Detecting Residual Root Canal Filling Material After Retreatment: Cone-Beam Computed Tomography and Digital Microscopy Compared with Microcomputed Tomography

Background/Objectives: Reliable detection of residual root canal filling material after retreatment is essential for comparing retreatment protocols. However, available methods quantify different clinical–physical dimensions and may not yield comparable estimates. This in vitro study compared cone-beam computed tomography (CBCT) and digital microscopy (DGM) for detecting residual obturation material after retreatment, using microcomputed tomography (micro-CT) as the reference standard. Methods: Fifteen extracted human mandibular premolars with single, straight canals were instrumented, obturated with gutta-percha and a calcium silicate-based sealer (AH Plus Bioceramic), and retreated with ProTaper Universal Retreatment files. Residual material was assessed in the coronal, middle, and apical thirds using CBCT (voxel size 0.10 mm), micro-CT (voxel size 60 µm), and DGM after longitudinal root splitting. Surface-based (DGM) and volumetric (CBCT and micro-CT) outcomes were analyzed separately using Wilcoxon signed-rank tests, diagnostic accuracy metrics (sensitivity, specificity, predictive values), and Cohen’s kappa for agreement. Results: DGM showed low median residual surface percentages across thirds (0.34–1.52%), whereas CBCT yielded higher median residual volume percentages (10.20–14.20%) than micro-CT (3.27–5.04%). The difference in the middle third between CBCT and micro-CT remained significant after Bonferroni correction (p = 0.002). For binary detection, CBCT showed higher sensitivity but lower specificity (overclassification of positive thirds), whereas DGM showed high specificity but limited sensitivity in the coronal and middle thirds. Conclusions: Within the limitations of this laboratory study, micro-CT was the most reliable reference method. CBCT tended to overestimate residual material, suggesting that clinical decisions based solely on CBCT may lead to unnecessary retreatment. DGM underestimated remnants because it assesses only the exposed split surface. These method-specific limitations should guide both clinical interpretation and future research design.