This study investigated the SBS of composite resin bonded using two adhesion protocols to primary dentin that had been pretreated with 38% SDF. The findings showed that delaying composite resin application for one week after pretreatment significantly improved SBS when compared to application immediately after pretreatment. Significant differences in SBS were also observed between the ER and SE subgroups when composite was applied immediately after pretreatment; however, this difference disappeared upon delaying application. Based on these findings, both null hypotheses were partially rejected.
The current study implemented a delay of one week between restoration and SDF pretreatment. Ng et al., [21] previously hypothesized that separation of pretreatment and restoration would allow for SDF solidification and, consequently, enhance the SBS of the final restoration. However, the findings of their study showed no statistically significant differences between groups where glass ionomer restorations were placed immediately, and one week after SDF pretreatment [21].
It has to be noted that the dentin bonding mechanisms of glass ionomer and composite resin differ considerably, with the former using chemical adhesion dependent on weak polyacrylic acid and the latter using micromechanical adhesion [23, 41]. A systematic review by Frolich et al. (2022) concluded that although application of SDF did not affect the SBS of glass ionomer, it caused deterioration of the bonding systems [22]. In light of this data, composite resin was considered to be the material of choice as a final restoration after SDF pretreatment in the present study.
The findings of the present study showed that the SBS of composite resin significantly deteriorated when it was applied immediately after SDF application compared to the control group (p-value < 0.001). This is consistent with the results obtained by Markham et al. [42], Koizumi et al. [34] who used KI/SDF as a pretreatment, Kucukyilmaz et al. [43] who used SDF and ammonium hexafluorosilicate on intact and demineralized dentin and Lutgen et al., who pretreated sound
dentin [33].
In contrast, Favaro et al. [44] and Quock et al. [19] reported no adverse effect of SDF on the composite micro-SBS and micro- tensile bond strength, respectively. Pérez-Hernández et al. [45] reported improvement in fissure sealant retention when applied after SDF application. This contradiction can be attributed to the fundamental differences between enamel and dentin nature, leading to the distinct impact of SDF and etching on each of the substrates [46–48].
SDF reaction with hydroxyapatite forms silver phosphate layer (Ag3PO4) [46]. The pretreatment also results in obstruction of dentinal tubules by silver particles [49]. The presence of this impermeable layer and inter-tubular precipitate acts as a phase separation interfering with intimate resin impregnation, which yields a suboptimal hybrid layer [22, 27, 33, 50]. Therefore, SBS of applied composite resin will be affected [22, 28]. Moreover, putting into consideration that SDF is a highly alkaline fluid (PH = 10-12.5) [51]. It works as a physical and chemical barrier to adhesion that necessitates a dry acidic media to create a route for adhesive resin [33].
The significant improvement in SBS observed upon delaying composite restoration bonding to one week after pretreatment can be attributed to a deeper penetration of silver ions into the dentinal tubules during the time interval [21, 43, 46, 52]. In order to test this hypothesis, the maximum depth of penetration of silver was assessed immediately and one week after pretreatment using EDS. The findings showed that while silver could be detected at a depth of up to 309.5 µm initially, this increased to be up to 1001.1 µm when examined again one week after pretreatment. Moreover, this time interval also seemed to cause ammonia preservatives to leach out as nitrogen content significantly decreased .
Silver ions have an inhibitory effect on matrix metalloproteinases (MMPs) and cysteine cathepsin enzymes that degrade dentinal collagen matrix proteins, leading to bonding loss [53–55]. The re-mineralizing and inhibitory properties of SDF suggest it may have an enhancing effect on the SBS of composite resin if time interval were further extended.
With regard to the adhesion protocols used, a significant difference in mean SBS was observed between the two subgroups in group 2 only (p-value = 0.03).The obtained result is in line with Lutgen et al. [33] .This could potentially be attributed to the fact that the SE technique relies solely on functioning acidic monomers in universal adhesives, which exhibit mild acidity (pH = 2.5) that is considerably weaker compared to the 37% phosphoric acid (pH = 0.1–0.4) used in the ER [56, 57]. Those weak monomers are more likely to be buffered by the alkaline ammonia content of the SDF. Hence, more debonding events were encountered [33, 58].
Performance of SE subgroup was enhanced in group 3 when bonding was delayed to be similar to the ER subgroup, which provides an additional evidence that alkalinity of SDF decreases when left to set.
Assessment of failure mode was done using a stereomicroscope. Currently, there is a lack of consensus in the literature with regard to the classification of failure modes and the methodology of interpretation. The current study opted to use the Scherrers et al. classification [40], and found that the three groups did not significantly differ with regard to the type of failures observed between the composite restoration and the dentin.
The current study was limited by the inability to apply a conductive coating during SEM examination as the teeth were examined on two separate occasions. It should also be noted that only intact dentin was used in the current study, and the SBS is expected to be lower and the silver ion atomic ratio is anticipated to be higher in carious dentin [59, 60]. Further clinical studies are necessary to confirm these findings.