European Journal of Cardiovascular Medicine

The foramen magnum is the largest opening in the base of the skull through which the spinal cord exits the cranial vault. It is found in the occipital bone and formed around the base of the brainstem, separating the brain above from the spinal cord below. The foramen magnum in humans is formed by the fusion of the four individual parts of the occipital bone. The foramen magnum is an important landmark of the skull base and is of particular interest for anthropology, anatomy, and forensic medicine. Due to the important relations of the foramen magnum and contents posing through it, the morphometric dimensions are important and also noteworthy. Foramen magnum indicates the exact angle of the spinal column which will decide whether our body is horizontal or vertical. The position of this passage has been used by several anthropologists to determine the ability to walk upright in human ancestors. Besides the spinal cord, spinal accessory nerve, vertebral arteries, anterior and posterior spinal arteries, and ligaments like membrana tectoria and alar ligaments also pass through it[1,2].

Measurements of the foramen magnum are relevant in cases of achondroplasia  where there is a high risk of spinal cord stenosis in the base of the skull, Arnold-Chiari malformation  foramen magnum meningioma, plagiocephaly, basilar invagination, and others cranial deformities[3]. The diameters and area of the foramen magnum are greater in males than in females, hence its dimensions can be used to determine sex in the medicolegal conditions, especially in the following circumstances, such as explosions, aircraft accidents and war fare injuries. Foramen magnum is about 3cm wide by 3.5cm anteroposteriorly. It is located midway between and on a level with mastoid processes. The foramen magnum is surrounded by different parts of the occipital bone, squamous part lies behind and above, basilar part in front and a condylar part on either sides. On each side its antero-lateral margin is encroached by occipital condyles, hence the foramen magnum is narrow anteriorly. The anterior edge of the foramen magnum is slightly thickened and lies between the anterior ends of the condyles. The posterior half of the foramen magnum is thin and semicircular. Upper ends of anterior and posterior atlanto-occipital membranes are attached to the anterior and posterior margins of the foramen magnum respectively, and their lower ends are attached to the superior surface of anterior and posterior arches of the atlas respectively [4].

Variations in the shape of the foramen magnum have clinical as well as radiological significance. The knowledge of the dimensions and shape of the foramen Magnum has important clinical implications in the prognosis and treatment of various neurological pathologies like Arnold Chiari syndrome, and posterior cranial fossa lesions. Similarly, dimensions of the foramen magnum are important in various surgical approaches to posterior cranial fossa through foramen magnum. Thus, understanding of the variations in its normal anatomy is essential for accurate diagnosis and treatment of same. The present study conducted to determine and analyse the morphological types and diameters of the foramen magnum in adult skull, the findings may help in improving the efficacy and minimize the failure rates in surgical procedures in posterior cranial fossa particularly those involving the approaches through foramen magnum, also helpful in forensic and neurosurgery practice.

The present study conducted with 154 adult dry skull, the gender was not determined, this skulls were collected in multiple medical and dental colleges in Karnataka.  The damaged skulls were not included in study. Only intact and with clear foramen magnum structure skulls were selected. Only fully ossified, dried, macerated and thoroughly cleaned skull were included in the study. We have observed the shapes of foramen magnum and classified as oval, round, tetragonal, pentagonal, hexagonal and irregular in shape. In addition, we have also noted incidence of shapes of foramen magnum. We have recorded morphometric measurements like anterior-posterior diameter and transverse diameter of foramen magnum with help digital Vernier sliding calliper which is accurate to 0.01 mm. Values of each measurement were expressed in the form of Mean+SD. We have recorded antero-posterior diameter of foramen magnum by measuring from the end of anterior border to the end of posterior border of foramen magnum. Transverse diameter measured from the point of maximum concavity on the right margin to the maximum concavity on left margin of foramen magnum.

The present study was conducted in 154 skulls with unknown gender skulls, as morphological features, oval shape foramen magnum was observed in 52 number of skulls and the incidence was 33.76%, tetragonal shape foramen magnum was observed in 31 number of skulls and the incidence was 20.12%, hexagonal shape foramen magnum was observed in 18 number of skulls and the incidence was 11.68%, round shape foramen magnum was observed in 26 number of skulls and the incidence was 16.88%, pentagonal shape foramen magnum was observed in 13 number of skulls and the incidence was 8.44%  and pear shape foramen magnum was observed in 14 number of skulls and the incidence was 9.12%(Table 1). The Anterior – Posterior diameter (mm) Foramen Magnum was 36.84+4.62mm, minimum anterior- posterior diameter was observed 33.46mm and maximum was 42.12mm. The transverse diameter (mm) foramen magnum 30.45+2.82mm, minimum diameter was 26.12mm and maximum was 94.86mm. the index of foramen magnum was 82.65+6.10 and minimum index was 78.06 and maximum was 82.76.

Table. 1. Shapes of foramen magnum

Table  2. Dimensions of Foramen Magnum.

The morphometric study of the human skull is a practice in research practice of anatomists, anthropologists, and forensic experts, as it is a structure of great interest since it possesses sexual dimorphic characters and ethnic differences. The foramen magnum is an anatomic region of vital importance. The skull alone can provide 90% of accuracy regarding the gender aspect. It is also known that the morphological features of the foramen magnum went through evolutionary changes[3].

In present study the Anterior – Posterior diameter (mm) Foramen Magnum was 36.84+4.62mm, minimum anterior- posterior diameter was observed 33.46mm and maximum was 42.12mm. The transverse diameter (mm) foramen magnum 30.45+2.82mm, minimum diameter was 26.12mm and maximum was 94.86mm. the index of foramen magnum was 82.65+6.10 and minimum index was 78.06 and maximum was 82.76. In study of Tubbs, R[6] found that the mean surface area of the foramen magnum was 558 mm2, the mean anteroposterior diameter was 3.1 cm, and the mean horizontal diameter was 2.7 cm. in their study for comparison, surface areas were classified into 3 types based on size. Type I foramina were identified in 20.8% of the dry skulls and exhibited a surface area of less than 500 mm2. Type II (66.6%) was applied to foramina of an intermediate size with surface areas ranging between 500 to 600 mm2. Type III (12.5%) was applied to large foramina with surface areas of more than 600 mm2.

Surgical procedures and approaches in foramen magnum region are essential due to the high mortality rate. For this reason, anatomical structures in these regions should be well known before surgery. The quantitative data presented in this study may assist in surgical procedures around the foramen magnum and the planning of these procedures

1. Mumal Nagwani, Archana Rani, Anita Rani. A morphometric and comparative study of foramen magnum in North Indian population. Journal of the Anatomical Society of India.2016:65(1):S11-S15. https://doi.org/10.1016/j.jasi.2015.11.001.
2. Askin Seker, Kohei Inoue, Shigeyuki Osawa, Akin Akakin, Turker Kilic, Albert L. Rhoton, Comparison of Endoscopic Transnasal and Transoral Approaches to the Craniovertebral Junction. World Neurosurgery. 2010;74(6):583-602. https://doi.org/10.1016/j.wneu.2010.06.033.
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5. Archana Singh, Preeti Agarwal, Arun Singh. Morphological and Morphometric Study of Foramen Magnum in Dry Human Skull and Its Clinical Significance. International Journal of Anatomy, Radiology and Surgery. 2019 Jul, Vol-8(3): AO10-AO12.
6. Tubbs, R. Shane MS, PA-C, PhD; Griessenauer, Christoph J. MD; Loukas, Marios MD, PhD; Shoja, Mohammadali M. MD; Cohen-Gadol, Aaron A. MD, MSc. Morphometric Analysis of the Foramen Magnum: An Anatomic Study. Neurosurgery 66(2):p 385-388, February 2010. | DOI: 10.1227/01.NEU.0000363407.78399.BA.