Morphometric Evaluation of Foramen Magnum and its Shape Variations using Computed Tomographic Images of Normal Adults

Introduction

The study of sizes of foramen magnum (FM) is highly interesting due to the complexity of the cranial base from a descriptive and topographic points of view, due to the important relation of FM with its contents.¹ Several scientific fields, involving forensic and physical anthropology, comparative anatomy, biology, surgery of the foramen structures, and the craniovertebral junction, have an affinity for the foramen magnum. Foramen magnum is the largest foramen in the skull.

The most noticeable feature in the floor of this aspect of the cranial base is a large central opening with an oval shape that is located in the deepest part of the posterior cranial fossa. It is found within the anteromedian aspect of the occipital bone, specifically.² Anteriorly, it is bounded by the basiocciput. Posteriorly, it is bounded by the supraocciput. The occipital condyles surround it laterally.³

The foramen magnum on the occipital bone has two mid-line cephalometric landmarks. The middle of the foramen magnum's posterior margin is known as the opisthion. On the anterior margin of the foramen magnum, the basion is situated in the centre. The foramen magnum is separated into an anterior smaller compartment and a posterior larger compartment by the alar ligament, which is connected on either side to the tubercle of the occipital condyle. It is a component of the occipital bone and is situated in the most inferior region of the cranial fossa.⁴

Physical anthropology is applied to the legal system in forensic anthropology to identify skeletonized remains and related legal structures. Identification of skeletonized human remains is crucial because it can reveal whether a crime was committed, who committed it, the victim's ancestry, probable age, sex, and stature of the individual, as well as the perpetrator. For skeletal identification, mainly sex determination, aging, and stature estimation, comparisons have been done in several criminal cases. Human male skulls exhibit significantly higher sagittal and transverse dimensions of foramen magnum than human female skulls, according to research.⁵

Computed Tomography (CT) has been used to study fossilized skulls and analyse bite marks. The foramen magnum and its various shapes can be evaluated using computed tomography scans, which also provide detailed information that cannot be obtained from conventional radiographs. As a result, foramen magnum CT measurements can be used to determine gender.⁶ In this study, sagittal diameter, transverse diameter and area of foramen magnum were correlated against gender. Also, different variations in the shape of foramen magnum were evaluated using computed tomography.

The main purpose of the study was to radiologically estimate the sagittal and transverse diameters and also the area of foramen magnum using CT in order to determine the gender of specific individuals to support the field of forensic anthropology.

Materials and Methods

Study design: Record-based observational study

Participants: Patients who underwent Computed Tomography (CT) scan of brain between February 2021 to June 2022 were included.

Sampling technique: Purposive sampling technique

Sample size: On basis of study conducted by Khalil Awadh Murshed et al. (2003),¹ in order to expect the mean difference of 26 mm in sagittal diameter and 23 mm in transverse diameter among males and females, assuming 95% confidence interval and 90% power with a pool standard deviation of 5, the sample size estimated for study was 156. Further assuming 10% known response rate, the final sample size estimated for study was 78 each. Hence, a total of 156 individuals were included in the study.

Inclusion criteria

Patients undergoing CT head in the age range of 18-85 years.

Exclusion criteria

Patients with previous history of surgery, trauma, or pathological lesions near the foramen magnum were not included in the study.

Study method

This retrospective study was designed based on CT head scan data from past three years (Record based study). Approval was acquired from the Institutional Ethical Committee. Data for the study were collected from the PACS (Picture Archiving and Communications System) of the hospital. Informed consent was taken from the patients. Plain head CT scan was performed using the multi-detector CT (Siemens somatom go top definition 128 slice) scans of brain plain protocol. Only the principal investigator had access to the confidential information that was collected during this research project. From the collection of old pictures using a 1 mm slice thickness, multi-planar reconstruction (MPR) images were obtained. After gathering each patient's raw data, the 2D CT scan axial images were converted using a post-processing (volume rendering) technique into 3D images. In syngo.via workstation, different foramen magnum measurements were measured on axial plane and from 3D volume rendering technique (VRT) images using the distance measurement tool. The method described by Abtehag A Taib et al.⁷ in their study to obtain foramen magnum's morphometry was applied to measure the sagittal diameter, transverse diameter, foramen magnum area, and various foramen magnum shapes. The largest anteroposterior dimension of the FM was measured as the FM sagittal diameter (FMSD). After using a 3D volume rendering analysis which was available in the syngo.via CT workstation to trace the bony margin of the FM on the CT image, the area (FMA) was automatically determined.

Data analysis

Z test was used to test significant difference in outcomes measured among males and females. Software from the Social Package of Statistical Science (SPSS, version 21) was used to analyze the data. By computing the mean and standard deviation of each foramen magnum measurement, statistical analysis was performed.

Results

The Table 1 depicts that the mean sagittal diameter was higher among males (33.5 ± 2.9 mm) as compared to females (31.4 ± 3.2 mm). Hence on using unpaired t test, high statistical significant difference was observed in the sagittal diameter among males and females (P <0.001).

It may be observed from the Table 1 that the mean transverse diameter was higher among males (29.1 ± 3.3 mm) as compared to females (26.7 ± 3.7 mm). Hence using unpaired t test, high statistical significant difference was observed in the transverse diameter among males and females (P <0.001)

Table 1 shows that the mean area of the part was more among males (642 ± 94.2) as compared to females (559.9 ± 115.5). Using Unpaired t test, statistically very high significant difference was observed in the mean area of the part among males and females (P <0.001).

Oval shape of foramen magnum was found to be most common among males. Round shape of foramen magnum was found to be equally distributed among males. Irregular shape of foramen magnum was found to be most common among females. Pentagon shape of foramen magnum was found to be most common among males. Hexagonal shape of foramen magnum was found to be most common among males. Tetragonal shape of foramen magnum was found to be most common among females (Figure 1).

Discussion

One hundred fifty six computed tomography (CT) heads (78 men and 78 women) were used in this study. The average age of study participants was 52.48 ± 19.04 years. The descriptive statistics of three variables, which include the mean and standard deviation for both genders were reported in the current study. Foramen magnum in male participants had larger overall mean values than females, indicating a significant difference between the two groups, according to the morphometric analysis of foramen magnum measurements. Sagittal diameter, transverse diameter, and foramen magnum area morphology were determined, and gender differences in these morphologies were established. Additionally, measurements were taken of various foramen magnum shapes. The results of this study showed a strong relationship between gender and foramen magnum. Males had significantly higher morpho metric measurements than females, such as sagittal diameter, transverse diameter, and area. Males had longer mean and SD values for the sagittal diameter (m = 33.5 ± 2.91), transverse diameter (M = 29.1 ± 3.3), and area (M = 642.2± 94.2, F = 559 ± 115.5) than females.

Foramen magnum morphometric measurements revealed a highly significant statistical difference between the male and female groups (P=0.001). An important task in forensic medicine is identifying the sex of incomplete or damaged skeletons. Such gender identification issues can be resolved with the aid of anthropometric measurements. Since it is a regular structure and is less likely to undergo significant morphological changes, the foramen magnum was chosen. The present study depicts sagittal value of 33.5 ± 2.91 mm in males and 31.4 ± 3.23 mm in females with the P value ˂0.001 which is lower than the study reported by Uthman et al., ⁷ (34.9±2.00 in males, 32.9±2.00 in females with the P value ˂0.001). It is also lower than the study reported by Radhakrishna SK8 (34.09 ± 2.36 mm in males and 31.72 ± 2.14 mm in females with the P value ˂0.005) and the study reported by Aghakhani, K et al.⁹ (37.7 ± 1.0 in males and 34.3 ± 1.4 in females with P value˂ 0.001).

The following findings were obtained when the results of this study's morphometric analysis were compared to those of other studies. The transverse diameter of foramen magnum in the present study was estimated as 29.1 ± 3.3 mm in males and 26.7 ± 3.7 mm in females with the P value ˂0.001 which is lesser than the study reported by Uthman et al.⁷ (29.5 ± 2.5 in males and 27.3 ± 2.2 in females) which is higher than the study reported by Radhakrishna SK8 (28.63 ± 1.89 in males and 26.59 ± 1.64 in females with P value ˂0.005), which is lesser than the study reported by Aghakhani K et al.⁹ (31.68 ± 1.2 in males and 28.3 ± 1.43 in females with P value ˂0.001).

The results of this study's morphometric analysis when compared to those of other studies were as follows:for the area of foramen magnum, the present study estimated 642.2 ± 94.2 in males and 559.5 ± 115.5 in females with the P value ˂ 0.001 which is lesser than the study reported by Aghakhani K et al.9 (946 ± 61.9 in males and 773.9 ± 70.3 in females, with P value lesser than 0.001) and also, was lesser than that reported by Uthman et al. ⁷ (765.2 ± 98 in males and 670.2 ± 93.7 in females with the P value ˂ 0.001). The findings showed that the foramen magnum exhibits sexual dimorphism. Metric analysis of the foramen magnum in incomplete skeletons may offer a statistically significant hint as to the sex of the unidentified head. The findings of this study suggest that the foramen magnum is significant in determining gender. Due to genetic, environmental, and dietary factors, the outcomes may vary geographically and can also depend on skeletal structure. The above study can be of aid for practical utilization to determine the gender in the field of forensic radiology by clinically evaluating and measuring the dimensions of foramen magnum using computed tomography. 

Conclusion

The present study concluded that the foramen magnum estimations can be useful with a high precision of accuracy in determining gender. Based on the results of this study conducted using CT scan images, the diameters of the foramen magnum and its area showed a high precision in sex determination.

Limitations

The above study has been conducted over a limited period of time in a single population; hence cannot be generalized to other populations or ethnicities.

Conflict of Interest

Nil