This study set out to compare the efficacy of laser-activated and ultrasonically activated root canal disinfection with conventional irrigation, specifically its ability to remove bacterial film formed on root canal walls. Methods: Seventy human premolars were shaped to an apical size #20, taper .07, sterilized, and contaminated in situ with oral bacteria for 1 week and incubated for 2 more weeks. Irrigation was done with 6% NaOCl (group 1), NaOCl ultrasonically activated with blunt inserts (group 2), or a pulsed erbium:YAG laser at nonablative settings (group 3) for a total of 60 seconds each. Positive and negative controls were also included. Aerobic bacterial sampling was performed, and the incidence of positive samples after 24 and 48 hours as well as bacterial counts (colony-forming units) were determined. Fixed and demineralized sections 1 mm and 4 mm off the apex were Brown-Brenn stained and assessed for remaining intracanal bacteria/biofilm and dentinal tubule penetration.

Results: All 3 canal disinfection protocols significantly reduced bacterial counts (P < .001). None of the 3 techniques predictably generated negative samples, but laser-activated disinfection was superior to the other 2 techniques in this aspect (P < .05). Histologic sections showed variable remaining bacterial presence in dentinal tubules at the 4-mm level and significantly less bacterial biofilm/necrotic tissue remaining at the 1-mm level after laser-activated irrigation (P < .05). Conclusions: Under the conditions of this combined in situ/in vitro study, activated disinfection did not completely remove bacteria from the apical root canal third and infected dentinal tubules. However, the fact that laser activation generated more negative bacterial samples and left less apical bacteria/biofilm than ultrasonic activation warrants further investigation. (J Endod 2011;37:1008–1012)

Root canal treatment aims at the elimination or prevention of periradicular periodontitis; bacteria and their toxins are the cause of this disease (1), and therefore the eradication, or reduction to a biologically acceptable number, of intracanal microorganisms is required (2). Enlargement of root canals with modern root canal instruments reduces bacterial counts even in the case of buccolingually wide root canals (3). However, preparation does not eliminate all microorganisms from the root canal system. Therefore, antimicrobial irrigants are commonly used, and it is believed that enhancement of the flushing action is effective in improving root canal cleanliness (4, 5). Different agitation techniques have been proposed to improve the efficacy of irrigation solutions, including agitation with hand files, gutta-percha cones, plastic instruments, and sonic and ultrasonic devices (6). One of the more recent suggestions is the use of laser energy to enhance irrigation. Lasers are used to activate photosensitizers that associate with bacteria(7, 8)and more recently to activate irrigation solutions by the transfer of pulsed energy (9, 10). It appears that irrigation enhanced by erbium:YAG laser light is effective in removing dentin debris(9)and also in smear layer removal(11). It appears that direct laser irradiation is less effective in killing Enterococcus faecalisthan 2.5% NaOCl(12), but laser activation of conventional irrigants might aid in debriding root canals (9). The latter might be achieved by the action of a pulsed erbium:YAG laser via photon-induced photoacoustic streaming (PIPS) (11).

However, to the best of our knowledge, the capacity of PIPS to disinfect root canals has not been established. Therefore this study aimed at comparing the efficacy of root canal disinfection with this technique with conventional syringe irrigation and ultrasonic activation, specifically regarding the ability to remove bacterial biofilm formed on root canal walls.

Materials and Methods In Situ Inoculation of Teeth with Oral Bacteria
From teeth that had been extracted for reasons unrelated to the current study, 70 human mandibular premolars were collected and stored in 0.1% thymol solution at 4
C until further use. Teeth were then decoronated and trimmed to a uniform length of 14 mm. Canals were checked for patency, and working length (WL) was determined by placing a size #10 K-file so that it was just visible and then reducing 0.5 mm from that length. Subsequently, canals were shaped with ProTaper rotaries (Dentsply Tulsa Dental, Tulsa, OK) to an apical size #20, taper .07 (Finishing File 1), according to manufacturer’s instructions. A coronal reservoir for irrigant placement was created with a Gates Glidden drill #5 placed 5 mm into the canal. Between every instrument, canals were irrigated with 6% NaOCl deposited with a 30-gauge Maxiprobe needle (Dentsply Tulsa Dental). After shaping was completed, teeth were irrigated with a sequence of 17% ethylenediaminetetraacetic acid (EDTA) for 1 minute and 6% NaOCl for 1 minute and then placed in an ultrasonic bath in 17% EDTA for 2 minutes for smear layer removal. Teeth were sterilized in an autoclave and then, with approval from the universities’ Internal Review Board, were prepared for in situ contamination. An in situ method to establish root canal infection was described earlier (13, 14); it involves individual sectional specimen holders that house 3–4 roots each. In brief, after impression taking from volunteers’ maxillary arches and pouring of plaster models, polymethylmethacrylate appliances were fabricated and fitted so that that they could be placed buccally in the maxillary molar region without interfering with occlusion and articulation. The accesses remained open to the oral cavity.

A total of 20 devices were prepared and continuously carried by volunteers for 6–8 days each. The appliances were incubated for a further time period of up to 15 days in tryptic soy broth (TSB) (MoBio, Carlsbad, CA) so that bacterial contamination for all teeth was equilibrated at 3 weeks. Then the 70 roots were removed from the appliances and individually mounted in heavy body silicone blocks to expose only the occlusal surface with the access cavity.

Disinfection of Root Canals with Photon-Induced Photoacoustic Streaming