## 4 Discussion  
Nowadays, ECC is one of the chronic infectious childhood diseases most widespread worldwide [2; 3]. The potential of the combined use of ozone with disinfect agents, i.e., chlorhexiidine, and remineralizing products, i.e., sodium fluoride and/or nano-hydroxyapatite [13; 23; 24; 25; 26] and/or glass ionomer cements (GICs) [27], in halting or slowing down caries progression in ECC-affected children is positively described in the literature, while its efficacy when used alone, as well as its ideal concentration and the time needed to reach a full antimicrobial effect, remain controversial [13; 23; 24; 25; 26; 27]. Ozone is an oxidizing agent whose activity is selective on pathogens and has no disadvantages or negative effects on human body cells [15; 16; 17; 18]. Its well-known disinfect and biostimulating properties are used in several medical fields, e.g., in chronic inflammatory disease management, thanks to its ability to activate the immune system and the antihypoxic and biosynthetic biological processes [15; 16; 17; 18]. In dentistry, ozone has several applications in dental caries and hypersensitivity treatment, the acceleration of mucosal healing, and the disinfection of root canals [15; 16; 17; 18]. The rationale of this study was that ozone, thanks to its bactericidal and analgesic properties [13; 15; 16; 17; 18; 19], improving the decayed dentin compactness and decreasing the salivary bacterial count, might arrest or slow down caries progression, reducing dentin hypersensitivity and thus improving patients' and families' quality of life. The lack of an evident gender prevalence among the enrolled patients (9 males vs. 11 females) contrasted with literature data reporting that boys are more affected than girls [2]. Data about the decayed dentin's compactness showed a significant decrease from T0 (mean value 3.2), when dentin was explorable by the dental explorer's tip, which produced marked grooves, to T1 (mean value 2.1), when it was only slightly explorable according to the ADS score description, remaining stable at the subsequent follow-ups. Thus, the primary null hypothesis tested was rejected. The increase in the decayed dentin's compactness between T0 and T1 may be explained by the ozone's oxidizing power, which can slow down the demineralization process and encourage dentin remineralization by inhibiting caroigenic bacteria and stimulating the production of a sound reactive denitnal barrier [12; 13; 14; 15; 16; 17; 18; 19; 20]. The maintenance of this improvement over time can be explained considering the stimulating effect of ozone on the formation of reactive dentin [12; 13; 14; 15; 16; 17; 18; 19; 20]. However, it is essential to specify that the advice given to families about at-home daily oral hygiene and diet may have biased the results, helping to improve the compactness of the treated decayed dentin over time. The authors should not have provided any advice to avoid this bias, but it would have been unethical. Ozone therapy also helped to reduce the ECC-related dentin hypersensitivity when present, leading to the rejection of the second null hypothesis. At T0, only 2 of the 20 enrolled patients complained of dentin hypersensitivity to compressed air spray (average value 3.5 measured according to the WBFPRS), and none of them complained about spontaneous symptoms. After the ozone therapy cycle, dentin hypersensitivity was resolved entirely and the improvement remained stable over time. According to the hydrodynamic theory, dentin hypersensitivity is due to the fluids' movement into the dentinal tubules, caused by external stimuli that stimulate the pulp pain receptors [31]. In this context, ozone can neutralize the acidic proteins produced by cariogenic bacteria, representing the osmotic stimulus responsible for the movement of fluids in dentinal tubules that leads to hypersensitivity [20; 21; 22]. It has to be reported that recording patients' dentin hypersensitivity was not always easy since they were not always able to provide reliable feedback due to their young age.