Photodynamic therapy (PDT) has been growing in dentistry for aiding the decontamination of the root canal system due to its antimicrobial effectiveness. PDT involves the association between a low-power laser light source and a nontoxic photosensitizer, which produces highly reactive oxygen species that induce bacterial cell damage [1]. However, despite its effectiveness in combating endodontic pathogens, there are reservations regarding the use of PDT and the quality of the adhesive bond strength [2] required for cementing intraradicular glass fiber posts (GFP) [3].

Intraradicular retainers are recommended when the tooth does not have sufficient crown structure to receive a final restoration [4,5], providing enough retention to the central structure [4]. However, the photosensitizer used in PDT is a viscous substance that significantly impregnates the dentin surface. Thus, a chemical smear layer may form, obliterating dentinal tubules and leading to microleakage and decreased bond strength of intraradicular retainers to dentin [6].

So, a series of studies involving the application of PDT prior to the adhesive cementation of prefabricated intraradicular posts have been developed in order to evaluate the influence of PDT on the adhesive strength of posts to root dentin, showing positive results. These studies demonstrate that the use of US over final irrigants (EDTA 17% and QMix) improved the bond strength of GFP cemented with resin cements in root dentin previously treated with PDT [7]. Later, it was demonstrated [8] that methylene blue at 50 mg/L can be applied in two PDT sessions, after biomechanical preparation and before glass-fiber post luting, for it presents no influence on root dentin bond strength in vitro conditions. Finally, it was observed [9] that the use of PDT with methylene blue at 50 mg/L associated with calcium hydroxide intracanal medication is a suitable alternative for root canals disinfection after biomechanical preparation and glass-fiber posts luting procedure, demonstrating no impact on bond strength of glass-fiber posts to intraradicular dentin.

Despite the above, it is known that a final irrigant must be used to effectively remove the photosensitizer from the root canal walls after PDT [10]. Ethylenediaminetetraacetic acid (EDTA) is the final irrigant most frequently used in endodontics due to its recognized ability to remove the smear layer [11], especially when associated with ultrasonic activation (US). The literature reports that US, performed with a device inserted in the root canal to promote the mechanical agitation of the auxiliary chemical substance or final irrigant, helps remove debris and microorganisms from the root canal system [12]. Despite the advantages of EDTA used alone or associated with US, it is a cytotoxic substance that reduces dentin microhardness and changes the dentin structure [11].

Seeking an alternative to EDTA as a final irrigant, Cecchin et al. [13] reported glycolic acid (GA), noting that it satisfactorily conditioned the enamel and dentin, significantly improving the adhesion of restorative materials to these surfaces. Conversely, Dal Bello et al. [14] state that GA can remove the smear layer and has low cytotoxicity and the same effects as EDTA on the mechanical properties of root dentin.

Therefore, this study aimed to evaluate the bond strength of glass fiber posts to root dentin and the influence of PDT and US on different final irrigants. We tested the null hypotheses that the use of PDT does not improve the bond strength of GFPs to root dentin, regardless of the final irrigant used and the use of US over final irrigants.