal transtentorial approach, retrosigmoid approach, posterior petrosal approach and relevant studies have been reported many times, the most appropriate choice for the operation is extremely difficult. In addition, due to reasons such as anatomical complexity, diversity of tumor spread and tumor recurrence, a uniform standard cannot currently be accepted to determine the superiority of one approach over the other [4, 5].

The anterior transpetrosal approach, introduced by T. Kawase et al. [6] in 1991, is an important surgical method used to access petroclival region lesions. This approach has the advantages of short operating time, appropriate tumor base resection, and less intraoperative bleeding, especially while providing access to lesions in this region [6]. Tumor if it extends laterally beyond the anterior part of the carotid interna, this approach is insufficient and additional bone drilling is required. In such cases, the extended anterior transpetrosal approach, defined as increasing the amount of bone removed from the apex petrosae, has been described [7].

The main purpose of drilling the pars petrosa in the extended anterior transpetrosal approach is to provide maximum exposure of the lesion within the bone, especially for meningiomas that have a broad base in the dura mater of the bone or extend to the clivus region. During this approach, apex petrosa, impressio trigeminalis, eminentia arcuata, and the posterior edge of the pars petrosa are important landmarks. At the same time, for this approach, the apex petrosa and its surrounding canalis semicircularis, labyrinthus and a. Multidimensional anatomical information of some neurovascular structures such as carotid interna is required [7, 8]. All these distances are very important in terms of not damaging the surrounding structures during the drilling of the apex petrosa during the extended anterior transpetrosal approach.

In this study, we aimed to contribute to creating a reference range by examining some morphometric parameters of the region, as well as to shed light on the path to be followed by surgeons dealing with the region.

Methods

Subjects

This retrospective study approved by the local Ethics Committee with an approval number 2024/797. The study performed using temporal CT images of 100 patients (50 female and 50 male, age range 20—65) who presented at the department of radiology between 2010 and 2024. In our study, g power analysis was used to determine this number. The patients who had under 20 years of age, temporal fractures, history of surgery in the region, and the imaging fles with low-resolution were excluded.

Imaging procedure

CT images were obtained using Somatom Drive (Siemens Healthineers, Germany) 256 multislice scanner with the following study parameters; exposure 120 kV, 74 mA, 60 mAs; rotation time 0.28 s; slice thickness 0.625 mm. Images obtained in DICOM format were uploaded to 3D-Slicer, an open-source software platform, and displayed in sagittal, coronal, and axial sections with the help of multiplanar reconstruction. During file scanning, the researcher was blinded to patient information.

Measurements

In our study, the parameters measured and recorded in the coronal sagittal plane are as follows (Figure):

Measurements of the parameters.

a: BTAp — red line, BTTI — yellow line, BTIAC — blue line; b: Ap-TI — blue line, Ap-IAC — red line; c: CCD1 — red line; d: CCD2 — red line.

Bone thickness at apex petrosa (BTAp): We measured the bone thickness at apex petrosa.

Bone thickness at trigeminal impressio (BTTI): We measured the bone thickness at trigeminal impressio.

Bone thickness at internal auditer canal (BTIAC): We measured the bone thickness at internal auditer canal.

Distance between apex petrosa and trigeminal impressio (Ap-TI): We measured the distance between apex petrosa and trigeminal impressio.

Distance between apex petrosa and internal auditer canal (Ap-IAC): We measured the distance between apex petrosa and internal auditer canal.

Distance between petrosa bone and carotid canal (CCD1): We measured the distance between superior margine of the petrosa bone and superior border of carotid canal at apex petrosa.

Distance between petrosa bone and carotid canal (CCD2): We measured the distance between superior margine of the petrosa bone and superior border of carotid canal at İnternal auditer canal

Statistical Analysis

SPSS 25 was used for statistical analysis. The mean, standard deviation, minimum, and maximum for each of measurements were calculated. For statistical comparisons unpaired t-test, and ANOVA were used. A p value <0.05 was considered signifcant.

Results

In our study, 7 different parameters of the pars petrosa of the temporal bone were measured and their average values were determined. The BTAp, BTTI and BTIAC values we found in our study were recorded as 4.40±0.91 mm for women and 4.86±1.20 mm for men, 7.80±2.26 mm for women, 7.76±1.91 mm for men, 9.81±2.27 mm for women and 9.53±1.61 mm for men, respectively. The Ap-TI and Ap-IAC values are recorded as 6.77±2.61 mm in women and 6.78±2.32 mm in men and 13.07±3.65 mm in women and 14.21±3.22 mm in men, respectively. And the CCD1 and CCD2 values are determined as 4.54±1.26 mm in women and 4.20±1.22 mm in men and 13.07±3.65 mm in women and 14.21±3.22 mm in men, respectively. The averages obtained were compared in terms of both gender and lateralization. As a result of the comparison between men and women, BTAp and Ap-IAC were found to be significantly (p<0.05) higher in men than in women (Table 1).

Table 1. Comparison of the obtained data according to gender (mean±SD)(mm)

BTAp: Bone thickness at apex petrosa, BTTI: Bone thickness at trigeminal impressio, BTIAC: Bone thickness at internal auditer canal, Ap-TI: Distance between apex petrosa and trigeminal impressio, Ap-IAC: Distance between apex petrosa and internal auditer canal, CCD1: Distance between superior margine of the petrosa bone and carotid canal at apex petrosa, CCD2: Distance between superior margine of the petrosa bone and carotid canal at internal auditer canal.

CCD2 was significantly (p<0.05) higher in women (Table 1). No statistically significant difference could be detected between BTTI, BTIAC, Ap-TI and CCD1 (p>0.05) (Table 1). The difference between the right and left side averages of the parameters is not statistically significant (p>0.05) (Table 2).

Table 2. Comparison of the obtained data according to lateralization (mean±SD)(mm)

BTAp: Bone thickness at apex petrosa, BTTI: Bone thickness at trigeminal impressio, BTIAC: Bone thickness at internal auditer canal, Ap-TI: Distance between apex petrosa and trigeminal impressio, Ap-IAC: Distance between apex petrosa and internal auditer canal, CCD1: Distance between superior margine of the petrosa bone and carotid canal at apex petrosa, CCD2: Distance between superior margine of the petrosa bone and carotid canal at internal auditer canal.

The averages and comparisons of the data obtained are given in detail in table 1 and table 2.

Discussion

The standard anterior transpetrosal approach is essentially an epidural procedure involving puncture of the petrous apex. This approach requires transection of the middle meningeal artery and dissection of the greater petrosal nerve, which are complex procedures within a narrow epidural corridor [9]. This approach is a sophisticated and compact approach to reach central skull base lesions, especially lesions located in the anterolateral brainstem from the trigeminal root to the porus acusticus [10].

Treatment of petroclival lesions with the anterior transpetrosal approach is difficult due to high mortality and morbidity, and combined surgical approaches and a combination of microscopy and endoscopy are used in the surgical treatment of this region. The anterior transpetrosal approach, introduced by Kawase, is a relatively less aggressive skull base approach to expose the petroclival space and has been used for basilar artery aneurysm clipping. However, if the tumor is large in volume, for example extending beyond the internal auditory canal, the anterior loop of the internal carotid artery, or into the sellar region, an extended intradural anterior transpetrosal approach is required. Epidural or intradural puncture of the petrous bone is performed to create further expansion. The petrous bone is closely associated with important neurovascular structures such as the brainstem internal auditory canal, auris interna, labyrinth and internal carotid artery [7].

The petrous part of the temporal bone is located between the occipital and sphenoid bones. Although it contains the acoustic and vestibular labyrinths, the jugular fossa also borders the facial and carotid canals. It has a base, apex and 3 surfaces. The apex is the angle between the greater wing of the sphenoid bone and the occipital bone, it also forms the edge of the medial opening of the carotid canal. The posterior surface of the petrous bone is adjacent to the posterior fossa and cerebellopontine angle and continues as the mastoid surface. The opening of the internal auditory canal is located on the posterior surface of the petrous bone, in the midline between the roof and base of the petrous bone [11].

Preoperative evaluation should always include medical, neurological, otolaryngological, and radiographic evaluation and should only be performed after careful consideration of the surgical objectives in the context of the risks of perioperative complications that may arise from the surgery [12]. In the transpetrosal approach, drilling is started at the intersection of the mandibular nerve and the petrous ridge using a 3- or 4-mm cutting burr. The cancellous bone within the premeatal triangle is then carefully removed. It is important to preserve the surrounding structures, especially the internal carotid artery [13, 14].

In our study, we measured bone thickness at three different points of the petrous bone, which are the drilling distances for the extended transpetrosal approach. The first of these was the bone thickness at the apex level of the petrous bone. We found this value to be 4.40±0.91 mm in women and 4.86±1.20 mm in men. The difference between men and women is statistically significant. In addition to the apex thickness, we also measured bone thickness at the level of the trigeminal impressio and internal audit canal. We determined the result of these measurements as 7.80±2.26 mm in women, 7.76±1.91 mm in men, 9.81±2.27 mm in women and 9.53±1.61 mm in men, respectively. The difference between these parameters was not statistically significant.

Again, in order to have information about the details of the petrous bone, we measured the distance from the petrous apex to the trigeminal impressio. This distance was found to be 6.77±2.61 mm in women and 6.78±2.32 mm in men. The difference was not statistically significant. The distance of the petrous apex to the internal auditory canal was determined as 13.07±3.65 mm in women and 14.21±3.22 mm in men. It was observed that the averages of men were significantly higher than those of women.

The carotid canal was described as a bony canal directed antero-inferiorly, transmitting the internal carotid artery within its internal curvature. The carotid canal had a rounded external opening lying at the base of skull in the petrous part of the temporal bone. It extended inside the bone forward and medially to open into the middle of the posterior wall of the foramen lacerum [15].

One of the most important issues during surgery is the preservation of intact neurovascular structures. The internal carotid artery is among the structures that must be protected first in many surgeries. Rupture of the internal carotid artery can have profound effects, with massive bleeding being the preliminary symptom. Abundant bleeding can lead to hypotension and shock, intraoperative disorientation, and an unclear surgical field. Sequelaes after injury include altered mental status, paralysis, cranial nerve lesions and even death [16]. In our study, we measured the distance of the canalis caroticus to the highest point of the petrous bone, at the level of the apex of the petrous bone and the internal audit canal. We determined this distance at the petrous apex level as 4.54±1.26 mm in women and 4.20±1.22 mm in men. We recorded the same distance at the level of the internal auditor canal as 7.27±1.57 mm in women and 6.41±1.82 mm in men.

When we scanned the literature, it was understood that the parameters we measured in our study were also measured by Fournier et al [1]. We compared the findings of Y. Cheng et al. [7] with the data obtained in our study in Table 3. The data we obtained are generally compatible with the data of Y. Cheng et al. [7].

Table 3. Comparison of parameters with our study and Cheng et al. (mean±SD)(mm)

BTAp: Bone thickness at apex petrosa, BTTI: Bone thickness at trigeminal impressio, BTIAC: Bone thickness at internal auditer canal, Ap-TI: Distance between apex petrosa and trigeminal impressio, Ap-IAC: Distance between apex petrosa and internal auditer canal, CCD1: Distance between superior margine of the petrosa bone and carotid canal at apex petrosa, CCD2: Distance between superior margine of the petrosa bone and carotid canal at internal auditer canal.

Conclusion

One of the most important issues in surgical interventions is to protect healthy structures, especially vital neurovascular structures. It is necessary to have detailed anatomical knowledge in order to plan a successful surgery and conclude this procedure in a healthy way. We aimed to draw attention to the anatomical details of the region with the results we obtained in our study. We believe that our study will give surgeons an idea in preoperative evaluation.

Participation of authors:

Concept and design of the study — Acar M., Karaköse S.

Data collection and processing — Karaköse S.

Statistical processing of the data — Acar M., Songül Ş., Şeker E.İ.

Text writing — Songül Ş., Şeker E.İ.

Editing — Acar M.