Influence of agitation methods of irrigants after methylene blue-mediated PDT on the bonding interface of a fiber post cementation system
Introduction
During post-space preparation, microorganisms from saliva may contaminate the root dentin, which can lead the endodontic therapy to fail [1,2]. To overcome this clinical issue, antimicrobial protocols such as photodynamic therapy (PDT) have been used. PDT is an adjunctive strategy also used to clean and decontaminate the post space. The main mechanism of PDT is based on the synergistic effect of a light-sensible drug (photosensitizer) and a light source under an appropriate wavelength. Briefly, the photosensitizer absorbs energy from the light source and generates reactive oxygen species (ROS) that trigger oxidative events leading to bacteria-killing [3,4].
Methylene blue in different concentrations is one of the most used photosensitizers in PDT [3,5]. However, methylene blue can cause tooth discoloration, hindering esthetic results, especially in anterior teeth [6]. Furthermore, the relatively high viscosity of methylene blue can facilitate its impregnation over the dentin surface, leading to obliteration of dentinal tubules and decrease of bond strength to resin-based materials [4,7]. In this context, a final irrigant must be used to effectively remove the methylene blue from the dentin surface after PDT. For this purpose, chemical substances and/or ultrasonic irrigation can be used to dissolute the pigmented substance [6,8,9].
Sodium hypochlorite solution is the most recommended irrigant for chemical-mechanical preparation of root canals. In addition, sodium hypochlorite is also recommended for methylene blue removal [10], and chromatic recovery of tooth color [11], due to the oxidizing activity provided by singlet oxygen, which is derived from the ionic dissociation of hypochlorous acid [1,12]. However, the free radicals released in this chemical reaction can negatively affect the degree of conversion of resin-based materials. Moreover, the free radicals can occupy the intratubular space and/or the space between the collagen fibrils, compromising the adhesion to dentin [10,13].
Antioxidant solutions can be used to minimize the harmful effects of the free radicals on adhesive interface. However, their action occurs in a punctual and localized manner, without significant diffusion of the solution into dentin [14]. To overcome this limitation, final irrigation with mechanical agitation of the irrigant can be used in an attempt to diminish the harmful effects of sodium hypochlorite residues and to assist the smear layer removal [9]. With the use of mechanical agitation is expected the re-establishment of adequate parameters for bonding to dentin with cementation systems that use etch-and-rinse adhesive and conventional resin cement [1,10].
XP Endo Finisher is a flexible instrument used in rotary motion to mechanically agitate the irrigant [15,16]. It is manufactured with a special NiTi alloy, which changes its molecular orientation from martensite to austenite phase when previously heated, providing greater flexibility and potential removal of residues from the root canal [15]. XP Clean is another agitation insert, similar to XP Endo Finisher, but there are no studies that evaluated its potential for removing residues from the root canal and post space. Easy Clean is a plastic instrument designed to produce mechanical agitation of the irrigating solution, with malleability and flexibility different from those provided by NiTi alloy [17]. This insert is driven by reciprocating motion, rotating only on its own axis, without increasing the amplitude of vibration [18]. Passive ultrasonic irrigation (PUI) is recommended in final root canal irrigation, since it can induce acoustic streaming and cavitation of the irrigant by ultrasonic waves, generating micro-bubbles that implode in contact with dentin and remove debris satisfactorily [19,20].
Therefore, the agitation of irrigants can be a complementary strategy to minimize the side effects of sodium hypochlorite on the adhesion interface of cementation systems to root dentin. However, there is a lack of studies that evaluate different protocols and instruments for these agitation techniques.
This study aimed to evaluate the effects of conventional final irrigation or agitation of distilled water with XP Endo Finisher, XP Clean or PUI, after 0.01% methylene blue-mediated PDT and neutralization with 2.5% sodium hypochlorite solution on the bond strength to post space after fiber post cementation with etch-and-rinse adhesive (Adper Scotchbond Multipurpose) and conventional resin cement (Relyx ARC). The null hypotheses tested were: H01 – Mechanical agitation of distilled water after PDT mediated by 0.01% methylene blue and irrigation with 2.5% sodium hypochlorite has no effect on bond strength of the resin cementation system and H02 - The failure mode of the cementation system at the bonding interface is similar after the irrigation protocols.
Section snippets
Material and methods
This study received approval from the Ethics Committee in Animal Use of the Araraquara School of Dentistry, São Paulo State University (UNESP) under the registration number 19/2017.
Bond strength
PUI, XPF e XPC that used final agitation of the distilled water showed the highest values of bond strength regardless of the post space third (p < 0.05), with no differences among them and compared to NCO (p > 0.05). CEI e ECL showed similar bond strength between them (p > 0.05) regardless of the post space third, but lower than other groups (p < 0.05).
Table 2 shows the mean and standard deviation of the bond strength values (in MPa) of the fiber post cementation system with etch-and-rinse and
Discussion
Agitation of distilled water using XP Endo Finisher, XP Clean and PUI instruments provided bond strength values for the cementation system with etch-and-rinse adhesive and conventional resin cement similar to NCO, but higher than Easy Clean instrument and conventional endodontic irrigation. Failure modes were different among the irrigation protocols. Thus, the null hypotheses (H01 and H02) were rejected in this study.
PDT is an adjunctive antimicrobial strategy to control bacterial contamination
Conclusion
Mechanical agitation of distilled water with XP Endo Finisher, XP Clean instruments or by PUI as final irrigants after methylene blue-mediated PDT and 2.5% sodium hypochlorite irrigation promoted bond strength of the resin cementation system in post space dentin comparable to control group. Thus, these methods appear to be effective to neutralize the negative effects of sodium hypochlorite residues on the bonding interface.
Declaration of Competing Interest
The authors declare that there is no conflict of interest in this article.
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