Volumetric Morphometric Analysis of Posterior Fossa Structures Following Posterior Fossa Decompression with Superficial Durotomy in Chiari I Malformation.

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<b>Background/Objectives:</b> Posterior fossa decompression (PFD) with superficial durotomy is used in Chiari I malformation to limit the CSF-related complications of complete durotomy and duraplasty, but quantitative volumetric evidence has relied on linear measurements, modest cohorts, and heterogeneous methods. We applied a fully automated deep-learning segmentation pipeline to characterize multi-structure volumetric change after PFD with superficial durotomy. <b>Methods:</b> Thirty-three patients (24 females and nine males; mean age, 39.2 ± 13.5 years; two pediatric) with symptomatic Chiari I malformation underwent PFD with superficial durotomy. Pre-operative and 6-month post-operative 3D T1-weighted 1.5T MRIs were segmented using nnU-Net, CERES, TotalSegmentator, and nnDetection. Posterior fossa volume (PFV), supratentorial volume, PFV/STV ratio, cerebellar volume, brainstem volume, fourth ventricle volume, basal cisterns' CSF volume, and tonsillar descent were computed and compared pre/post using paired t-tests or Wilcoxon tests, Cohen's d with bootstrapped 95% confidence intervals, and Benjamini-Hochberg correction. The pipeline was developed and validated exclusively on 1.5T Siemens Aera data and is not presented as ready for out-of-box use on other scanner platforms without re-validation. <b>Results:</b> PFV increased from 166.0 ± 12.9 to 172.5 ± 13.1 mL (Δ + 6.5 mL; <i>p</i> < 0.001; d = 1.72). Tonsillar descent decreased from 8.0 ± 2.8 to 4.9 ± 2.8 mm (Δ -3.1 mm; <i>p</i> < 0.001; d = -2.74). Basal cisterns' CSF volume increased from 8.8 ± 1.5 to 11.0 ± 2.4 mL (Δ + 2.2 mL; <i>p</i> < 0.001; d = 1.16). Fourth ventricle volume increased (<i>p</i> < 0.001; d = 1.58). Cerebellar volume increased modestly (<i>p</i> < 0.001, q < 0.001; d = 0.86), while brainstem volume was unchanged (<i>p</i> = 0.22, q = 0.22; d = -0.11). Seven of eight primary outcomes remained significant after false-discovery-rate correction. Twenty-nine of 33 patients (87.9%) achieved tonsillar ascent of at least 2 mm. <b>Conclusions:</b> Automated volumetric analysis demonstrated significant posterior fossa expansion, basal cisterns' CSF augmentation, fourth ventricle enlargement, and tonsillar ascent after PFD with superficial durotomy. These effect sizes provide a quantitative benchmark for sample-size estimation in future comparative surgical trials and for patient counseling on expected structural change at 6 months.

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