Introduction

The capabilities of fixed orthodontic appliance (FA) treatment paved the way to making it the most popular orthodontic appliance [1]. However, acceptance by the patients is often hindered by the appliance’s appearance and the patient’s ability to maintain proper oral health [2,3,4]. The rapid transformation in the dental sector hastens the shift toward patient-centered approaches and makes it mandatory to introduce new orthodontic appliances that meet both treating physician and patient needs. Clear aligner (CA) therapy has recently been introduced in orthodontics as a more esthetic and comfortable alternative to FA. The Align Technology 3D planning software “ClinCheck” enables practitioners to digitally plan treatment and preview all teeth movements up to the final result. Recently, CA has been used to treat a wide range of orthodontic cases, such as mild or moderate crowding, deep overbite, distalization, closing or opening of space, rotation, arch expansion, and others [5]. Nonetheless, its accuracy in clinical practice and compatibility with the designed movements on the software is still debatable in the literature [6]. Despite the limitations of both appliances, previous literature has shown that both appliances could be effectively used to treat mild and moderate crowding cases [7].

The concept of orthodontic tooth movement (OTM) in terms of both applications is based on bone remodeling theory and is mediated by the action of osteoclasts and osteoblasts through bone resorption and formation. The process, also known as “anabolic and catabolic therapeutic actions of the bone,” starts with the catabolism of bone on one side followed by anabolism on the opposite side of the intended-to-move tooth [9, 10]. Therefore, the proper understanding of physiological changes accompanied by OTM will prevent any iatrogenic sequelae in the periodontium, such as dehiscence and fenestration [11].

Another common iatrogenic effect of OTM that may jeopardize treatment success and tooth longevity is apical root resorption (ARR), which is defined as a physiological or pathological process that results in the shortening of the root apex [12, 13]. Orthodontically induced inflammatory root resorption (OIIRR) occurs in almost all cases, and its extent varies from one tooth to another [14]. Previous literature has shown an increased tendency for resorption in maxillary incisors and second premolars due to extensive tooth movement [15,16,17]. In addition to type and degree of OTM, other factors, such as genetics, previous trauma, age, nutrition, certain features of malocclusion, extraction/non-extraction treatment, type of appliance, type of applied force, and duration of treatment, may contribute to root resorption [12, 18]. Although many investigators have studied the prevalence and severity of OIIRR in orthodontic patients, it is still a complicated process that is not fully understood [19, 20].

According to a recently published umbrella review, it was found that there is an increase in OIIRR in maxillary incisors with FA therapy [21]. In comparison with CA, a recent systematic review by Gandhi et al. reported a non-significant difference between the two systems except for the maxillary right lateral incisors that showed less root resorption with CA [22], while other studies found that CA has a lower prevalence and severity of root resorption than FA [23,24,25]. These results should be interpreted with caution, as there was a difference in treatment duration, used method of detection (periapical, CBCT, cephalometric radiographs, orthopantomogram, and/or microscopic examination), and mechanism of action [7].

Diagnostic accuracy differs according to the used method of detection [26]. Previous studies reported that conventional two-dimensional (2D) radiography has inherent limitations due to magnification and distortion in contrast to CBCT, which helps to quantitatively assess alveolar bone and root length with high accuracy and precision [27], besides being highly reproducible [28] and exhibiting excellent accuracy and sensitivity [29]. Nonetheless, when using CBCT, special precautions should be taken, such as selecting a limited field of view CBCT and using protective devices to ensure patient safety [34].

In terms of the inter-group comparison, current study findings showed a significant palatal and total ABT reduction in both central and lateral incisors in the FA group when compared with the CA group. The study results suggest that the forces exerted by the FA group were heavier because of its nature as bodily movement with more control over root torque, while in the CA group, the applied forces were much less, which occurred as a result of the tip** movement.

Regarding ABH changes after orthodontic treatment with both FA and CA, this study’s findings showed that labial and palatal ABH significantly increased in both groups. No statistically significant difference was found in the inter-group comparisons, indicating marginal bone level was compromised in both groups. These findings match the results of previous studies [35, 36]. The highest marginal bone resorption in this study was found in the labial side of lateral incisors in the CA group (2.01 ± 3.21 mm). Increased alveolar bone height above 2 mm is termed bone dehiscence. According to Sheng et al., post-treatment maxillary alveolar bone dehiscence increased by 19% in patients treated with fixed appliances [35]. Zhang et al. [36] found in their recent study that the mandibular labial and lingual alveolar bone resorption in CA patients was 0.78 and 0.34 mm, respectively, indicating that the labial alveolar bone loss was greater than lingual alveolar bone loss, which is consistence with our study findings. In this study, the labial and lingual alveolar bone loss in the CA group was 1.64 and 0.57 mm, respectively, while it was 0.89 and 1.43 mm, respectively, in the FA group. The possibility of a spontaneous repair of the alveolar bone defect caused by orthodontic treatment is still unknown. Lee et al. [37] found that if the post-orthodontic treatment marginal bone resorption was less than 1 mm, regeneration of bone to its original condition is possible. Therefore, preventing the occurrence or aggravation of alveolar bone resorption is crucial.

Previous studies have reported a correlation between the development of alveolar bone defects and alveolar bone thickness on the corresponding side and a correlation with the type of tooth movement and inclination [33]. For example, continuous tooth movement in the case of maxillary incisors retraction at the point at which the tooth apex is near or in contact with the bone cortex (thin labial bone) might result in the creation of an alveolar bone defect if the remodeling rate of the alveolar bone is slower than the rate of tooth movement. Therefore, it is essential to ensure that the root apex remains within the alveolar bone during orthodontic movement of the maxillary incisors; if not, labial bone fenestration and lingual alveolar bone resorption might occur during retraction of the incisors. Hence, prior to starting the treatment, the incisor position within the alveolar bone must be determined, and the tooth movement type must be carefully planned.

The inclination changes and the anteroposterior root apex movement are factors associated with labial and lingual marginal alveolar bone resorption. When the root apex moves closer to the cortical bone, greater alveolar bone loss on the same side tends to occur [36]. In our study, the root apex of the maxillary incisors moved more labially in the CA group, and average tooth inclination decreased significantly, which was associated with more labial marginal bone resorption. While in the fixed appliance group, the root apex moved lingually and average tooth inclination slightly increased, resulting in more lingual marginal bone resorption.

Concerning OIIRR, the intra-group comparison showed significant root resorption in central and lateral incisors in both groups (p < 0.000). At the same time, inter-group comparisons showed that the prevalence and severity of OIIRR in the CA group (68.50% and 0.31 ± 0.42 mm, respectively) were significantly less than those in the FA group (82.50% and 0.62 ± 0.54 mm, respectively). Previous studies reported mean OIIRR for comprehensive treatment with FA ranging from 1.36 to 1.42 mm [16, 17, 38]. A study used periapical radiographs found that the mean external root resorption of maxillary incisors treated with FA was 2.26 mm [39]. Another study used CBCT found that patients who received FA had mean resorption in maxillary incisors of 0.59 mm [40]. In our findings following FA, maxillary central and lateral mean root resorption values were 0.62 mm. Regarding CA, 2 mm maxillary incisor ARR was reported on the periapical, panoramic, and cephalometric radiographs [41]. While using CBCT, another study reported a mean value of maxillary incisor root resorption of 0.51 mm [42], and another study found that maxillary incisors length was shortened by 0.13 mm [23]. Our study found that after CAT, the mean value of maxillary incisor root resorption was 0.31 mm. The conflicting results concerning the outcome of OIIRR might be due to the difference in the selection of imaging tools, the magnitude of applied force, sample size, and the type of selected appliance.

Compared with FAs, previous studies have reported that CAs are associated with a lower risk of OIIRR due to the applied light and intermittent forces programmed in the aligners [24, 25]. FAs apply heavier forces than CAs and control the labiolingual inclination of the incisor by controlling the incisor root torque, a significant predictor for external root resorption [43]. Therefore, the probability of root healing by encouraging the cementum repair process is greater with CAT than with FAT. Additionally, treatment with CAs is subject to individual compliance, in which noncompliance leads to more intermittent force delivery and a shorter duration of force application, resulting in less root resorption [25].

CBCT imaging provides a great diagnostic and assessment modality for alveolar bone dimensional changes and root resorption. A study compared buccal bone height and thickness measurements on both CBCT and direct measurements on cadavers found strong agreement [27]. Another study found a simple non-significant risk of fenestration and dehiscence overestimation on CBCT [44]. However, this finding has no conflict with our study since it reminds us that even if the situation is not so bad, we must remain cautious. Generally, external root resorption is hard to detect with 2D radiographs when the root resorption is minimal or resorption at one aspect (mesial, distal, buccal, or mid-apical) [45]. Furthermore, root resorption values were found to be overestimated and less accurate with 2D than with CBCT [22, 45]. It has been reported that panoramic radiography may overestimate the prevalence of ARR by 20% compared with periapical radiography [16]. Moreover, according to Gandhi et al. [22] CBCT shows a decreased magnitude of ARR than 2D radiographs, and that is why 2D radiographs may overestimate the amount of ARR with orthodontic treatment. Also, it was reported by Deng et al. [45] that the ARR value of CBCT was lower than the 2D as a result of magnification and distortion in 2D images.

One of the limitations of this study was the sample size; although calculated in advance and the resulting sample size was used, the main challenge that prevented increasing the sample size was equating the baseline characteristics of the two groups based on the ABO DI and matching of pre-treatment measurements. Hence, future studies with a larger sample size may provide more information on the dimensional changes of alveolar bone and root resorption during orthodontic treatment with FAs and CAs. Another limitation of this study is that data analysis was carried out on immediate post-treatment CBCT images; therefore, we cannot state whether or not the orthodontic treatment sequel of both treatment modalities undergoes spontaneous reparative changes over time unless a comprehensive long-term assessment is done; this is recommended for future similar research. Although the 3D identification of the apical root area is accurate and valid [8, 26, 46, 47], volumetric measurements using manual segmentation might give a better presentation of root resorption and are recommended in any further research.

Conclusion

Within the limitations of this study, it was concluded that

  • FA treatment causes a significant decrease in alveolar bone thickness of both maxillary incisors, particularly on the palatal side, while CA treatment resulted in a significant labial bone thickness reduction of the maxillary lateral incisors.

  • Treatment with FA and CA causes significant labial and lingual marginal alveolar bone loss around maxillary incisors, with the highest found in the labial side of lateral incisors in the CA group.

  • FA and CA treatments resulted in statistically significant increased OIIRR in the maxillary incisor region with higher prevalence and severity of OIIRR in patients receiving FAs. However, the increased OIIRR values in most of both groups’ cases were not clinically significant.

  • The post-treatment buccolingual inclination of maxillary incisors in the CA group significantly decreased due to labial root movement, indicating a compromised capability of CA to achieve adequate palatal root movement; such an issue may increase the possibility of labial bone fenestration.

Finally, different types of tooth movement using different appliance designs and orthodontic biomechanics might have different side effects than those reported in this study.