## 4 Discussion  
Although numerous works can be found in the literature comparing these two orthodontic therapies, answers to several questions are still missing, including whether the amount of apical root resorption is predictable, and whether therapy with aligners brings advantages in terms of apical resorption compared to fixed therapy. Therefore, a broader review is important. One of the most undesirable effects of orthodontic therapy is apical root resorption (ARR), which is a permanent loss of hard tissue on the root apex of a tooth. The incidence of ARR in orthodontic patients ranges from 20% to 100% [60]. Severe ARR is uncommon, with a frequency of 1 to 5%; however, resorption can be greater than 5 mm, or one-fourth of the root length [60]. ARR can result in an uneven crown-to-root ratio in the afflicted teeth, as well as tooth loss, thus compromising patients' quality of life and orthodontic treatment outcome [63]. Clear aligners are becoming increasingly popular for orthodontic treatment; nevertheless, ARR during clear aligner therapy is still poorly understood. Figure 4: Risk of bias: red indicates high risk of bias, and green indicates low risk of bias. [PERSON] et al. [64], [PERSON] et al. [60], [PERSON] et al. [19], [PERSON] et al. [31], [PERSON] et al. [44], [PERSON] et al. [35], [PERSON] et al. [62], [PERSON] et al. [63], [PERSON] et al. [63]. The ARR in patients treated with clear aligners and conventional fixed appliances was evaluated and compared in this study [63]. The type of tooth movement can contribute to an increased occurrence of root resorption [65; 66]. The removal of the hyaline zone is believed to be necessary to initiate natural tooth movement, but it can also lead to the absorption of the root's cement, making the exposed dentine more vulnerable to resorption by scavenger cells. To avoid or lower the chances of root resorption, it is suggested to use light and regular orthodontic forces instead of shorter and stronger ones. Some argue that intermittent forces, such as those exerted by aligners, versus continuous forces from fixed appliances, result in distinct types of pressure [67; 68; 69]. According to certain sources, intermittent pressure may permit the cementum around the root to repair itself. In a randomized controlled clinical trial, [PERSON] et al. compared the amount of root resorption six months into orthodontic treatment between fixed appliances and orthodontic aligners (OA) [63]. Intraoral X-rays of the front teeth demonstrated that both methods resulted in comparable root resorption rates that did not impact tooth longevity. Similar findings were reported by [PERSON] et al., who considered various clinical, genetic, and radiographic factors [44]. However, [PERSON] et al. suggested that external apical root resorption (EARR) was lower in non-extraction patients treated with OA compared to those treated with fixed appliances, indicating that OA might be more effective in safeguarding teeth from root resorption [61]. Given that ARR is a three-dimensional topographical change, the accuracy of ARR assessment is limited in two-dimensional radiography, such as in panoramic and periapical radiographs [60]. Cone beam computed tomography (CBCT), on the other hand, has demonstrated considerably higher accuracy in the diagnosis and quantification of ARR [60]. [PERSON] et al. conducted a study analyzing the prevalence and severity of root resorption using fixed orthodontic treatment and aligners. They used cone beam computed tomography (CBCT) to measure root resorption, defined as the difference, in millimeters, between tooth lengths before and after orthodontic treatment [35]. A total of 373 roots from 70 subjects were considered. The prevalence of root resorption was significantly lower in the clear aligner group (56.30%) compared to the fixed appliance group (82.11%).