Abstract
<title>Abstract</title> <p> <italic> <bold>Purpose:</bold> </italic> Myelomeningocele-associated Chiari type II malformation is characterized by posterior cranial fossa (PCF) hypoplasia and craniocervical junction (CCJ) alterations, but controlled quantitative MRI data remain limited. We aimed to quantitatively evaluate PCF dimensions and volume by manual and segmentation-based methods in operated MM patients compared with age-matched controls, characterize CCJ morphometry through 12 standardized measurements, and identify morphometric predictors of below-C1 tonsillar herniation. <italic> <bold>Methods:</bold> </italic> In this retrospective two-center study, 62 operated MM patients (median age 5 days) and 57 age-matched controls were included. PCF linear dimensions, manual and segmentation-based volumes, and seven CCJ parameters were measured on T2-weighted MRI and evaluated by ROC and multivariate logistic regression analysis. <italic> <bold>Results:</bold> </italic> All PCF dimensions and volumes were significantly smaller in MM patients than controls (p<0.05), with vermis–dens (13 vs. 21 mm) and pons–dens axis distances (19 vs. 26 mm) most affected (p<0.001). Vermis–dens axis showed the highest diagnostic accuracy for patient–control discrimination (AUC 0.881) and below-C1 herniation prediction (AUC 0.903). Independent predictors of below-C1 herniation were segmentation-based PFV ≤29 cm³ (aOR 14.64), craniocaudal diameter ≤30 mm (aOR 26.50), and McRae line >21 mm (aOR 9.65; all p<0.01). <italic> <bold>Conclusion:</bold> </italic> PCF hypoplasia and CCJ morphometric alterations in MM-associated Chiari type II malformation can be quantitatively assessed using routine MRI. CCJ-related measurements, particularly vermis-dens axis distance, showed stronger discriminatory performance than volumetric parameters alone. The morphometric thresholds identified here may support objective grading of anatomical severity and inform clinical decision-making. </p>