Key-words:
Anatomic landmarks - foramen ovale - greater wing of the sphenoid - middle cranial fossa - skull base
Introduction
Foramina in the floor of middle cranial fossa are very important as they allow passage of as nerves and blood vessels. The greater wing of sphenoid contains three consistent foramina and other small variable foramina. Foramen ovale (FO) is located in the posterior part of the greater wing of sphenoid bone for the transmission of the mandibular nerve, accessory meningeal artery, lesser petrosal nerve, and emissary vein which opens into the infratemporal fossa.[[1]]
There are few studies documented the abnormal morphology of the FO associated with ossified ligaments of pterygospinous (ligament of Civinini) and pterygoalar (ligament of Hyrtl),[[2]],[[3]] or its venous part may be compartmentalized by a bony spur resulting in doubled FO.[[4]] Knowledge of variations of FO will help in distinguishing potentially abnormal foramina from normal during computed tomography and magnetic resonance imaging. Moreover, it is of greater surgical and diagnostic importance in procedures such as percutaneous trigeminal rhizotomy, transfacial fine-needle aspiration technique in perineural spread of tumor, and electroencephalographic analysis for seizure.[[5]]
The incidence in the anomaly of FO varies in different regions of the world, as reported by various authors. The knowledge of three-dimensional topographic anatomy of skull structures and their morphometric values is needed, but it is not sufficient for performing safe treatment. Therefore, detailed knowledge of morphological variants and morphometric details of the FO in adult human skulls of Indian origin is essential for clinicians who perform various invasive procedures on head and neck.
Materials and Methods
The materials for the present study included 62 (124 sides) dry adult, Indian human skulls of unknown sex and origin were obtained from Bone Bank of the Department of Anatomy. Skulls damaged in the floor of the middle cranial fossa and skulls with gross evident deformities were excluded. Existence of the FO was ascertained bilaterally by observing the posterior part of the greater wing of the sphenoid bone, and its patency was confirmed by inserting a probe through each foramen.
The anteroposterior diameter (length) or “L” and transverse diameter (width) or “W” of the FO of both sides are determined using digital vernier calipers.[[6]] Each dimension was measured, while mean figures were recorded. Area “A” of the FO was calculated using the formula: (π × L × B)/4 or (3.142 × L × B)/4.[[5]]
Statistical analysis
Various measurements between the right and left sides were analyzed statistically, using SPSS Statistics Version 16 (IBM, Chicago, USA) by testing student “t” test method, and P < 0.05 was considered statistically significant.
Results
Morphology
The present study was conducted on a total of 124 sides in 62 dry adult skulls. Various shapes of the FO were observed [[Table 1]], of which it was typically oval in 75 sides (40 right, 35 left); almond shape in 35 sides (19 right, 16 left), typically round in 10 sides (6 right, 4 left); and irregular shaped in four sides (1 right, 3 left). The incidence of oval-, almond-, round-, and irregular-shaped FO [[Figure 1]] observed in our study was 60.4%, 28.22%, 6.45%, and 3.22%, respectively [[Table 1]].
Table 1: Variations in appearance of foramen ovale
Figure 1: Types of FO shape, (a) oval type, (b) almond-type, (c) round-type, and (d) irregular-type. FO - Foramen ovale
In the present study, the mean length of FO was 7.74 ± 1.94 mm on the right side and 7.60 ± 1.25 mm on the left side, whereas the observed maximum and minimum length on the right side was 12.5 mm, 4.95 mm and on the left was 12.1 mm, 4.3 mm, respectively [[Table 2]] and [[Figure 2]]. However, the difference between the length of right and left sides was not statistically significant [[Table 3]]. The maximum and minimum width of FO was 8.0 and 3.5 mm on the right and 8.8 and 3.5 mm on the left side [[Table 2]], and the mean width on the right side was 5.18 ± 0.98 mm and on the left side was 5.4 ± 0.85 mm [[Table 2]] with no significant difference between the sides [[Table 3]]. When the mean area (A) of FO was calculated, based on the formula obtained by previous studies, it was found 32.54 ± 7.54 mm 2 (55.73–14.5 mm 2) on the right side and 31.07 ± 9.54 mm 2 (58.3–15.1 mm 2) on the left side [[Table 2]] and [[Figure 3]], with no statistically significant difference between the two sides [[Table 3]]. Other features noted are bony spicule, spur, bony bridge, confluence etc as shown in [[Figure 3]] and [[Figure 4]]. There was a positive correlation between lengths, breadths and areas between right and left sides of the FO [[Table 4]].
Table 2: Dimensions of foramen ovale
Figure 2: FO anteroposterior length (L) and transverse width (W). FO -Foramen ovale
Table 3: Side comparative results of foramen ovale
Figure 3: FO spur/tubercle (yellow star) (a), osseous bridging (black arrow) (b). FO - Foramen ovale
Figure 4: Confluence (yellow star) between FO and FL. FO - Foramen ovale, FL - Foramen lacerum
Table 4: Comparison between length, breadth, and area on the right and left sides of the foramen ovale
Discussion
The skull foramina exist as the ossification of the skull base occurs around preexisting vessels and cranial nerves. After the 8th week of gestation, the cartilage development begins around the already apparent vessels and nerves.[[7]] The cartilaginous ossification of the greater sphenoidal wing begins at the 15th gestational week and until the 22nd week; the FO appears in the greater wing of the sphenoid.[[8]],[[9]]
The FO is an important landmark for middle cranial fossa surgery as well as diagnostic procedures, such as electroencephalographic analysis of the seizure for patients undergoing selective amygdalohippocampectomy,[[10]] microvascular decompression by percutaneous trigeminal rhizotomy for trigeminal neuralgia,[[11]] and percutaneous biopsy of cavernous sinus tumors.[[12]] The technique of computed tomography-guided transfacial fine-needle aspiration technique through the FO is used to diagnose squamous cell carcinoma, meningioma, Meckel, etc.;[[13]] Other than CT- guided transfacial fine-needle aspiration technique, several key landmarks that could be used radiologically and surgically to guide an endoscopic, endonasal, transzygomatic, transmaxillary, transpterygoid approach to the infratemporal fossa, to access pathologies arising in and around the FO of middle cranial fossa and thus helping to decrease patient morbidity and significantly decrease the cost involved.[[14]]
In the present study, the most common variations in the shape of FO were oval shape (60.4%), followed by almond shape (28.22%), round shape (6.4%), and irregular shape (3.2%), similar to the studies conducted in the past.[[15]],[[16]] Further, various bony outgrowths of the FO such as spine on the margin of FO, tubercle protruding from its margin, and bony bar extending to divide the foramen into two complete compartments were noticed in the present study [[Figure 2]].
Mean length of FO in our study was 7.74 ± 1.94 mm on the right and 7.60 ± 1.25 mm on the left side, and however, the difference was not significant similar to various other studies, i.e., the mean length of FO was 7.46 ± 1.41 mm on the right and 7.01 ± 1.41 mm on the left side.[[3]] In addition, according to Lang et al.,[[17]] the mean length of FO is 7.2 mm, whereas in a fluoroscopically-assisted laser-targeting of the FO, it was 6.9 mm on the right side and 6.8 mm on the left, less than the present values. In our study, the maximal length of FO was 12.1 mm and its minimal length was 4.3 mm, whereas these values in previous studies such as Arun [[18]] and Osunwoke et al.[[6]] were 9.8 and 2.9 mm and 9.5 and 5.0 mm, respectively.
According to a study conducted by fluoroscopically-assisted laser-targeting of FO, the mean width was 3.4 mm on the right side and 3.8 mm on the left side.[[13]] In this study, the maximum and minimum width of FO on the right side was 8.0 and 3.5 mm whereas on the left side was 8.5 mm and 3.8 mm. Further, the mean width on the right side was 5.18 ± 0.98 mm and on the left side was 5.44 ± 0.85 mm, and this difference was not statistically significant. The mean area of FO calculated in the present work was 31.07 ± 9.54 mm 2 (55.3–14.5 mm 2) and 32.54 ± 7.54 mm 2 (55.73–14.5 mm 2) on the right and left sides, and in a previous study, the combined area of FO ranged from 25.99–67.40 mm 2 in adult male skulls to 19.58–67.40 mm 2 in case of female skulls.[[5]]
Conclusion
Variations in the shapes of the FO are found due to developmental reasons, which may seriously hamper clinical and diagnostic procedures. The detailed knowledge of anatomy and morphology including variations of FO, as observed in the current study, is of great importance to the clinicians in diagnosis and management of various conditions where microneurosurgical and microvascular approach is a requisite.
