## 1 Introduction  
Composite resins are the preferred restorative materials for anterior and posterior direct restorations due to their repairability and optical properties [1]. With advancing technology, the color match of composite resins has improved, color stability has increased, and it has become easier to mimic the natural tooth form [2]. The filler content and filler particle size affect the mechanical properties of composite resins. Nanocomposites and microhybrid composites are widely used composites due to their esthetic and mechanical properties.  
One of the main challenges for composite resins is discoloration over time [3]. The intrinsic factors affecting the discoloration of composite resins are related to the properties of filler particles and resin matrix, degree of conversion during polymerization, and water absorption by the material [4]. Moreover, microcracks and microvoids that occur between the filler and resin matrix also lead to stain accumulation over time and cause discoloration. Extrinsic reasons for discoloration are mostly related to polishing procedures [5]. Extrinsic reasons for discoloration are related to oral hygiene, smoking, or eating habits [6; 7]. Coffee, in particular, is one of the beverages that most commonly causes discoloration due to its widespread consumption. According to some studies, coffee immersion showed the most discoloration, followed by tea and red wine. Calorimetry, a spectrophotometer, and a digital camera are used to measure the discoloration of teeth and composite resins [8; 9]. CIE Lab* and CIEDE 2000 are the most commonly used color systems for dental color assessments. CIEDE 2000 formula contains three weighting functions: chroma, lightness, and hue, and is a good indicator of color changes [10].  
The color stability of materials and teeth is important in line with increasing aesthetic expectations. Many chemical and mechanical whitening techniques are used to remove the discoloration [11; 12]. In addition, many patients prefer whitening toothpaste for its reasonable price and easy application without a dentist's supervision [13]. Chemical agents such as hydrogen peroxide, physical agents such as abrasives, optic agents such as blue covarine, surfactants, and calcium chelators, and enzymes such as bromelin are added to the formulas of whitening toothpaste to remove discoloration [14]. Toothpaste containing blue covarine modifies the apparent color of teeth by providing blueish pigment on the tooth surface and making teeth whiter [15]. The di-structured form of pyrophosphate, a calcium chelator, is effective in removing dental plaque and whitening [16]. Abrasive particles, which are also added to toothpaste to provide the whitening feature, may change the surface properties of composite resins with tooth brushing and cause a rougher surface. The most commonly used abrasive particle in toothpaste is silica. During brushing, abrasive particles of toothpaste lodge between the bristles of the toothbrush and the tooth surface, and the external discolorations on the tooth are removed by the brushing action. However, since the bristles do not abrade or smooth the composite during brushing, the surface hardness of the composite resin may deteriorate and a rougher surface may occur. A recently introduced polishing toothpaste (PRO Polishing, R.O.C.S., Moscow, Russia) was recommended by the manufacturers to be used once a week in combination with other kinds of toothpaste and was stated to have whitening properties while reducing surface roughness.  
This study aimed to assess the level of bleaching and abrasiveness for different kinds of toothpaste with various relative dentin abrasion (RDA) values on nanohybrid and microhybrid composite samples using a novel Press-on Force-Guided (PFG) brushing simulator. The (h\({}_{1}\)) hypothesis of the study was that the use of polishing toothpaste reduces surface roughness while creating a whitening effect on discolored composites.