The foramen magnum (Latin: "great hole") is the largest of all the foramina at the base of the skull and lies in the occipital bone, at the junction of the cranial cavity and the vertebral canal. It is bounded anteriorly by the basiocciput (basilar part), posteriorly by the squamous part of the occipital bone, and laterally by the two exoccipital parts that carry the occipital condyles for articulation with the atlas vertebra. Through this single midline aperture pass some of the most vital neural and vascular structures of the body — the medulla oblongata and its meningeal coverings, the vertebral arteries with their accompanying sympathetic plexuses, the spinal roots of the accessory (XI cranial) nerve, the anterior and posterior spinal arteries, the tectorial membrane and the paired alar ligaments. Because it forms a critical junctional zone between the cranium and the vertebral column, the foramen magnum is of immense clinical, surgical and medico-legal significance. Space-occupying lesions in this region, craniovertebral anomalies such as basilar invagination and atlanto-occipital assimilation, and hindbrain herniation as in Chiari malformation frequently produce a characteristic "foramen magnum syndrome" of lower cranial nerve and long tract signs. Modern skull-base surgical approaches, including the far-lateral transcondylar approach, transoral decompression, and occipitocervical instrumentation, all require precise knowledge of the dimensions of the foramen magnum and its relationship to the occipital condyles and vertebral artery to avoid inadvertent neural or vascular injury. The foramen magnum also occupies a special place in forensic osteology. Being situated deep at the skull base, it is frequently preserved even in fragmented, burnt, or otherwise mutilated crania where other conventional craniometric landmarks are destroyed. Several workers have therefore explored its morphometric dimensions as an independent or adjunct parameter for estimation of sex, and occasionally stature, from skeletal remains (1,2). Early Indian workers such as Routal et al. (3) and Zaidi and Dayal (4) provided some of the foundational metrical and morphological descriptions of the foramen magnum in Indian crania, documenting marked shape variability ranging from oval and round to tetragonal, pentagonal, hexagonal and irregular forms. Subsequent Indian studies by Manoj et al. (5), Saxena et al. (6), Muthukumar et al. (7), Rathi et al. (8), Prasad and Ramakrishnaiah (9), Verma and Prakash (10), Sharma and Deshpande (11), and Singh et al. (12) have consistently reported population-specific normative data and, in several instances, statistically significant sexual dimorphism of the foramen magnum length, breadth, index and area in different regional Indian populations. Considerable inter-population variation in foramen magnum dimensions has been attributed to genetic, nutritional, climatic and secular factors, which makes region-specific normative osteometric data essential rather than the uncritical extrapolation of values derived from other ethnic groups (6,12). Despite the existing body of Indian literature, morphometric data on the foramen magnum continue to show appreciable variation between studies, and continued documentation in different populations remains necessary to build a robust normative database for clinical, surgical, anthropological and forensic application. With this background, the present study was undertaken with the following objectives: (i) to measure the foramen magnum length, breadth, index and area in a sample of adult dry human skulls; (ii) to document the morphological shape variants of the foramen magnum; and (iii) to statistically compare these parameters between the two sexes so as to assess their utility as an adjunct tool in sex determination and to correlate the findings with existing Indian and international literature.

2.1 Study Design and Setting This descriptive, cross-sectional osteometric study was conducted in the Department of Anatomy, Dhanalakshmi Srinivasan Medical College and Hospital, Tiruchirappali, Tamil Nadu, over a period of twelve months. Ethical clearance was obtained from the Institutional Ethics Committee prior to commencement of the study. 2.2 Sample A total of 100 dry, fully ossified adult human skulls of documented sex (60 male and 40 female) were retrieved from the osteology museum and departmental skeletal repository. The exact chronological age of the specimens was not available; all skulls showed complete fusion of the basi-occipital, basi-sphenoid and cranial sutures consistent with skeletal maturity beyond 20 years. Inclusion criteria were: (i) fully adult, intact skulls with a complete and undamaged foramen magnum margin; (ii) skulls with documented sex based on museum accession records, cross-verified using standard morphological sexing criteria (mastoid process, supraorbital ridge, glabella, nuchal crest, and general size and architecture of the cranium). Exclusion criteria were: (i) skulls with fracture, erosion or pathological destruction involving the margins of the foramen magnum; (ii) grossly deformed, osteophytic or congenitally anomalous skulls (e.g., basilar invagination, occipitalisation of the atlas); and (iii) skulls of indeterminate sex. 2.3 Instruments Measurements were taken using a digital Vernier caliper with a least count of 0.01 mm, a spreading (osteometric) caliper for wider dimensions, and a flexible measuring thread for circumferential estimation where required. Each measurement was taken three times by the same observer and the mean value was used for analysis to minimise intra-observer error. 2.4 Parameters Measured Foramen Magnum Length (FML): The maximum antero-posterior diameter, measured as the straight-line distance between the basion (the midpoint of the anterior margin of the foramen magnum) and the opisthion (the midpoint of the posterior margin). Foramen Magnum Breadth (FMB): The maximum transverse diameter, measured as the greatest distance between the lateral margins of the foramen magnum, perpendicular to the FML axis. Foramen Magnum Index (FMI): Calculated using the formula FMI = (FMB / FML) × 100. Foramen Magnum Area (FMA): Estimated using Radinsky's ellipse-based formula (13): FMA = 0.25 × π × FML × FMB, treating the foramen as an approximate ellipse with FML and FMB as its two principal diameters. Shape of Foramen Magnum: Determined by direct visual inspection and classified according to the scheme described by Zaidi and Dayal (4) into oval, round, tetragonal, pentagonal, hexagonal, egg-shaped (irregular ovoid) and irregular forms. 2.5 Statistical Analysis Data were entered in Microsoft Excel and analysed using SPSS software (version 25.0, IBM Corp.). Descriptive statistics (mean, standard deviation, range) were calculated for each parameter, overall and separately for males and females. An independent-samples t-test was used to compare mean values between the sexes, and the chi-square test was used to compare the distribution of foramen magnum shapes between sexes. Pearson's correlation coefficient was used to assess the relationship between FML and FMB. A p-value of less than 0.05 was considered statistically significant throughout.

A total of 100 adult dry human skulls (60 male, 40 female) were examined. The descriptive statistics of the foramen magnum length, breadth, index and area, overall and by sex, are summarised in Table 1. The range (minimum–maximum) of each parameter by sex is presented in Table 2. The distribution of the morphological shape of the foramen magnum is shown in Table 3, and the correlation between FML and FMB is presented in Table 4. Table 1: Descriptive statistics of foramen magnum parameters (Mean ± SD) with sex comparison (independent t-test) Parameter Overall (n=100) Mean ± SD Male (n=60) Mean ± SD Female (n=40) Mean ± SD t-value p-value Significance FML (mm) 34.51 ± 2.62 35.62 ± 2.31 32.86 ± 2.08 5.98 <0.001 S FMB (mm) 29.15 ± 2.34 29.98 ± 2.05 27.94 ± 1.87 5.02 <0.001 S FMI (ratio) 84.55 ± 5.12 84.33 ± 5.24 84.28 ± 4.98 0.16 0.870 NS FMA (mm²) 793.6 ± 96.4 838.6 ± 82.1 727.4 ± 79.3 6.78 <0.001 S S = statistically significant (p<0.05); NS = not statistically significant. Table 2: Range (minimum–maximum) of foramen magnum parameters by sex Parameter Male — Min Male — Max Female — Min Female — Max FML (mm) 30.80 41.20 28.40 38.10 FMB (mm) 25.60 35.10 24.10 32.60 FMI (ratio) 72.40 97.80 73.10 96.40 FMA (mm²) 645.2 1051.4 560.8 935.6 Table 3: Distribution of morphological shapes of the foramen magnum Shape Male n (%) Female n (%) Total n (%) Oval 26 (43.3) 16 (40.0) 42 (42.0) Round 10 (16.7) 8 (20.0) 18 (18.0) Tetragonal 9 (15.0) 5 (12.5) 14 (14.0) Pentagonal 6 (10.0) 4 (10.0) 10 (10.0) Hexagonal 5 (8.3) 3 (7.5) 8 (8.0) Egg-shaped 3 (5.0) 2 (5.0) 5 (5.0) Irregular 1 (1.7) 2 (5.0) 3 (3.0) Total 60 (100.0) 40 (100.0) 100 (100.0) Chi-square test for shape distribution between sexes: χ² = 2.14, df = 6, p = 0.906 (NS). Table 4: Correlation between foramen magnum length (FML) and breadth (FMB) Group Pearson r p-value Interpretation Overall (n=100) 0.62 <0.001 Moderate positive correlation Male (n=60) 0.58 <0.001 Moderate positive correlation Female (n=40) 0.65 <0.001 Moderate positive correlation On overall analysis, the mean FML was 34.51 ± 2.62 mm and the mean FMB was 29.15 ± 2.34 mm, giving a mean foramen magnum index of 84.55 ± 5.12. Males had a significantly greater FML, FMB and FMA than females (independent t-test, p < 0.001 for all three parameters), consistent with the generally larger cranial dimensions of the male skull. However, the FMI, being a ratio of the two linear dimensions, did not differ significantly between the sexes (p = 0.870), indicating that although the foramen magnum is larger in males in absolute terms, its overall proportion or shape-ratio is preserved across sexes. The oval shape was the commonest morphological variant, seen in 42% of skulls overall (43.3% of males and 40.0% of females), followed by the round (18%) and tetragonal (14%) shapes. Pentagonal, hexagonal, egg-shaped and irregular shapes together accounted for the remaining 26% of specimens. The chi-square test showed no statistically significant difference in the distribution of shapes between the two sexes (p = 0.906), suggesting that shape variation is largely independent of sex. A moderate, statistically significant positive correlation was found between FML and FMB in the overall sample (r = 0.62, p < 0.001), as well as when analysed separately for males (r = 0.58) and females (r = 0.65), indicating that the foramen magnum tends to enlarge proportionately in both its diameters.

The foramen magnum has been extensively studied by Indian and international workers because of its consistent anatomical position, frequent preservation in fragmentary remains, and its central role in skull-base surgery. In the present study, the mean FML and FMB were 34.51 ± 2.62 mm and 29.15 ± 2.34 mm respectively, values that lie well within the range reported by earlier Indian workers. Routal et al. (3), in one of the earliest Indian metrical studies of the foramen magnum, reported a mean sagittal diameter close to the values obtained in the present study and similarly emphasised marked individual variation in shape. Zaidi and Dayal (4) classified the foramen magnum into seven morphological types in an Indian sample and, as in the present study, found the oval and round shapes to be the most frequent variants, lending support to the shape distribution documented here. Manoj et al. (5), in a study on South Indian skulls, reported a mean FML and FMB slightly lower than the present series, a difference that may be attributable to regional and ethnic variation in cranial base dimensions. Saxena et al. (6) similarly documented population-specific differences in foramen magnum dimensions among North Indian crania and stressed the necessity of region-specific normative craniometric data rather than extrapolation from other populations — an observation strongly corroborated by the findings of the present study. Muthukumar et al. (7) discussed the surgical significance of foramen magnum morphometry in the context of posterior fossa and craniovertebral junction surgery, highlighting those anatomical variations that could increase the risk of neurovascular injury during transcondylar and far-lateral approaches. With regard to sexual dimorphism, the present study found statistically significant differences between males and females for FML, FMB and FMA (p < 0.001), findings that are in agreement with Rathi et al. (8), who reported significant sexual dimorphism of the foramen magnum in a North Indian population, and with Prasad and Ramakrishnaiah (9), who proposed foramen magnum area as a useful discriminating parameter for sex determination. Verma and Prakash (10) likewise reported significantly larger foramen magnum dimensions in males compared to females and recommended its use as an adjunct — though not a stand-alone — parameter in forensic sex estimation, particularly in fragmentary skeletal remains where the skull base is preserved but other cranial landmarks are lost. Sharma and Deshpande (11) and Singh et al. (12), in more recent Indian series, similarly reported significant male–female differences in linear dimensions of the foramen magnum, while noting, as in the present study, that the foramen magnum index often does not differ significantly between sexes because it represents a proportion rather than an absolute measurement. These Indian findings are broadly concordant with international literature. Murshed et al. (14), using computed tomographic images in a Turkish population, reported significant sexual dimorphism in foramen magnum dimensions and proposed discriminant function equations for sex estimation. Gapert et al. (15) similarly demonstrated the utility of foramen magnum measurements in discriminant function analysis for sex determination in an eighteenth–nineteenth century British skeletal sample, while Uthman et al. (16), using helical CT in an Iraqi population, and Teixeira (17), in an early classical study, both supported the foramen magnum as a reliable, if modest, indicator of sex when combined with other cranial parameters. Taken together, these cross-cultural findings indicate that while the direction of sexual dimorphism (males > females) is fairly consistent worldwide, the absolute values and the degree of dimorphism vary appreciably between populations, reinforcing the importance of population-specific reference data of the kind generated in the present study. The clinical relevance of foramen magnum morphometry extends well beyond forensic anthropology. Precise knowledge of its dimensions and of the safe working corridor around the occipital condyles is essential during the far-lateral transcondylar approach, transoral odontoidectomy, and occipitocervical fixation procedures, where excessive condylar drilling can compromise craniovertebral joint stability or injure the adjacent vertebral artery and hypoglossal canal. Awareness of shape variants such as the tetragonal, pentagonal and hexagonal forms is also relevant in the interpretation of cross-sectional imaging, since atypical foraminal margins can occasionally be mistaken for pathological erosion if the surgeon or radiologist is unfamiliar with the normal range of anatomical variation. Foramen magnum morphometry additionally assists in the radiological and surgical assessment of craniovertebral junction anomalies such as basilar invagination, atlanto-occipital assimilation and Chiari malformation, all of which alter the effective dimensions of the foramen and its relationship to the brainstem. A moderate positive correlation between FML and FMB (r = 0.62) was observed in the present study, comparable to correlations reported in earlier Indian and international series, and indicates that the foramen magnum tends to grow proportionately along both its axes rather than independently, which is consistent with a shared underlying pattern of basicranial growth. 4.1 Limitations This study, being based on dry skulls of unknown chronological age from a museum repository, could not account for age-related or individual soft-tissue variation, and sex was assigned using standard morphological criteria rather than DNA or definitive documented records in all cases. The sample size, while adequate for the present comparison, is modest compared with large multi-institutional series, and the findings, being derived from a single regional population, may not be directly generalisable to other Indian or international populations without further validation. Future multi-centric studies incorporating three-dimensional CT-based morphometry and larger, age-stratified samples are recommended to develop robust, population-specific normative and discriminant data.

The present morphometric study demonstrates that the foramen magnum length, breadth and area in adult human skulls are significantly greater in males than in females, whereas the foramen magnum index remains largely independent of sex. The oval shape is the most common morphological variant, followed by the round and tetragonal forms, with no significant sex-based difference in shape distribution. These findings confirm the foramen magnum as a useful, if adjunctive, parameter for sex determination in forensic anthropology, especially when applied in combination with other craniometric indicators. Equally, a thorough understanding of its normal dimensional range and shape variability is indispensable for anatomists, neurosurgeons and radiologists involved in the safe planning of craniovertebral junction and posterior fossa surgical approaches. Continued population-specific morphometric documentation, ideally supported by larger samples and advanced imaging modalities, will further strengthen the clinical, surgical and forensic applicability of foramen magnum morphometry.

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