Morphometric Analysis of Foramen Magnum for Sex Determination in South Indian Population

Introduction

Sex determination from skull morphology is important in medico-legal cases. In most of the forensic studies the skeleton will be incomplete and makes gender identification difficult. The foramen magnum (FM) is one of the primary centers of ossification on the cranial base during growth and development, and is located inferior to the sagittal suture, on the cranial base. Characteristics of FM and cranial base have identifying features for sexing^1–3. Many authors have reported the usefulness of the FM in gender determination^4–9. Significant craniometry differences exist in the FM between the two sexes within a restricted geographical region and historical period^10-. Gunay & Altinkok¹³ reported that the dimensions of the FM were useful in sex determination but the total area was not a good indicator. The FM measurements in males are greater because they tend to have larger heads than females¹⁴. Uysal¹⁵ using Computed Tomography scans of living individuals reported all dimensions of FM were larger in males than females, with the length and width of the right occipital condyle and the width of the FM reflecting the greatest differences. Murshed¹⁴ classified FM into eight different shapes: oval, egg, round, tetragonal, pentagonal, hexagonal, irregular A, and irregular B. Updating temporal changes of skull morphometry is essential for forensic study and anthropologists. Hence, the focus of this research was to evaluate the morphological differences of foramen magnum for sex determination in human skulls of South India.

Materials and methods

100 skulls (55 males and 45 females), from the Department of Anatomy, Yenepoya Medical College, Yenepoya University were used for the study. Only skulls with good conditions with gender identified in book record were included for the study. The skull samples which were damaged, incomplete or without identification were not included in the study. All the 102 cranial bases were visually assessed for foramen magnum shape classification. Each foramen magnum was classified into one of the four shapes, oval, round, tetragonal, pentagonal (Fig 1)

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Fig. 1

Shapes of foramen magnum

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Morphometry (Anterioposterior diameter (APD) & Transverse diameter (TD)) of the foramen magnum was measured using Vernier Caliper (Forbes) to an accuracy of 0.5mm. APD of the foramen magnum is the distance between opisthion to basion along the midsagittal plane¹⁶ (Fig 2), while TD is the maximum distance along the transverse plane. (Fig 2) Differences in length and width of foramen magnum between males and females were assessed by t-test¹⁷.

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Fig. 2

Anterioposterior diameter (APD) and Transverse diameter (TD)

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Results

Table 1 details the number of skulls in various shapes. Oval shape was the most common followed by round, tetragonal and pentagonal in both males and females. The dimensions of the foramen magnum for 100 skulls showed statistically significant differences. The APD (34.04 vs 31.72) and TD (28.63 vs 26.59) were significantly higher in males than in females (P<0.05) (Table 2) (Fig 3).

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Fig. 3

Morphometry of foramen magnum correlated with gender

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Discussion

The sex determination of incomplete or damaged skeletons is an important task in forensic medicine. Anthropometric measurements can aid in solving such problems of gender identification. The foramen magnum was used since it is a regular structure and less likely to major morphological changes. The morphometry of the skull can be used for assessing sexual dimorphism has been reported by Gunay¹³, Westcott and Moore-Jansen¹⁸ and Murshed et. al.,¹⁴. The comparison of the morphometric analysis obtained in this study with the results of other studies had the following results: the length (APD) of the foramen magnum of South Indian male skulls (34.04±2.36) was lower than the Brazilian male skulls (35.7 ± 0.29)⁷ the Turkish (37.2 ± 3.43)¹⁴, Spanish (36.2 ± 0.3)¹⁹, English populations (35.91 ± 2.41)²⁰, and the Indian population (35.5 ± 2.8)²¹. Similarly APD of the female skulls of the South Indian population was (31.72±2.14) lower than Brazilian population (35.1 ± 0.33)⁷, than those of the Turkish (34.6 ± 3.16)¹⁴, Spanish (34.30 ± 0)¹⁹, Indian (32.0 ± 2.8)²¹, and English populations (34.71 ± 1.91)²⁰. Regarding the width (TD) of the foramen magnum, the values of the South Indian male skulls (28.63± 1.89) was lower than Brazilian male skulls (30.3 ± 0.20)⁷, Indians (29.6 ± 1.9)²¹, the Turkish (31.6 ± 2.99)¹⁴, Spanish (31.1 ± 0.3)¹⁹, and English populations (30.51 ± 1.77)²⁰. The same measure for the female skulls of the South Indian population (26.59±1.64) was lower than the Brazilian population (29.4±0.23)⁷ Indian (27.1 ± 1.6)²¹ and Turkish populations (29.3 ± 2.19)¹⁴ and lower than Spanish (29.6 ± 0.3)¹⁹ and English populations (29.36 ± 1.96)²⁰. The results demonstrated that sexual dimorphism is present in the foramen magnum. In incomplete skeletons, metric analysis of the foramen magnum may provide a statistically useful indication as to sex of the unknown skull.