Efficacy of Photon-induced Photoacoustic Streaming in the Reduction of Enterococcus faecalis within the Root Canal: Different Settings and Different Sodium Hypochlorite Concentrations Barbara Skrlj Golob, MD,* Giovanni Olivi, MD, DDS, † Mirijam Vrabec, PhD, ‡ Rita El Feghali, DDS,* Steven Parker, BDS MFGDP,* and Stefano Benedicenti, DDS* Abstract Introduction: The purpose of this study was to deter- mine the effectiveness of laser-activated irrigation by photon-induced photoacoustic streaming (PIPS) in the reduction of Enterococcus faecalis in root canal disin- fection, varying laser energy output, and sodium hypo- chlorite (NaOCl) concentration. For effective removal of the smear layer, the sequence and resting time of the final irrigation steps were modified compared with the stan- dard PIPS protocol. Methods: Eighty-six extracted single-rooted teeth were mechanically prepared, steril- ized, and inoculated with E. faecalis for 4 weeks. Teeth were divided into 9 groups and treated with an Er:YAG laser using a PIPS 600/9 tip at the following parameters: 10 mJ or 20 mJ, 15 Hz, and a 50-microsecond pulse dura- tion at 0.15 W or 0.3 W average power, respectively. Root canals were irrigated with different concentrations of NaOCl (ie, 1%, 3%, and 5% and activated using the adjusted PIPS protocol). The bacterial count was per- formed immediately after and 48 hours after decontami- nation and new incubation on an agar plate. Results: A statistically significant difference in bacterial counts (P < .05) was detected in all groups before and directly after the treatment and in groups treated with 5% NaOCl 48 hours after treatment. Scanning electron microscopic imaging showed an absence of bacteria and biofilm in the scanned areas after treatment with 5% NaOCl. Conclusions: Laser-activated irrigation using 5% NaOCl and a modified PIPS protocol resulted in effective eradica- tion of the bacterial biofilm and removal of the smear layer. (J Endod 2017;-:1–6) Key Words Disinfection, Enterococcus faecalis, irrigation, laser, photon-induced photoacoustic streaming T he main cause of pulpi- tis and apical peri- odontitis is bacteria in planktonic and/or biofilm form (1). Biofilm provides protection for microor- ganisms against immune responses, stress, antibacterial agents, and antibiotics (2) and enables better resistance to external influence (3). Until recently, mechanical instrumentation was the most important part of end- odontic treatment. Despite different file systems and improved metallurgical properties of materials, more than 35% of the root canal surface remained untouched by instru- mentation (4). Furthermore, root canal instrumentation has several disadvantages including dentin debris and smear layer production, iatrogenic errors (eg, apical trans- portation, crack formation, and stripping), and weakening of the root structure (5–7). Long-term success and resistance to fracture are related to the amount of residual tooth structure (8). Increasing diameter and taper size are counter to the concept of mini- mally invasive dental treatment. Root canal irrigation enables chemical dissolution of organic and inorganic tissue, mechanical detachment and flushing of microorganisms and their products, dentin debris, and the smear layer. Different irrigation techniques have been developed. Laser-activated irrigation (LAI s.l. ) is based on the activation of liquid irrigants by medium-infrared lasers (2780 nm and 2940 nm). Two different techniques are sug- gested: the tip is placed inside the root canal (LAI s.s. ) or located inside the pulp chamber only (photon-induced photoacoustic streaming [PIPS]). The latter technique uses a radial firing and stripped tip, allowing lateral emission of laser energy in the liquids. The use of subablative energy (20 mJ) delivered in a very short time (pulse duration of 50 microseconds) produces a high peak power of 400 W, causing an explosion- implosion phenomenon within the irrigant solution. The result is a strong photoacous- tic shock wave that induces irrigant streaming 3-dimensionally throughout the entire root canal system while avoiding any direct laser irradiation on the dentin and conse- quent unwanted thermal effects (9, 10). Compared with other techniques, many studies reported better cleaning and more efficient smear layer removal using the PIPS technique as confirmed by scanning From the *University of Genova, Genova, Italy; † Inlaser, Studio Medico Dentistico Olivi and Genovese, Roma, Italy; and ‡ University of Ljubljana, Slovenia. Address requests for reprints to Dr Barbara Skrlj Golob, Department of Surgical Sciences and Integrated Diagnostics, University of Genova, Largo Rosanna Benzi 10, Pad IV, 16132 Genova, Italy. E-mail address: barbara@dentalnistudio.com 0099-2399/$ - see front matter Copyright ª 2017 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2017.05.019 Significance Laser-activated irrigation by photon-induced pho- toacoustic streaming is effective in the reduction of Enterococcus faecalis in root canal disinfection with widely used sodium hypochlorite. Basic Research—Technology JOE — Volume -, Number -, - 2017 Photon-induced Photoacoustic Streaming 1