## 4 Discussions
Moreover, the normal movement of the oral cavity causes friction between the hard and irregular surface of the metal brackets, which can cause microlesions that can then cause an increase in oxidative stress and the production of free radicals [16]. Finally, during sleeping hours, both types of devices come into intimate contact with the oral mucosa, being pressed into it by the normal sleeping position. While the thermoplastic aligners have a smooth and regulated surface that is better tolerated by the oral mucosa, the metallic brackets can cause mechanical lesions to the mucosa that, ultimately, lead to inflammation. Aligner wearers registered similar values compared to the control group except for a non-significant increase in the inducible nitrite. Since inducible nitrite is a quantification of free radicals and, thus, a measure of oxidative stress [17], the results suggest the minimal invasiveness of aligner devices when compared to fixed metal devices. The level of oxidative stress expressed by the level of inducible nitrite returns to values close to baseline after a period of 60 days for both treatments. Research investigating rapid nitrite quantification identified a positive correlation with salivary flow and caries incidence, especially in patients with high levels of daily stress [18; 19]. A recent study assessed thermoplastic aligners that have self-ligating brackets with low friction and force exertion, regarding the oxidative stress of the oral cavity. The authors evaluated the patients at 30 and 90 days of treatment and highlighted an increase in oxidative stress in the first 30 days of treatment, followed by a return to base values after 90 days. The authors concluded that both methods impact salivary oxidative status similarly and note the importance of friction and other forces exerted on the saliva oxidative status [20]. Another recent study assessed the manifestation of oxidative stress in patients treated with fixed metallic appliances. The authors measured ROS in the patients' blood and the antioxidant defence (AD) and found that orthodontic treatments with fixed metal appliances can induce short-term systemic oxidative stress. This study highlights that fixed metallic appliances can cause microlesions that can lead to the formation of free radicals and shows that the shift of the oxidative status is traceable in other bodily fluids [21]. As microlesions form, there is a shift in the oxidative status of the oral cavity which favours the release of oxygen and nitrogen reactive species. As this phenomenon continues in time, it is possible that the oxidative status shift can be detected in other body fluids [21]. Additionally, metal brackets are also known to release nickel [22] and chromium [23]. This phenomenon further contributes to the shift of the oxidative status towards the formation of free radicals, further impacting all oxygen and nitrogen reactive species levels. The alternatives to orthodontic treatment with vestibular appliances (those used in research) and aligners are the lingual technique and the self-ligation system. They also involve appliances with brackets bonded to the tooth using different mechanisms. We consider that aligners are the only real replacement of the classic system. **Limitations:** A nitrate-rich diet together with the activity of commensal nitrate-reducing bacteria also increase salivary nitrate/nitrite levels by excretion or local production. The confounding effect of diet-derived nitrate could not be eliminated, as none of the patients followed a low-nitrate diet prior to saliva sampling. The small sample size was another limiting factor, as well as the overall lack of high-quality clinical trials working with saliva for the quantification of reactive oxygen and nitrogen species. In our study, the rapid test showed an increase in total nitrite in metal appliances wearers similar to ELISA for both the T\({}_{1}\) and T\({}_{2}\) moments, as demonstrated by the statistical analysis.