Fluid Dynamics and Biofilm Removal Generated by Syringe-delivered and 2 Ultrasonic-assisted Irrigation Methods: A Novel Experimental Approach Gillian Layton, BSc, DDS,* Wen-I. Wu, PhD, † Ponnambalam Ravi Selvaganapathy, MS, PhD, † Shimon Friedman, DMD,* and Anil Kishen, BDS, MDS, PhD* Abstract Introduction: Thorough understanding of fluid dy- namics in root canal irrigation and corresponding anti- biofilm capacity will support improved disinfection strategies. This study aimed to develop a standardized, simulated root canal model that allows real-time anal- ysis of fluid/irrigation dynamics and its correlation with biofilm elimination. Methods: A maxillary incisor with an instrumented root canal was imaged with micro– computed tomography. The canal volume was recon- structed in 3 dimensions and replicated in soft lithography-based models microfabricated from polyeth- ylene glycol–modified polydimethylsiloxane. Canals were irrigated by using a syringe (SI) and 2 ultrasonic- assisted methods, intermittent (IUAI) and continuous (CUAI). Real-time fluid movement within the apical 3 mm of canals was imaged by using microparticle image velocimetry. In similar models, canals were inoculated with Enterococcus faecalis to grow 3-week-old bio- films. Biofilm reduction by irrigation with SI, CUAI, and IUAI was assessed by using a crystal violet assay and compared with an untreated control. Results: SI gener- ated higher velocity and shear stress in the apical 1–2 mm than 0–1 and 2–3 mm. IUAI generated consistently low shear stress in the apical 3 mm. CUAI generated consistently high levels of velocity and shear stress; it was the highest of the groups in the apical 0–1 and 2–3 mm. Biofilm was significantly reduced compared with the control only by CUAI (two-sample permutation test, P = .005). Conclusions: CUAI exhibited the high- est mechanical effects of fluid flow in the apical 3 mm, which correlated with significant biofilm reduction. The soft lithography-based models provided a novel model/ method for study of correlations between fluid dynamics and the antibiofilm capacity of root canal irrigation methods. (J Endod 2015;-:1–6) Key Words Biofilm, fluid dynamics, irrigation, root canal, ultrasonic E limination of microbial biofilms from root canals critically depends on root canal irrigation (1, 2). To disrupt surface-adherent biofilms and to reach anatomic complexities within root canal systems (3, 4), the fluid mechanical parameters of irrigation need to be optimized such as velocity, shear stress, and overall flow patterns (5, 6). The common method of irrigation is syringe-based (SI), which generates low fluid velocity with little interaction between the irrigant and canal walls (5–8). Ultrasonic- assisted irrigation (UAI) has been introduced to enhance irrigant-wall interaction (9) by transmitting acoustic energy from an oscillating wire/file to the irrigant, causing acoustic microstreaming and transient cavitation (9–11). Acoustic microstreaming, comprising rapid movement of fluid in a vortex motion, generates shear stresses that enhance debridement (9, 10). Transient cavitation generates bubbles that, when collapsing, produce radiating shock waves and temperature rise (11). The antibiofilm efficacy of UAI compared with SI has been disputed, with reports of enhancement (12–15) and no enhancement (10, 16, 17). Further research is warranted into the fluid/irrigation dynamics associated with UAI, both intermittent (IUAI) and continuous (CUAI), with the aim to improve their antibiofilm efficacy. The antibacterial efficacy of root canal irrigation has been conventionally studied by determining the final static condition implying irrigant penetration (18). Computa- tional fluid dynamics models, which are used to determine the physical parameters associated with root canal irrigation, are limited because of their virtual nature and inability to assess antibacterial efficacy (5–8, 18). To overcome the limitations of current methodologies, there is a need for a dual-purpose experimental model that replicates root canal systems and supports real-time assessment of both fluid/irrigation dynamics and biofilm removal to explore their correlation. Soft lithography (SL) is a three-dimensional microfabrication technique that can accurately replicate channel geometries with high resolution within transparent sub- strate constructs that allow visual imaging of fluid dynamics (19). SL constructs that replicate the anatomy of root canal systems imaged by micro–computed tomography (mCT) (20) could enable real-time study of fluid/irrigation movement through the use of microparticle image velocimetry (PIV) (21). In addition, by growing microbial biofilms within the root canals of the same SL constructs, correlations could be explored between fluid/irrigation dynamics and biofilm removal. From the *Discipline of Endodontics, University of Toronto, Toronto, Ontario, Canada; and † Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada. Address requests for reprints to Dr Anil Kishen, Discipline of Endodontics, University of Toronto, 124 Edward Street, Toronto, Ontario MSG 1G6, Canada. E-mail address: akishen@gmail.com 0099-2399/$ - see front matter Copyright ª 2015 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.01.027 Basic Research—Biology JOE — Volume -, Number -, - 2015 Fluid Dynamics, Biofilm Removal by Syringe, Ultrasonic-assisted Irrigation 1