The Effect of Needle-insertion Depth on the Irrigant Flow in the Root Canal: Evaluation Using an Unsteady Computational Fluid Dynamics Model Christos Boutsioukis, DDS, MSc,* ‡ Theodor Lambrianidis, DDS, PhD,* Bram Verhaagen, MSc, § Michel Versluis, PhD, § Eleftherios Kastrinakis, PhD, † Paul R. Wesselink, DDS, PhD, ‡ and Lucas W.M. van der Sluis, DDS, PhD ‡ Abstract Introduction: The aim of this study was to evaluate the effect of needle-insertion depth on the irrigant flow inside a prepared root canal during final irrigation with a syringe and two different needle types using a Computational Fluid Dynamics (CFD) model. Methods: A validated CFD model was used to simulate irrigant flow from either a side-vented or an open-ended flat 30-G needle positioned inside a prepared root canal (45 .06) at 1, 2, 3, 4, or 5 mm short of the working length (WL). Velocity, pressure, and shear stress in the root canal were evaluated. Results: The flow pattern in the apical part of the root canal was similar among different needle positions. Major differences were observed between the two needle types. The side- vented needle achieved irrigant replacement to the WL only at the 1-mm position, whereas the open-ended flat needle was able to achieve complete replacement even when positioned at 2 mm short of the WL. The maximum shear stress decreased as needles moved away from the WL. The flat needle led to higher mean pressure at the apical foramen. Both needles showed a similar gradual decrease in apical pressure as the distance from the WL increased. Conclusions: Needle- insertion depth was found to affect the extent of irrigant replacement, the shear stress on the canal wall, and the pressure at the apical foramen for both needle types. (J Endod 2010;36:1664–1668) Key Words Computational Fluid Dynamics, insertion depth, irriga- tion, needle I rrigation of root canals with antibacterial solutions is an integral part of chemome- chanical preparation, aiming at the removal of bacteria, debris, and necrotic tissue, especially from areas of the root canal that have been left unprepared by mechanical instruments (1). Irrigants are commonly delivered using a syringe and needle (2, 3), even before passive ultrasonic activation of the solution (4). The significance of the needle position in relation to the apical terminus of the preparation, also described as needle insertion depth or penetration, has been highlighted in a series of in vitro (5) and ex vivo studies (6-9). It has been hypothesized that positioning the needle close to the working length (WL) could in fact improve the debridement and irrigant replacement (6, 10). However, previous studies have mainly focused on the removal efficiency of debris and bacteria and provided little understanding of the etiology (ie, the flow pattern developed in the root canal that leads to debridement and irrigant replacement). Limited insight in the fluid dynamics of the flow inside the root canal has been presented using thermal image analysis (9) because this approach could only provide a coarse estimation of the irrigant flow. A Computational Fluid Dynamics (CFD) model was recently introduced as a method to study root canal irrigation (11). This model was subsequently validated by comparison with experimental high-speed imaging data (12) and used to evaluate the effect of needle tip design on the flow (13). In these previous studies, needles were positioned at 3 mm short of the WL. A similar approach has also been reported (14, 15), but the effect of needle insertion depth on the irrigant flow has not been studied in detail. The aim of this study was to evaluate the effect of needle insertion depth on the irrigant flow inside a prepared root canal during final irrigation with a syringe and two different needle types using the validated CFD model. Materials and Methods The root canal and apical anatomy were simulated similarly to a previous study (11), assuming a length of 19 mm, an apical diameter of 0.45 mm (ISO size 45), and 6% taper. The apical foramen was simulated as a rigid and impermeable wall, cor- responding to a closed system. Two different needle types, a side-vented and an open-ended flat needle, were modeled using commercially available 30-G needles as references, similar to a previous study (13). The external and internal diameter and the length of the needles were stan- From the *Department of Endodontology, Dental School and † Chemical Engineering Department, School of Engineering, Aristotle University of Thessaloniki, The- ssaloniki, Greece; ‡ Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam (ACTA), Amsterdam, The Netherlands; and § Physics of Fluids Group, Faculty of Science and Technology, and Research Institute for Biomedical Technology and Technical Medicine MIRA, University of Twente, Enschede, The Netherlands. Supported in part by a Scholarship for Excellent PhD Students from the Research Committee of Aristotle University of Thessaloniki, Greece (CB), and through Project 07498 of the Dutch Technology Foundation STW (BV). Address requests for reprints to Mr Christos Boutsioukis, 29, Kimis Str, 551 33 Thessaloniki, Greece. E-mail address: chb@dent.auth.gr. 0099-2399/$0 - see front matter Copyright ª 2010 American Association of Endodontists. doi:10.1016/j.joen.2010.06.023 Basic Research—Technology 1664 Boutsioukis et al. JOE — Volume 36, Number 10, October 2010