The aim of this study was to evaluate the bond strengths of composite restorations made with different filler amounts and resin composites that were photoactivated using a light-emitting diode (LED). Thirty bovine incisors were selected, and a conical cavity was prepared in the facial surface of each tooth. All preparations were etched with Scotchbond Etching Gel, the Adper Scotchbond Multipurpose Plus adhesive system was applied followed by photoactivation, and the cavities were filled with a single increment of Filtek Z350 XT, Filtek Z350 XT Flow, or bulk-fill X-tra fil resin composite (n = 10) followed by photoactivation. A push-out test to determine bond strength was conducted using a universal testing machine. Data (MPa) were submitted to Student's t-test at a 5% significance level. After the test, the fractured specimens were examined using an optical microscope under magnification (10x). Although all three composites demonstrated a high prevalence of adhesive failures, the bond strength values of the different resin composites photoactivated by LED showed that the X-tra fil resin composite had a lower bond strength than the Filtek Z350 XT and Filtek Z350 XT Flow resin composites.

Background Due to the enormous demand for conservative and aesthetic restorations [1], fiber-reinforced composites (FRC) have recently been advocated as an alternative to fixed metal framework prostheses. Compared to metal prostheses, FRC restorations are lighter and more esthetic [1]. In addition, the restorations can be effectively adhered to dental tissues and cause less damage to remaining teeth [2]. Although the durability of this type of prosthesis is inferior to metal frameworks, much less time and cost are associated to its placement [2]. FRC's durability has been reported differently in the related studies, so that the overall durability rate of 75-94.75 percent has been reported after three to 5 years [1, 3-5]. The use of composite materials in metal-free fixed partial dentures (FPDs) became feasible following the introduction of fiber reinforcement [6]. FRC-based FPDs have good resistance to masticatory forces [6] in addition to their low cost, improved aesthetics, reduced weight, and favorable elastic modulus [7, 8]. Prosthetics manufactured from ceramic material have superior color stability and wear resistance, exhibit marginal adhesion to tooth structure, and have the potential to damage unrestored opposing teeth [9]. FRC materials are currently used not only for crowns and inlays, but also