## 4 Discussion
On the other hand, \(X\)-tra fil behaved differently with silane application prior to Admira Bond. Silane acted presumably as an intermediary that enhanced bonding between the two materials having dissimilar resin matrices, as stated previously. \(X\)-tra fil resin composite showed higher immediate repair bond strength compared to Admira Fusion X-tra, either at 24 h or 6-month storage periods, except when Admira Bond was used. It was previously reported that shear bond strength between dental materials depended on the substrate material strength, onto which the other material was applied [45]. It was shown that \(X\)-tra fil had superior mechanical surface properties compared to Admira Fusion X-tra [46; 47]. As tensile stresses generated near the application load point in shear test using the wire loop [48; 49], the strength of the material to resist such stresses could be of importance. This could interpret why the repair bond strength of \(X\)-tra fil was significantly higher than that of Admira Fusion X-tra. Admira Bond is an acetone containing adhesive with 30 s application time (Admira Bond technical product profile, VOCO GmbH). This relatively long clinical application time might result in a rapid evaporation of acetone, which has a high vapor pressure [43], and consequently a thick viscous adhesive layer [21] might have been formed on the surface of substrate resin composites. This thick layer might act as stress absorbing layer that resisted the tensile stresses generated during testing, leading to insignificant repair bond strength results between the tested bulk fill resin composites. Immediate repair bond strength dropped significantly after six months of storage, even with the use of silane prior to the adhesive application, except for Admira Fusion X-tra when treated with silane/Cimara adhesive. The hydrophilic nature of the adhesives used in this study could encourage the absorption of water [50; 51], which could lead to a degradation of the adhesive layer during storage [19]. Furthermore, silane bond could be degraded in the presence of water, which could be due to its hydrolytic instability [41]. The use of silane before the application of Cimara adhesive preserved the repair bond strength of Admira Fusion X-tra after six months of storage. It could be hypothesized that the innovation in the matrix technology of ormocer-based resin composite (Admira Fusion X-tra, technical product profile, VCOG GmbH), besides the long clinical application time of both silane (2 min) and Cimara adhesive (20 s) might provide an adhesive layer capable to resist the degradation during storage. This hypothesis may need further investigation. It was obvious that the percentage drop in bond strength was lower in Admira Fusion X-tra (1.6-50.6%) compared to X-tra fil (20.73-61.48%). This could encourage further research with longer storage time to detect if further deterioration in the repair bond strength of ormocer-based resin composite would occur. The results of this study confirmed that \"there is no one optimum repair technique that fits for all composites [22]\". It seemed that the universal adhesive used in this study can optimally repair methacrylate-based resin composites; while the ormocer-based adhesive is preferably used to repair ormocer-based resin composite. The application of silane is not necessary in immediate repair of methacrylate-based resin composite, but it enhanced the repair bond strength of ormocer-based adhesive to repair ormocer-based resin composite. Clinicians believe that immediate repair of resin composites is as simple as the application of resin composites in the layering technique. This concept should be reconsidered as the success of immediate repair of resin composite differs from one type to another and from an adhesion protocol to another.