The modern concepts of adhesive dentistry always have driven dental resin composite (RC) to lead as an esthetic restoration serving both anterior and posterior teeth [1,2]. However, servicing RC restoration that can fulfill the criteria of success in both esthetics and functionality is the target for all dental professionals. Unfortunately, the technique sensitivity and the adaptability of composite resin to the prepared cavity walls have been always an obstacle faced by dental professionals.

Preheating of RC is a technique that is used to reduce the viscosity of the material, making it easier to adapt to cavity preparation walls, increase the degree of conversion in addition to the rate of polymerization, aiding in better cross-linking abilities and decreasing the polymerization shrinkage of the RC material [3,4]. By decreasing the viscosity and securing the free movement of monomer, preheating of RC reduces the shrinkage stress that occurs through the polymerization process [5]. Also, this significant impact on the rheology of the RC material, in turn, secures better monomer mobility in the resin matrix, which elevates the polymerization reaction rates with better curing quality. This high level of polymerization always enhances both mechanical and physical properties [5], [6], [7], [8]. Despite these findings, dentists are hesitant to use this technique because there are no enough clinical studies that can support this technique in real functioning RC restorations besides the believe that preheating procedure may adversely affect the properties of the RC [9,10].

Clinical based studies have always been created to cover different aspects that are not covered in the in vitro studies. The combined effect of occlusal load, saliva, pH, changes in temperature, different enzymes, and bacterial colonization, on dental restorations, could alter or suggest changes to the protocol dental professionals are dealing with different restorative materials. However, finding a protocol to standardize the way how dental professionals can assess the clinical performance and success of various dental restorations, was one of the main targets to achieve predictable and standardized dental service. Fortunately, the innovation of the U.S. Public Health Service (USPHS) criteria and its modification always play a significant role in the assessment of the dental restorations' clinical performance and further judgment of the applicability of different dental restorations [11], [12], [13], [14].

Therefore, this study aimed to contribute additional evidence to the existing studies by evaluating the effect of different numbers of preheating cycles “ten times”, that may be equal to the average number of cases that could consume a single RC syringe, on the clinical performance of class II RC restorations over a 12-month period. The designated null hypothesis postulated that there would be no notable discrepancy in the clinical outcomes of preheated RC compared to non-heated RC over a 12-month period in class II restorations. This research addressed the following question: Does the clinical performance of preheated RCs in class II restorations surpass that of non-heated RCs using the modified United States Public Health Service criteria USPHS?