## 1 Introduction  
Nowadays, due to the abandonment of amalgam use, direct resin composites have become one of the most widely used restorative materials to restore damaged tooth structures [1]. Many efforts have constantly been deployed to improve resin composite material quality in order to increase the longevity of such restorations. Researchers and manufacturer developers seek not only to improve the quality of the material but also to achieve an acceptable clinical time. In this regard, simplified adhesives [2], flowable [3] and regular viscosity bulk fill resin composites [4] have been launched in the market to decrease the clinical procedural steps and accordingly shorten the restoration time. Flowable and regular viscosity bulk fill resin composites can frequently be used to restore class II cavities, as they showed equivalent marginal quality compared to conventional ones [5].  
Restoration defects can be detected immediately in the form of sub-margins, under contours or in the presence of voids [6] that would necessitate instant repair, which could be deemed a \"minimallyinvasive approach\" [7] to correct such defects. Studies on adhesion should not be limited to restorative material/hard tissue interface, but they should also be extended to evaluate the adhesion between restorative materials [8], as defects are not always at the restoration/tooth interface--they could also be confined within the restoration itself.  
Different surface treatments, mechanical, physical, chemical or a combination of all were evaluated extensively in previous studies [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Among the techniques proposed to improve the repair bond strength of composite resins, mechanical roughening with diamond burs was recommended to eliminate the superficial layer and create irregularities in order to increase the bonding surface area [11; 15; 16; 21]. Since the old composite no longer has its oxygen inhibited layer, the use of a silane primer as an intermediary agent enhances the wetting of fresh resin composites to aged composite substrates, promotes chemical bonding of resin to filler particles and increases the flow of low-viscosity adhesives on irregular surfaces [9; 17; 19; 21]. Additionally, the application of bonding agents could be responsible for chemical bonding to organic matrix of the repaired resin composite [11; 13; 14; 19]. Universal adhesives were recently introduced to enable bonding to various substrates, such as dental hard tissues, ceramics, resins and metals, without the need for separate primers [13; 21]. However, it was reported that there was no universal surface treatment to repair defective resin composite materials, as it was shown to be material dependent [22]. Furthermore, the variously proposed repair protocols were based on studies with \"low level of evidence\" [23].  
Recently, several studies [24; 25; 26; 27; 28] focused on the repair of methacrylate-based bulk fill resin composites, either with conventional resin composites or with the same bulk fill materials. They concluded that methacrylate-based bulk fill resin composites could be efficiently repaired with both materials. On the other hand, and to the best of our knowledge, the repair potential of ormoreer-based bulk fill resin composite did not gain much attention in former studies.  
The aim of this study was to evaluate the effect of different surface treatments and storage time on immediate bond strength durability of methacrylate- and ormoreer-based bulk fill resin composites. The null hypothesis tested was that neither surface treatments nor storage time has any effect on immediate bond strength durability of methacrylate- and ormoreer-based bulk fill resin composites.