## 1 Introduction  
In restorative dentistry, there is a growing trend of repairing defective resin composite restorations rather than removing and replacing the complete restoration. According to FDI criteria, which were improved and modified in 2010, composite restoration repairs may be considered the treatment of choice for several intraoral occurrences, such as chipping; partial or marginal fracturing of restorative material; unacceptable severe staining; secondary caries in accessible locations; the replacement of only one approximinal box of Class II MOD restorations; and instances in which the complete removal of an extensive composite restoration would endanger the health of a tooth, such as in composite laminates [1]. Furthermore, restoration repair has gained additional importance due to the worldwide acceptance and implementation of minimally invasive dentistry. There is, however, a possibility that the repair procedure may result in an unacceptably weak restoration. Therefore, an excellent bond between the old and the fresh resin composites is necessary to successfully repair the restorations. In general, bonding between two fresh composite layers is accomplished in the presence of an oxygen-inhibited layer. However, aged restorations do not have this layer of unreacted monomers on their surfaces. Therefore, repairing aged composites using fresh composites remains a challenge due to the exhaustion of free radicals in the aged composite [2; 3]. Several surface conditioning methods have been suggested to improve the composite-to-composite bond strength when adhesive systems are used [3; 4; 5; 6]. These methods include the use of bur roughening of the aged composite surface, silane pretreatment, air abrasion, and application of silane agents, as well as combinations of these procedures [4; 5; 6]. All these methods increase the reactive surface area for adhesive resins, thereby improving bond strength. Recently, universal adhesives have been developed and become popular among dentists due to their wide range of application modes, such as self-etch mode, etch-and-rinse mode, or selective enamel etching [7; 8; 9]. Most of these adhesives contain 10-methacryloyloxy-decyl-dihydro-phosphate (10-MDP) as a functional monomer and provide very intense and stable interactions with calcium in hydroxyapatite. Some universal adhesives have silane incorporated into their formulations, in addition to 10-MDP [10]. Silanes rebuild the inhibition layer for safe bonding between old and new resin composites. They also improve surface wetting and chemical bonding between the aged composite and the new composites by activating the inhibition layer. Although the use of so many additional primers with universal adhesives is not suggested when bonding to tooth structures, incorporating silane into these adhesives may help to increase the repair bond strength and eliminate the need for silane pretreatment [11; 12]. The complexity of achieving a strong bond between aged and new composites calls for an in-depth exploration of the forces at play, which is where finite element analysis (FEA) becomes an invaluable tool. FEA allows researchers to simulate and analyze the stress distributions within restorative materials, highlighting potential failure points that could compromise the integrity of the repair [13]. By understanding these distributions, modifications in material choices or application techniques can be made to optimize bond strength and reduce failure rates [14]. In this way, FEA supports the investigation of universal adhesives' effectiveness and helps identify promising avenues for improving restorative practices. Even though there are several studies [15; 16; 17; 18; 19; 20; 21; 22; 23] that present alternative methods of composite repair, there is still no consensus as to the best treatment. Thus, the present study aimed to assess the efficiency of different adhesive systems on the repair bond strength of new resin composite bonded to aged resin composite and evaluate the silane effect on the repair bond strength.