Morphological Variations and Morphometric Analysis of Foramen Ovale in South Indian Population

Foramen ovale is present medial to the foramen spinosum and foramen lacerum is located medial to the foramen ovale. It transmits the mandibular division of the trigeminal nerve, accessory meningeal branch of the maxillary artery, lesser petrosal nerve and an emissary vein which connects the pterygoid venous plexus in the infratemporal fossa to the cavernous sinus ¹.

The foramen ovale is located in the area where intracranial and extracranial structures meet ². Procedures involving Trigeminal Neuralgia and mandibular nerve anaesthesia require an understanding of the foramen ovale's morphometry and anatomy ³. Foramen Ovale is also used for patients undergoing selective amygdalohippocampectomy, percutaneous biopsy of cavernous sinus tumours, and micro vascular decompression by percutaneous trigeminal rhizotomy for trigeminal neuralgia to analyse electroencephalographic seizure data. Hence it is used for diagnostic and surgical procedures ⁴.

Our present study was conducted to define mean values and anatomical variations in foramen ovale. This work intends to highlight such prior knowledge of changes in foramen ovale, which may be useful for forensic, anthropological, and surgical purposes.

The retrospective cross-sectional study was carried out on100 foramen ovale using 50 dry adult human skull bones of unknown sex, the bones were taken from department of Anatomy, AIIMS Bibinagar Hyderabad. Skulls which were fractured at the surroundings of foramen ovale were excluded from the study.

a) Maximum transverse diameter, antero-posterior diameters of the foramen were measured with the help of vernier calipers (Carbon Fiber Composites Digital Caliper(RoHS)) ( Picture 1& Picture 2)

Picture 1.Measurement of vertical length

Picture 2.Measurement of Horizontal length

b) Different shapes of foramen were noted. (Picture 3)

Picture 3.Showing different shapes of Foramen Ovale

c) Margins of foramen were carefully observed for any bony projections.

d) The area of foramen ovale is calculated by using the formula 𝝿 x Length x Breadth/4 ⁵

e) Unpaired t’ test was used for statistical analysis.

Out of 100 foramen ovale, minimum to maximum anteroposterior diameter on left side were 3.9 mm 10 mm and on right side were 4.25 mm and 10.5 mm. The mean anteroposterior diameter on left side was 6.59±1.37 mm and on right side was 6.99±1.44mm. (Table 1)

Minimum transverse diameter on left side was 2.03mm and on right side was 2.45mm. Maximum transverse diameter was 6.16mm and 6.3mm on left and right sides respectively. The mean transverse diameter on the left is 4.09±0.74 mm and 4.17±0.76 mm on the right side. The mean area of the foramen ovale is 23.03+6.73 mm² and 21.42+6.52 mm² on the right and left sides respectively as shown in Table 1.

Majority of the foramen were oval shaped and it was seen in 70 sides (left 36, right 34), almond shape was seen in 11 sides (4 left, 7 right), round shape was seen in 9 sides (6 left, 3right), pear shaped foramen was observed in 2 sides one on each side. 6 elongated shaped foramina (2 left and 4 right) were noted along with 2 irregular shaped foramina (1 left and 1 right) as shown in picture 3.

Incidences of various shapes of the foramen ovale ( Fig:1)were oval 70%, almond 11%, round 9%, elongated 6%, pear shaped 2 % and irregular 2%.

The ossification of the base of the skull happens round existing arteries and cranial nerves, creating the skull foramina. The growth of cartilage around the already evident arteries and nerves starts after the eighth week of gestation.The greater sphenoidal wing's cartilaginous ossification starts from the 15th gestational week and lasts until the 22nd week, during which the foramen ovale develops.^{6, 7}

The morphometric and developmental aspects of the foramen ovale have been the subject of a number of studies around the world. In our study, the mean anteroposterior diameter (length) on the left side was 6.59±1.37 mm and on the right side was 6.99±1.44mm; there is no statistically significant difference between the right and left side as shown in Table 1.. A similar study done in Japan demonstrated that the foramen ovale has an average maximum length of 7.48 mm and an average minimum length of 4.17 mm ⁸. In New York, laser targeting of the foramen ovale with the aid of fluoroscopy revealed mean lengths of 6.9mm on the right side and 6.8mm on the left ⁹. These studies have reported lower values when compared to our study where the maximum length on the right and left sides 10.5 mm and 10 mm respectively. Our study is similar to other Indian studies done by Patil et al., and Prakash et al., who documented that maximum length of foramen ovale was 9.5mm and 12.1 mm ^{10, 11}. Another German study postulated that the foramen ovale in an adult's skull measures 7.2 mm in length and 3.7 mm in breadth ¹². None of these studies have documented any significant statistical difference between the right and left sides. A radiological study done by Bhattarai et al., the mean length of foramen ovale to be 7.79±1.10 mm ¹³.

The maximum transverse diameter in our study was 6.16mm and 6.3mm on left and right sides respectively (Table 1) This is lower when compared to another Indian study where the maximum width of the foramen ovale was found to be 8.8 mm ¹⁰. On the contrary to our study, a radiological study documented the width of the foramen ovale to be lower than our findings which is only about 3.68±0.64 mm ¹³. Another Indian study documented that the the maximum and minimum length of foramen ovale on the right and left was 10.1 mm, 4.3 mm and 9.1 mm, 3.2 mm. On the contrary, a study in the New York population documented the foramen ovale's average width to be 3.4 mm on the right side and 3.8 mm on the left side ⁹. None of these studies have documented any significant statistical difference between the right and left sides. However, Berge et al., Shapiro et al., in their studies postulated that right sided foramen ovale is narrower than the left sided foramen ovale in most of the people ^{15, 16}. Studies have shown that a small foramen ovale has been linked to primary trigeminal neuralgia relapse ^{17, 18, 19}. Classical trigeminal neuralfia may be ascribed to the trigeminal nerve compressed by blood vessels and a narrow foramen ovale meanwhile. The mandibular nerve may enlarge as a result of neurovascular compression, which causes compression from a narrow foramen ovale and makes the nerve vulnerable to entrapment during its passage through a small foramen ^{20, 21}. The mean area of the foramen ovale in our study is 23.03+6.73 mm² and 21.42+6.52 mm² on the right and left sides respectively. Bhattari et al., reported the mean area of foramen ovale to be 22.80±6.18 mm² which is similar to our findings ¹³. On the contrary, Prakash et al., documented higher mean area of foramen ovale which is around 30.808 ± 7.545 mm² and 31.310 ± 8.262 mm² on the right and left respectively ¹⁴. The presence of increased length, width and area of the foramen ovale could be attributed also to more number of emissary veins passing through it ²².

In our study, as shown in picture 3, majority of the foramen were oval shaped and it was seen in 70 sides (left 36, right 34), almond shape was seen in 11 sides (4 left, 7 right), round shape was seen in 9 sides (6 left, 3right),pear shaped foramen was observed in 2 sides one on each side. 6 elongated shaped foramina (2 left and 4 right) were noted along with 2 irregular shaped foramina (1 left and 1 right) (picture 3). Similar other studies have also documented that oval shaped foramina are predominantly noted followed by almond shaped and round shaped foramina ^{14, 23, 24, 25, 26}. Developmental factors lead to variations in foramen ovale forms, which can substantially impair clinical and diagnostic processes. Even though the foramen ovale's shape might vary in a healthy state, a careful examination of these foramina can help to diagnose abnormalities in the middle cerebral fossa and nasopharynx. It is tempting to think of a fifth nerve neurinoma when the foramen ovale enlarges ²⁶. The authors also compared the values of present author with the previous studies. The comparison is represented in Table 2. There is a lot of variation among the parameters of different authors. The values of present author are more or less equal with Karishma et al ¹⁴

These results can serve as a principal reference for future studies on FO morphological variation and its clinical applications. When conducting different diagnostic and surgical operations through the foramen ovale, neurosurgeons will find great utility in the differences seen in this study as well as the information regarding its location and dimensions.

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