# Microbiological Changes during Orthodontic Aligner Therapy: A Prospective Clinical Trial
## 1 Introduction  
Oral microbiota consists of more than 700 different kinds of microorganisms, including bacteria, viruses, mycoplasmas, fungi and protozoa, which inhabits the human mouth establishing an equilibrium with consequent benefits both for the microorganisms themselves and for the guest [1]. One of the major advantages brought by the resident microbiota is represented by the prevention of colonisation by pathogenic microorganisms, which, conversely, would lead to a state of \"dysbiosis\"; this event is favoured by different conditions, such as plaque accumulation, the alteration of immune defences of the subject and antibiotics therapies [2].  
Bacterial plaque accumulation is the leading cause of gingivitis, an inflammatory process of the gums that, if untreated, might evolve into periodontitis affecting the soft and hard tooth-supporting tissues. The latest classification of periodontal diseases dates back to 2017 [3] and it proposes the diagnosis of periodontitis according to stages (from stage Ito stage IV, based on the severity and complexity of management) and grades (from grade A to grade C, according to the evidence or risk of rapid progression).  
The bacteria most related to periodontitis development have traditionally been considered to be _Porphyomonas gingivalis_, _Troponema danticola_ and _Tannerella forsythensys_, constituting the so-called \"red complex\", despite the fact that this classification is quite no more actual [4].  
The insertion of fixed orthodontic appliances in the mouth, aimed at the replacement of teeth in a correct position, actually provides the presence of more sites for bacterial adhesion and biofilm formation (plaque accumulation) [5], with an eventual progression toward the abovementioned inflammatory processes [6]. In addition to that, the increase in bacteria is also referable to the difficulty in realising proper oral hygiene caused by the presence of the appliances [7]. Accordingly, the use of removable clear aligners has also been proposed with the pros of facilitating teeth brushing in orthodontic patients avoiding the risk of white spot lesions as well as gingivitis and periodontitis [8; 9]. Despite contrasting results, studies have generally shown a reduced plaque accumulation and periodontal risk in patients who underwent CAT (Clear Aligner Treatment), if compared to fixed appliances therapies [10; 11; 12]. However, it has not been widely investigated if the risk of the abovementioned factors is similar in patients undergoing CAT with respect to patients not under treatment.  
Therefore, the purpose of this clinical trial is to compare the variations induced after two months of CAT on periodontal status and microbiological composition, compared to people not under orthodontic treatment. The first null hypothesis is that there is no statistically significant difference, neither for periodontal nor for microbiological parameters, between T\({}_{0}\) and T\({}_{1}\). The second null hypothesis is that no significant difference occurs between the trials and the controls considering the abovementioned parameters.