## 4 Discussion
The \(\upmu\)TBS results have been associated with in vivo outcomes, mainly when the specimens are subjected to storage and longitudinal assessment [52]. The longer the specimens of bonding procedures are stored, the higher is the correlation with in vivo results [53]. Indeed, \(\upmu\)TBS after six months of storage in water correlates with marginal discoloration of restorations. Unfortunately, during literature screening, we have noticed that there was no longitudinal assessment of bonding after eugenol application on dentin. The most extended time of specimens' storage was 45 days [41] but this study did not perform a longitudinal analysis for dentin bonding strength. Thus, a further in vitro longitudinal analysis, possibly including in vivo outcomes, would be beneficial in order to avoid overestimating the results found in immediate \(\upmu\)TBS. However, to the best of our knowledge,the present review provides the most complete view of the impact of eugenol in resin restorations on coronal dentin. In this context, bond strength tests using a reduced specimen size were preferred based on [PERSON]'s defect theory, which suggests a decrease in the tensile strength of large materials in comparison to small ones [54]. The bond strength of large specimens could be low or involve too many defects. Such test could not show the proper performance of the tested materials [54; 55; 56]. The subgroup analysis considered for the type of method applied (\(\upmu\)tBS or \(\upmu\)SBS) showed \(\mathrm{I}^{2}=0\) in the test for subgroup difference and supported the findings of the pooled results. Thus, eugenol jeopardized the bonding to coronal dentin regardless of the adhesive systems used and the method of application. Through this meta-analysis, it was possible to combine data across multiple studies and better estimate the differences between single studies. The percentage value defined by \(\mathrm{I}^{2}\) showed the magnitude of the difference among the studies, which can be explained by heterogeneity and not by chance [57]. Although eugenol reduced the bond strength in all meta-analyses performed here, high heterogeneity could be observed in the subgroups and in the pooled data analysis. Therefore, other factors presented here should explain the model and the obtained results. Among 10 studies included, 4 showed a high risk of bias, and 5 a moderate risk of bias. The resulting bias can be accredited to the lack of a proper report of methodologies. It can also be hypothesized that a link exists between the lack of a proper report of methodologies and the high heterogeneity found here. Moreover, the studies differed mainly regarding the method used for the removal of the temporary eugenol-based material (Table 3). The different instruments applied and the duration of each step (e.g., time for cleaning with a pumice-water slurry) may have influenced the results. Some studies reported similar cleaning steps for the control group (no contact with eugenol prior to bonding), while other papers did not mention cleaning (Table 3). Variation in the cleaning steps was also found among several studies. Different cleaning steps can lead to changes in the dentin's surface, which might impact the bonding performance. Finally, the type of ZOE-based material could also be an influencing factor for high heterogeneity. Different powder/liquid ratios for the ZOE cements classifications could increase eugenol release if a higher concentration of liquid with respect to that of powder is used [18]. One of the studies included in our analysis tested the effect of a root canal filling paste with eugenol instead of provisional eugenol-cement on the \(\upmu\)SBS of an adhesive system [6]. We have included this study because its methodological design was very similar to those applied for testing eugenol-containing cements. Regardless of the high heterogeneity observed, it is possible to conclude that this study corroborates a previous one showing that eugenol jeopardizes not only the bonding to root dentin [29] but also the bonding to coronal dentin.