## 5 Discussion
Polymerization shrinkage causes material shrinkage in all directions, and most often, the dentin margins are unprotected against microleakage [39]. However, RMGI and alkasite exhibited the opposite result; higher microleakage scores were observed at the enamel margin than at the gingival margin in the current study. For RMGI, adhesion is mainly due to a physicochemical reaction with dentin and enamel due to the polar nature of the polyacrylates and minerals in dental hard tissues. Alkasite is deficient in adhesive potential and does not contain any acidic monomers or water. Thus, it was applied after universal dentin adhesive application to class V cavities. For both materials, compensation mechanisms, such as water sorption and the existence of a special stress reliever, might reduce polymerization shrinkage and stress. Selective enamel etching was not performed in the absence of shrinkage stress, which might have increased the tendency of microleakage in the enamel margin. An intragroup analysis of microleakages in the gingival margin of the RMGI and alkasite restorations showed significant differences between the pH 4 and pH 7 storage specimens (Table 3). In the gingival margin, pH 4 resulted in a significantly lower microleakage than pH 7. This result suggests that the strong buffering capacity with ion-releasing property of these materials might counteract a low pH environment. The current results indicated that the bond strength to dentin is not affected according to the type of restorative materials. Although the pH condition did not influence to the bond strength of RMGI and alkasite, that of giomer significantly decreased in acidic pH, which suggested a relatively lower ion-releasing capacity (esp, hydroxyl ions) of giomer than RMGI or alkasite. The microleakage scores were affected by three experimental factors (the type of materials, the location of margin and pH). Related to the types of materials and the location of margin, it is assumed that the experimental result is mainly due to polymerization shrinkage, which occurred more in giomer. RMGI and alkasite showed a lower microleakage in pH 4 than pH 7 (\(p=0.028\)). It is assumed that the ion-releasing capacities of the materials can be activated under acidic conditions. Several attempts have been made to develop hybrid restorative materials that combine the advantages of the resin composite and glass ionomer cement families. In this study, although the three materials showed no significant difference in bond strength to the dentin surface, the bond strength of the giomer decreased significantly at pH 4. RMGI and alkasite exhibited a lower microleakage than the giomer, especially under acidic pH conditions, and the microleakage under low-pH conditions was decreased compared to when under neutral pH conditions. It is suggested that the ion-releasing capacity of these materials is activated under acidic conditions. RMGI and alkasite might be adopted in generalized or localized low pH clinical conditions (e.g., oral pathologic conditions, and lowered pH induced by reflux episodes). In this study, thermocycling was not considered in the microleakage test. Microleakage specimens were stored at 37 \({}^{\circ}\)C in a buffer solution at various pH values to control a variable,linear coefficient of thermal expansion (LCTE)', which influences the volumetric change of restorative materials in the oral cavity. The LCTE of resin-based materials is affected by the amount of filler [40]. Beautiful flow plus has a 47 vol% of filler [41], RMGI has 55 vol% filler [28], and alkasite has a 58 vol% (78 wt%) filler content in the final mass [28; 42]. Thus, the harsh oral environment could not be simulated, which is a limitation of this study. The buffer solution was stored for 24 h. The short storage period was intended to limit the hygroscopic expansion of the resin-based restorative materials.