Impact of molar teeth distalization by clear aligners on maxillary alveolar bone thickness and root resorption: a three‑dimensional study

Skeletal Class II malocclusions, considered the most common in orthodontic patients, can affect the patient’s facial appearance, masticatory function, and mental health [1]. During the correction of malocclusion, orthodontic force may influence the alveolar bone remodeling. Therefore, the anatomy of the alveolar bone plays a crucial role in orthodontic treatment, as it dictates the treatment’s direction through coordination of resorption and apposition [2]. When there is a significant conflict between the bone remodeling and the amount of tooth movement occurring at a 1:1 ratio, the tooth remains within the alveolar housing “with the bone” [3]. Molar distalization is one of the most critical orthodontic movements for correcting skeletal Class II malocclusions [4, 5]. This technique can move the teeth away from the alveolar bone housing “through-the-bone” [3], potentially causing root resorption, alveolar bone loss, dehiscence, fenestration, and gingival recession [6].

Fixed appliances have been mainstreamed in orthodontic treatment [7]. However, patient acceptance is often hindered by challenges in maintaining proper oral health and the aesthetic concerns associated with the appliances [8]. Clear aligners, such as the Invisalign system, have become increasingly popular due to their advantages in comfort and aesthetics compared to conventional fixed appliances [9, 10]. Sequential distalization movement with clear aligners may be programmed in the maxillary arch for correction of a Class II relationship, with high predictability of molar distalization (88%) when supported by attachments on the tooth surface [11, 12] with or without posterior interproximal reduction and/or elastic simulation [13]. In addition, it is crucial to include bone thickness, dehiscence, fenestration, and other intra-bony defects in the orthodontic diagnosis and treatment plans [14, 15]. The accuracy of measurements may also be influenced by the tools used for assessment [7]. Prior research has primarily focused on assessing alterations in the alveolar bone and identifying orthodontically induced inflammatory root resorption (OIIRR) before and after orthodontic treatment, typically using periapical or bitewing radiographs [16, 17]. However, the reliability of these findings is constrained by the limitations of two-dimensional images, such as magnification, geometric distortion, and structural overlap [18].

To the authors’ knowledge, there are few published studies on using CBCT to assess changes in the alveolar bone thickness and root length in anterior teeth after distalization movements with conventional fixed orthodontic appliances [19]. Moreover, there is a lack of data evaluating three-dimensional changes in alveolar bone and OIIRR following maxillary molar distalization movement with clear aligners. Therefore, this study aims to assess the changes in alveolar bone thickness and OIIRR three-dimensionally following maxillary molar distalization with clear aligners in patients with Class II malocclusions using CBCT.

A total of one hundred forty molar teeth roots from 35 patients who met the inclusion criteria were analyzed, with a mean age of 24.4 ± 7.1 years. The reliability of intra- and inter-observer assessments for repeated measurements showed non-significant differences, where both intra- and interobserver R and ICC values were excellent to very good (see Table 1). The positive values indicated bone losses and root resorption, while the negative values denoted bone deposition in the horizontal plane.

The morphology of the maxillary alveolar cortex plays an essential role in orthodontic treatment, particularly in cases where there is a significant discrepancy between the amount of available space in the dental arches and the volume of teeth. Distal movement of the alveolar cortex should be considered in molars during treatment [22]. Excessive movement can lead to undesirable side effects for the periodontal tissue, including attachment loss, bone loss, gingival recession, and root resorption [23].

The primary purpose of the current study was to evaluate the impact of clear aligners on the thickness of buccal and palatal alveolar bone, as well as root length, following horizontal movement of molars in skeletal Class II malocclusion. The anatomical thickness of alveolar bone in molar teeth after distalization by clear aligner therapy was measured using CBCT, which provides distortion-free slice images [24]. These images enhance the possibility of evaluating more tooth surfaces compared to conventional radiography [25]. Clear aligner therapy is widely used by clinicians worldwide as an alternative to conventional fixed orthodontic treatments, particularly among adult patients who prioritize aesthetic considerations. In our study, the patient cohort consisted of adults undergoing a standardized treatment protocol for maxillary sequential molar distalization, specifically targeting a minimum of 2 mm actual molar distalization, in non-extraction cases. The maxillary first molars were successfully displaced distally by an average of 2.23 mm. This finding aligns with previous studies demonstrating the feasibility of posterior teeth distalization to address Class II malocclusion using clear aligner therapy [20, 26]. Simon et al. [27] reported a high precision (88%) in the upper molar’s bodily movement with aligners, with a prescribed mean distalization movement of 2.7 mm. Conversely. Brezniak et al. [28] suggested that bodily movement with aligners is not achievable, even when composite attachments are utilized. While various studies have explored the biomechanical mechanisms of clear aligners and their treatment outcomes, the side effects exerted by these treatments remain insufficiently understood [29, 30].

In the present study, the mean change in alveolar bone thickness on the buccal surface of all maxillary molars demonstrated a reduction at S1. This result aligns with findings by Juliana et al. [31], who showed a significant decrease in maxillary buccal alveolar bone thickness after treatment with Damon 3MX brackets without extractions, particularly around the 1st molar at S1 (0.6 ± 0.6). Similarly, our findings revealed a reduction in bone thickness on the palatal surface at S2 of the molars, corroborating a CBCT study by Maspero et al. [32], which indicated a decrease in palatal bone thickness at the same location (0.0 ± 0.4). Another study highlighted a significant reduction in the bone thickness for maxillary anterior teeth in the palatal area at the middle of the root length (0.11 ± 0,58) [23]. This was study confirmed these observations, showing a loss of bone thickness on the palatal surface at S2 of both molars (0.07 ± 0.30, 0.12 ± 0.38).

Regarding the average differences in T0-T1 values for maxillary molars, our results found that alveolar bone reduction was more pronounced on the buccal and palatal surfaces of the maxillary 2nd molars and the buccal surface of the 1st molar. Conversely, there was a significant increase in bone thickness on the palatal surface of the 1st molar (p < 0.005). These findings contrast with those of Fayed et al. [33] and Swasty et al. [34] reported a significant increase in buccal and palatal bone thickness in both the anterior and posterior regions of the maxilla. The discrepancy may be attributed to the unique type of tooth movement induced by aligners. Prior to the establishment of sequential distalization techniques, the most accurate achieved by aligners is the buccolingual tipping. This is due the materials of the appliance primarily bending in the buccolingual direction, which is in line with the logical mechanics of tooth movement. As a result, arch expansion and buccal tilting of the maxillary molars may lead to palatal tip drooping, causing resorption of the alveolar bone thickness [35, 36].

A previous study found that clear aligners effectively treated simple malocclusions with minimal OIIRR in Class I malocclusion patients [8]. Our study observed significant OIIRR in the 1st molar roots (0.45 ± 0.61, p < 0.000). Although our research didn’t directly compare aligners to fixed appliances, recent studies focusing on anterior teeth have found that the clear aligner experienced lower prevalence and less severe root resorption (68.50% and 0.31 ± 0.42 mm, respectively) compared to the fixed appliance (82.50% and 0.62 ± 0.54 mm, respectively) [37]. Additionally, prior research has documented that the average OIIRR during comprehensive treatment with fixed appliances ranges between 1.36 and 1.42 mm [38]. An analysis of periapical radiographs indicated that the average OIIRR for maxillary incisors treated with fixed appliances was 2.26 mm [39]. Conversely, a CBCT study found that patients with fixed appliances experienced an average resorption of 0.59 mm in their maxillary incisors [40]. Regarding clear aligners, 2 mm maxillary incisor OIIRR was reported based on the panoramic, periapical, and cephalometric radiographs [41]. However, when using CBCT imaging, one study reported an average root resorption of 0.51 mm in maxillary incisors [42], while another noted a reduction in root length of 0.13 mm for the same teeth [37].

To our knowledge, only one recent study has evaluated OIIRR in posterior teeth following clear aligner treatment. Our study indicated an average maxillary molar root resorption of 0.45 mm after clear aligner therapy, which aligns with the study of Elfouly et al. [26] who reported that the amount of root resorption observed in all studied molars’ roots (< 1 mm) was not clinically significant. The divergent results regarding OIIRR outcomes may stem from differences in the imaging tools selection, sample size, the magnitude of applied force, and the type of appliance used. Unlike fixed appliances, previous research has suggested that clear aligners are associated with a lower risk of OIIRR, likely due to the gentle and intermittent forces exerted during treatment [43, 44]. Recent progress in three-dimensional imaging and rapid prototyping technologies is revolutionizing the workflow in orthodontic clinical practices. Recent research indicates that the specific type of prototyping technology used, as well as the complexity and features of the components in three-dimensional printers, can significantly affect the precision of orthodontic models created for manufacturing clear aligners [45, 46]. These advancements suggest a critical evaluation of the technology and methods used in order to ensure the highest accuracy in orthodontic treatments.

We must acknowledge certain limitations of this study, including the analyses conducted immediately post-treatment without considering follow-up images. As a result, it is unclear if the observed defects resolve spontaneously over time. Additionally, the assessment of significant changes in alveolar bone thickness was hindered by an unequal gender distribution in our study cohort. Variations in hormonal changes between males and females and across different age groups might affect bone remodeling during orthodontic tooth movement. To better understand the changes in bone morphology due to clear aligner treatment, further clinical studies are recommended. Future research should include a comprehensive evaluation of morphology changes across all types and surfaces of teeth and examine the effects of different malocclusions on bone changes during orthodontic treatment.