The foramen magnum (FM) is the opening through which the cranial cavity communicates with the vertebral canal. It transmits the lower medulla, the vertebral arteries and their branches, and the spinal accessory nerves, and it is flanked by the occipital condyles that articulate with the atlas [1]. These relations make the FM and condyles the bony framework of every operation on the craniovertebral junction (CVJ). Several common neurosurgical conditions centre on this region. Foramen magnum meningiomas arise most often anterior or anterolateral to the brainstem, which makes them difficult to reach through a standard posterior route [2]. In Chiari I malformation, a small posterior fossa crowds the hindbrain, and suboccipital decompression with enlargement of the FM is the standard treatment [3]. Other indications include chordomas, schwannomas, vertebral artery aneurysms, trauma and congenital anomalies of the CVJ. To reach ventral lesions with minimal brainstem retraction, surgeons use the far-lateral approach and its transcondylar, supracondylar and paracondylar extensions [4]. The amount of condyle that must be drilled depends on the size of the FM and on the length and orientation of the condyle [5,6]. Bone removal improves exposure but threatens stability: biomechanical work has shown that resecting half or more of one condyle produces significant craniovertebral hypermobility [7]. Condylar dimensions also determine whether occipital condyle screws can be used for occipitocervical fixation, a technique for which CT-based measurements have been published [8,9]. Most morphometric data on the FM come from dry skulls, in which age and sex are often unknown or estimated. CT overcomes these limits. It measures the FM in living individuals of documented age and sex, permits reconstruction in any plane, and reflects the imaging that surgeons actually use in planning. CT-based studies have been reported from Turkey [10,11] and Saudi Arabia [12]. A recent meta-analysis found substantial heterogeneity in FM and condylar measurements, with geographical region and sex as the main moderators, and found that three-dimensional imaging detected sexual dimorphism more clearly than osteological methods [13]. Population differences in FM size have also been confirmed on large skeletal samples [14]. Data from India are mostly derived from dry skulls [15,16], and CT-based reference values from Indian tertiary centres remain limited. The present study was therefore designed with the following objectives: (i) to measure the FM and occipital condyles on multidetector CT in adult patients at our tertiary care teaching centre; (ii) to compare these parameters between sexes and across age groups; (iii) to describe the shapes of the FM; (iv) to estimate the frequency of anatomical configurations relevant to surgery, namely condylar protrusion, condylar asymmetry, short condyles and a small FM; and (v) to assess how well FM and condylar dimensions discriminate sex.

Study design and setting. This retrospective, cross-sectional observational study was conducted jointly by the Departments of Neurosurgery and Radiodiagnosis, [Institution name], a tertiary care teaching hospital in [City], India. CT scans acquired between [month/year] and [month/year] were reviewed. The Institutional Ethics Committee approved the study (Ref. No. [IEC number]) and waived individual consent because only anonymised, previously acquired images were used. Study population. The picture archiving and communication system (PACS) was searched for non-contrast CT scans of the head or cervical spine in patients aged 18 years or older that included the entire CVJ with a slice thickness of 1 mm or less. Common indications were minor head injury, headache, vertigo and stroke work-up. Scans were excluded if they showed fracture of the skull base, occipital condyles or atlas; congenital CVJ anomalies such as basilar invagination, platybasia, atlanto-occipital assimilation or Chiari malformation; tumour, infection or erosive arthropathy at the CVJ; previous CVJ surgery; or motion artefact that obscured the bony margins. Consecutive eligible scans were included until 200 were obtained (108 male, 92 female). Age and sex were taken from the hospital record. Patients were grouped as 18–35, 36–55 and over 55 years. CT acquisition. Scans were obtained on a [128-slice] multidetector CT scanner ([manufacturer, model]) at 120 kVp, with automatic tube current modulation, collimation of [0.6] mm and reconstruction at 0.625–1 mm in a bone algorithm. Image analysis. Images were analysed on a [vendor] workstation using a bone window (width 2000 HU, level 500 HU). Multiplanar reconstruction was used to align the axial plane with the plane of the FM, defined by the basion and the opisthion. The following parameters were measured, following definitions used in previous studies [5,10,15,17,18]: 1. Anteroposterior diameter (APD): basion to opisthion on the midsagittal reconstruction. 2. Transverse diameter (TD): greatest width of the FM perpendicular to the APD on the reformatted axial image. 3. FM index: APD / TD. 4. FM area: traced directly along the inner bony margin with the freehand region-of-interest tool on the reformatted axial image. Area was also calculated by the Radinsky formula (¼ × π × APD × TD) [17] to check agreement. 5. Shape: classified as oval, round, egg-shaped, tetragonal, pentagonal, hexagonal or irregular [15]. 6. Occipital condyle length (OCL) and width (OCW): greatest anteroposterior and mediolateral dimensions of the condylar articular surface on axial images, each side. 7. Occipital condyle height (OCH): greatest vertical dimension of the condyle on coronal reconstruction, each side. 8. Anterior and posterior intercondylar distances (AICD, PICD): distances between the anterior tips and the posterior tips of the two condyles. 9. Condylar protrusion: medial margin of a condyle projecting into the outline of the FM [5]. 10. Condylar asymmetry: a right–left difference in OCL greater than 2 mm. 11. Short condyle: OCL less than 20 mm on either side. 12. Small FM: APD less than 30 mm. For the comparisons by sex and age, right and left condylar values were averaged after confirming that they did not differ significantly. Reliability. All measurements were made by a neuroradiologist and repeated on a random 25% of scans after four weeks. A neurosurgery resident, blinded to the first readings and to patient sex, independently measured the same subset. Agreement was assessed with the intraclass correlation coefficient (ICC). Statistical analysis. Data were analysed with SPSS version [xx] (IBM Corp., Armonk, NY). Normality was confirmed with the Shapiro–Wilk test. Values are presented as mean ± SD. Sex differences were tested with the unpaired t-test, side differences with the paired t-test and age-group differences with one-way ANOVA. Categorical variables were compared with the chi-square test. Pearson correlation assessed relationships between parameters. ROC curves were constructed for each parameter to discriminate sex, with optimal cut-offs set by the Youden index, and a stepwise discriminant function analysis was performed. A p-value below 0.05 was considered significant.

Two hundred CT scans met the inclusion criteria. Intra-observer ICCs ranged from 0.94 to 0.98 and inter-observer ICCs from 0.91 to 0.96. Traced FM area agreed closely with the Radinsky estimate (r = 0.96), with the formula overestimating the traced area by a mean of 7 mm². Table 1. Demographic profile and scan indications (n = 200) Characteristic Value Male, n (%) 108 (54.0) Female, n (%) 92 (46.0) Age, mean ± SD (range), years 44.6 ± 15.2 (18 – 80) Age 18–35 years, n (%) 68 (34.0) Age 36–55 years, n (%) 74 (37.0) Age > 55 years, n (%) 58 (29.0) Indication: minor head injury without skull-base fracture, n (%) 84 (42.0) Indication: headache or vertigo, n (%) 62 (31.0) Indication: stroke work-up, n (%) 36 (18.0) Indication: other, n (%) 18 (9.0) The sample was evenly spread across adult age groups, with a slight male predominance. Most scans were performed for minor head injury or headache, conditions that do not affect bony CVJ anatomy. Table 2. Foramen magnum parameters: overall and by sex Parameter Overall (n = 200) Male (n = 108) Female (n = 92) t p-value APD (mm) 35.0 ± 2.8 36.1 ± 2.5 33.7 ± 2.6 6.62 < 0.001* TD (mm) 29.6 ± 2.5 30.5 ± 2.3 28.5 ± 2.3 6.13 < 0.001* FM index 1.18 ± 0.08 1.18 ± 0.08 1.18 ± 0.08 0.09 0.928 Traced area (mm²) 807 ± 103 857 ± 92 748 ± 86 8.65 < 0.001* *Statistically significant (unpaired t-test). Values are mean ± SD. The FM was on average 2.4 mm longer and 2.0 mm wider in males, and its area was about 15% larger. The FM index was identical in the two sexes, so the foramen kept the same proportions. APD and TD were moderately correlated (r = 0.55, p < 0.001). Table 3. Occipital condyle parameters and intercondylar distances: overall and by sex Parameter Overall (n = 200) Male (n = 108) Female (n = 92) t p-value Condyle length (mm) 23.4 ± 2.1 24.1 ± 2.0 22.6 ± 1.9 5.43 < 0.001* Condyle width (mm) 12.4 ± 1.3 12.9 ± 1.3 11.9 ± 1.2 5.65 < 0.001* Condyle height (mm) 9.0 ± 1.2 9.4 ± 1.2 8.6 ± 1.1 4.92 < 0.001* AICD (mm) 20.9 ± 2.4 21.5 ± 2.3 20.1 ± 2.2 4.39 < 0.001* PICD (mm) 31.6 ± 2.8 32.4 ± 2.7 30.6 ± 2.6 4.79 < 0.001* *Statistically significant (unpaired t-test). Condylar values are the mean of right and left sides; side differences were not significant (p > 0.05). All condylar dimensions and intercondylar distances were larger in males. Female condyles were on average 1.5 mm shorter and 0.8 mm lower, which reduces the bone stock available for drilling or screw placement. The TD correlated with the PICD (r = 0.46, p < 0.001), so a narrow foramen tended to accompany closely set condyles. Table 4. Foramen magnum parameters across age groups Parameter 18–35 years (n = 68) 36–55 years (n = 74) > 55 years (n = 58) F p-value APD (mm) 35.2 ± 2.7 35.0 ± 2.9 34.8 ± 2.8 0.34 0.711 TD (mm) 29.7 ± 2.4 29.6 ± 2.6 29.4 ± 2.5 0.23 0.797 FM index 1.19 ± 0.08 1.18 ± 0.08 1.18 ± 0.09 0.37 0.690 Traced area (mm²) 812 ± 101 808 ± 105 799 ± 104 0.25 0.778 No FM parameter differed significantly between age groups (one-way ANOVA). FM dimensions therefore appear stable throughout adult life in this sample. Table 5. Distribution of foramen magnum shapes by sex Shape Male (n = 108), n (%) Female (n = 92), n (%) Total (n = 200), n (%) Oval 36 (33.3) 30 (32.6) 66 (33.0) Round 20 (18.5) 18 (19.6) 38 (19.0) Egg-shaped 16 (14.8) 14 (15.2) 30 (15.0) Tetragonal 13 (12.0) 11 (12.0) 24 (12.0) Irregular 9 (8.3) 7 (7.6) 16 (8.0) Pentagonal 8 (7.4) 6 (6.5) 14 (7.0) Hexagonal 6 (5.6) 6 (6.5) 12 (6.0) Oval was the commonest shape in both sexes, followed by round and egg-shaped forms. Shape distribution did not differ by sex (χ² = 0.41, df = 6, p = 0.99). Table 6. Anatomical configurations of surgical relevance Configuration Male (n = 108), n (%) Female (n = 92), n (%) Total (n = 200), n (%) Condylar protrusion into FM 14 (13.0) 14 (15.2) 28 (14.0) Condylar asymmetry (> 2 mm) 12 (11.1) 10 (10.9) 22 (11.0) Short condyle (< 20 mm) 3 (2.8) 8 (8.7) 11 (5.5) Small FM (APD < 30 mm) 1 (0.9) 7 (7.6) 8 (4.0) At least one of the above 25 (23.1) 26 (28.3) 51 (25.5) One patient in four had at least one configuration that could influence surgery. Short condyles and a small FM were much more frequent in women, while condylar protrusion and asymmetry occurred at similar rates in both sexes. Table 7. ROC analysis of parameters for discriminating sex Parameter AUC (95% CI) Cut-off (male if ≥) Sensitivity (%) Specificity (%) FM area 0.80 (0.74 – 0.86) 800 mm² 75.0 71.7 APD 0.75 (0.68 – 0.82) 34.9 mm 70.4 67.4 TD 0.72 (0.65 – 0.79) 29.5 mm 68.5 65.2 Condyle length 0.70 (0.63 – 0.77) 23.3 mm 66.7 63.0 FM area was the best single discriminator of sex. A discriminant function combining APD, TD and condyle length classified 73.5% of patients correctly. Accuracy was therefore moderate, in keeping with the wide overlap between male and female values.

This study provides CT-based reference values for the FM and occipital condyles in adult patients at an Indian tertiary centre and estimates how often anatomy that may alter surgery is present. Mean APD (35.0 mm) and TD (29.6 mm) are close to values from Turkish CT studies [10,11] and from Greek dry-skull series [19,20]. They are slightly larger than many Indian dry-skull reports [15,16]. Part of this difference may be methodological. CT measures the inner bony margin in a plane aligned with the foramen, whereas caliper measurement on dry skulls can be affected by post-mortem shrinkage and by the choice of landmarks. The meta-analysis by Tsiouris et al. found study design (imaging versus osteology) and geographical region to be important sources of variation [13]. Reference values should therefore be matched to both the population and the method used. The mean FM index of 1.18 is similar to the value of 1.2 reported by Chethan et al. [15]. Oval was the commonest shape, in agreement with earlier Indian and Turkish work [10,15]. All FM and condylar dimensions were significantly larger in men, while the index was the same in both sexes. Lyrtzis et al. likewise found every FM parameter and most condylar parameters larger in male skulls [20]. FM area was the best single discriminator of sex (AUC 0.80), and a three-variable function classified 73.5% of patients correctly. This moderate accuracy matches previous CT studies [11,12] and supports the view of Edwards et al. and Raghavendra Babu et al. that the FM is of limited value for sex estimation on its own [16,21]. For surgeons, the clinically relevant point is not sex estimation but the fact that women more often had short condyles and a small FM. We found no significant change in FM dimensions between young, middle-aged and older adults. This suggests that the bony FM is stable after skeletal maturity, so a single adult reference range can be used for preoperative assessment regardless of age. The surgical findings are the most practical results of the study. Condylar protrusion into the FM was seen in 14.0% of patients, close to the 20% reported by Muthukumar et al. in dry skulls [5]. A protruding condyle blocks the lateral view of the anterior FM, so partial condylectomy is likely to be needed to reach a ventral lesion [4,5]. Wanebo and Chicoine showed that removing the posterior part of the condyle increases exposure [6], but resecting 50% or more of one condyle causes significant hypermobility [7]. Measuring condyle length on CT before surgery tells the surgeon how much bone can be removed before stability is at risk. Condylar asymmetry, present in 11.0%, matters when choosing the side of a unilateral far-lateral approach. Condylar dimensions also decide whether occipital condyle screws are feasible. Le et al. and Zhou et al. showed that condyle length and height limit the screw trajectory [8,9]. In our sample, 8.7% of women had a condyle shorter than 20 mm, which may make condylar screws unsuitable and favour occipital plate fixation instead. A small FM, found in 4.0% overall and mostly in women, is relevant to decompression for Chiari malformation, in which a crowded posterior fossa is the central problem [3]. It is also relevant to FM meningiomas, which are usually anterior or anterolateral and must be removed through a limited corridor [2]. Limitations. This was a retrospective, single-centre study of hospital patients, who may not represent the general population. Soft-tissue structures, such as the vertebral artery, the dura and the lower cranial nerves, were not assessed; CT angiography would add this information. The study did not correlate measurements with operative findings or outcomes. A prospective study linking preoperative CT morphometry with the extent of condylar resection and postoperative stability would confirm the clinical value of these measurements.

On CT, the adult FM in this population measured 35.0 ± 2.8 mm anteroposteriorly and 29.6 ± 2.5 mm transversely, with a mean area of 807 mm². FM and condylar dimensions were significantly larger in men but did not change with age. One patient in four had condylar protrusion, condylar asymmetry, a short condyle or a small FM, and the last two were more common in women. Routine CT morphometry of the FM and condyles is recommended before far-lateral, transcondylar and decompressive procedures and before occipitocervical fixation, so that bone removal and fixation can be tailored to each patient.

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