MRI-Based Morphometric Analysis of the Sacral Hiatus and Its Exploratory Value for Predicting Caudal Epidural Block Success: A Retrospective Observational Study

<b>Objectives:</b> In this retrospective observational study, we aimed to evaluate sacral hiatus morphometry in detail using MRI and investigate whether morphometric parameters can predict the success of a caudal epidural block. We hypothesized that reduced hiatus width and altered morphometric features may increase the likelihood of block failure. <b>Methods:</b> MRI scans of 240 adult patients (mean age: 51 years; 50% female) were retrospectively analyzed at a single center. Hiatus height and width, canal depth, and sacrococcygeal angle were measured. Gender- and age-related differences were assessed. Multivariate regression and ROC curve analysis were used to determine predictive thresholds for block failure. <b>Results:</b> Hiatus height was slightly higher in women (16.97 ± 7.25 mm) than in men (15.99 ± 7.55 mm, <i>p</i> < 0.05, Cohen's <i>d</i> = 0.25). The mean hiatus width was 9.9 ± 3.0 mm. Canal depth (5.9 ± 2.2 mm) and sacrococcygeal angle (40.8 ± 11.1°) did not differ significantly between genders (<i>p</i> > 0.05). A hiatus width threshold of 8 mm was identified as a predictor of block failure (AUC = 1.00, 95% CI 0.98-1.00), though this exceptionally high value should be interpreted cautiously due to the risk of overfitting in regard to a single-center cohort. <b>Conclusions:</b> MRI-based morphometric assessment of the sacral hiatus provides clinically relevant information. A narrow hiatus (<8 mm) is strongly associated with block failure, while other parameters showed limited predictive value. These findings are hypothesis-generating and require prospective, multicenter validation before clinical application.