The concept of lateral skull base is currently adopted by the van Huijzen zoning method [1] at home and abroad, that is, an extension line is made under the skull base along each of the infraorbital fissure and the rock-occipital fissure, which intersects inwardly at the nasopharyngeal roof and points outwardly to the zygomatic bone and the posterior border of the mastoid process, respectively, and the triangular area between the two lines is called the lateral skull base. It includes the parapharyngeal gap, infratemporal fossa, pterygopalatine fossa and the important structures within them, and involves neurosurgery, maxillofacial surgery, otorhinolaryngology-head and neck surgery, plastic surgery and other multidisciplinary disciplines, and has been a hot spot and a difficult area in skull base surgery. Therefore, more microscopic anatomical and imaging studies have been performed on this region [2-6]. However, it is difficult to observe and measure the deep foramina and irregular cavities in cadaveric anatomical studies alone, and it is difficult to reflect the complex spatial relationships in 2D imaging studies such as CT and MRI. In this study, the Destroscope virtual reality system was used to study the important bony structures of the lateral skull base and explore their significance as surgical landmarks.
I. Materials and methods
1. Study materials: 15 cases (30 sides) of national adult cranial wet specimens adequately fixed by 10% formalin.
2. Imaging equipment: PHILIPS Brilliance 64-row CT, GE 3.0T MRI, Destroscope virtual reality system (Volume Interaction, Singapore, software: RadioDexter TM 1.0).
3. Image scanning and 3D reconstruction methods: cranial CT scan (FOV 26, Rotation Time 0.75s, Matrix 512×512, window position: 40, window width: 300, layer thickness: 0.67mm). The raw data of CT and MRI scans in Dicom format were imported into the Destroscope virtual reality system on CD-ROM for 3D reconstruction, fusion, observation and measurement.
II Results
The important bony structures of the lateral skull base can be summarized as 1 point, 2 spines, 3 fissures, 4 lines, and 5 foramina.
Point 1: The intersection of the temporal pterygoid suture and the infratemporal crest was defined as point “O”, and the distance from point “O” to each bony landmark was measured, and there was no statistically significant difference between the left and right sides (Figure 1, Table 1).
Table 1: The distance from the “O” point to the important bony landmarks at the lateral skull base (x±s, unit: mm)
Item
Mean value (n=30)
Range
Left(n=15)
Right side(n=15)
P-value
O point-front edge of the root of the outer wing plate
20.83±2.63
15.14 to 27.79
21.37±2.46
20.29±2.77
0.05
O point – posterior edge of the root of the outer wing plate
22.61±2.22
19.04～27.90
22.40±2.18
22.83±2.32
0.48
O point – anterior border of the inferior temporal crest
17.00±2.47
14.21 to 24.89
17.10±2.94
16.90±1.98
0.70
O point – anterior border of temporomandibular joint
17.17±3.22
11.67 to 27.66
16.80±3.44
17.54±3.05
0.32
O point – oval hole
22.01±3.02
16.20-28.05
21.54±3.19
22.48±2.86
0.12
O-point-spinaculum
24.42±2.79
19.09 to 29.97
23.96±2.88
24.89±2.72
0.20
O point – root of butterfly spine
26.34±2.62
20.82～31.43
25.88±2.83
26.79±2.41
0.17
2 spines: the butterfly spine and stem spine were the two obvious bone spines at the lateral cranial base, and the distances from both to each bone foramen were measured, and the differences were not statistically significant when comparing the left and right sides, except that the distance from the stem spine to the posterior edge of the rupture foramen was smaller on the left side than on the right side (Figure 2, Table 2).
Table 2:The distance between the stem protrusion and the pterygoid spine to each bone foramen (x±s, unit: mm)
Item
Mean value (n=30)
Range
Left side(n=15)
Right side(n=15)
P-value
Stromal-pterocarpus
19.18±2.25
15.18 to 23.83
19.43±2.58
18.94±1.94
0.50
Stromal-carotid canal
12.03±3.45
7.08 to 27.50
11.03±1.53
13.03±4.50
0.08
Stromal-jugular foramen
5.78±1.83
2.35～9.22
6.13±1.80
5.44±1.86
0.17
Stromata- foramina
20.80±2.27
16.35～24.73
20.72±2.37
20.89±2.24
0.81
Stromal-oval pore
26.24±2.35
20.41 to 30.41
26.49±2.00
26.00±2.70
0.50
Stromal-rupture pore
32.07±2.26
28.02～35.89
31.54±1.86
32.60±2.55
*0.02
Pterygoid- carotid canal
9.04±1.64
6.10 to 12.83
9.12±1.60
8.95±1.72
0.73
Pterygoid spine-jugular foramen
14.85±2.36
11.01 to 20.41
15.24±2.60
14.46±2.11
0.30
Pterocarpus-oval pore
7.67±1.61
4.47 to 10.35
7.61±1.80
7.73±1.45
0.81
Pterygostoma-rupture pore
17.18±1.92
13.59～21.08
16.69±1.69
17.67±2.08
0.09
* The difference was statistically significant
3 fissures: infraorbital fissure, squamous bulbar fissure, and occipital fissure, respectively. The extension line of the infraorbital fissure and the occipital fissure intersected inwardly at the nasopharyngeal apex with an angle of 86.08 degrees, and the triangular area between the two lines was the lateral skull base. The angle between the orbital fissure and the occipital fissure is 21.33 degrees, and between the two fissures is the rock bone and its internal structures. The angle between the infraorbital fissure and the squamous bulla fissure is 67.48 degrees, and the area between the two fissures corresponds to the underside of the base of the middle cranial fossa and the top of the inferior temporal fossa (Figure 3). The difference was not statistically significant when comparing the left and right sides of the three angles.
Line A is the line from the root of the external pterygoid plate to the outer edge of the foramen ovale, which is lateral to the structures in the infratemporal fossa and medial to the maxillary nerve, middle meningeal artery, eustachian tube, internal carotid artery, internal jugular vein, etc. Line B is the line from the root of the internal pterygoid plate to the pterygoid spine, which corresponds to the position of the eustachian tube. The line C is the line from the midpoint of the rupture foramen to the foramen caudalis, which roughly bisects the rock bone along its long axis, and contains three important foramina: the rupture foramen, the carotid canal, and the foramen caudalis, arranged from anterior to posterior. Between lines C and D are the rupture foramen, carotid canal, jugular foramen, and foramen caudatum.
The five foramina: foramen ovale, foramen spinosum, foramen rupture, foramen jugularis and foramen caudatum, respectively. The rupture foramen, foramen ovale, foramen spinosum, foramen caudalis, and foramen jugulare are connected sequentially to form an approximate ellipse, with the external opening of the carotid canal located approximately in the center of the ellipse. If the rupture foramen, foramen ovale, foramen spinosum, carotid canal, and jugular vein foramen are connected sequentially, they form an approximately circular arc, and the external opening of the subungual nerve canal is also located on this arc (Figure 5). The distance between the foramina was measured (Table 3), and the difference was not statistically significant when comparing the left and right sides.
Table 3: Distances between the bony foramina of the lateral skull base (x±s, unit: mm)
Item
Mean value (n=30)
Range
Left side(n=15)
Right side(n=15)
P-value
Foramen ovale – carotid canal
10.80±1.97
7.18 to 15.31
10.83±2.10
10.76±1.90
0.89
Foramen spinosum-jugular foramen
16.85±2.61
12.07～22.82
17.07±2.73
16.63±2.57
0.49
Spiny pore-oval pore
5.17±1.70
2.46～8.73
5.19±1.79
5.16±1.67
0.96
Echinocore-rupture hole
15.29±2.61
10.21～20.19
15.02±2.49
15.57±2.79
0.41
Foramen ovale – carotid canal
13.73±1.94
9.76～18.50
14.26±2.26
13.20±1.45
0.10
Foramen ovale – jugular foramen
21.11±2.32
17.62 to 26.35
21.47±2.91
20.74±1.55
0.33
Ovular pore-rupture pore
10.06±2.39
6.16～14.18
9.91±2.39
10.20±2.46
0.69
Rupture hole – carotid artery duct
15.04±2.11
10.81～18.71
14.97±2.58
15.10±1.59
0.79
Rupture hole-jugular vein hole
21.06±2.16
16.26～24.75
20.46±2.56
21.66±1.52
0.09
III Conclusion
The use of 1 point, 2 spines, 3 fissures, 4 lines, and 5 foramina to describe the important bony structures of the lateral skull base facilitates the localization of the complex anatomical relationships in this region and provides anatomical landmarks for surgery.
IV DISCUSSION
The anatomical structure of the lateral skull base region is extremely complex, and it is difficult to reflect the spatial relationship of each structure on 2D images such as cross-sectional specimens, CT and MRI, and the application of computer 3D reconstruction technology to obtain 3D images to study its spatial structure has been reported [7-11]. However, due to the limitations of computer hardware and software performance such as image processing, previous studies could not achieve interactive and real-time operation in a virtual reality environment. Moreover, studies using clinical patients or volunteers as subjects have limited the scan layer thickness or scan range because of the amount of radiation, which affects the image quality and shows poorly for fine structures. In this study, we use the most advanced Destroscope virtual reality system that has been used in the medical field, which can fuse MRI, CT and other images, and after computer reconstruction technology, we get 3D three-dimensional objects, which can be moved and rotated with random control, and can perform fine operations such as abrasion and measurement (length, width, height, volume, area, angle). The complex anatomical structure of the lateral skull base can be observed and measured in all directions, from all angles and at all levels without destroying the specimen, which can save the cost of cadaveric head specimens and labor and improve the research benefit. Of course, it also has its limitations, such as the image imaging quality is constrained by the original images such as CT and MRI, which require high-resolution and thin-scan images; artificial intelligence needs to be improved, and it cannot truly realize biomimetic simulation operation.
In this study, we use the natural markers of the lateral skull base to measure the relevant distances, angles, delineate the boundaries and describe the interrelationships. The concept of “O” point was proposed, and the distance from “O” point to the anterior edge of the root of the outer pterygoid plate was measured from the lateral cranial base at 20.83 mm, beyond which the pterygopalatine fossa was entered; the distance from O point to the anterior edge of the inferior temporal crest at 17.00 mm, beyond which the inferior orbital fissure was entered; the distance from O point to the anterior edge of the temporomandibular joint at 17.17 mm, beyond which the inferior orbital fissure was entered. O point to the anterior margin of the temporomandibular joint 17.17 mm, beyond which it enters the temporomandibular joint cavity; O point to the foramen ovale 22.01 mm, which is the distance to the maxillary nerve; O point to the foramen spinosum 24.42 mm, which is the distance to the middle meningeal artery. Therefore, using the “O” point as a marker can help to locate these important structures intraoperatively and reduce the risk of surgery. The present study emphasizes the role of the stria and sphenoid process as markers. As an important surgical anatomical landmark in the parapharyngeal infratemporal region, the stoma provides the starting point for the stoma hyoidis, stoma lingualis, and stoma pharyngeal muscles. The stalk musculature and septum divide the parapharyngeal space into two anterior and posterior gaps. The pterygoid spine is located anterolaterally to the carotid canal and medially to the posterior aspect of the foramen spinosum, and is the starting point of the pterygomandibular ligament. Using the caudate and pterygoid spine as references, measuring their distances to each foramen can help guide intraoperative identification and protection of important structures. In this study, four lines were marked approximately parallel to the long axis of the rock bones using important bony landmarks, with the lateral side of line A being the structures in the infratemporal fossa and the medial side being important structures such as the maxillary nerve, middle meningeal artery, eustachian tube, internal carotid artery, and internal jugular vein. This line serves as a “warning line”, and beyond this line is a relatively safe area for intraoperative exposure, while within this line, care should be taken to protect the important neurovascular structures [12]. Line D serves as the demarcation line between the rock bone and the base of the occipital bone. By studying these points, spines, fissures, lines and foramina of the lateral skull base, it helps to locate the important foramina of the lateral skull base and their corresponding nerves and blood vessels intraoperatively to improve the safety of surgery.