Evaluation of the effects of irrigation solutions on isthmuses at different levels using computational fluid dynamics

The success of root canal treatment depends on the effective removal of infected tissues and thorough disinfection of the root canal system. Mechanical instrumentation contacts only a limited portion of the canal walls; therefore, effective cleaning within the complex anatomy of the root canal, particularly in isthmus regions, relies on the efficiency of irrigation. Consequently, understanding the flow behavior of irrigation solutions and their internal canal dynamics is of critical clinical importance. The present study aimed to evaluate the flow characteristics of different irrigation solutions within the root canal system, with a particular focus on their hydrodynamic effects in isthmus regions located at different levels. Using Computational Fluid Dynamics (CFD), the flow patterns of 5% NaOCl, 1% NaOCl, and 17% EDTA solutions were compared. The mesial root canal of a mandibular first molar was modeled as two parallel canals, each 15 mm in length, with coronal and apical diameters of 0.9 mm and 0.3 mm, respectively. These canals were connected by three isthmuses, each 0.08 mm in width, positioned 2 mm, 4 mm, and 6 mm from the apex. Irrigation was performed using a 30-G side-vented closed-end needle at a constant inlet velocity of 8.6 m/s. Each irrigant (5% NaOCl, 1% NaOCl, and 17% EDTA) was evaluated separately for 5 seconds. Simulations were conducted under transient conditions using ANSYS Fluent 2023 R1, and analyses were performed for velocity, pressure, and flow vector distributions. CFD analyses revealed that flow behavior in the isthmus regions changed markedly over time for all solutions. The 17% EDTA solution exhibited a more homogeneous flow profile and provided a more stable velocity distribution near the canal walls compared to the other solutions. In contrast, the 5% NaOCl solution generated the highest local velocity and pressure peaks, resulting in a more unsteady and potentially riskier flow pattern. The 1% NaOCl solution displayed moderate flow and pressure characteristics. The findings demonstrated that the viscosity and surface tension properties of irrigants play a decisive role in the flow dynamics within the isthmus regions. The 17% EDTA solution, owing to its lower surface tension and higher wettability, produced a more balanced and effective flow pattern.