###### Abstract  
The external apical root resorption that occurs during orthodontic treatment was the focus of this study, using either fixed appliances (FA) or clear aligners (CA). Using the Boolean keywords \"APICAL ROOT RESORPTION\" and \"CLEAR ALIGNERS\", the study searched PubMed, Scopus, and Web of Science, with a restriction to English-language publications. A total of 50 publications were found by the computerized search, but after eliminating duplicates, completing reviews, and determining eligibility, only 9 papers could be used in the study. In conclusion, apical root resorption (ARR) is a frequent consequence of orthodontic tooth movement that mainly affects the lower incisors, which may compromise the success of the procedure and the health of the patient's teeth. Several variables, including the type of tooth, have an impact on the severity of ARR. apical root resorption; clear aligner; CBCT; fixed orthodontic appliances; root resorption +
Footnote †: journal: selected topics in Applied sciences  
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Footnote 0: _Applied Sciences_  
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Footnote 0: _Applied Sciences_  
resorption, it is suggested to use light and regular orthodontic forces instead of shorter and stronger ones. If root resorption occurs, it should be considered an irreversible process that can lead, in severe cases, to the extraction of the tooth (Figure 1) [19; 20; 21]. The type of orthodontic movement, depending on the degrees and directions of forces, and on the treatment duration, can influence apical root resorption [22]. External apical root resorption (EARR) consists of a loss of cementum or dentine accompanied by irreparable damage to the root structure, resulting in a reduction in the length of the root apex. This adverse event could cause tooth mobility and can reduce the viability of teeth [23; 24]. In fact, orthodontic tooth movement may result in a concentration of forces on the periodontium, mostly on the apical third of the root, causing a loss of protective cells on the surface layer which consequently causes a loss of root structure that, in some cases, can be linked with episodes of orofacial pain [25; 26; 27; 28; 29; 30]. In the first phase, the damage to this structure leads to exposed denuded mineralized tissue; in the second phase, this tissue is colonized by inflammatory cells with consequent bone resorption. If the inflammatory stimulus persists, root structures are damaged, as revealed by radiographic evidence [31; 32; 33; 34]. Cone beam computed tomography (CBCT), a three-dimensional radiography tomography, has a demonstrably higher accuracy in diagnosis and measurement of ARR (Figure 2) [35; 36; 37; 38; 39]. Being an irreversible process, it is fundamental to study this process carefully, along with its protective and risk factors [40; 41; 42; 43]. Apart from the potential advantages of better aesthetics and comfort, the potential of predictability, reproducibility, and objectivity of aligners will potentially allow for controlling the stress derived from orthodontic forces, especially in the third apical of the radicular area [44; 45; 46]. The best strategy for treating root resorption is to take risk factors into account, talk to the patient in need of orthodontic treatment about the factors that were found, and include these factors in the treatment consent form. Among these risk factors is the length of the course of treatment. The longer a treatment is administered, the higher the risk of root resorption [47; 48; 49]. Resorption is more likely to occur in roots with a thin, tapered, and dilated morphology [50; 51; 52; 53]. Furthermore, the risk of root resorption is elevated in cases of past trauma related to the anterior teeth [54]. There is a chance that root resorption from prior orthodontic treatment will cause additional root shortening [55]. In these cases, conservative orthodontic re-treatment should be used, and the scope of the treatment should be constrained. Root resorption may be more likely in people who have a history of chronic bruxism, occlusal trauma, or thumb sucking [56; 57].