Effect of ultrasound streaming on the disinfection of flattened root canals prepared by rotary and reciprocating systems

Abstract New technical and scientific developments have been advocated to promote the success of the endodontic treatment. In addition to rotary and reciprocating systems, irrigating solution agitation has been suggested and passive ultrasonic irrigation (PUI) is the most used. Objective: To evaluate, in vitro, the effect of ultrasound streaming (US) in the disinfection of flattened root canal systems prepared by the ProTaper, BioRaCe and Reciproc systems, utilizing the microbiological culture. Methodology: Extracted human mandibular incisors (n=84) were used. Suspensions of Enterococcus faecalis (ATCC 29212) were standardized and inserted along with the teeth immersed in brain-heart infusion (BHI) broth. The contamination was made following a protocol during 5 days. The teeth were randomly divided into six groups: G1, ProTaper Universal; G2, ProTaper Universal with US; G3, BioRaCe; G4, BioRaCe with US; G5, Reciproc; and G6, Reciproc with US. Irrigation was performed with saline solution. After biomechanical preparation, microbiological samples were performed with sterilized paper points, which were diluted and spread on BHI agar; after 48 h, the colony forming units (CFU/mL) were counted for each sample. Results: Groups using ultrasonic agitation presented a greater antibacterial effect than the other ones, even using saline solution as irrigant. The ProTaper Universal system showed the best antibacterial activity of the tested systems (median of 0 CFU/mL with and without surfactant or ultrasonic activation [PUI]). Even with PUI, Reciproc (median of 2.5 CFU/mL with PUI and 5 without it) could not reduce as many colonies as ProTaper Universal without US. The BioRaCe system had greater bacterial reduction when using US (median of 0 CFU/mL with PUI and 30 without it). Conclusions: US promoted greater reduction in the number of bacteria in the flattened root canals prepared with nickel-titanium mechanized systems. Regarding the instruments used, the ProTaper Universal system was the most effective in reducing the bacterial number.


Introduction
Biomechanical preparation plays an important role in eliminating bacteria and reducing their population inside the root canal system. Teeth that have a complex anatomy can shelter, in the crevices and isthmus areas, remaining necrotic pulp tissue and of persistent infections, resulting in the failure of endodontic treatment 23 . In these cases, Gram-positive microorganisms are the most frequent, and among these, Enterococcus faecalis 5 is the most commonly used. This bacterial species has the ability to endure many ecological conditions and it can adjust to lethal challenges such as high levels of alkalinity 23 , requiring few nutrients, adhering to dentine 21 and penetrating deeply into the dentinal tubules 11,14 , which makes it a resistant pathogen 12  is made by machining. The instruments of the BioRaCe system (FKG, La Chaux-de-Fonds, Switzerland), also machined, were launched with electrochemical surface treatment, providing the removal of surface defects that can initiate a fracture when the instrument is subjected to a cyclic fatigue process 13

Instrumentation procedures
The sterilized specimens were divided into six groups according to the instrumentation system used for root canal preparation, as follows:  (Table 1).
In the group of BioRaCe system without ultrasonic agitation there was one specimen without bacterial growth, while when the ultrasound was used with the same system 7 specimens did not present bacterial growth. In ProTaper U group, 7 specimens did not present bacterial growth, but when the ultrasound was used this number raised to 9. The Reciproc system group presented 2 specimens without bacterial growth without ultrasound agitation and 3 specimens with no bacterial growth when the ultrasound was used.
No specimens of negative control presented bacterial growth. Only 3 specimens of the BioRaCe system group without ultrasonic agitation presented more than 100 CFU/mL, as well as all the specimens from the positive control ( Figure 1).

Discussion
This study evaluated the effect of the ultrasound streaming in the disinfection of root canals.

Thus, mandibular human incisors were chosen for this
In this study there were a great number of canals with negative cultures. The probable reason for this is that the sample collection was made only in the main root canal with absorbent paper points, so, bacteria may still remain inside dentine deep tubules.
PUI is more effective than conventional irrigation in cleaning the root canal system 25,29 . This kind of irrigation has the potential to remove dentin debris and organic tissue from areas inaccessible to instrumentation 29 24 (2010) showed that PUI was more effective than irrigation with a syringe and a sonic system in the removal of debris in agreement to other studies 9,18 . Besides promoting greater dentin removal, this system has more instruments than the others tested, which leads to a greater amount of irrigating solution and ultrasound used during the preparation.
All the systems have the same 0.25 mm apical results. In the ProTaper U system, the F2 instrument has a taper of 0.8, the same as the Reciproc system's wore away a smaller dentin area, resulting in less decontamination. In addition, when Reciproc was compared with ProTaper U, the latter showed a greater removal of dentin 7 , which can explain the results.