MRI-Based Foramen Magnum Morphometry in Tibetan Adults: Sexual Dimorphism and Findings Across Adult Age Groups

Objective: This study aimed to establish MRI-based normative morphometric data for the foramen magnum in adult Tibetans, assessing sexual dimorphism and age-related variation in its size and proportional shape. Methods: High-resolution cranial MRI scans from 186 Tibetan adults (74 men, 112 women) aged 18 to 76 years (mean age: 35.88±13.10 y) were analyzed cross-sectionally. The sagittal and transverse diameters of the foramen magnum were measured using standardized multiplanar reconstructions. The foramen magnum index (transverse-to-sagittal diameter ratio × 100) was used to describe proportional shape. Sex differences were evaluated using the Mann-Whitney U test. Age-related variation within each sex was assessed across 4 age groups (18–29, 30–39, 40–49, and ≥50 y) using the Kruskal-Wallis test. Results: Men exhibited larger foramen magnum diameters than women. The median sagittal diameter was 3.86 cm in men versus 3.58 cm in women ( P <0.001), and the median transverse diameter was 3.33 versus 3.11 cm ( P <0.001). No significant difference in the foramen magnum index was found between sexes (P >0.05). The large index-based morphology was the most common, accounting for 54.8% of the cohort. No significant differences in sagittal diameter, transverse diameter, or index were observed across adult age groups within either sex (all P >0.05). Conclusions: In Tibetan adults, the foramen magnum exhibits clear sexual dimorphism in linear size but not in proportional shape. These measurements remained stable throughout adulthood. These MRI-derived normative data may assist in the assessment of the craniovertebral junction and in forensic applications for Tibetan populations.