## 4 Discussion  
Based on the results of the current research, H\({}_{0}\)1, which stated that after artificial aging, no microgap formation would be detected at the restorative/dentin interface, was rejected. In the current study, the thermocycling protocol that was followed allows for the simulation of the oral environment over almost 6 months [27]. During the process of dimensional changes in restorative materials and tooth tissues, which are different due to the discrepancies in their thermal expansion coefficients, stresses are developed at the restorative/tooth interface deteriorating the bond between them, depending on the number of applied cycles [28]. Although no standard protocol regarding thermocycling has been established for evaluating microgap formation, in the present investigation, the selected protocol was adopted similarly to previous reports [28; 29; 30]. It has been claimed that the weakening in bonding between the restorative material and tooth tissues may also be attributed to the decomposition of the components at the interface area through hydrolysis [31; 32]. Although it is not certain that the influence of thermocycling on bond strength is equal to that of long-term water storage, limited data are available regarding the synergistic effect of thermocycling and long-term water storage on microgap formation. The formation of microgaps raises concerns because they let the fluid or bacteria invade the dentin-pulp complex, which may lead to postoperative sensitivity and the formation of tooth caries [33]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Experimental Groups**} & **Mean and SD of the** & **Free Microgap Interfaces (\%)** \\  & **Microgap Size (\(\upmu\)m)** & **Free Microgap Interfaces (\%)** \\ \hline CR & 170.72 \(\pm\) 79.06 \({}^{\text{A}}\) & 0\% \\ \hline SB1 & 6.83 \(\pm\) 3.02 \({}^{\text{B}}\) & 25\% \\ \hline SB2 & 2.31 \(\pm\) 1.61 \({}^{\text{B}}\) & 50\% \\ \hline HF & 22.46 \(\pm\) 9.29 \({}^{\text{C}}\) & 25\% \\ \hline LR & 163.44 \(\pm\) 59.23 \({}^{\text{A}}\) & 0\% \\ \hline \hline \end{tabular}  
\({}^{\text{Same}}\)uppercase superscripts in the middle column indicate no statistically significant difference (\(p<\) 0.05).  
\end{table} 
Table 2: Means and standard deviations (SDs) of the width of microgaps of each experimental group formed after thermocycling. CR: control; SB1: air abrasion with 29 \(\upmu\)m Al\({}_{2}\)O\({}_{3}\) particles; SB2: air abrasion with 53 \(\upmu\)m Al\({}_{2}\)O\({}_{3}\) particles; HF: hydrofluoric acid gel + silane; LR: Er,Cr:YSGG laser treatment. Figure 4: Representative SEM images (\(\times\)100 magnification) for each experimental group at the material/dentin interface revealing the microgap formation: **(a)** CR: control; **(b)** SB1: air abrasion with 29\(\upmu\)m Al\({}_{2}\)O\({}_{3}\) particles; **(c)** SB2: air abrasion with 53 \(\upmu\)m Al\({}_{2}\)O\({}_{3}\) particles; **(d)** HF: hydrofluoric acid gel + silane; **(e)** LR: Er,Cr:YSGG laser treatment. The size of the microgaps is affected by the magnitude of the internal stresses that are developed during the contractions and expansions of the materials and tissues when temperature fluctuations take place. Thadathil Varghese et al. [34], who focused on the effect of thermal stimuli on the restorative/dentin interface, found significant changes in the developed stresses in this area. These stresses impair the bond strength at the adhesive interface, and when they exceed a critical threshold, they may result in fracture, fatigue damage, interfacial debonding, and microgap formation [35]. The physical-mechanical properties of teeth and restoratives such as the elastic modulus, the coefficient of thermal expansion, and thermal conductivity, as well as oral conditions, can crucially affect the progress of these defects, which may lead to the failure of tooth restoration [34].