Risk Factors for Maxillary Central Incisors Torque Loss in Orthodontic Treatment of Premolar Extraction Cases with Clear Aligners：a retrospective study

doi:10.21203/rs.3.rs-3955626/v1

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Risk Factors for Maxillary Central Incisors Torque Loss in Orthodontic Treatment of Premolar Extraction Cases with Clear Aligners：a retrospective study

https://doi.org/10.21203/rs.3.rs-3955626/v1

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Background：Torque loss of maxillary central incisor teeth occurs frequently during space closure with clear aligners (CA).The aim of this study was to analyse the percentage reduction in upper incisor lingual inclination and risk factors for upper incisor angle after Invisalign treatment in extraction cases.

Methods：Records were obtained from 52 patients (average age 27.6 years) who initiated with Invisalign at four different orthodontic practices. Data was collected from cephalometric radiographs, doctors' case notes, and CA Web site databases. All patients who were treated with CAs were divided into maxillary incisors Lingual tipping (L) and non-maxillary incisors Lingual tipping (NL) groups based on whether U1-NA(°) was less than 17.1 (Mean -1 SD). Cephalometric data before and after therapy were analysed using a paired t-test. Differences between the L and NL groups were examined through independent sample t-tests, chi-square tests, and Wilcoxon rank sum tests. Correlation factors, including CC difficulty scores, age, gender, pre-treatment upper central incisor inclination, skeletal classification, divergence, orthodontist, treatment length, and refinements number, were identified using univariate and multivariate linear regression analysis.

Results：After treatment, 61.5% of the patients experienced upper incisor lingual inclination. Significant differences were observed between the groups in terms of the number of refinements, treatment length, and age (P＜0.05). The number of refinements was directly proportional to the duration of treatment. A statistical difference was observed between orthodontists and the number of refinements. However, adolescents had significantly less refinement. A multivariate linear regression analysis revealed negative correlations between age (P=0.002), treatment length (P=0.002), and different orthodontists (P=0.013) with the reduction of the upper incisor angle after treatment.

Conclusions：The upper central incisors exhibit torque loss in extraction cases treated with Invisalign. The risk of a reduction in the angle of the upper incisors after treatment is increased by factors such as the patient's age (older patients), longer treatment times, and young orthodontists. It is recommended that these factors be closely monitored and guided during clinic visits.

Extraction

Invisalign

Maxillary incisor torque

Influencing factors

Clear aligners (CA) are increasingly being used because of the advantages they offer over traditional fixed appliances (FA), including better aesthetics, comfort, shorter treatment times, and fewer oral health problems.[1–4] Initially, clear aligners were introduced to treat mild malocclusions.[3]However, with the rapid development of materials science and improvement of design methods, they are now also used to treat patients with more complex dentofacial abnormalities, including extraction cases.[5, 6] Despite the increasing range of clear aligner treatments, movements such as anterior torque control in premolar extraction cases remain challenging.[7]

The control of incisor torque is crucial in orthodontic treatment, particularly in extraction cases.[8] Achieving optimal torque is essential for both smile aesthetics and proper anterior guidance.[9, 10]Anterior torque loss refers to the lingual tipping and extrusion of incisors, which can cause occlusal interferences and mandibular clockwise rotation. Such complications can lead to more severe chin retrusion and jeopardise facial aesthetics. Additionally, the proximity of the upper anterior tooth root to the bone cortex increases the risk of fenestration and dehiscence.

A recent study revealed that the accuracy of anterior teeth lost more labial torque than predicted during space closure with CA.[5–7]From a biomechanical perspective, the limitations of CA materials cause retraction forces to be applied to the crowns of anterior teeth, passing through the occlusal side of the centre of resistance, resulting in lingual tipping and incisor extrusion. Recent research has confirmed that adding a power ridge and designing overtreatment are highly effective for central torque control.[11–13]However, in some cases, multiple stages of intermediate refinement may be used, and fixed orthodontics may be required to address the bowing effect.[14, 15]Although these methods can complete the treatment, they may extend the correction time. Consequently, assessing the efficacy of incisor torque could help determine the amount of movement required before treatment and reduce the frequency of refinements.

While much of the current literature discusses the insufficiency of upper incisor torque control with CA in extraction cases, there are few studies on the probability of lingual tipping and the factors that influence the correction of the upper incisor. The primary purpose of this research was to determine the percentage of upper incisor lingual inclination after treatment. Additionally, patient characteristics were investigated, including the number of refinement scans and treatment time to investigate possible risk factors for a reduction in the upper incisor angle after therapy. This information could help clinicians select appropriate patients for CA in extraction cases and develop effective plans to minimise the risk of lingual inclination of upper incisors.

The treatment was performed at the Department of Orthodontics, the, by four experienced orthodontists. The inclusion criteria for patients were as follows: age ≥ 12 years; ANB>0°; extraction of bilateral maxillary premolars(first or second);treatment with the Invisalign;compliant with prescribed Invisalign wear protocols as reported by the treating orthodontist. The exclusion criteria were as follows:incomplete pre- or post-treatment clinical data; missing permanent teeth; receiving auxiliary orthodontic appliances to facilitate tooth movement, and for example, fixed appliances, segmental archwire; craniofacial defects, syndromes or skeletal deformities; and history of orthodontic treatment.

The sampling flow chart is shown in Fig. 1. All patients started treatment in 2016 or later and were instructed to wear their aligners 20–22 h/d and to change aligners every two weeks. The study sample consisted of 52 patients (8 males and 43 females, aged 27.6years). Ten patients were excluded because of fixed appliances or segmental archwire used during treatment,twenty-two patients had incomplete pre- or post-treatment clinical data, eight patients were missing permanent teeth, and three patients were excluded because history of orthodontic treatment. The expression capacity of the upper incisors' buccolingual inclination was evaluated at U1-NA (angle formed by the upper incisal axis with the N-A planes), with all patients divided into two groups according to whether or not they had maxillary incisors Lingual tipping in post-treatment (based on the normal mean (22.8) and standard deviation (5.7), the maxillary incisors Lingual tipping groups (L) (n = 32) who post-U1-NA (°)<17.1 (Mean − 1 SD); and the non-maxillary incisors Lingual tipping groups (NL) (n = 20), who post-U1-NA (°) ≥ 17.1.

For each participant, data were collected from pre-therapy (T0) and post-therapy (T1) cephalometric radiographs, treatment records, and ClinCheck(Align Technology, Santa Clara, California, USA). Demographic and clinical data were collected regarding age, gender, extraction position, orthodontists, treatment length (mo) and number of refinements. Computerised cephalometric measurements were performed using Uceph software. Using ANB (to assess the skeletal class: skeletal class I 0–5°, skeletal class II >5°); SN-MP (to identify the divergence: low angle <29°, intermediate angle29-40°, high angle ≥ 40°); L1-NB(to assess the lower incisor inclination); UL-EP、LL-EP༈to assess the upper and lower lip positions༉. Based on the pre-treatment maxillary incisor inclination (Pre-U1-NA°༉, data was divided into three groups: labial tipping ≥ 28.5°, normal = 17.1° -28.5° and lingual tipping <17.1°. Mathematical model measurements were performed using ClinCheck software, and CC Difficulty Scores (CAT-CAT) were calculated.[16]

All measurements were repeated one month after the initial measurements by a single operator. The intraoperator agreement was evaluated using ICC. The normality of the data was tested by the Kolmogorov-Smirnov test. The skeletal pattern, dental parameters, and facial profile before and after orthodontic treatment, as well as their variations, were compared using the paired t-test. Differences between the L and NL groups were analysed. Continuous variables were compared using an independent sample t-test. Categorical variables were compared using the chi-square test. The Wilcoxon rank sum test was used if the data distribution was not normal. A univariate and multivariate linear mixed model was applied to examine the influence of variables (age, pre-treatment upper central incisor inclination, gender, skeletal classification, divergence, orthodontists, treatment length and refinements number) on post-U1-NA° decreases with CA. The variables with a significance level of P<0.05 in the univariate analyses were included in the multivariate model. SPSS (version 26.0; IBM Corp, Armonk, NY) was used to analyse the data. Differences with P<0.05 were considered statistically significant.

ICC showed high reliability for all measurements. The primary characteristics of 52 patients with CA are described by n (%) in Fig. 2. The maxillary incisor lingual tipping rate is 61.5% in post-treatment with CA. The sample was composed of 43 females (84.3%) and 8males (15.7%). The patients were classified as Skeletal Class I (24, 46.2%) and Skeletal Class II (28, 53.8%) malocclusion by skeletal classification. There were 10 hypo-divergent patients (19.2%), 29 normo-divergent patients (55.8%) and 13 hyper-divergent patients (26.5%) in this study. The average age of the sample is 27.6years, and the largest age group consisted of patients aged above 30 years. The second largest age group comprised patients 18–30 years. The treatment was carried out by four experienced orthodontists, based on working experience, ranging from high to low for Dr. A-D.

The skeletal, dental and soft tissue parameters from before and after treatment are shown in Table 1. The SNA decreased by 0.98°, and ANB decreased by 0.84° following treatment (P<0.001). The angles of both the upper and lower incisors were decreased(P<0.001). The changes of U1-NA (°) ranged from 26.4 to 14.72. The average of post-L1-NB (°) decreased by 10.23. The linear retractions of the upper and lower lip positions showed significance different (P<0.001). The SNB、SN-MP、SN-FH displayed no significant differences (P >0 .05).

Table 1

Descriptive statistics of pretreatment (T0), post-treatment (T1) and treatment variations (T0-T1)
Parameters	T0	T1	T0-T1	P
SNA(°)	82.13 ± 3.56	81.15 ± 3.35	0.98 ± 1.27	<0.001*
SNB(°)	77.08 ± 3.94	76.94 ± 3.98	0.14 ± 1.29	0.422
ANB(°)	5.04 ± 2.01	4.20 ± 1.71	0.84 ± 1.05	<0.001*
MP-SN(°)	36.31 ± 6.93	35.48 ± 6.88	0.83 ± 3.05	0.055
MP-FH(°)	26.82 ± 6.65	26.36 ± 6.86	0.46 ± 2.49	0.187
U1-NA(°)	26.40 ± 6.92	14.72 ± 6.50	11.68 ± 7.65	<0.001*
U1-NA(mm)	5.87 ± 1.96	1.86 ± 1.39	4.01 ± 2.20	<0.001*
L1-NB(°)	34.83 ± 6.19	24.57 ± 5.19	10.23 ± 7.40	<0.001*
L1-NB(mm)	8.11 ± 2.36	4.08 ± 1.75	4.03 ± 2.30	<0.001*
UL-EP(mm)	1.43 ± 1.97	-0.78 ± 1.79	2.21 ± 1.48	<0.001*
LL-EP(mm)	3.31 ± 2.58	0.03 ± 2.06	3.28 ± 1.98	<0.001*
Note. Data are presented as mean ± standard deviation. *Paired t test, significant at P<0.05.

Descriptive statistics of subjects in the L and NL groups and the differences between them are shown in Table 2 and Fig. 3. The differences in treatment length (P<0.001)、number of refinements (P<0.001), and age(P<0.001)differences between the groups were statistically significant. The treatment duration was shorter in the NL groups (37.3 ± 5.33mo) than in the L groups(49.53 ± 13.06mo). The L groups had twice as many average refinements as the NL groups. Age was lower in NL groups (23.05 ± 7.93y) than in L groups (30.44 ± 5.67y). Notably, while there is no statistical difference in these groups among different doctors, the lingual tipping rate of maxillary incisors by Doctor C is higher than that of other doctors. Data analysis also shows no significant differences in gender, Skeletal classification,extraction position, divergence and pre-U1-NA°(P>0.05).

Table 2

Descriptive statistics of subjects treated with Invisalign (n = 52)
Variables	L(n = 28)	NL(n = 19)	P
Gender			0.999
Male	5(15.6)	3(15)
Female	27(84.4)	17(85)
Skeletal classification			0.155
Skeletal Class I	12(37.5)	12(60)
Skeletal Class II	20(62.5)	8(40)
Extraction position			0.999
Maxillary first premolar	29(90.6)	18(90)
Maxillary second premolar	3(9.4)	2(10)
Divergence			0.289
Hypodivergent	4(12.5)	6(30)
Normodivergent	19(59.4)	10(50)
Hyperdivergent	9(28.1)	4(20)
orthodontists			0.086
DrA	9(28.1)	11(55)
DrB	2(6.2)	3(15)
DrC	11(34.4)	4(20)
DrD	10(31.2)	2(10)
Pre-U1-NA°			0.211
Labial tipping	9(28.1)	10(50)
normal	18(56.3)	9(45)
lingual tipping	5(15.6)	1(5)
Treatment length (mo)	49.53 ± 13.06	37.3 ± 5.33	<0.001*
Refinements number	3.16 ± 1.44	1.65 ± 0.99	<0.001*
Age	30.44 ± 5.67	23.05 ± 7.93	0.002*
Note. Categorical variables are described by number (percentage).Continuous variables are described by mean 6 standard deviation.*P<0.05.

On average, each treatment required 2.58 refinement scans. Almost half of the patients required more than two refinement scans, and no patient completed their treatment without requiring refinement. The duration of treatment was directly proportional to the number of refinements (Table 3). The statistical analysis reveals that Dr. A's treatment required fewer refinement scans than that of Dr. C and D. While there was no statistical difference in age, adolescents required fewer refinement scans than adults. (Fig. 4B).

Table 3

No. of refinement scans per treatment
Refinement scans per treatment	(N = 52)%	Treatment length per treatment(M)
0	0(0%)	-
1	13(23.4%)	25
2	18(36.2%)	34.61
3	9(19.1%)	17.31
>4	12(8.5%)	23.08

There is no linear relationship between the CC Difficulty Scores and the number of refinement scans and Post-U1-NA°. However, the CC Difficulty Scores were negatively correlated with treatment length. In other words, the higher the CC index, the shorter the treatment time.(Fig. 5).

After univariate linear regression analysis, six risk factors among eight independent variables were identified with P<0.05:Pre-U1-NA°、age、refinements number、treatment length 、orthodontists、and skeletal classification. The percentage of risk factors for post-U1-NA (°) described by the regression model is 62.1%. Multivariate linear regression analysis for these six potential factors is revealed that age、treatment length, and different orthodontists are independent risk factors for post-U1-NA (°) reduction. The risk increases as a patient's age increases (β = -0.316, P = 0.001), treatment length is prolonged (β = -0.176, P = 0.006), or Dr.D, young orthodontist with less experienced(β = -4.302, P = 0.013) compared to Dr.A, highly experienced orthodontist (Table 4). The other parameters are not significant risk factors for post-U1-NA (°) reduction with CA in extraction cases.

Table 4

Risk factors for reduction of central incisor angulation after treatment
Variables	Univariate		Multivariate
Variables	β(95%CI)	P	β(95%CI)	P
Pre-U1-NA°	0.331 (0.082,0.581)	0.010*	0.174(-0.031,0.379)	0.095
Age	-0.460(-0.669, -0.251)	<0.001*	-0.316(-0.498,-0.134)	0.001*
Refinements number	-1.864(-3.001,-0.728)	0.002*	-0.692(-1.909,0.525)	0.258
Treatment length (mo)	-0.236(-0.371, -0.102)	0.001*	-0.176(-0.300,-0.052)	0.006*
Gender
Male	Reference
Female	0.491(-4.866,5.848)	0.855
orthodontists
DrA	Reference
DrB	0.037(-1.985,2.059)	0.971
DrC	-2.170(-4.322,-0.019)	0.048*	0.053(-1.786, 1.893)	0.954
DrD	-6.359(-10.766,-1.952)	0.006*	-4.302(-7.645, -0.958)	0.013*
Skeletal classification
Skeletal Class I	Reference
Skeletal Class II	-3.993(-7.482, -0.503)	0.026*	-2.488(-5.137, 0.161)	0.065
Divergence
hypodivergent	Reference
normodivergent	-2.304(-7.053,2.445)	0.334
hyperdivergent	-1.967(-4.764,0.830)	0.164
β, Standardized regression coefficient, “+ ”positive impact, “- ” negative impact. *P<0.05.

Although CA meets the aesthetic and comfort needs of patients[17][18] it has been shown to lack control over the anterior teeth compared to fixed appliances, especially with regard to incisor torque.[7]Therefore, one of the major concerns of the orthodontic community is to investigate how CA can improve the rate of incisor torque and to identify the factors influencing the lingual inclination of the central incisor. This study includes 52 patients who underwent bilateral maxillary premolar extraction with CA, based on strict inclusion and exclusion criteria. Cephalometric measurements were used to determine the probability of lingual inclination of the upper incisor. Data were collected intraoperatively, and multiple linear regression analysis was used to identify the risk factors for reducing the post-U1-NA (°).

In the current study, most patients treated with CA were adult females who were more likely to choose transparent aligners for orthodontic treatment to improve their appearance. More than half of the patients had upper incisors lingual tipping after treatment, indicating that most of the cases experienced "the roller-coaster effect" during correction. This is consistent with the present study, where incisor torque was the least predictable movement.[19–21]Recent studies considering the reasons for these phenomena suggest that CA has more crown movement than root movement, which may cause a tipping movement of the anterior teeth during CA of retraction forces.[22] Due to the material properties of CA (stiffness is reduced at the gingival margin of aligners) and interruption of extraction sites, when solo retraction forces were applied, it was easy to cause aligner deformation, with the anterior teeth displaying lingual inclination combined with extrusion.[23]

Several studies have shown that the cephalometric radiograph should be approached with caution for assessing incisor torque.[20, 24]Conversely, other studies concluded that torque measurements obtained from cephalometric are predictable for clinical purposes.[7] The cephalometric analysis (T0-T1) shows that CAs are effective in retracting the maxillary incisors and improving the profile, which is consistent with previous studies. [7]However, the angle changes of the maxillary incisors decreased by 11.68° from T0-T1, and the post-U1-NA(°)was less than the standard deviation of the normal mean, indicating that CAs were not effective in controlling incisor torque in extraction cases. It is possible that the overtreatment design or movement steps were not considered.The current study results indicate that there is greater upper anterior retraction in CAs.When the crown is lingually inclined ,the root will be labially inclined,which increases the risk of creating or deteriorating bone fenestration during orthodontic treatment.[25]Such outcomes place higher demands on upper anterior torque control to prevent periodontal complications.Recently,the majority of methods to achieve optimal incisor torque control involve use a double power ridge or attachments on central incisors,[12, 26]and different clinical designs.[22, 27]

The lingual inclination rate of the upper incisor is 62.5% in the Skeletal Class II group and 37.5% in the Skeletal Class I group, demonstrating an increased impact for patients with Skeletal Class II on the central incisors lose torque with CAs. In general, to achieve the ideal profile in orthodontic treatment, most patients with Skeletal Class II require bodily movement of the anterior teeth. Consequently, it is more challenging than for patients with dental protrusion who require inclination movement.[19] Incisor torque loss is more pronounced in adults than in adolescents. The data are consistent with previous studies.[5]Incisor torque accuracy decreases slightly with age, possibly due to growth potential and periodontal remodelling in adolescents. In addition, compared with 37.3 months for NL groups, a more extended treatment duration was obtained for L groups (49.53 months). Notably, the longer the treatment time, the more restarts are required (Table 3). The number of refinements in the L group is nearly twice as high as in the NL group, reminding us that most orthodontists do not fully realise the design concept of "beginning to end", indicating that it is difficult to specify the target position before treatment in the CA extraction cases. Only in those cases with an upper incisor lingual and extrusion would orthodontists consider addressing the clinical situation with a restart. However, restarting is a means of correction, while not being efficient, and multiple restarts can result in longer correction times than a fixed correction.[7, 28] The extraction pattern in our study varied, with the extraction of premolars being one of the most common strategies to retract proclined incisors and reduce lip protrusion.[8, 29, 30]One study reports that more significant tipping occurs around the extraction of the second premolar than around the first premolar[31] because extraction of the second premolars involves more anchorage loss. The moment-to-force ratio applied to the teeth by CA produces more tipping in molars because they are larger than premolars and canines. The current study shows that the lingual inclination of the upper incisor is not statistically different for extractions of the first and second premolars. Perhaps this outcome could be attributed to only having five patients who experienced the extraction of second premolars. The protrusion degree of maxillary incisors has a different effect on CA retraction. Although no statistical difference occurred in the current study between the different protrusion degrees of maxillary incisors, the lingual inclination rate of upper incisors in the labial tipping groups was lower than the typical groups, reminding us that more tipping movements for severely protruded maxillary incisors.[7]There is no statistical difference in incisor lingual rate between different orthodontists, indicating that younger orthodontists can achieve the correction goals with digital-assisted design using CAs.

All patients in the current study underwent refinement, and nearly half of the patients had three or more restarts, with an average of 2.57 refinement scans required to complete treatment. As shown in Fig. 4A, although age was not correlated with the number of restarts, adolescents had fewer restarts than adult patients, which may be related to the differences in growing phases between adolescents and adults.[32] There is a statistical difference in the number of restarts between the different orthodontists ( Fig. 4B), perhaps due to differences in clinical design and experience. Orthodontists with more years of experience required fewer refinements to complete the correction than younger orthodontists. Tooth movement with aligners is more complex than it is with fixed appliances, especially in extraction cases.[33]Orthodontists must remember, particularly young ones, that preventive measures must be taken in advance, such as the position of the attachment,[26]the amount of activation in each aligner[28]and the sequence of motions,[21]and related auxiliary techniques. [34, 35]Otherwise, the correction time will be increased.

The CC Difficulty Assessment Index is currently the only scoring system to assess the difficulty of clear aligner treatment.[16] In this study, the CC score of the patients ranged between 20–60, which reflects medium difficulty cases. Correlation analysis shows that the CC Difficulty Assessment Index does not have a linear correlation with the degree of post-treatment upper incisor protrusion. Considering that the CC score is the criterion established for all malocclusion, the patients in the current study were all premolar extraction patients, proving the sample consistencies.

Multivariate linear regression analysis revealed that age、treatment time, and young orthodontist are three independent risk factors for post-U1-NA (°) reduction. The characteristics of older age, longer treatment time increased the complexity of treatment and influenced the accuracy of CA.The outcomes suggest that patients possessing these features should receive treatment that includes the early controlling of root movement to prevent the incisor lingual. In addition, more refinement numbers are observed in the L groups, but multivariate linear regression analysis shows it does not increase the risk of post-U1-NA (°) reduction. It is related to different orthodontic clinical habits, which have high variability.

Several limitations of this retrospective study can be identified. First, the parameters were obtained using 2D radiographs for measuring the inclinations of the maxillary incisors may not be very accurate. Further studies using three-dimensional analysis are still required in the future. Additionally, a clincheck setup, including the attachment designs and staging of tooth movement, was not considered in the current study. Finally, the current study had a small sample size, so the study results may underestimate the probability of upper incisor lingual inclination because patients who used braces or segmental archwires during treatment with aligners were excluded from this study.

More than half of the patients will experience an upper incisor torque loss in extraction cases with aligners. Adolescents exhibit superior expression of torque of incisors compared to adults in CA extraction cases. Orthodontists with less clinical experience may experience difficulty in maintaining upper anterior torque when using Invisalign for extraction cases. Therefore, it is recommended that young orthodontists increase their study of extraction cases and take preventive measures to avoid complications.The treatment time of CA is proportional to the number of restarts, so future research needs to reduce the number of restarts and improve correction efficiency.

CA Clear aligners

FA Fixed appliances

L Maxillary incisors Lingual tipping

NL Non-maxillary incisors Lingual tipping

Ethics approval and consent to participate

This study was approved by the ethics committee of the Affiliated Stomatology Hospital, Southwest Medical University (no.20211214001).Because this is a retrospective study that does not interfere with the patient's treatment regimen and does not pose a physiologic risk to the patient, the need for informed consent was waived by the ethics committee of the Affiliated Stomatology Hospital, Southwest Medical University.

Consent for publication

Not applicable.

Availability of data and material

All data generated or analysed during this study are included in this published article and its Additional files.

Competing interests

Not applicable.

Funding

This study was supported by Angelalign Technology Inc."Connect A+" Program Project(Grant number EARD20220208021).

Authors' contributions

JW and RZ contributed to methodology, data curation, data analysis and original draft writing. VL performed data curation and draft review & editing. MC and TW provided resources, software and professional technical support. LL and RS involved in data curation. GW provided resources support.XW contributed to conceptualization, funding acquisition,draft review & editing and supervision. All authors read and approved the final manuscript.

Acknowledgements

Not applicable.

Supplementary Information

Supplementary materials associated with this article can be found in an attached file which include Supplementary raw data file 1-3.

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No competing interests reported.

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Risk Factors for Maxillary Central Incisors Torque Loss in Orthodontic Treatment of Premolar Extraction Cases with Clear Aligners：a retrospective study

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