## 4 Discussion  
The bonding performance of dental adhesives and luting agents to dentin \"contaminated\" with eugenol [1, 17, 18, 19, 30, 31, 39] has been the subject of a plethora of investigations during the last two decades. Nevertheless, the controversial results, inconsistencies, and inaccuracies of these studies have not provided the dentists with any evidence to support a judicious clinical decision considering the adverse effect of eugenol on adhesive dentistry. In order to increase the quality of the current systematic review and meta-analysis, we systematized all the selected studies into subgroups: (1) conventional ER adhesive system or SE adhesive system and, (2) evaluation through \(\upmu\)SBS or \(\upmu\)TBS. For both subgroups analyses as well as for the global analysis, it was found that the presence of residual eugenol jeopardized the bond strength of resin-based materials. Previous in vitro studies highlighted the detrimental effect of eugenol-containing materials on dental bonding outcomes of resin-based materials [1, 20], although there was no general agreement in the literature about such issue. Several studies have suggested that the effect of eugenol on the bond strength depended on the adhesive system employed [17]. While ER adhesive systems require the use of phosphoric acid [43], SE adhesives act through acid monomers, which can demineralize and consecutively infiltrate the dentin surface. Accordingly, the conventional ER bonding approach involves a water-rinsing step after the application of phosphoric acid to remove the residual acid containing a dissolved smear layer [43], while SE adhesives modify and incorporate the smear layer within the adhesive layer [44]. Therefore, after the removal of eugenol-containing provisional cement, the following cleaning steps for dentinal surface treatment can change the concentration of eugenol present on dentin and affect the final bond strength of resin-based materials [17, 18, 19]. In a previous study [18], it was emphasized that the use of phosphoric acid during the application of the conventional ER system could remove the smear layer contaminated with eugenol. Nevertheless, the pooled outcome of the meta-analysis performed here showed that the eugenol-contaminated dentin presented lower bond strength, regardless of the adhesive system applied. Surface treatments such as orthophosphoric acid or ethylenediaminetetraacetic acid (EDTA) were investigated, and both significantly reduced the quantity of eugenol on the dentin surface [1]. However, it was previously observed that, even after acid application, residues of provisional cements could remain on the dentin surface [22]; hence, this justifies the outcomes of the current study. The phenolic component of eugenol can react with free radicals in the monomeric conversion from C = C bonds to C-C bonds, reducing the reactivity of the resin monomers and their polymerization. In this way, the remaining eugenol can bind to hydroxyl groups of the monomers [23, 24, 45, 46] and reduce the degree of conversion of the adhesive system [47]. A reliable degree of conversion is essential for polymer-based materials, as it is related to the achievement of proper physical properties (e.g., high tensile strength [48] and microhardness [49]) and lower water sorption and solubility [48, 50]. Thus, resin adhesive systems that show a high polymerization rate achieve better bonding performance and stability over time [51]. Here, by analyzing only immediate \(\upmu\)TBS and \(\upmu\)SBS, it was possible to observe that eugenol could reduce the adhesion values significantly. Therefore, it is possible to consider that, in terms of long-term results, this scenario could be even worse due to water sorption and degradation of the hybrid layer over time, mainly for a system with its monomer conversion impaired. Bonding tests, mainly those through \(\upmu\)TBS, are important for the study of resin-based restorative materials.