Background: Determination of sex of the skeletal remains plays a vital part in the identification of an individual. This study is focused on the morphometric measurement of the foramen magnum region and examining the accuracy of sexual dimorphism in the Nepalese population.   

Methods: Measurements were obtained from 3D computed tomography (CT) scan of 261 Nepalese adult cranial bases with known age and sex. Length and breadth of the foramen magnum, length and breadth of right and left occipital condyles and maximum and minimum intercondylar distance were measured on the base of the skull CT images.

Results: The mean values for all parameters were higher in males than females except for the maximum intercondylar distance. Sex prediction done with discriminant function scores could classify the skull with an overall accuracy of 70.5%.

Conclusions: It can be concluded from the results that the morphometric study of the foramen magnum is not suitable for sex determination in the Nepalese population.

The present study was a descriptive cross-sectional study conducted upon the three-dimensional computed tomography (3DCT) images of the base of the skull of 261 Nepali individuals whose age and sex were known. All the CT images during the study period were from adults who presented to the Department of Radiology of Lumbini Medical College Teaching Hospital, Palpa for the CT of the head as indicated in the course of their management for diagnostic purposes during the period between February and September 2021.

The study was conducted on a normal human skull (more than 20 years of both sexes and matched ages) who had a CT scan for various diagnostic medical or surgical indications. Only the high-quality reconstructed CT images were included and low-quality blurred images or those with artefacts and those which did not cover the entire area of the foramen magnum were excluded. Non-contrast CT head was done on SIEMENS SOMATOM scope 16 slice CT machine with 0.5 slice thickness and studied with OsiriX MD software. Scans of subjects where there was gross pathology or injury or any deformity in the skull base region were excluded.

For the purpose of the study 3DCT images were analyzed and various parameters were measured on the console of the scanner on a centimetre scale (Figure 1). The measurements less than 1 cm were expressed in millimetres which were converted to centimetres during data analysis. The parameters that were measured were:

1. Foramen magnum length (FML): the distance between the basion and opisthion was taken as the antero-posterior length of the foramen magnum in the mid-sagittal plane
2. Foramen magnum width (FMW): the distance between the lateral margins of the foramen magnum at the point of the greatest lateral curvature on either side was taken as the transverse diameter of the foramen magnum; perpendicular to the mid-sagittal plane
3. Length of right occipital condyle (LROC): the distance between the most anterior to the most posterior point on the margin of the right occipital condyle along the long axis
4. Length of left occipital condyle (LLOC): the distance between the most anterior to the most posterior point on the margin of the left occipital condyle along the long axis
5. Width of right occipital condyle (WROC): the distance between the most lateral to the most medial point on the margin of the right occipital condyle perpendicular to the long axis
6. Width of Left occipital condyle (WLOC): the distance between the most lateral to the most medial point on the margin of the left occipital condyle perpendicular to the long axis
7. Minimum intercondylar distance (MnICD): the distance from the most medial point on the margin of the right and left occipital condyles
8. Maximum intercondylar distance (MxICD): the distance from the most lateral point on the margin of the right and left occipital condyles

Two radiologists from the Department of Radiology obtained the data independently. The observers were allowed to manipulate the contrast and brightness features and to use the zoom tool of the software for optimal visualization.

The intra- and interobserver reliability were assessed by intraclass correlation coefficient (ICC). The measurements of FM and occipital condyles of nearly 10% sample (twenty-five scans) were taken twice by one observer with a time interval of 15 days and once by a second observer. The recalculated measurements were tested, and the ICC values were compared.

The measurements thus obtained were then noted into a proforma and then entered into IBM SPSS Statistics software v 25 and analyzed. The normal distribution of the data was determined by normality plots. The continuous variables were presented as mean ± standard deviation (SD). Male-female differences were tested using an independent t-test and the significance level of the test was defined at p<0.05. Pearson test was applied to examine the correlation between the measured foramen magnum dimension in both sexes. The foramen magnum dimensions were further analyzed using discriminate function analysis using a step-wise method for sex predictability.