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 Table of Contents  
Year : 2021  |  Volume : 33  |  Issue : 1  |  Page : 71-76

Foramen magnum as determinant of sexual dimorphism in Sri Ganganagar population: A radiographic study

1 Department of Oral Medicine and Radiology/ Oral Pathology, Dr Ziauddin Ahmad Dental College and Hospital, Aligarh Muslim University; Aligarh, Uttar Pradesh, India
2 Dental Surgeon, District Hospital, Auraiya, Uttar Pradesh, India
3 Department of Prosthodontics, Karpaga Vinayaga Institute of Dental Sciences, Tamilnadu, India
4 Private Practitioner, Patna, Bihar, India
5 Department of Prosthodontics, MM College of Dental Sciences and Research, Ambala, Haryana, India

Date of Submission08-Oct-2020
Date of Decision13-Jan-2021
Date of Acceptance14-Jan-2021
Date of Web Publication26-Mar-2021

Correspondence Address:
Dr. Pradhuman Verma
Professor and Chairman, Department of Oral Medicine and Radiology/ Oral Pathology, Dr. Ziauddin Ahmad Dental College and Hospital, Aligarh Muslim University Campus, Medical Road, Civil Lines, Aligarh - 202 002, Uttar Pradesh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jiaomr.jiaomr_205_20

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Background: In explosions, mass disasters, and warfare, human remains are got in fragmented states. Only strong bones that resist fracture can be used for personal identification in forensics. The foramen magnum (FM) dimension at the base of the skull is one such region that can be used for sex discrimination using radiographs. Aim and Objectives: To test evaluate sexual dimorphism in FM morphometric measurements and shape using digital submentovertex (SMV) radiograph among the Sri Ganganagar population. Materials and Methods: 100 digital SMV radiographs (50 males and 50 females) were randomly selected from the daily radiographic record of the patients with an age range of 25 to 65 years who visited the OPD and require SMV radiographs for diagnosis. The selected SMV radiographs were assessed for FM dimensions (longitudinal and transverse diameters, circumference, area, and FM index). Also, one particular shape was assigned to FM based on the radiographic image according to Chethan et al. classification. All the measurements were performed by two maxillofacial radiologists independently, and the data thus obtained were subjected to discriminant analysis for the evaluation of sexual dimorphism. Results: Values of four parameters out of five (transverse diameter, circumference, area, and foramen index) were found statistically significantly higher in males. The most common morphology of FM was an egg shape (50%), whereas round (3%) was the least common morphology. Circumference FM was the best indicator of gender determination followed by area, transverse diameter. and foramen index. The SMV radiographs for FM showed an overall accuracy of 69.73% in sex determination. Conclusion: The study showed that SMV radiographic FM morphometric measurements can be used for sex determination among the Sri Ganganagar population.

Keywords: Base of the skull, foramen index, forensics, gender determination

How to cite this article:
Verma P, Gupta N, Sameera Y, Faraz SA, Sharma P, Sharma B. Foramen magnum as determinant of sexual dimorphism in Sri Ganganagar population: A radiographic study. J Indian Acad Oral Med Radiol 2021;33:71-6

How to cite this URL:
Verma P, Gupta N, Sameera Y, Faraz SA, Sharma P, Sharma B. Foramen magnum as determinant of sexual dimorphism in Sri Ganganagar population: A radiographic study. J Indian Acad Oral Med Radiol [serial online] 2021 [cited 2021 Sep 19];33:71-6. Available from: https://www.jiaomr.in/text.asp?2021/33/1/71/312204

   Introduction Top

Morphometric features of skull bones are of great help in the determination of gender, age, and ethnicity in mass disasters, explosions, warfare, where identification may be extremely complicated because of skeletal fragmentation.[1] At the base of the skull, there is a large three-dimensional opening anteromedially , known as foramen magnum (FM), which is formed mainly by fusion of the four individual parts of the occipital bone (pars squama, left and right pars lateralis and pars basilaris) and acts as a communicator of the vertebral column with posterior cranial fossa.[2] The dimensions of FM have proven to be important in anthropology, forensics, and other medical fields.[3] The cranial base has been noted for its ability to remain intact where the rest of the cranium has been compromised and researchers have made use of that fact by analyzing sexually significant dimorphic traits for this anatomic region.[4] The FM completes its growth by early childhood. It is unresponsive to secondary sexual changes, with no influence of musculature on its size and shape, making it considerably stable beyond adolescence.

The FM contains the lower end of the medulla oblongata, meninges, vertebral arteries, the apical ligament of dens, spinal accessory nerve, and the tectorial membrane that passes through it to attach to the internal basiocciput.[5],[6] Because of these vital structures passing through FM, it becomes essential to measure their dimensions and morphology as these structures might get compressed in various conditions like meningioma, FM stenosis, cerebral herniation, and achondroplasia.[7],[8] These may cause life-threatening palsies and paresis of upper and lower extremities., The transcondylar approach is commonly used to assess the lesions which are ventral to the brainstem and at the cervicomedullary junction; hence, understanding the bony anatomy of the condylar region is important for this approach.[7] Assessment of FM dimensions about the hindbrain gives a clue for the initiation and propagation of Chiari I symptomatology.[8]

The sexual dimorphism in FM dimensions is population specific[9] and highly influenced by environmental, socioeconomic, and genetic factors. Teixeria[10] in 1982 reported the first time that the measurements regarding FM can be useful in the estimation of sex, and since then the various studies have been published about the evaluation of FM dimensions for sex estimation in different populations. The forensic scientist and anthropologist reported that radiographic measurements of FM may be more accurate and reliable than direct bone measurements for sex determination. The underlying reason is that these measurements provide clear images of the skeletal features where flesh is present.[11],[12] Also, it is a common practice to get radiographs as a part of post-mortem examinations to locate foreign bodies or document fractures or other injuries. In general submentovertex (SMV), reverse Towne's view, and lateral skull radiographs are used for viewing FM, but the reliability of SMV was proved maximum. With this background, the present study was undertaken to make use of a digital SMV radiograph to establish sexual dimorphism in FM by morphometric measurements in the Sri Ganganagar population.

   Material and Methods Top

The present study was conducted in the department of Oral Medicine and Radiology of a dental institute, Sri Ganganagar, Rajasthan. The estimation of the study sample size was done statistically using sensitivity and specificity formulae.

The two formulae were considered for sample size calculation for the present diagnostic efficacy of SMV radiographs. 1. Using sensitivity and 2. Using specificity


n = required sample size,

SN = anticipated sensitivity,

SP = anticipated specificity,

α = size of the critical region (1-α is the confidence level),

Z1-α/2 = standard normal deviate corresponding to the specified size of the critical region (α),

L = absolute precision desired on either side (half-width of the confidence interval) of sensitivity or specificity.

Taking sensitivity and specificity both as 90% as per previous studies [1],[3] and expected maximum patient reporting for SMV radiography as 50%, L = absolute precision as 10% and Z = 2 (for 95% confidence interval), from the above formula, the sample size was found to be = 97. Considering error @5%, the sample size was increased to n = 100.

100 subjects were selected who reported to the department for radiographic examination by SMV radiograph (zygomatic arch/skull base fractures, Temporomandibular joint (TMJ), or submandibular pathologies, etc). The written informed consent from each patient & the Institutional ethical committee clearance (SDCRI/IEC/2018/004 dated 08/11/2018) from Dental College Ethical Board was obtained before the conduct of the study. The study was conducted as per ethical standards declared by the revised 2000 Helsinki Declaration for Medical research. Inclusion criteria of the study comprised. 1. The age group between 25 and 65 years. 2. Patients requiring SMV radiograph for diagnosis. The Exclusion criteria included 1. Pregnant female. 2. Patients with cervical vertebral fracture. 3. Patients who were not willing for radiographic investigation. 4. Patients who were terminally ill or unconscious. 5. Patients with cervical pain. 6. Poor quality radiographs.

The growth of FM continues till early childhood; hence, patients under 25 years of age were excluded. Patients above 65 years of age are more likely to have cervical problems and less likely to need SMV; hence, they were also excluded from the study. The patients who reported to the OPD were seated comfortably on the dental chair and a thorough clinical examination was done. Following examination, the patients requiring SMV radiographs for diagnosis were selected and then the radiographic procedure was explained to them. Each selected patient was then asked to sit comfortably on a steel chair with their neck extended in the supine position and head tipped backward and touching the cassette. The Frankfort horizontal plane was made parallel to the floor [Figure 1]. To standardize the procedure, all SMV radiographs were taken by the same operator and through a single operational machine (Kodak 8000 C digital OPG/Ceph machine) at 80 kV tube voltage, 15 mA tube current, and 15 s exposure time.
Figure 1: Digital submentovertex radiograph with analysis of the foramen magnum dimensions using Kodak 8000 C digital OPG/Ceph machine

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The exposure parameters were standardized and the image was displayed on a computer screen. The following measurements of the FM were recorded in millimeters [Figure 2].
Figure 2: Seven morphometric types of foramen magnum

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  1. Longitudinal diameter/length of FM (LD): Maximum anteroposterior dimension measured from the highest point on the anterior border up to the inferior most point of FM.
  2. Transverse diameter/width of FM (TD): The maximum mesiodistal dimension measured from the point of highest convexity on the medial border up to the point of highest convexity on the distal- most margin of FM.
  3. Circumference of FM (C): Perimeter got by tracing the entire border of FM using software (IMAGE PROCESSING AND ANALYSIS IN JAVA).
  4. Area (A) of FM: Calculated by substituting LD and TD in Radinsky formula[13]

  5. Radinsky formula: Area = ¼ π × LD × TD

  6. Foramen index (FI) was calculated by dividing LD to TD. According to Krishna et al.[14] if the value of FI ≥1.2, FM morphological shape was considered to be oval.
  7. Shape of FM (S): Subjective assignment of one of the seven radiographic morphological shapes to FM was recorded as per Chethan et al.[15] classification. The different shapes of FM were classified as oval, round, egg, tetragonal, pentagonal, hexagonal, and irregular shapes. As a part of interpretation, only brightness and contrast was changed to aid in better visualization of the image by observers. To minimize interobserver bias the observations were recorded by two observers separately and independently. All the findings were tabulated and subjected to the statistical analysis using IBM Statistical Package for the Social Sciences (SPSS 20.0).

   Results Top

100 subjects were studied (50 males and 50 females) with an overall mean age of 35.79 ± 9.12 years [Table 1]. The mean and standard deviation for all the five measurements were obtained to derive the FM dimensions in the study population, which showed that except LD all other parameters (TD, circumference, area, FI) were noted higher in males, highlighting sexual dimorphism in FM dimensions. To substantiate this, P- value was calculated using Student's t-test and it was seen that P- value was <0.05 for TD, circumference, area, and FI suggesting statistically significant differences between the two genders [Table 2].
Table 1: Distribution of subjects among both genders

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Table 2: Sexual dimorphism in the foramen magnum dimensions among various parameters

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For evaluating inter-observer agreement, correlation (r -value) was calculated for all the FM dimensions as measured by two observers. The r- value showed a strong correlation for all, suggesting good interobserver agreement. The interobserver agreement was found to be 85% [Table 3].
Table 3: Foramen magnum dimensions in males and females as assessed by observers

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To assess the accuracy of digital SMV radiograph, a formula was derived using discriminate function analysis:

Gender accuracy formula: [(-0.263 x LD) +0.156 x TD) + (0.437 x C) + (0.659 x A)] – 102.17

By applying the data to the derived equation, canonical variables were derived for all the parameters of FM dimensions. Also, an attempt was made to assess the efficiency of all five parameters of FM in sex determination. The overall accuracy of 69.7% was got when 100 subjects were considered. The maximum accuracy was got for C and the least for LD [Table 4].
Table 4: Accuracy of foramen magnum dimension in sex determination

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Regarding the radiographic assessment of morphological shapes of FM, it was observed that egg shape was the most common shape observed in both genders, whereas hexagon or round (both 6%) was the least common shape observed in males and round (none) shape in females [Table 5].
Table 5: Shapes among males and females

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   Discussion Top

Gender determination in forensic sciences helps to channelize the investigations by deducing the search to half the population, thus conserving both resources and the time required for identification. The FM is a vital and prominent structure of the skull base, the dimensions of which are influenced by genetics, environment, and social factors.[12] Taking this into account, we conducted the present study on the North-Western (N-W) Indian subpopulation of Sri Ganganagar. In the literature, many authors have showed sexual dimorphism in the dimensions of FM among different Indian sub-populations, but they have used dry skulls to derive their results. This limits the study sample. Also, archiving skulls of known age and gender for a study is an added drawback. To overcome this, the present study was performed with the help of digital SMV radiographs and perhaps the first of its kind among the N-W Indian subpopulation.

The FM measurements in our study showed statistically significant (p < 0.05) differences between the gender with all values significantly greater in males than females except LD which was noted slightly higher among females (30.6 ± 2.86 mm). Babu et al.[16] found the mean LD values to be 35.68 mm in males and 32.57 mm in females, whereas Kanchan et al.[3] reported values of 34.51 mm in males and 33.6 mm in females. This difference of result can be because of different populations studied, different study methods (the present study was conducted using SMV radiographs, whereas mentioned studies were conducted on dried skulls).

In the present study, the area of FM among both genders (male = 659.98 ± 111.61 mm2 and female = 627.63 ± 62.06 mm2) was found comparable to Kanchan et al. and Babu et al. studies with statistically significant gender difference. The FM area was calculated by using Radinsky's formula as in a review of literature also, researchers had used two formulae to calculate the FM area, Texeira formula, and Radinsky's formula. Among the studies conducted in the Indian population, the value for area obtained by Radinsky's formula is a better evaluator of sex.

A significant statistical difference regarding FM circumference (male = 83.26 ± 5.22 mm and female = 79.15 ± 10.49 mm) was found in the present study, the results are following Raikar et al. and Uthman et al., studies where the mean C values were male = 106.15 mm; female = 99.95 mm and male = 99.3; female = 92.6 mm respectively.

The FI was calculated (if FI ≥ 1.2) which helps us in determining the shape of FM when values of circumference and area cannot be assessed.

In the present study, the most common morphological FM shape was found to be Egg shape (25%) in both genders, whereas the least common shape was round (no case) in females and hexagon and round (3%) in males. Raikar et al.,. observed similar findings.[13] But according to Chethan et al.[15] the incidence rate of round shape was most common followed by egg and tetragonal. This difference among the results of studies might result from racial differences or visualization techniques.

The strong interobserver correlation in our study implies that FM dimensions are minimally affected by subjective variations and hence are highly reproducible in determining sexual dimorphism. It was seen that FM dimensions tested using digital SMV were 69.73% accurate in differentiating sex and 68.92% and 70.54% in male and female determination, respectively. The results were following the Gapert et al.[17] (70.3%), Texeira et al. (70%), and Suazo et al.[18] (66.5%). However, the accuracy achieved by Uysal et al. in their studies on a sexual determination by FM using CT was found to be 81%. In CT though contrast can be enhanced for better results, but the disadvantage of high radiation dose to patients (20 μSv) compared to SMV radiographs (0.6 μSv) limits its use.[19],[20]

Limitations and future prospects

Limitations of the present study include a small study sample that cannot give accurate supportive evidence to the results got so far, a bigger sample size would easily substantiate the results got in our study. There can be errors in the identification of shapes of FM due to difference in the visualization by the observers.

   Conclusion Top

The study recommends the use of SMV radiographs in elucidating FM morphometric variations for the identification of unknown individuals and may act as a guide to the anatomists, neurosurgeons, and in other medical fields as well.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

Abo El-Atta HMH, Abdel-Rahman RH, El-Hawary G, Abo El-Al-Atta HM. Sexual dimorphism of foramen magnum: An Egyptian study. Egypt J Forensic Sci 2020;10:1-12.  Back to cited text no. 1
Avci E, Kara E, Ozturk NC, Uluc K. Anatomical variation of the foramen magnum, occipital condyle and jugular tubercle. Turk Neurosurg 2011;2:181-90.  Back to cited text no. 2
Kanchan T, Gupta A, Krishan K. Craniometric analysis of foramen magnum for estimation of sex. Int J Med Health Biomed Pharm Eng 2013;7:111-3.  Back to cited text no. 3
González-Colmenares G, Sanabria-Medina C, Rojas-Sánchez MP, Leóna K, Malpud A. Sex estimation from skull base radiographs in a contemporary Colombian population. J Forensic Legal Med 2019;62:77–81.  Back to cited text no. 4
Toneva D, Nikolovaa S, Harizanov S, Georgiev I, Zlatareva D, Hadjidekov V, et al. Sex estimation by size and shape of foramen magnum based on CT imaging. Legal Med 2018;35:50–60.  Back to cited text no. 5
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Sharma S, Sharma AK, Modi BS, Arshad M. Morphometric evaluation of the foramen magnum and variation in its shape and size: A study on human dried skull. Int J Anat Res 2015;3:1399-403.  Back to cited text no. 7
Furtado S, Thakre DJ, Venkatesh PK, Reddy K, HedgeAS. Morphometric analysis of foramen magnum in dry human skulls. Int J Anat Res 2015;3:1015-23.  Back to cited text no. 8
Galdames ICS, Russo PP, Matamala DAZ, Smith RL. Sexual dimorphism in the foramen magnum dimensions. Int J Morphol 2009;27:21–3.  Back to cited text no. 9
Teixeria WR. Sex identification utilizing the size of foramen magnum. Am J Forensic Med Pathol 1982;3:203-6.  Back to cited text no. 10
Madadin M, Menezes RG, Al Saif HS, Abu Alola H, Al Muhanna A, Gullenpet AH, et al. Morphometric evaluation of the foramen magnum for sex determination: A study from Saudi Arabia. J Forensic Legal Med 2017;46:66–71.  Back to cited text no. 11
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Krishna K, Nag AR, Prasad R. Morphometric study of foramen magnum and variation in its shape. IOSR J Dent Med Sci 2016;15:120-3.  Back to cited text no. 14
Chethan P, Prakash KG, Murlimanju B, Prashant KU, Prabhu LV, Saralaya VV. Morphological analysis and morphometry of the foramen magnum: An anatomical investigation. Turk Neurosurg 2012;22:416-9.  Back to cited text no. 15
Babu R, Manjunath S, Rastogi P, Mohan KTS, Bhat VJ, Yoganarasimha K. Determination of sex by foramen magnum morphometry in South Indian population. J Med Singapore 2013;54:458-62.  Back to cited text no. 16
Gapert R, Black S, Last J. Sex determination from the foramen magnum: Discriminant function analysis in an eighteenth and nineteenth century British sample. Int J Legal Med 2009;123:25-33.  Back to cited text no. 17
Suazo GIC, Russo PP, Zavando MD, Smith RL. Sexual dimorphism in the foramen magnum dimensions. Int J Morphol 2009;27:21-3.  Back to cited text no. 18
Tambawala SS, Karjodkar FR, Sansare K, Prakash N, Dora AC. Sexual dimorphism of foramen magnum using Cone Beam Computed Tomography. J Forensic Legal Med 2016;44:29e–34e.  Back to cited text no. 19
Gargi V, Prakash SM, Malik S, Nagaraju K, Goel S, Gupta S. Sexual dimorphism of foramen magnum between two different groups of Indian population: A cross-sectional cone-beam computed tomography study. J Forensic Sci Med 2018;4:150-5.  Back to cited text no. 20
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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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