# Air-Polishing Powders' Effect on the Color of CAD/CAM Restorative Materials
## 1 Introduction  
In recent years, patients' interest and expectations about prosthetic restorations have increased tremendously. In order to meet these expectations, the application of metal-free fixed restorations became very popular due to their superior biocompatibility, chemical stability, high color durability, lower surface roughness, and esthetical advantages [1, 2]. As a result, there has been a significant advance in material development. One of these advances is CAD/CAM systems, which offer a faster and more compatible indirect prosthetic treatment [3], and another one is the diversification of tooth-colored CAD/CAM blocks to develop the ideal material. Consequently, manufacturers introduced CAD/CAM restorative materials of glass ceramics, and feldspar ceramic was the first member of this group with its good aesthetic properties, high hardness, and low thermal conductivity [1]. Then, to strengthen the physical properties of feldspar ceramics, zirconia-, leucite-, or lithium disilicate-reinforced ceramics have become prominent. However, glass ceramics have many disadvantages, including wearing the antagonist tooth, the necessity of firing adequate strength, the difficulty of occlusal adjustment, and chipping [4, 5]. Therefore, CAD/CAM blocks of resin-matrix ceramics, which overcome these disadvantages and have further advantages, are produced [2]. For instance, they are easier to process, polish, and repair, have better marginal quality, and are more elastic than dental ceramics [6, 7, 8]. Resin-matrix ceramic blocks typically consist of methacrylate-based polymer matrices with a high ceramic content, including glasses, porcelains, and ceramics, thereby beingcategorized as a type of ceramics by the American Dental Association [9]. They can be categorized into two primary classes related to their micro arrangement: (1) resin with scattered fillers and (2) polymer-infiltrated ceramic networks [2; 10]. The first class consists of resin composite blocks, manufactured by integrating inorganic fillers, such as silica, zirconium, and barium, into an organic matrix composed of methacrylate monomers. The second class consists of blocks with polymer-infiltrated ceramic networks, which include mainly inorganic parts and the polymers infiltrated into them [2; 7; 10; 11; 12]. The aesthetic appearance of the restorative materials is very important for the patients, and color stability is crucial for an aesthetic smile. Most patients treated prosthodontically regularly undergo professional teeth cleaning, in which the stains and the plaque deposits are removed [13]. During this procedure, polishing is usually performed to sustain color stability and to make a more glossy and smoother surface after initial periodontal therapy [14; 15]. In addition, polishing is also applied to remove the biofilm layer during supportive periodontal therapy (SPT) [16]. SPT includes all parts of a typical dental recall examination, periodontal re-evaluation, removal of bacterial plaque and calculus, and re-treatment of any parts with recurrent disease [17], and patients with high risk are advised to be recalled three to four times a year for preventive care [15; 18]. During these recall appointments, apart from conventional techniques like using ultrasonic scalers and curette, air-polishing can be applied to remove bacterial plaque and stains [19; 16]. Air-polishing provides a safer, faster, more compatible, and non-heat-producing option for patients [16; 20; 21; 22]. Air-polishing devices work with air pressure, water, and abrasive powders [14]. When these abrasive air-polishing powders (APPs) are sprayed using the air-polishing instrument, dental plaque and stains are removed with kinetical energy [23; 24; 25]. Among the APPs, sodium bicarbonate, with a particle size of up to 250 \(\upmu\)m, was the first to be introduced to the market [26]. However, according to the manufacturer's instructions, its use should be avoided in patients with kidney disease and those who have inconveniences with a salty diet, and its taste is not pleasant.