## 4 Discussion
This unique property of the bulk fill resin composites was not observed in the non-bulk fill (conventional) resin composites, as the latter showed no significant difference in the degree of conversion initially or after 24 h [33] and exhibited a fast transformation from gel phase to glass phase during photo-polymerization [40]. The use of silane in this study prior to the application of the universal adhesive did not improve the repair bond strength of \(X\)-tra fill, but it had a significant positive influence on the repair bond strength of Admira Fusion X-tra compared to universal adhesive without silane pre-treatment. The difference in filler size between both resin composites might be the reason of these findings. \(X\)-tra fil is a micro-hybrid resin composite with its fillers reaching up to 10 \(\upmu\)m in size [40]. On the other hand, Admira Fusion X-tra is a nano-hybrid resin composite with fillers size ranging from 0.1 \(\upmu\)m up to 5 \(\upmu\)m [39]. The smaller the filler size, the higher the surface area available for bonding, which might enhance the interaction of silane with the exposed fillers. Subsequently, this could also improve the adhesion of the universal adhesive to resin composite surface. The proposed explanation still warrants further investigation. The bond strength results of Admira Fusion in this study were in disagreement with the results of [PERSON] et al. [19; 21] It was reported that silane enhanced the immediate repair bond strength of nano-hybrid resin composite, when its surface was wet rather than dry condition [19]. In the presence of water, silane could efficiently react with the glass filler's hydroxyl groups to form highly cross-linked siloxane groups [41], which might lead to improvement in the chemical bond between substrate and repair composites via the intermediate adhesive layer [21]. Although the surface of Admira Fusion was repaired in dry condition in the study of [PERSON] et al. [21] as in the present study, the application of silane yielded different results. The lower immediate degree of conversion of Admira Fusion X-tra [33] results in slow transformation rate from gel phase to glass phase during photo-polymerization with slow rate of polymerization initiation [39]. This might be the reason behind the presence of monomer sites of attachment just after photo-polymerization; that could lead to strong bonding between silane and resin matrix, resulting in significantly higher bond strength. The results of methacrylate-based (\(X\)-tra fil) composite came in disagreement with the results of [PERSON] et al. [42]. They reported that silane pretreatment of bur treated methacrylate-based bulk fill resin composite enhanced the repair bond strength, and the use of abrasive bur/silane surface treatment could be an alternative treatment in situations where intraoral sandblasting is not available. The difference in the tested bulk fill resin composites might be the reason of such disagreement. Interestingly, silane showed a significant reduction in repair bond strength of Admira Fusion X-tra when applied before Admira bond. However, it showed a numerical increase in bond strength of X-tra fil. According to manufacturer's instructions, Admira Bond was applied over the surface and left undisturbed for 30 s. Admira Bond contains acetone as solvent, which has a high vapor pressure and evaporation rate [43]. Furthermore, air dryness of Admira bond was performed gently for 5 s. Leaving the adhesive layer for 30 s could be considered a sufficient time to permit the evaporation of the highly volatile acetone from the adhesive layer. This might have created a thick adhesive layer over the silane layer. The presence of thick multilayer formed from silane and adhesive might create a multiphase thick layer, resulting in a weak bond [44]. It could be also assumed that the application of an intermediate layer (silane) had restricted the direct chemical interaction between Admira Bond and Admira Fusion X-tra.