The foramen magnum (FM) is the largest foramen of the skull and lies in the basal part of the occipital bone. It transmits the medulla oblongata with its meninges, the spinal roots of the accessory nerves, the vertebral arteries, the anterior and posterior spinal arteries, and the apical and alar ligaments with the tectorial membrane [1]. The basiocciput bounds it anteriorly, the occipital condyles laterally and the squamous occipital bone posteriorly. Because of these relations, the FM is the central landmark of the craniovertebral junction (CVJ), where small errors in surgical orientation can injure the lower cranial nerves, the vertebral artery or the brainstem [1,2].

Lesions in and around the FM include meningiomas, schwannomas, chordomas, vertebral artery aneurysms, and congenital anomalies such as Chiari malformation and atlanto-occipital assimilation. They pose major surgical challenges, especially when they lie ventral to the brainstem [1,3]. The far-lateral approach and its transcondylar, supracondylar and paracondylar extensions were developed to reach these lesions with less brainstem retraction [2,4]. How much condylar drilling these approaches need depends on the size of the FM, the length and orientation of the occipital condyles, and the position of the hypoglossal canal [3,5,6]. A narrow foramen, or a condyle that projects into it, narrows the working corridor and may call for more bone removal, at the cost of craniovertebral stability [3,7].

Morphometric studies have shown wide variation in FM dimensions between populations and between the sexes [8-10]. Mean anteroposterior and transverse diameters differ among Turkish [11], Greek [5,6], Eastern European [12] and Indian [3,10] series. The shape of the FM also varies: oval, round, egg-shaped, tetragonal, pentagonal, hexagonal and irregular forms have been described [13]. Chethan et al. reported this full range of shapes in South Indian skulls, with a mean FM index of 1.2 [13], while Zaidi and Dayal found the oval form to be most common in Indian skulls [14]. Sexual dimorphism of the FM is well documented and is used for sex estimation in forensic practice [9,10,15].

Despite this literature, regional data from Indian tertiary teaching centres remain limited. Few studies have reported FM dimensions together with the condylar parameters that guide neurosurgical decisions. Population-specific reference values help the neurosurgeon anticipate the size of the operative corridor, plan condylar resection, and interpret preoperative imaging. They also help radiologists assessing FM narrowing in Chiari malformation and skeletal dysplasias, and anatomists and forensic experts who use the FM for identification.

The present study was therefore undertaken with the following objectives: (i) to measure the anteroposterior and transverse diameters, index and area of the FM in dry adult human skulls; (ii) to document the shapes of the FM; (iii) to measure the dimensions of the occipital condyles and the intercondylar distances, and to record condylar protrusion into the FM; (iv) to compare these parameters between male and female skulls; and (v) to discuss their relevance to neurosurgical approaches to the CVJ at our tertiary care teaching centre.

Study design and setting. This descriptive cross-sectional anatomical study was conducted in the Department of Anatomy, in collaboration with the Department of Neurosurgery, [Institution name], a tertiary care teaching centre in [City], India, between [month/year] and [month/year]. Approval was obtained from the Institutional Ethics Committee (Ref. No. [IEC number]). The skulls belonged to the departmental osteology collection used for teaching; no identifiable personal data were associated with them.

Sample. A total of 120 intact dry adult human skulls of Indian origin were examined. Adulthood was confirmed by complete eruption of the third molars and fusion of the spheno-occipital synchondrosis. Skulls with damage to the margins of the FM or the occipital condyles, gross deformity, healed fractures, signs of pathology at the CVJ, or atlanto-occipital assimilation were excluded. Sex was assigned from standard cranial non-metric traits (supraorbital ridge, mastoid process, external occipital protuberance, nuchal crest, mental eminence and gonial angle), assessed independently by two anatomists. Only skulls on which both observers agreed were included, giving 72 male and 48 female skulls.

Instruments. Measurements were taken with a digital Vernier caliper (Mitutoyo, Japan) with an accuracy of 0.01 mm. Each skull was placed upside down on a skull stand with the Frankfort horizontal plane parallel to the table.

Parameters measured. The following parameters were recorded, following definitions used in earlier studies [3,13,16]:

1. Anteroposterior diameter (APD): distance between basion and opisthion in the midsagittal plane.
2. Transverse diameter (TD): greatest distance between the lateral margins of the FM, perpendicular to the APD.
3. FM index: APD / TD.
4. FM area: calculated by the formula of Radinsky [17], Area = ¼ × π × APD × TD, and by the formula of Teixeira [15], Area = π × [(APD + TD)/4]².
5. Shape of FM: classified by visual inspection as oval, round, egg-shaped, tetragonal, pentagonal, hexagonal or irregular, as described by Chethan et al. [13].
6. Occipital condyle length (OCL): greatest distance between the anterior and posterior margins of the condylar articular surface, on each side.
7. Occipital condyle width (OCW): greatest distance between the medial and lateral margins of the articular surface, perpendicular to the length, on each side.
8. Anterior intercondylar distance (AICD): distance between the anterior tips of the two condyles.
9. Posterior intercondylar distance (PICD): distance between the posterior tips of the two condyles.
10. Condylar protrusion: recorded as present when the medial margin of a condyle projected beyond the outline of the FM, narrowing its lateral part, and graded as unilateral or bilateral [3].

An APD below 30 mm was taken as a “small FM” for the purpose of describing skulls with a potentially restricted surgical corridor.

Reliability. All measurements were taken twice by the same observer at an interval of two weeks, and the mean was used. A second observer re-measured a random 20% of skulls. Intra- and inter-observer reliability was assessed with the intraclass correlation coefficient (ICC); values above 0.90 were considered acceptable.

Statistical analysis. Data were entered in Microsoft Excel and analysed with SPSS version [xx] (IBM Corp., Armonk, NY). Normality was checked with the Kolmogorov–Smirnov test. Continuous variables are expressed as mean ± standard deviation (SD) with range. Male and female values were compared with the unpaired Student t-test, and right and left condylar values with the paired t-test. The association between APD and TD was assessed by Pearson correlation. Shape frequencies are given as number and percentage. A p-value below 0.05 was considered statistically significant.

Of the 120 skulls, 72 (60%) were male and 48 (40%) female. Intra-observer and inter-observer ICCs ranged from 0.93 to 0.97 for all parameters.

Table 1. Overall morphometric parameters of the foramen magnum (n = 120)

The APD exceeded the TD in every skull, so the FM was longer anteroposteriorly than it was wide. The two area formulae gave closely similar results, with the Teixeira formula giving values about 6 mm² higher on average. APD and TD showed a moderate positive correlation (r = 0.58, p < 0.001).

Table 2. Comparison of foramen magnum parameters between male and female skulls

*Statistically significant (unpaired t-test).

All linear and area measurements were significantly larger in male skulls. On average, male APD was 2.3 mm longer and male TD 2.0 mm wider than in females, and male FM area was about 15% greater. The FM index was almost identical in the two sexes, showing that the foramen is larger in males but has the same proportions.

Table 3. Distribution of foramen magnum shapes (n = 120)

Oval was the most common shape, seen in nearly one-third of skulls, followed by round and egg-shaped forms. Together these three regular forms made up two-thirds of the sample. Angular (tetragonal, pentagonal, hexagonal) and irregular forms accounted for the remaining third. Shape distribution did not differ significantly between sexes (χ² = 0.93, df = 6, p = 0.99).

Table 4. Occipital condyle parameters and intercondylar distances (n = 120)

Condylar length and width showed no significant side-to-side difference. The posterior intercondylar distance was about 10 mm greater than the anterior, reflecting the anteromedial convergence of the condyles towards the basion.

Table 5. Anatomical findings of surgical relevance (n = 120)

Condyles projected into the FM in 19 skulls (15.8%), bilaterally in more than half of these. A small FM (APD below 30 mm) was found in 7 skulls, six of them female. These are the configurations in which the lateral working corridor is narrowest and in which more condylar drilling may be needed during a transcondylar approach.

This study provides reference values for the FM and occipital condyles from a tertiary teaching centre in India and relates them to neurosurgical approaches to the CVJ. The mean APD (34.2 mm) and TD (28.6 mm) fall within the range reported for Indian skulls [3,10,13]. They are also close to values from Greek and Turkish series [5,6,11]. Gruber et al. showed that FM size varies widely between populations and over time, reflecting genetic, environmental and secular influences [8]. This variability supports the use of local reference data rather than values imported from other populations.

The mean FM index of 1.20 matches the value reported by Chethan et al. in South Indian skulls [13]. An index above 1 in every skull confirms that the foramen is consistently longer than it is wide. This matters surgically, because the transverse dimension, not the sagittal one, limits the lateral corridor used in far-lateral approaches [2,3].

All linear and area measurements were significantly larger in male skulls. This agrees with Catalina-Herrera [9], with Raghavendra Babu et al. in an Indian population [10], and with Lyrtzis et al., who found every FM parameter larger in male Greek skulls [6]. The FM index did not differ by sex, so the foramen is scaled up in males without a change in proportion. Male and female ranges overlapped widely, which is why FM dimensions alone give only moderate accuracy for sex estimation [10,15]. For the neurosurgeon, the more useful finding is that six of the seven skulls with a small FM (APD under 30 mm) were female. A narrow corridor should therefore be anticipated more often in women.

Oval was the commonest shape (31.7%), followed by round and egg-shaped forms. This agrees with Zaidi and Dayal, who found oval to predominate in Indian skulls [14], and with the spectrum of shapes reported by Chethan et al. [13] and Avci et al. [11]. Angular and irregular forms, seen in one-third of our skulls, often reflect bony projections from the condyles or the posterior rim. Such projections can narrow the lateral recess of the foramen and alter the line of sight to the ventral brainstem.

Mean condylar length (about 23 mm) and width (about 12.5 mm) were comparable to those reported by Muthukumar et al., Naderi et al. and Kizilkanat et al. [3,16,18], with no significant side difference. Muthukumar et al. found that the condyle protruded significantly into the FM in 20% of skulls [3]. Our figure of 15.8% is similar. When the condyle projects medially, it blocks the lateral view of the anterior FM, and partial condylectomy is usually needed to expose ventral lesions [2,3]. Spektor et al. and Wanebo and Chicoine quantified the extra exposure gained by drilling the posterior part of the condyle [7,20]. Biomechanical work has shown that resecting 50% or more of one condyle causes significant craniovertebral hypermobility [21]. Knowing the length of the condyle before surgery therefore helps the surgeon judge how much can be removed safely.

These data are relevant to FM meningiomas, which arise most often anterior or anterolateral to the brainstem [19]. They also bear on decompression for Chiari malformation and on surgery for other CVJ lesions. Correlating these dry-skull values with preoperative CT can help choose between a posterior midline, far-lateral or transcondylar approach.

Limitations. Sex was assigned from morphological traits rather than documented records, and the age of the individuals was unknown. Dry skulls do not show soft tissue, the vertebral artery or the dural relations that also shape the operative corridor. The study was conducted at a single centre, and the sample of female skulls was smaller than that of male skulls. A CT-based study in living patients would complement these findings.

In this sample of 120 dry adult skulls, the FM measured 34.2 ± 2.6 mm anteroposteriorly and 28.6 ± 2.3 mm transversely, with a mean index of 1.20. All dimensions were significantly larger in males, while the index was the same in both sexes. Oval was the commonest shape. Occipital condyles protruded into the FM in about one skull in six, and a small FM was found mainly in female skulls. These population-specific values can help neurosurgeons anticipate a narrow corridor and plan the extent of condylar resection in far-lateral and transcondylar approaches to the CVJ.

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