## 1 Introduction  
Composite resins are extensively used in dentistry for both posterior and anterior restorations. Despite being more aesthetic with respect to previous materials, resin composite restorations still suffer from several shortcomings [1, 2]. In addition to polymerization shrinkage and secondary caries, plaque accumulation and color stability are some of the most relevant drawbacks [3]. Discoloration of composite resins can be caused by intrinsic or extrinsic factors. In light-curing composite resins, camphorquinone is generally used as the photoinitiator. However, if curing is inadequate, unconverted camphorquinone could cause a yellowish discoloration. Moreover, additional components of the photoinitiator system, e.g., tertiary aromatic or aliphatic amines acting as synergists/accelerators, also tend to cause yellow or brown discoloration under the influence of light or heat [4]. These internal discolorations are permanent and depend on intrinsic factors such as polymer quality, filler type, and quantity, as well as the synergism added to the photoinitiator system [5; 6; 7]. In addition to that, the discoloration can derive from a superficial degradation or a slight penetration and adsorption of staining agents at the superficial layer of composite resins [8]. The resin's affinity for extrinsic stains is influenced by its conversion rate and physico-chemical characteristics, with the water sorption rate being of particular importance [9; 10]. Additional important factors affecting stainability are surface roughness, surface integrity, and polishing technique. Previous studies concerning color stability have shown that drinks (such as coffee, tea, red wine, and cola) and mouth rinses have different degrees of staining effect on auto- and light-cured composite resin restorative materials; the staining potential of these substances differs on the basis of their composition and properties [11; 12; 13]. In recent years, ozone therapy has been extensively used in dentistry to manage wounds healing, dental caries, oral lichen planus, gingivitis and periodontitis, halitosis, osteonecrosis of the jaw, post-surgical pain, plaque and biofilms, root canal treatment, dentin hypersensitivity, temporomandibular joint disorders, and teeth whitening [14; 15]. Different in vitro studies have demonstrated an excellent anti-bacterial action for ozonized products against different microorganisms, such as bacteria, viruses, protozoa, and fungi [16; 17]. Moreover, ozone has shown an immunomodulatory, anti-hypoxic, biosynthetic, and anti-inflammatory action [18]. One of the most relevant applications of ozone in dentistry is represented by its use for the treatment of periodontal conditions, i.e., gingivitis and periodontitis. Scaling and root planning (SRP) is regarded as the gold standard non-surgical treatment, whose goal is to remove dental plaque and calculus and also to smooth the root surfaces infected by pathogenic microorganisms [19]. Besides this conventional mechanical therapy, local chemical agents can be used as antimicrobial substances, and these should be favored to systemic treatments such as antibiotics which are characterized by a systemic effect besides the risk of emergence of resistance. In light of these considerations, the application of ozone as an antimicrobial agent has been demonstrated to be very useful. Broad-spectrum antiseptics such as chlorhexidine have been generally considered, but some shortcomings are associated with this latter substance, such as tooth discoloration. Conversely, ozone guarantees a broad antimicrobial spectrum as well but with lower toxicity. It has been shown in vitro that the exposition of periodontopathogens to ozone causes the oxidation of the phospholipids and lipoproteins of the bacterial cell envelope, causing the destruction of the cell integrity, thus allowing ozone to infiltrate the microorganisms and oxidizing glycoproteins and glycolipids, with a final block of the enzymatic functions of bacterial metabolism [19].