## 3 Results  
The search strategy identified 18 relevant studies [2; 7; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31] with only 4 randomized studies [15; 24; 28; 29], 1 of which was of high quality and had a low risk of bias [28]. In turn, over 90% of the non-randomized studies included in the review had a high level of quality according to the Newcastle-Ottawa scale (NOS). Of these, half had a serious risk of bias. Only one study [19] had a low risk of bias. A total of 1090 participants took part in the analyzed studies. In 14 studies, Invisalign(r) clear aligners were used (Align Technology, San Jose, CA, USA). In the case of FA treatment, conventional brackets [2; 7; 14; 15; 18; 19; 20; 21; 24; 25; 26; 27; 28; 29; 30; 31], self-ligating brackets [21; 26; 27; 28; 29; 30; 31] and lingual systems [23] were applied. The studies were carried out in China [18; 19; 21; 27], Korea [2], India [20], USA [7; 28; 31], Germany [22; 23; 30] and Italy [14; 15; 24; 25; 26; 29]. Among participants, in four studies [15; 23; 28; 29], there were also minors included. The following malocclusions were analyzed in the studies: crowding in the front area [15; 17; 19; 24; 25; 26; 28], PTM [2], class II division 2 [21] and skeletal class II or III [27]. There were also studies in which malocclusion was not specified [7; 14; 18; 20; 22; 23; 29; 30; 31]. Only one study assessed people with diagnosed periodontal diseases [2]. In the other studies, participants had healthy periodontitis. Four studies assessed root apex resorption after orthodontic treatment [18; 19; 20; 21], and one study assessed occlusion stability [31]. The others assessed the periodontal condition, six of which also performed microbiological examinations [7; 15; 24; 25; 26; 29]. The available literature was analyzed in terms of the occurrence of root apex resorption, the possibility of treating \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Study** & **Confounding** & \begin{tabular}{c} **Selection of** \\ **Participants** \\ **for the** \\ **Study** \\ \end{tabular} & \begin{tabular}{c} **Classification** \\ **of the** \\ **Interventions** \\ \end{tabular} & \begin{tabular}{c} **Deviations** \\ **from Intended** \\ **Interventions** \\ \end{tabular} & \
\begin{tabular}{c} **Missing** \\ **Data** \\ **of Outcomes** \\ \end{tabular} &
\begin{tabular}{c} **Measurements** \\ **of Outcomes** \\ **Results** \\ \end{tabular} & **Total** \\ \hline [PERSON] et al. [18], 2020 & Moderate & Serious & Moderate & Low & Low & Low & Low & Serious \\ \hline Almagrami I & & Low & Low & Low & Low & Low & Low & Low \\ \hline [PERSON] et al. [20; 2021] & Moderate & Low & Low & Low & NI & Low & Moderate \\ \hline [PERSON] et al. [21] & Moderate & Low & Low & Low & NI & Moderate & Low & Moderate \\ \hline [PERSON] [2], 2015 & Moderate & Serious & Moderate & Low & Low & Low & Serious \\ \hline [PERSON] et al. [7], 2013 & Moderate & Low & Low & Low & Moderate & Moderate & Low & Moderate \\ \hline [PERSON] & & & & & & & & \\ ACP et al. [14], 2020 & & NI & NI & Low & Low & NI & Low & NI \\ \hline [PERSON] & & & & & & & & \\ and [PERSON] [22], 2005 & Serious & NI & Low & Low & NI & NI & Nl & Serious \\ \hline [PERSON] & & & & & & & & \\ and [PERSON] [23], 2007 & Serious & NI & Low & Low & NI & NI & Nl & Serious \\ \hline [PERSON] & & & & & & & & \\ et al. [25; 2019] & Serious & Low & Low & Low & Low & Low & Serious \\ \hline [PERSON] & & & & & & & & \\ et al. [26], 2020 & Serious & Low & Low & Low & Low & Low & Serious \\ \hline [PERSON] & & & & & & & & \\ FHK et al. [27], 2020 & & NI & Low & Low & Low & NI & Low & NI \\ \hline [PERSON] et al. [30], 2015 & Serious & Serious & Serious & Low & Low & Serious & Low & Serious \\ \hline [PERSON]. & & & & & & & & \\ et al. [31], 2022 & Serious & Low & Low & Serious & Moderate & Low & Serious \\ \hline \hline \end{tabular}  
NI—no information.  
\end{table} 
Table 6: Risk of bias of non-randomized studies according to ROBINS-I tool.