## 3 Results
The meta-analysis conducted on 10 studies with subgroups in ER and SE adhesive systems resulted in 36 comparisons (Figure 3): 21 in the ER adhesive system and 15 in the SE adhesive system. The random effects model was used, and the pooled global analysis showed a significant influence of eugenol (\(p<0.00001\)). Eugenol had a negative effect on the bond strength of resin-based restorations (5.79 (3.31-8.28) MPa). Within the subgroup analyses, a previous contact with eugenol-based materials also negatively affected the bond strength (ER: 3.63 MPa (1.22-6.05), \(p=0.003\); SE: 9.16 (4.08-14.23) MPa, \(p=0.0004\)). The heterogeneity was calculated via Tau\({}^{2}\), Cochrane's Q value, and I\({}^{2}\). For the ER subgroup, Tau\({}^{2}\) was 22.08 (Cochrane's Q value \(p=0.00001\)), and I\({}^{2}\) was 86%. For the SE subgroup, Tau\({}^{2}\) was 75.34 (Cochrane's Q value \(p<0.00001\)), and I\({}^{2}\) was 90%. For the global analysis, Tau\({}^{2}\) was 42.18 (Cochrane's Q value \(p<0.00001\)), and I\({}^{2}\) was 90%. Therefore, significant heterogeneity was detected within comparisons and in the global analysis. The test for subgroup difference showed I\({}^{2}=73.1\%\). Figure 2: Funnel plot of the included studies. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Study** & \begin{tabular}{c} **Teeth Free of** \\ **Caries** \\ \end{tabular} & \begin{tabular}{c} **Blinded** \\ **Researcher** \\ \end{tabular} & \begin{tabular}{c} **Teeth** \\ **Randomization** \\ \end{tabular} & \begin{tabular}{c} **Manufacturer’s** \\ **Instructions** \\ \end{tabular} & \
\begin{tabular}{c} **Storage** \\ **Time** \\ \end{tabular} &
\begin{tabular}{c} **Pre-Test** \\ **Failure** \\ \end{tabular} & **Classification** \\ \hline [PERSON] et al., 2005 [39] & + & - & - & + &? & - & High \\ \hline [PERSON] et al., 2007 [18] & + & - & - & + & + & - & Moderate \\ \hline [PERSON] et al., 2007 [17] & + & - & - & + & + & - & Moderate \\ \hline [PERSON]; [PERSON], 2009 [40] & + & - & - & - & + & - & High \\ \hline [PERSON] et al., 2011 [19] & + & - & + & + & + &? & Moderate \\ \hline [PERSON] et al., 2011 [20] & + & - & + & + & - & - & Moderate \\ \hline [PERSON] et al., 2013 [1] & - & - & + & - & + & - & High \\ \hline [PERSON] et al., 2014 [41] & - & - & - & + & + & - & High \\ \hline [PERSON] et al., 2018 [6] & + & + & + & + & + & - & Low \\ \hline [PERSON] et al., 2018 [42] & + & - & + & + & + & - & Moderate \\ \hline \hline \end{tabular}  
* not reported;? unclear.  
\end{table} 
Table 2: Individual risk of bias of the included studies. Besides the meta-analysis comparing ER and SE adhesive systems, the differences between subgroups for uSBS and uTBS were compared (Figure 4). Starting from 36 comparisons, only 6 were used for uSBS and 30 for uTBS. For both subgroups, the presence of eugenol before bonding negatively affected the bond strength of the tested adhesives (uSBS: 4.21 (0.91-7.51) MPa; uTBS: 6.39 (3.39-9.38) MPa). Intragroup heterogeneity was observed for uSBS(Tau\({}^{2}\) = 10.38,(Cochrane's Q value \(p\) = 0.009) and I\({}^{2}\) = 67%) and for uTBS (Tau\({}^{2}\) = 51.06 (Cochrane's Q value \(p\) < 0.00001) and I\({}^{2}\) = 92%. The test for subgroup differences showed no heterogeneity between them (I\({}^{2}\) = 0%, Cochrane's Q value = 0.34). Figure 3: Forest plot of global and subgroups (etch-and-rinse (ER) or self-etch (SE) adhesive system) meta-analyses. The bond strengths of adhesive systems/resin-based restorative materials to coronal dentin after contact with eugenol-based materials and without contact were compared. Table 3 shows a qualitative description of the methods applied to remove the cements and to clean the surfaces before performing any bonding procedure. There was high variability among the techniques. For instance, some of them used Al\({}_{2}\)O\({}_{3}\) sandblasting, and others a pumice-water slurry, a scaler, or a combination of these methods.