The success of oral restorations involving the use of glass fiber posts (GFPs) for reconstructing teeth with significant crown loss is directly linked to several factors: the mechanical characteristics of root dentin, successful adhesion between the GFPs, luting agents, and dentin substrates, as well as the overall success of endodontic treatment [1,2]. Achieving proper and thorough elimination of pathogens is critical for the efficacy of endodontic interventions and direct restorative care [3,4]. Therefore, various methods and practices have been developed to minimize the occurrence of microorganisms and inflammatory processes. These protocols include root canal biomechanical instrumentation [5] and the utilization of irrigation fluids (sodium hypochlorite and chlorhexidine) at different concentrations [6,7]. Although, in vitro and in vivo studies suggest that conventional instrumentation combined with irrigation solutions may not completely eliminate bacteria and their remnant toxins, such as endotoxins and apical biofilms, in case of persistent infections. In addition to their potential negative effects on periapical tissues [8,9], these solutions could also interfere with the adhesion of luting agents, such as dental adhesives and resin-based cements, to the root dentin substrate [[10], [11], [12]].

By reason of its antimicrobial properties and natural properties, medication containing calcium hydroxide (CH) is widely applied enhancing microbial load reduction after biomechanical instrumentation [13,14]. CH medication is an alkaline substance having a pH of around 12.5, enabling it to dissociate into hydroxyl ions and calcium when in a watery medium [13,14]. The medication possesses distinct beneficial characteristics, including antimicrobial activity, inhibition of tooth resorption, the ability to dissolve biofilms and tissues, and stimulation of hard tissue formation [15,16]. These attributes make it the major option for intracanal medication in different therapeutic scenarios. However, CH may have restricted antimicrobial efficacy in certain clinical situations of persistent infection [17], such as Enterococcus faecalis infection, leading to exploration of alternative approaches for eliminating endodontic pathogens, such as antimicrobial photodynamic therapy (aPDT) [2,18].

Furthermore, several studies have demonstrated that aPDT by itself may not achieve complete eradication of biofilms [19,20]. Biofilms are formed by a diverse consortium of microorganisms, and their resilience is 10 to 1000 times more resistant to conventional endodontic therapies [19,21]. Authors have suggested that the simultaneous application of aPDT alongside CH medication enhances bacterial eradication [22,23]. Therefore, the application of photochemotherapy is not meant to replace conventional endodontic approaches but rather to complement them, working in tandem to eliminate persistent infections [19,[24], [25], [26]].

Considerable works have examined the impact of distinct photosensitizers (PSs) at various concentrations, whether irradiated by LED or laser sources or not, on the mechanical characteristics of root dentin, such as microhardness and elastic modulus [1,2,27,28]. They have also explored their effects on the adhesive strength of luting agents used to lute GFPs to root dentin [1,2,11,27,28]. However, there is a scarcity of research in the literature evaluating the effect of methylene blue (MB)-mediated aPDT in varying the PS concentrations, whether activated by red laser or not, in conjunction with CH as intracanal cure along the root dentin. Strazzi-Sahyon et al. conducted a study investigating the effect of CH medication along with two application sessions of MB-mediated aPDT, whether light-activated by a red laser or not, on the mechanical properties of intraradicular dentin [29]. However, there is currently no consensus on how the combination of aPDT with CH medication may influence the mechanical properties, degree of conversion, and integrity of the bonding interface between GFPs and root dentin.

Therefore, this research sought to assess the impact of CH as intracanal medication and MB PS-mediated aPDT, irradiated or not by laser – 660 nm, on the mechanical characteristics [Martens hardness (MH) and elastic modulus (Eit)], in addition on morphology and chemical composition of intraradicular dentin in different depths. Additionally, effects on MH, Eit, degree of conversion (DC), and integrity of the bonding interface layer of GFP to different third regions of intraradicular dentin were also assessed. The null hypotheses tested were: 1) The use of MB-mediated aPDT, whether irradiated or not, in conjunction with CH intracanal medication would not lead to differences in the mechanical features of intraradicular dentin, nor would it affect the mechanical properties, DC, or integrity of the adhesive interface; and 2) distinct depth regions of root dentin submitted to MB-mediated aPDT and CH medication would not result in differences on MH and Eit of root dentin, as well as on MH, Eit, DC, and integrity of the bonding interface of GFPs.