Foramen Magnum: A Morphometric Study in Dried Human Skulls

Introduction: The foramen magnum is the largest foramen or opening present in the base of skull. The dimensions of foramen magnum are clinically important because many vital structures are passing through it. Objective: The aim of the study was to determine the different shapes of foramen magnum and its antero-posterior diameter and transverse diameter. Methods: This study was conducted on 32 dry human skulls of unknown age and sex. All the important parameters were studied using vernier caliper. Results: The mean transverse diameter (TD) was found to be 27.75 ± 2.47mm (Mean ± SD) and mean antero-posterior diameter (APD) was found to be 34.62 ± 3.58mm (Mean ± SD). The various shapes of foramen magnum were observed. The most common shape was oval 46.9%, followed by round18.8%, tetragonal 15.6%, hexagonal 12.5% and irregular 6.3%. Conclusions: In our study, the most common shape of foramen magnum was oval; this can help the surgeons to perform post cranial surgery and surgery near foramen magnum as oval shape indicating the narrow operative field. The study may also be helpful for anatomist for study purpose and forensic experts to determine identity from different populations when compared to other studies.


INTRODUCTION
The foramen magnum is the largest foramen in the skull. 1 Foramen magnum is a wide opening in the posterior cranial fossa connecting the cranial cavity with the vertebral canal. It allows the passage of vital structures of brain through it. 2 The position of foramen magnum in the skull is antero-median and it lies in the posterior cranial fossa. Its shape is oval, it is wider behind and the largest diameter is antero-posterior. It contains lower end of medulla oblongata, meninges, cerebro-spinal fluid, vertebral arteries and veins, and spinal accessory nerve. 3 The dimensions of the foramen magnum are clinically important because of the above-mentioned vital structures passing through it may endure compression such as in cases of foramen magnum herniation, foramen magnum meningiomas and foramen magnum achondroplasia. 4 Configuration and size of the foramen magnum play an important role in the pathophysiology of various disorders of the cranio-vertebral junction. Thus, a fundamental knowledge of normal anatomy is important to the clinician who diagnoses disorder affecting this region or the surgeon who operates on this anatomy. 1 Medical College, Chisapani, District Banke, Nepal. The study illuminates the clinical importance of this region and also collects data which may be used in further studies.

METHODS
A study of foramen magnum was conducted on total 32 dry adult human skulls of unknown age and sex from the collection of the Department of Anatomy, Nepalgunj Medical College, District Banke, Nepal. Only above mentioned numbers of skulls with good condition were available in the department. Deformed and damaged bones related with foramen magnum were excluded from the study. The ethical clearance was taken dated on 16 th May 2021 from NGMCTH-IRC. The study was conducted for the period of 6 months from June 2021 to December 2021.
The instrument used in the present study.
1. Vernier caliper: To measure transverse diameter and antero-posterior diameter of the foramen magnum, a verner caliper of 0.1mm accuracy made by Aerospace is used.
Each skull was numbered serially with numbering sticker to help in identification. The antero-posterior diameter and transverse diameter of foramen magnum for each skull was measured twice with a sliding vernier caliper of 0.01mm accuracy and an average was recorded. Antero-posterior diameter (APD) was measured from basion to opsthion. To transverse diameter (TD), the point of maximum concavity from left margin to point of maximum concavity from right margin was taken.
All the above parameter were carefully tabulated and analyzed. The dimensions of various parameters of foramen magnum were measured and recorded. SPSS version 20.0 was used for statistical analysis and to find the Minimum, Maximum, Mean and Standard Deviation of all the parameters.

RESULTS
The measurement of various parameters of foramen magnum of skull is shown in Table 1 and 2. In the present study various shapes of foramen magnum were observed. The most common shape was oval in 15 (46.9%) skulls, followed by round 6 (18.8%), tetragonal 5 (15.6%), hexagonal 4 (12.5%), and irregular 2 (6.3%) skulls as shown in Table 1.
The mean transverse diameter (TD) was found to be 27.75 ± 2.47 mm (Mean ±SD) and mean anteroposterior diameter (APD) was found to be 34.62 ± 3.58 mm (Mean ± SD).
All the parameter of foramen magnum is depicted in Table 1

DISCUSSION
The anatomical knowledge of foramen magnum is helpful to understand the clinical sign and symptoms in various cranio-cervical diseases. Because foramen magnum allows a passage to various important structures such as lower end of medulla oblongata, vertebral arteries, meninges, spinal acessosory nerve, apical ligament of dens and membrena rectoria. It is formed by interaction of bony ligaments and muscular structures forming the complex craniovertebral junction. Knowledge about the bony parameters of foramen magnum is also needed for trans-condylar approach. 5 The comparison of morphometry of foramen magnum with previous authors is shown in tabulated form in Table 4 and 5.
The present study shows that antero-posterior diameter of foramen magnum is found to be 34.62mm, whereas Sharma et al 6    Thus a detailed study is required to establish the reason of differences in shape of foramen magnum.

Limitation
The major limitation of the study was that the gender and ethnic origin of the skulls were not known. Thus a study with known gender and ethnicity of skulls is required to establish more conclusive opinion to help the clinicians and others like anatomists and forensic experts.

CONCLUSIONS
In our study, the most common shape of foramen magnum was oval. This can help the surgeons to perform post cranial surgery and surgery near foramen magnum as oval shape indicating the narrow operative field. The study may also be helpful for anatomist for study purpose and forensic experts to determine identity from different populations when compared to other studies.