This prospective study was approved by the Clinical Research Ethics Committee of Karadeniz Technical University Faculty of Medicine (protocol number: 2021/2) and the Turkish Pharmaceuticals and Medical Devices Agency (approval number: 2022-074, E-68869993-511.06-820892) and conducted in accordance with the tenets of Declaration of Helsinki. Informed consent was obtained from all patients.

GPower 3.1.0 software package (Universitat Düsseldorf, Düsseldorf, Germany) was used to determine the number of individuals required for this study. Sample size evaluation was based on the standard deviation of a similar study performed by Saif et al.¹⁶ The adequate minimum number of patients required was 22, which was calculated by setting the alpha error to 0.05, beta error to 0.20, and effect size to 0.65. Ultimately, 24 patients were included in this study to increase the strength of the study. All patients underwent therapy using the Orthero Clear Aligner (İstanbul, Turkey) system.

A total of 24 patients (16 females and 8 males) who underwent orthodontic treatment at Karadeniz Technical University Faculty of Dentistry Hospital between July 2022 and August 2023 formed our study population. The patients were randomly divided into two groups and randomization was performed using www.randomization.com. In total, 12 patients (five males and seven females) were assigned to the 33% sequenced distalization group (group 1), and 12 (three males and nine females) to the 50% sequenced distalization group (group 2). The mean age of study participants belonging to groups 1 and 2 were 22.9 ± 0.7 years and 25.83 ± 0.5 years, respectively.

Inclusion criteria were:

Exclusion criteria were:

The treatment plans for this prospective study were designed by a single orthodontist, who also performed the treatments. The same technician assisted with the planning stage.

Distalization in individuals belonging to group 1 was performed by means of 33% sequential alignment using CAs. Vertical rectangular attachments were added on U5, U6, and U7. The average number of aligners used in group 1 was 16.27 and the mean treatment duration was 8.12 ± 0.75 months. For individuals belonging to group 2, distalization of U5, U6, and U7 was performed in a 50% sequence with vertical rectangular attachments and CAs. The average number of aligners used in group 2 was 22.75 and the mean treatment duration was 10.6 ± 0.78 months.

In particular, sequential distalization of U5, U6, and U7 was planned for all patients, and the total amount of distalization planned for both groups was determined as 2.5 mm. To control the distalization movement, rectangular vertical attachments were placed on U5, U6, and U7, and the movement of only these teeth was planned throughout the distalization process in the upper arch. To strengthen the anchorage, rectangular horizontal attachments were also placed on upper first premolars. No attachments were added to any other tooth, and no movement of any other tooth was planned.

In this study, we evaluated the anchorage loss that may occur in U1 after the distalization of U5, U6, and U7. Therefore, Class II intermaxillary elastics or any additional mechanics were not used during distalization. Similarly, no attachments were placed on the anterior teeth, and interproximal reduction was not included in the treatment plan. All patients were treated with this standard procedure without any other auxiliary mechanics and were required to wear aligners for at least 22 hours per day.

Cephalometric radiographs and digital model recordings of all patients before treatment (T0) and after completion of distalization (T1) were obtained.

Reference planes used in this study included:

Twelve linear dentoalveolar and four angular measurements were performed on the cephalometric radiographs (Figures 1 and 2). Fourteen linear and six angular measurements were performed using digital models (Figures 3–5).

All the statistical analyses were performed using SPSS software (SPSS v23; IBM Corp., Armonk, NY, USA). Conformity to normal distribution was examined using Shapiro–Wilk test. Independent samples t test was used to compare the parameters normally distributed by groups. Mann–Whitney U test (for intergroup comparisons) and Wilcoxon test (for intragroup comparisons) were used to compare the parameters that were not distributed normally. Intraclass correlation coefficient was used to examine compliance between parameter and repeat measurements. Level of significance was set at P < 0.05.

Table 1 shows the comparison between the two groups in terms of cephalometric parameters during pre-treatment (T0) periods, time-dependent change differences (T1-T0), and changes during post- (T1) and pre-observation (T0) periods. No significant differences were observed between the two groups at T0 (P > 0.05). Analysis of changes in cephalometric dentoalveolar measurements from T0 to T1 in group 1 revealed an increase in U1/HR (7.15°; P < 0.001) and a decrease in U5/HR (5.92°; P < 0.01), U6/HR (8.19°; P < 0.001), and U7/HR (11.35°; P < 0.01) angles. In dentoalveolar linear measurements, there were significant decreases in U5-VR (1.91 mm; P < 0.001), U6-VR (2.30 mm; P < 0.001), U7-VR (2.41 mm; P < 0.001), and U1-HR (0.19 mm; P < 0.05) along with a significant increase in U1 VR (1.56 mm; P < 0.001). There was also an increase in overjet (1.30 mm; P < 0.001) and a decrease in overbite (0.15 mm; P < 0.05) after distalization. Additionally, it was determined that the upper lip was protruded (0.45 mm; P < 0.01) (Table 1).

Analysis of the changes in cephalometric dentoal veolar angular measurements from T0 to T1 in group 2 revealed an increase in U1/HR (5.33°; P < 0.001) and a decrease in U5/HR (5.96°; P < 0.001), U6/HR (8.45°; P < 0.001), and U7/HR (10.96°; P < 0.001) angles. Linear measurements showed significant decreases in U5-VR (1.86 mm; P < 0.001), U6-VR (2.32 mm; P < 0.001), U7-VR (2.42 mm; P < 0.001), and U7-HR (0.15 mm; P < 0.05), along with a significant increase in U1-VR (0.94 mm; P < 0.001). There was also an increase in the overjet value (0.85 mm; P < 0.001). Meanwhile, it was determined that the upper lip was protruded (0.29 mm; P < 0.01) (Table 1).

When the differences in time-dependent changes in cephalometric measurements between the two groups were examined, it was found that there were greater in creases in U1/HR (1.82°; P < 0.001), U1-VR (0.62 mm; P < 0.001), and overjet (0.45 mm; P < 0.001) values in group 1 (Table 1).

Table 2 shows the comparison of digital model parameters between the two groups in terms of pre-treatment (T0) periods, time-dependent change differences (T1-T0), and changes during post- (T1) and pre-observation (T0) periods. No significant differences were observed between the two groups at T0 (P > 0.05). In the context of sagittal model parameters for groups 1 and 2, significant increases were observed in all the sagittal and transverse (P < 0.001) measurements from T0 to T1. In contrast, for angular model measurements, significant decreases were found from T0 to T1 in both the groups (Table 2).

When the differences in time-dependent changes in cephalometric measurements between the groups were examined, although greater increases were found for group 1 in the distance from incisor tip of 11 to G line (0.16 mm) and the distance from incisor tip of 21 to G line (0.01 mm), the difference was not statistically significant (P > 0.05). In addition, there were no significant differences between the two groups over time in terms of all the model measurements (P > 0.05) (Table 2).

Ravera et al.⁴ reported that the maxillary first and second molars moved 2.25 mm and 2.52 mm distally, respectively, without any significant distal tipping. They showed that the presence of rectangular vertical attachments on the buccal surface of the teeth were responsible for small amount of tipping. Similarly, Caruso et al.¹⁴ demonstrated that the maxillary molars moved distally with bodily movement of the CAs. These findings are consistent with those of Simon et al.,¹⁸ who showed distal displacement of the molars by 2–3 mm. However, studies on traditional distalization applications indicate that molar teeth move significantly by tipping distally.^19-28 In our study, distal tipping was also observed during distalization of U5, U6, and U7 in both the groups, and these findings are consistent with those of previous studies that investigated distalization using CAs.^4,12-18 At the same time, we did not observe any significant difference between the two groups in terms of amount of distalization. Additionally, in this study, 2.5 mm distalization was planned for each tooth in all patients, and it was determined that comparatively less distal movement occurred in the posterior teeth than originally planned in both the groups. In both groups, the amount of distal movement occurred the most in U7 and the least in U5. This can be explained by two factors. First, the mechanical forces produced by CAs are typically distributed between the teeth and along the periodontal ligament. The periodontal ligament area of maxillary first molars is generally larger than that of maxillary second molars, and the need for anchorage during movement is greater. Second, aligners are made of plastic, and as the number of teeth to be moved distally increases, the deformation of aligners increases, which may cause subsequent movements to be less than predicted.⁴

Moreover, in our study, cephalometric measurements in the sagittal plane and digital model measurements were consistent. Moreover, the amount of distalization measured in the digital model and cephalometric radiography was compatible in both the study groups. These findings are consistent with those of previous studies.^22,24-27

In treatments involving CAs, anchorage loss may occur in anterior region due to reciprocal force that arise in response to distalization force.⁴ Caruso et al.¹⁴ investigated the effect of distalization of molars using CAs and found no anchorage loss during treatment and that root torque control of U1s were perfectly ensured. Saif et al.¹⁶ assessed digital models and reported different amounts of anchorage loss in almost every patient. Patterson et al.²⁹ stated that Class II elastics should be used to avoid overjet increase during upper molar distalization. Other researchers have also confirmed that Class II elastics prevent uncontrolled proclination of anterior teeth.^4,15

The findings of this study showed that 33% sequence distalization caused more anchorage loss in anterior teeth than 50% sequence distalization. This means that more proclination and protrusion of U1s were observed in the 33% sequence group. This finding can be explained as follows. While a maximum of two teeth moved simultaneously when 50% sequence was applied to the posterior teeth, three or more teeth participated in simultaneous movement when 33% sequence was applied. This complicates anchorage control and causes more opposing inverse force in the anterior teeth, which serves as the anchorage unit, while the posterior teeth move. In this study, the faster 33% sequence distalization protocol was found to be preferable because Class II elastics were not used to evaluate the absolute response of anterior tooth movements. Moreover, it is likely that these posterior anchorage losses were also seen with increased protrusion and proclination in the anterior region, since the molar teeth (which were initially distalized in the posterior segment) lost anchorage during the movement of U5 and also slightly mesialized and preserved the length of the CAs.

In summary, in cases requiring upper molar distalization, Class II intermaxillary elastics can be used in early stages to prevent loss of anchorage. If distalization with CAs is to be performed in patients with Class II malocclusion and greater overjet, the 50% sequence protocol would be preferable.

It has been reported that mandible rotates posteriorly with downward movement of the mesial cusp and that the vertical dimensions and angles increase when the molars are tipped distally by traditional intraoral distalization appliances.^26-28 Studies conducted using CAs have reported no significant change in vertical direction values after distalization.^4,14,30 Thickness of the CAs in the occlusal plane and the ‘bite block effect’ that occurs with it are responsible for the absence of change in the vertical dimension. Traditional applications have been reported to decrease the overbite with intrusion of U1s after distalization. It has been found that the resulting intrusion develops due to the proclination of U1s.^4,14,16,18

In the current study, in accordance with the literature, no significant change was observed in the angle between SN and Steiner's mandibular plane in either group, and no significant vertical movement occurred after distalization in U5, U6, or U7. In addition, it was confirmed that the right-direction angle of the aligners did not change due to the bite-block effect. Therefore, we found that CAs could provide an effective alternative for average distalization of posterior teeth by 2–3 mm, especially in patients with hyperdivergent growth pattern or open bite. In addition, in the present study, a small amount of U1 intrusion was observed in group 1, but this value was not statistically significant in group 2. This intrusion in group 1 was thought to be related to the amount of tooth proclination.

In a normal arch form, the width of the posterior region in the transverse direction gradually increases distally. However, if the normal arch form is not followed, a posterior crossbite may occur due to distal movement of posterior teeth. It can be argued that an increase in the transverse widths between the teeth during distalization of upper posterior teeth using CAs is advantageous in terms of harmony of the arches.³¹

In this study, we observed that the width between U5, U6, and U7 increased in the transverse direction in both the groups. Therefore, distalization using CAs may be preferred, especially in Class II cases with a tendency towards posterior crossbite.

In this study, we found that U5, U6, and U7 underwent distopalatal rotation in both the groups. The post-distalization rotation amounts were similar in both the groups, with least and maximum amount of rotation occurring in U5 and U7, respectively.

Previous studies using traditional intraoral distalization headgears have reported that rotations in molars occur in accordance with the size and direction of application of the distalization force.^27,28,32,33 Gulati et al.³² reported 2.4° distopalatal rotation in molar teeth after performing distalization using the Jones Jig appliance. On the other hand, Wilmes and Drescher³³ elicited 3.4° distobuccal rotation with the Beneslider Appliance. Henry³⁴ in the year 1956 stated that the permanent U6 in malocclusions are rotated in 83% of the cases and that the axis of rotation passes through the palatal root and mesiopalatal cusp. In CA systems, it is important to apply distopalatal rotation to the posterior teeth during digital planning.³⁵ Therefore, distopalatal rotation occurred in teeth that moved distally, and this was in accordance with the virtual planning in this study.

Previous studies using intraoral molar distalization appliances have reported a significant level of protrusion in the upper lip after distalization, which may be due to increased U1 proclination resulting from the loss of anchorage in the anterior region.^28,36 In our study, significant amount of protrusion was observed in the upper lip in both the groups, which was associated with the movement of U1s toward the lips.

As in all dynamic appliance studies, an obvious limitation of this study was that it required excellent patient cooperation. In terms of controlling aligner–patient compliance, it would be beneficial to place indicators on aligners in the future. In addition, long-term results should be evaluated in subsequent studies. Although our results are encouraging, this issue needs to be further investigated in future randomized clinical trials.