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

## TL;DR

This study uses MRI scans to analyze the shape of the sacral hiatus and finds that a narrow hiatus is linked to failed caudal epidural blocks.

## Contribution

The study introduces MRI-based morphometric analysis of the sacral hiatus to predict caudal epidural block success.

## Key findings

- A hiatus width below 8 mm is strongly associated with block failure.
- Hiatus height was slightly higher in women compared to men.
- Canal depth and sacrococcygeal angle showed no significant gender differences.

## Abstract

Objectives: 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. Methods: 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. Results: Hiatus height was slightly higher in women (16.97 ± 7.25 mm) than in men (15.99 ± 7.55 mm, p < 0.05, Cohen’s d = 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 (p > 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. Conclusions: 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.