Integrated Micro-Computed Tomography and Computational Fluid Dynamics Analysis of Stepwise Enlargement in Mesial Root Canals of Mandibular Molars

Introduction To evaluate how stepwise enlargement in the mesial root canals of mandibular first molars affect shaping outcomes and irrigant dynamics. Methods The shaping ability and irrigant flow patterns in mesial canals of mandibular first molars enlarged with ProTaper Next instruments (25/.06v, 30/.07v, 40/.06v) were evaluated using micro-computed tomography (micro-CT) and computational fluid dynamics (CFD). Instrument design and metallurgical properties were assessed by 3D scanning, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry to ensure manufacturing consistency. An extracted mandibular molar with two curved mesial root canals was selected and scanned by micro-CT to generate 3D models for CFD simulation. Root canals were prepared stepwise with the ProTaper Next. Digital canal and needle geometries were reconstructed from micro-CT and scanning electron microscopy data. CFD simulations were conducted in ANSYS Fluent to calculate irrigant velocity, wall shear stress, and apical pressure. For shaping analysis, ten mesial canals from 5 mandibular molars were scanned before and after each preparation step. Canal volume, surface area, unprepared surfaces, and pericervical dentine thickness were quantified. Data were analyzed using repeated measures ANOVA with Bonferroni correction ( P < .05). Results Excluding for dimensions, all instruments shared similar metallurgical features and design characteristics, with minor surface imperfections. The open-ended needle consistently delivered irrigant to the working length, unlike the side-vented needle. Enlargement slightly increased apical velocity and irrigant density, particularly with the open-ended needle, which also generated higher wall shear stress and apical pressure. Canal volume and surface area increased significantly with larger files ( P < .05), while unprepared areas decreased. Progressive canal enlargement significantly reduced minimal dentine thickness at the distal aspect of the mesial canals (most at 1–2 mm below the furcation), dropping from preoperative means of ∼1 mm to 0.60 mm after the 40/.06v instrument ( P < .05), with the proportion of sections with dentine <0.5 mm increasing from 0% to 26%, while those >1 mm decreased markedly. Conclusions Progressive apical enlargement improved shaping and irrigant dynamics, but additional increases offered limited benefits. Irrigant effectiveness was more influenced by needle design than by size alone, suggesting that optimizing delivery systems may enhance cleaning efficiency while minimizing unnecessary dentine removal.