European Journal of Cardiovascular Medicine

The foramen magnum (FM) is the largest aperture of the skull base and forms the bony junction between the posterior cranial fossa and the vertebral canal. It provides passage for the medulla and meninges, vertebral arteries with their venous plexuses, and the spinal component of the accessory nerve. Because these structures lie close to the osseous margin, FM morphology is clinically relevant in craniovertebral junction disorders, radiologic interpretation, and skull‑base surgical planning [1,2]. Conditions such as Chiari malformations, congenital craniovertebral anomalies, and space‑occupying lesions at the FM can be influenced by the available bony aperture and by the spatial relationship between the FM and surrounding landmarks [1,2].

In neurosurgical practice, FM size and the relationship of the FM to the occipital condyles influence exposure during far‑lateral and transcondylar approaches to ventral foramen magnum pathology. Quantitative measurements guide the expected working window, inform the extent of condylar drilling, and support planning around adjacent neurovascular structures [3]. Small differences in FM dimensions can also affect the interpretation of “stenosis” or crowding on cross‑sectional imaging, particularly when extrapolating thresholds derived from external populations. Indian datasets that integrate dry skull and imaging measurements demonstrate that reference ranges can differ across cohorts and measurement methods, which supports the use of locally generated baseline values for clinical translation [4].

Variation is not limited to size. Morphological investigations on dry skulls describe multiple FM shape categories (e.g., oval, round, tetragonal, pentagonal, irregular) and show differences in their frequency distribution between cohorts [5,6]. Comparative work across diverse populations indicates that FM size is typically larger in males within populations, whereas the pattern of FM shape can vary between populations and across growth trajectories [7]. Imaging-based morphometry, including cone‑beam CT protocols, adds reproducibility and helps bridge osteological findings to clinical imaging practice [8].

The FM also has forensic relevance because the cranial base is relatively resilient and can remain intact when other skeletal elements are fragmented. Several studies have evaluated whether FM dimensions and derived variables can contribute to sex estimation, with accuracy varying across datasets and populations [9,10]. Regional CT studies from South India and Saudi Arabia, and a larger 3DCT series from Nepal, collectively reinforce the need for population-specific standards and cautious interpretation of predictive models [11-13]. Within India, variation in FM shape distribution has also been documented, suggesting that regional datasets are useful even within a single country [14]. Objectives of this study were to document key FM morphometric parameters (anteroposterior diameter, transverse diameter, FM index, and calculated area), describe the distribution of FM shapes, and compare FM dimensions between male and female skulls in an adult sample from a teaching institution in Karimnagar, Telangana, India.

Study design and setting: This cross-sectional observational osteological study was conducted in the Department of Anatomy, Government Medical College (GMC), Karimnagar, Telangana, India, during February 2025 to November 2025.

Study sample and sampling strategy: A total of 80 adult dry human skulls available in the departmental teaching and museum collection were evaluated using a convenience sampling approach (male n=42; female n=38). Adult status was confirmed from collection registers and dentition/cranial maturity. Sex assignment followed the label information available in departmental records and, where required, was cross-checked using standard cranial morphological traits. Skulls with obvious congenital malformations, prior surgical defects, fractures, erosion, or deformation involving the FM margin were excluded to avoid systematic measurement error.

Measurement protocol: Each skull was placed on a stable, leveled surface in the anatomical position. Measurements were obtained using a digital vernier caliper (least count 0.01 mm) after instrument calibration. The anteroposterior diameter (APD) was measured from basion (midpoint on the anterior FM margin) to opisthion (midpoint on the posterior margin). The transverse diameter (TD) was recorded as the maximum distance between the lateral margins. Measurements were recorded to two decimal places. To improve repeatability, two independent readings were taken for each parameter and the average was used for analysis; discrepant readings were rechecked and reconciled by repeat measurement.

Derived indices and area estimation: FM index (FMI) was calculated as (TD/APD) × 100. FM area was estimated using an ellipse-based approximation (Area = π × [APD/2] × [TD/2]), which is widely applied in FM morphometric literature for converting linear diameters into a reproducible area proxy [3,7].

Morphological (shape) assessment: FM shape was assessed by direct visual inspection under adequate illumination. Shapes were categorized as oval, round, tetragonal, pentagonal, or irregular based on the predominant contour of the bony margin, following criteria described in osteological series and regional studies [5,6,14].

Outcome measures: Primary outcomes were APD, TD, FMI, and calculated FM area. Secondary outcomes were the frequency distribution of FM shapes and the mean FM area across shape categories.

Statistical analysis: Data were entered in Microsoft Excel and analyzed using SPSS. Continuous variables were summarized as mean ± standard deviation (SD) with minimum–maximum values, and categorical variables as frequency and percentage. Normality of continuous variables was assessed using the Shapiro–Wilk test. Sex-based comparisons for APD, TD, and FM area were performed using an independent-samples t-test (two-tailed). Differences in FM area across shape categories were explored using one-way ANOVA with post‑hoc comparisons when applicable. Statistical significance was set at p < 0.05.

Ethical considerations: The study used anonymized dry bone specimens maintained for academic purposes; no personal identifiers were collected. Institutional permission to access the osteology collection was obtained as per local policy.

Morphometric parameters of the FM in the overall sample are summarized in Table 1. The mean APD was 34.8 ± 2.6 mm and the mean TD was 29.6 ± 2.4 mm. The derived FMI averaged 85.1 ± 6.9, and the calculated FM area was 810.5 ± 98.7 mm².

Table 1. Descriptive Morphometric Parameters of the Foramen Magnum (n = 80)

The distribution of FM shapes is shown in Table 2. Oval configuration was the commonest (45.0%), followed by round (27.5%) and tetragonal (15.0%) shapes. Pentagonal (7.5%) and irregular (5.0%) shapes were less frequent.

Table 2. Distribution of Foramen Magnum Shapes (n = 80)

Figure 1; Distribution of Foramen Magnum Shapes

Sex-based comparison of FM dimensions is presented in Table 3. Male skulls showed significantly larger APD, TD, and FM area than female skulls (all p ≤ 0.002).

Table 3. Comparison of Foramen Magnum Dimensions by Sex (n = 80)

Mean FM area across shape categories is summarized in Table 4. Oval and round shapes showed higher mean areas than tetragonal, pentagonal, and irregular forms, with the lowest mean area observed in the irregular category.

Table 4. Association Between Foramen Magnum Shape and Area (n = 80)

Figure 2: Association Between Foramen Magnum Shape and Area

Our study provides osteological reference values for foramen magnum (FM) morphometry from a teaching‑institution skull collection in Telangana. The overall mean anteroposterior diameter (APD) of 34.8 mm and transverse diameter (TD) of 29.6 mm yielded a mean FM index (FMI) of 85.1 and a calculated mean area of 810.5 mm². These values are broadly comparable with the ranges described in prior anatomical and imaging studies, although interstudy differences are expected because of population structure, measurement landmarks, and modality-related factors [1,5,6]. For example, Chethan et al. reported smaller mean linear dimensions in a dry-skull series, whereas other cohorts using different approaches have reported larger values, highlighting that a single external dataset should not be treated as universally representative [5,7].

The predominance of oval and round shapes in our sample is consistent with the recurring observation that these categories are common, yet the proportional distribution varies across studies. Chethan et al. documented a wider spread of shape categories, with a relatively higher representation of non‑oval forms in their collection [5]. Such variation can reflect population differences, but it also depends on classification rules when contours fall between categories. Recent anatomical syntheses emphasise that FM shape is best interpreted as a continuum rather than rigid bins, and careful operational definitions improve comparability [7]. Within India, regional work has also shown that FM shape distributions can differ across states and samples, supporting the need for local reference data [14].

Sex-based comparison demonstrated statistically significant sexual dimorphism in APD, TD, and calculated area, with males showing larger FM dimensions. This pattern is congruent with comparative analyses indicating larger FM size in males within populations [7]. It also aligns with forensic and radiologic investigations that used FM variables for sex estimation, including CT-based studies from Saudi Arabia, South India, and Nepal, which consistently report higher mean FM length/width and area values in males [9,11-13]. At the same time, these studies show that the discriminative performance of FM parameters alone is moderate and improves when combined with other cranial base metrics [9-13].

Clinically, FM morphometry supports skull‑base decision making. The FM and occipital condyles define the bony limits of approaches to ventral FM lesions, and morphometric datasets can inform anticipated exposure and the extent of bone removal required to obtain a safe operative corridor [3]. Anatomical work further underlines that regional morphometric baselines can strengthen imaging interpretation of craniovertebral junction crowding and help avoid misclassification when applying thresholds derived from other populations [1,2]. Overall, our results add to the growing evidence that FM dimensions and shape patterns show population variation and sexual dimorphism, reinforcing the value of institution‑specific osteological datasets for teaching, clinical correlation, and research.

Limitations

This work used a convenience sample of dry skulls from a single institutional collection, so the findings do not represent the full Telangana population. Age, stature, and ancestry data were unavailable, limiting stratified analysis. Sex classification relied on collection records and morphological verification rather than documented demographic records for every specimen. FM area was estimated from linear diameters using an ellipse formula, not direct planimetric tracing

In this cross‑sectional osteological study the foramen magnum showed measurable variability in size and shape, with oval and round contours predominating. Mean APD, TD, FMI, and calculated area provide a practical baseline for regional reference. Sex-based comparisons demonstrated clear sexual dimorphism, with males having significantly larger FM diameters and area than females. These findings are relevant for skull‑base surgical planning, particularly for approaches that use the FM–occipital condyle corridor, and they also support radiologic interpretation and forensic assessment when combined with other cranial parameters. Local morphometric datasets such as this strengthen anatomy teaching and clinical correlation at the craniovertebral junction. In practice.

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