## 5 Discussion
It is speculated that the thin ABRZ might not resist acid attacks on the bonding interface, which decreases the bonding durability of the giomer stored at pH 4. In this study, universal dentin adhesive with self-etch application was used, for the theoretical ability to etch and infiltrate simultaneously, thus preventing discrepancies between demineralization and infiltration [27] and the capability of ABRZ formation [26]. The ABRZ in self-etch mode of universal adhesives might not be enough to confront the acidic changes of environment in giomer restoration in this study. Universal dentin adhesive with self-etch application was also used in alkasite restoration. However, there was no significant difference among various pH conditions. It is assumed that alkasite has high alkalizing potential with the release of hydroxyl ions [28]. [PERSON] et al. reported that the RMGI and alkasite showed a significant increase in the pH following acidification of the environment [28]. Three factors ('pH', 'location of margin', and'materials') significantly affected the microleakage scores of in vitro class V restorations. There were significant differences in the microleakage scores between the three materials (\(p=0.001\)). Giomers showed a higher microleakage score than RMGI and alkasite. Beautiful Flow Plus, which was used in this study, has a relatively low (47%) volume of the filler and a higher volume of the resin matrix, which makes it susceptible to polymerization shrinkage. The resin matrix of giomer is similar to that contained in a composite, with a base around bis-GMA and other monomers added for modifying its rheological and polymerization properties [29]. The setting reaction of the giomer is performed only by photopolymerization, which is the same as that of a composite. This involves the creation of a resin network and covalent bonds with inorganic (especially S-PRG) fillers. [PERSON] et al. reported that bulk-fill giomers exhibit significantly higher volumetric shrinkage than bulk-fill composite resins [30]. RMGI and alkasite have a compensatory mechanism against polymerization shrinkage of the matrix, which is a hygroscopic expansion with water sorption [7; 31]. In addition, the presence of a special filler (isofiller) in the alkasite, which acts as a stress reliever, may minimize the shrinkage force [32]. In the case that the composite material has the lower-molecular weighed monomer (e.g., triethylene glycol dimethacrylate, TEGDMA; 2-hydroxyethyl methacrylate, HEMA) in the matrix, the tendency of polymerization shrinkage decreases [33]. However, in addition to the composition of composite matrix, the amount of matrix and the existence of the functional monomer or modulator to reduce the polymerization shrinkage can influence the final volume of polymerization shrinkage. Although polymerization shrinkage stress must be considered a multi-factorial phenomenon [34], in this study, it is speculated that higher volume of the matrix might have mainly contributed the higher contraction and consequential higher microleakage of the giomer. [PERSON] et al. exhibited that giomer showed high microleakage, especially in enamel margin with self-etch adhesive [6]. [PERSON] et al. reported reduced marginal adaptation and increased marginal discoloration of class V restorations with giomer in a two-year clinical study [35]. In addition, for giomer, a higher microleakage was observed at the gingival margin than at the occlusal margin (\(p=0.001\)). This may be related to a difference between the bonding ability of enamel (occlusal) and of dentin (gingival). Dentin provides a weaker bond than enamel, and polymerization shrinkage in one increment may cause debonding at the weaker interface [36]. Previous studies reported that several composite resin restorations on non-cervical caries lesions (class V cavities) exhibited more microleakage on the gingival margins than on the occlusal margins because the flexural stresses at the cervical margins are much higher than those at the occlusal margins [37; 38].