Molar distalization and stability following maxillary anterior teeth alignment with class II elastics and a 10-day aligner protocol

Abstract

This study compares the anticipated maxillary anterior teeth alignment and molar distal movement with the actual results. The molar stability after anterior teeth alignment has also been investigated. The sample consisted of 30 individuals. Three digital models were obtained using an intra-oral scanner before treatment (T0), after maxillary molar distalization (T1), and at the treatment end, before refinement (T2). The digital models were registered over the palatal rugae area. 120 maxillary molars were investigated for distal movement and anchorage loss. Also, 180 upper anterior teeth were evaluated for anterior teeth alignment. After maxillary molar distalization, the predicted distalization of maxillary molars significantly differed from the actual clinical values (P < 0.001) with a mean percentage accuracy of 75.3% (T0-T1). During T0-T2 stage, anterior teeth showed a significant difference between the two variables except for the labial movement of the lateral incisor (p = 0.056) and canine (p = 0.167). Also, lingual torque of the lateral incisor (p = 0.190). For anterior tooth alignment, the overall mean accuracy was 54.79%. At the treatment end before refinement, the average percentage of posterior anchorage loss was 34.02% and the average percentage accuracy of molar distalization decreased to 48.25% (T0-T2). While clear aligners are an effective modality for maxillary molar distalization in non-growing adults, their accuracy in achieving anterior tooth alignment is limited. Clinicians should account for significant posterior anchorage loss and anticipate challenges in achieving precise lingual movement of the anterior teeth during treatment planning.

Introduction

The advent of the clear aligner has presented a promising alternative to traditional fixed braces for dental correction. Notably, the use of Invisalign for maxillary molar distalization has garnered substantial attention owing to its potential to address complex malocclusions while offering patients an aesthetic and comfortable treatment experience¹.

Several studies have highlighted the efficiency of the Invisalign appliance for molar distalization. Saif et al.² reported an accuracy of 73.8% for molar distalization, which can be achieved with a mean distalization movement of 2.6 mm. Simon et al. also reported an even higher accuracy of 87% for upper molar distalization with clear aligners³. Ravera et al. and Garino et al. both reported the effectiveness of Invisalign in distalizing maxillary molars for adults requiring up to 3 mm of distalization without negatively affecting lower facial height^4,5. A recent systematic review concluded that maxillary molar distalization of 2–3 mm can be achieved across all selected studies, regardless of attachment use⁶.

Anterior teeth alignment is a fundamental aspect of orthodontic treatment. Efficient anterior teeth alignment not only enhances aesthetic appeal but also contributes to achieving functional occlusion. Assessing the efficiency of anterior teeth alignment involves evaluating the effectiveness of the aligners in achieving the desired tooth movement within the predicted timeframe. Regarding anterior tooth movement, Nguyen⁷ reported a mean accuracy of 56%. Grünheid et al. concluded that although the predicted and achieved tooth positions still differed in all tooth types in non-extraction cases, the Invisalign appliance can achieve the predicted outcome with acceptable accuracy⁸. Haouili et al.‘s study also concluded that although Invisalign appliances continue to improve, they still lack accuracy with certain types of tooth movements, such as canine rotation and upper incisor intrusion, with an overall mean accuracy of 50%⁹. Similarly, Sachdev et al.‘s study showed that an overall accuracy of 56.18% for anterior tooth movement can be achieved with clear aligners¹⁰.

Ensuring molar stability after distalization is essential to maintain treatment success over the long term. Quantifying anchorage loss precisely requires obtaining an oral scan at a specific time during treatment. Liu et al., in their finite element analysis, explain the anterior anchorage loss as greater mesial movement and protrusion of the maxillary anterior teeth during molar distalization¹¹. Saif et al. observed a statistically significant relationship between the upper molar distalization and the anterior anchorage loss, where central and lateral incisors were mostly affected². Loberto et al. also reported that significant mesial movement of the maxillary canines occurred during the maxillary molar distalization process¹². However, the posterior anchorage loss associated with clear aligners has not been investigated in depth.

This study aims to assess the accuracy of maxillary molar distalization and anterior teeth alignment using the Invisalign system; besides evaluation of posterior anchorage loss at the treatment end, it presents a comprehensive analysis of Invisalign effectiveness and limitations in achieving desired dental movements. Additionally, it offers valuable considerations for orthodontists and researchers alike to enhance treatment planning and patient care.

Materials and methods

This prospective study conducted following the Declaration of Helsinki and ethically approved by the Stomatological Hospital of Xi’an Jiaotong University Medical Ethics Committee (No.45). To evaluate molar distalization movement and posterior anchorage loss, this study initially enrolled 30 participants (4 males and 26 females), whose digital models included 120 upper molars (60 first molars and 60 second molars) for analysis. Additionally, 180 upper anterior teeth from the digital models of these 30 participants were assessed to evaluate tooth alignment. The inclusion criteria for patient selection include (1) Participants over 18 years old who are eligible for Invisalign treatment. (2) Requiring maxillary molar distalization and subsequent anterior teeth alignment. (3) Extraction of maxillary third molars if present. Exclusion criteria: (1) Growing patients. (2) Patients presented with systemic conditions. (3) Patients with periodontitis. (4) Re-treatment cases. The average age of the participants was 24.6 years, ranging from 18 to 39 years. Every patient received Invisalign therapy at the Department of Orthodontics at the Stomatological Hospital of Xi’an Jiaotong University - Shaanxi, China, by a proficient orthodontist trained in Invisalign treatment. The orthodontic evaluation process is divided into the first stage of molar distalization and the second stage of anterior teeth alignment and molars stability assessment, which are carried out sequentially. In the first stage, a sequential molar distalization protocol was applied to all patients in this study, in which the process was initiated by distalizing the 2nd molar to halfway to its final position; then the 1st molar was moved distally until it stopped. The second stage is also a sequential anterior teeth retraction and alignment after the first premolars reach halfway to their final position. On average, 22 aligners were employed to successfully achieve molar distalization movement, while a mean of 38 aligners was utilized for anterior teeth alignment. The average number of aligners across the entire treatment regimen was determined to be 60. In terms of treatment time, the average duration to achieve maxillary molar distalization was 7.2 months, whereas the average time for anterior tooth alignment was 12.5 months. Consequently, the overall mean duration for the complete treatment process was 19.7 months.

The patients were instructed to wear the aligners full-time, except for eating and cleaning, and to switch to the next aligner every 10 days. Class II elastics (size of elastics ¼, 3.5 Oz.) between precision cut on the area of the maxillary canines to buccal button on the mandibular first molars should also be worn full-time during the molar distalization stage and anterior teeth alignment process. The patients were informed at the initial visit that they were part of a research study and signed informed consent.

The actual pre-treatment model (T0), the digital model after molar distalization (T1), and the final treatment model before refinement (T2) for each patient were acquired using the iTero Element 2 intraoral scanner (Align Technology Inc.) and exported as STL files for future measurements. In contrast, the Clincheck tooth movement table was used to record the predicted tooth movement. Actual models were then imported into the GOM Inspect software suite (version 2022; Carl Zeiss AG, Oberkochen) for superimposition, inspection, and measurement. By selecting the palatal rugae area and applying the local best-fit method based on the iterative closest point (ICP) registration algorithm for digital model alignment, the initial model (T0) was aligned with the second model acquired after maxillary molar distalization was achieved (T1), as shown in (Fig. 1).

Fig. 1.

Digital models alignment. A Initial oral scan model (T0); B Second scan after maxillary molar distalization (T1) and selection of palatal rugae area for local best-fit alignment; C The two models after alignment; D Color map inspection.

The target tooth was selected to construct the center point, which is the origin of the coordinate system. Two coordinate systems were constructed on the same tooth of the two models (T0 and T1). Then, the measurement principles are applied, and the two coordinate systems are connected to see the measurements (Fig. 2).

Fig. 2.

Measurement of molar distalization. A Selection of molar crown area on (T0); B Construction of best-fit point; C Construction of coordinate system; D–F Apply the previous steps to (T1) model; G Link the two coordinate systems; H Check the result; I lateral view.

Posterior anchorage loss has also been evaluated by examining the immediate position of the molar after distalization on the (T1) digital model relative to the (T2) digital model (Fig. 3).

Fig. 3.

Evaluation of posterior anchorage loss. A: Superimposition of (T1) and (T2) models; B: Construction of coordinate system over the molar tooth on the (T1) model. C: Construction of coordinate system over the same molar tooth on the (T2) model; D: Show the results.

For anterior tooth alignment evaluation, the initial oral scan (T0) and the end-of-treatment (T2) digital models were aligned as performed for molar distalization evaluation. Two coordinate systems are constructed over the intended tooth and linked together to see the results. Three linear and three angular measurements were examined to evaluate the anterior teeth alignment. Linear measurements are as follows: (1) Mesiodistal movement, which is the movement along the X-axis. (2) Vertical movement is the movement along the Y-axis that shows the intrusive and extrusive anterior teeth movement. (3) Sagittal movement is the movement along the Z-axis that shows the labiolingual movement. (4) Inclination, which is the labiolingual rotational movement around the X-axis representing torque movement. (5) Rotation, the rotational movement around the Y-axis. (6) Angulation, the rotational movement around the Z-axis that represents the crown tipping (Fig. 4).

Fig. 4.

Evaluation of anterior teeth alignment. A Selection of incisor crown area on (T0) model; B Construction of best-fit point; C Construction of coordinate system over the point; D–F Repeat the same steps on the (T2) model; G Link the two coordinate systems; H Result inspection.

To account for a model registration error, the anticipated linear and angular movements of less than 0.2 mm and 1.0° were not included for analysis.

Statistical analysis

To assess the accuracy of the measurement method, the first examiner remeasured 10% of digital models and a different examiner two weeks later, to evaluate intra- and inter-rater reliability. Normality of the data was confirmed by the Shapiro-Wilk test, allowing paired t-tests to determine differences between clinically achieved and Clincheck-predicted tooth movement values for each tooth. Statistical significance was defined as P < 0.05 for assessing the difference between predicted and achieved values.

Based on a formula proposed by Pandis¹³, we conducted a sample size calculation with a statistical power of 80% and a significance level set at 0.05. The objective was to detect a difference of 0.5 mm in linear movement between the predicted and achieved outcomes, assuming a standard deviation of 0.7 mm. Therefore, a sample size of thirty subjects would be sufficient for our study.

The following equation was used to calculate the percentage of tooth movement accuracy: percentage of accuracy = 100% - [(predicted-achieved)/predicted) *100%]¹⁴, The Pearson correlation test was employed to analyze the association between the distal movement of the maxillary molars and the subsequent posterior anchorage loss. All statistical analyses were performed using SPSS software (version 23; IBM Corp., Armonk, NY).

Results

A total of 120 maxillary molars were investigated for distal molar movement and posterior anchorage loss. Also, 180 upper anterior teeth were evaluated for alignment. The intraclass correlation coefficient (ICC) results demonstrated excellent agreement regarding intra-rater reliability with a score of 0.976 (95% confidence interval [CI], 0.961–0.985) for linear measurements, and 0.947 (95% [CI], 0.915–0.967) for angular measurements. The inter-rater reliability values also showed excellent agreement, 0.953 and 0.924 for linear and angular measurements, respectively.

During T0-T1 stage, the predicted distal movement of the maxillary 1st molar (P < 0.001) and maxillary 2nd molar (P < 0.001) differed significantly from the achieved outcome. The percentage accuracy for maxillary molar distalization was 75.70% for the 1st molar and 75.01% for the 2nd molar. The mean, standard deviation, and the difference between the predicted and clinically actual maxillary molar distalization were presented in Table 1.

Table 1.

Paired t-test used to evaluate the differences between the predicted and achieved values for maxillary molar distalization during T0-T1 stage.

Tooth		Predicted	Achieved
	n	Mean ± SD	Mean ± SD	Difference	Accuracy (%)	P-value
Maxillary 1st molar	60	2.59 ± 1.02	1.78 ± 0.98	0.80655	75.70%	0.0001†
Maxillary 2nd molar	60	2.75 ± 1.13	1.93 ± 1.09	0.81189	75.01%	0.0001†

^†Statistically significant difference (P ≤ 0.0001).

For anterior teeth alignment, acceptable sample sizes were included for all tooth movements, except for the labial movement of the central incisor (n = 8) and lingual inclination of the lateral incisor (n = 7). When comparing the mean predicted and the achieved tooth movement, results showed that all anterior teeth showed a significant difference between the two variables in all six types of tooth movement, except for the labial movement of the lateral incisor (p = 0.056) and canine (p = 0.167). Also, the lingual torque of the lateral incisor (p = 0.190) (Table 2).

Table 2.

Paired t-test compares the differences between the predicted and achieved values of anterior teeth movement.

				Predicted		Achieved
		Tooth	n	Mean	SD	Mean	SD	Mean difference	P-value
Mesiodistal	Mesial	Central incisor	16	1.21	0.61	0.76	0.49	0.44	0.01**
		Lateral incisor	16	1.89	0.95	0.84	0.65	1.05	0.001†
		Canine	17	2.54	1.01	0.75	0.71	1.79	0.001†
	Distal	Central incisor	39	1.04	0.62	0.51	0.45	0.53	0.001†
		Lateral incisor	39	1.71	0.91	0.64	0.46	1.06	0.001†
		Canine	41	2.31	1.20	0.85	0.77	1.45	0.001†
Vertical	Extrusion	Central incisor	11	1.90	1.05	0.55	0.57	1.34	0.001†
		Lateral incisor	13	0.83	0.54	0.41	0.36	0.42	0.01**
		Canine	14	1.37	0.97	0.66	0.59	0.71	0.012*
	Intrusion	Central incisor	46	2.09	1.18	0.56	0.61	1.53	0.001†
		Lateral incisor	42	1.84	0.86	0.56	0.81	1.28	0.001†
		Canine	46	0.88	0.62	0.59	0.54	0.28	0.013*
Labiolingual	Labial	Central incisor	8	1.46	1.32	0.65	0.58	0.81	0.039*
		Lateral incisor	15	0.88	0.88	0.73	0.71	0.15	0.056
		Canine	13	0.61	0.36	0.46	0.27	0.14	0.167
	Lingual	Central incisor	50	1.75	1.21	0.42	0.74	1.33	0.001†
		Lateral incisor	44	1.34	1.10	0.51	0.42	0.83	0.001†
		Canine	43	1.27	1.14	0.51	0.48	0.76	0.001†
Inclination (Torque)	Labial	Central incisor	40	7.41	5.94	4.22	4.32	3.18	0.001†
		Lateral incisor	52	6.28	4.05	3.94	3.48	2.34	0.001†
		Canine	48	4.79	3.84	3.61	3.31	1.18	0.001†
	Lingual	Central incisor	16	3.45	3.34	1.89	2.96	1.56	0.01**
		Lateral incisor	7	3.57	2.36	1.72	3.04	1.84	0.190
		Canine	12	7.27	7.44	4.63	5.04	2.64	0.045*
Angulation (Tip)	Mesial	Central incisor	22	3.30	3.73	2.78	3.48	0.51	0.017*
		Lateral incisor	33	3.25	2.29	2.02	1.89	1.22	0.001†
		Canine	42	4.64	2.91	2.71	1.99	1.93	0.001†
	Distal	Central incisor	36	3.40	2.09	2.26	1.95	1.14	0.01**
		Lateral incisor	22	4.88	3.92	3.29	2.98	1.58	0.001†
		Canine	19	5.05	5.32	3.54	4.39	1.51	0.014*
Rotation	Mesial	Central incisor	11	7.17	6.89	5.41	5.66	1.76	0.01**
		Lateral incisor	22	9.25	6.37	5.49	3.85	3.75	0.001†
		Canine	35	11.9	9.11	7.38	5.98	4.51	0.001†
	Distal	Central incisor	50	15.9	12.3	9.93	7.97	5.99	0.001†
		Lateral incisor	38	9.28	7.75	6.77	6.04	2.51	0.01**
		Canine	26	11.2	10.9	6.03	4.76	5.09	0.001†

SD, standard deviation.

*Statistically significant difference (P ≤ 0.05); **Statistically significant difference (P ≤ 0.01); ^†Statistically significant difference (P ≤ 0.001).

The overall mean accuracy of anterior teeth alignment was found to be 54.79%. The highest accuracy was achieved during distal tipping (68.51%) and labial movement (68.20%), followed by mesial tipping (67.90%) and mesial rotational movement (65.21%), while the lowest accuracy was lingual movement (34.12%).

The most accurate tooth movements were the mesial tipping of central incisors (82.41%), followed by labial movement of lateral incisors (82.95%), then labial movement of canines (76.85%) and mesial rotational movement of central incisors (75.20%). In contrast, the least accurate tooth movements were the lingual movement of central incisors (24.29%), followed by intrusion (26.68%) and extrusion (29.24%) movement of central incisors, and intrusion of lateral incisors (30.49%) (Table 3).

Table 3.

Percentage accuracy of anterior teeth alignment.

Tooth	Mesiodistal		Vertical		Labiolingual		Inclination		Angulation		Rotation
	M	D	E	I	La	Li	B	L	M	D	M	D
C.Incisor	63.06	48.79	29.24	26.68	44.80	24.29	57.07	56.29	84.41	66.05	75.20	62.49
L.Incisor	44.37	37.67	49.34	30.49	82.95	37.87	62.65	44.43	60.90	67.29	59.40	73.47
Canine	29.51	37.14	48.34	67.38	76.85	40.21	74.57	63.62	58.40	72.20	62.02	54.07
Total	45.65	41.20	42.31	41.52	68.20	34.12	64.76	54.78	67.90	68.51	65.21	63.34

C: central, L: lateral. M: mesial, D: distal, E: extrusion, I: intrusion, La: labial, Li: lingual.

For posterior anchorage loss, mesial movement of the molars was observed during the T1-T2 stage, with the first and second molars exhibiting displacements of 0.57 mm and 0.69 mm, respectively. Pearson correlation test results showed a statistically significant correlation between the amount of maxillary molar distal movement and the amount of the posterior anchorage loss (0.76, P < 0.001) for the 1st molar, and (0.74, P < 0.001) for the 2nd molar (Table 4). The average percentage of posterior anchorage loss was 34.02%, where the maxillary 1st molar showed slightly less anchorage loss (32.27%) than the maxillary 2nd molar (35.77%).

Table 4.

Posterior anchorage loss.

	Mean	SD	Pearson Correlation	P-value
1st Molar	0.57	0.41	0.760**	0.001
2nd Molar	0.69	0.41	0.737**	0.001

**Correlation is significant at the 0.01 level.

Discussion

The use of Invisalign for maxillary molar distalization has been a subject of research in recent years. This study was established to understand two phases of Invisalign treatment: measurements were obtained at the end of the distalization movement to determine the exact distalization amount accurately, and additional measurements were performed at the end of treatment to examine molar stability after the distalization phase and to evaluate subsequent anterior tooth alignment.

This study’s results demonstrated that Invisalign is an effective method for maxillary molar distalization, with a percentage accuracy of 75.70% for the first molar and 75.01% for the second molar after maxillary molar distalization. These results indicated that Invisalign can achieve approximately two-thirds of the predicted maxillary molar distalization outcome, consistent with previous investigations. Simon et al. indicated an even higher accuracy of 87% for upper molar distalization with clear aligners. Their research found no statistically significant difference between the groups with and without attachments, suggesting that these attachments may not significantly impact treatment outcomes³. Similarly, Saif et al. have reported that an accuracy of 73.8% can be achieved with a mean distalization movement of 2.6 mm. Their study also found no significant difference between the attachment and non-attachment groups².

Antò et al.‘s investigation in the same area showed that, with a mean distalization of 2 mm, the Invisalign appliance can achieve 69.3% accuracy for the 1st molar and 75.2% for the 2nd molar¹⁵. Additionally, Ravera et al. and Garino et al. both highlighted the effectiveness of Invisalign in distalizing maxillary molars for adult patients requiring up to 3 mm of distalization without negatively affecting lower facial height^4,5. Systematic reviews performed by Rossini et al.¹⁶, Papadimitriou et al.¹⁷, and Lopez et al.¹⁸ also showed that maxillary molar distalization was highly predictable.

The results of these studies collectively emphasize the effectiveness of Invisalign for maxillary molar distalization, with accuracy rates ranging from 69% to 87%. However, a critical distinction must be made when interpreting these reported accuracy rates within a comprehensive treatment plan. The aforementioned rates, including our initial finding of approximately 75% measured immediately after the distalization phase (T0–T1), reflect initial distalization efficiency. The clinically decisive outcome, however, is the net distalization ultimately available for anterior alignment at the end of active treatment (T0–T2). This study reveals that this final value decreased to approximately 48%, a consequence of significant posterior anchorage loss, expressed as mesial drift of the molars observed during the subsequent anterior teeth alignment and retraction phase. Therefore, the space created by distalization is dynamically contested and not statically held. Consequently, while the aligner system demonstrates a reliable capacity to achieve distal movement, a substantial portion of the initially gained space is inherently consumed to compensate for anchorage loss during final anterior teeth alignment. This fundamental clinical reality must be meticulously considered in virtual treatment planning to accurately anticipate the final spatial envelope for anterior tooth alignment.

Achieving successful molar distalization is only one aspect of the orthodontic journey; ensuring the stability of the achieved results is equally crucial. The ability of the treated molars to maintain their newly acquired positions during distalization throughout the treatment is vital to allow the use of the obtained spaces for tooth alignment as planned. Viewing the treatment procedure virtually in Clincheck software provides an overview of the dental movement status that mimics the clinical situation; however, it does not mean the clinical situation will be achieved in the same way as the virtual plan, and the clinician should have sufficient knowledge of the treatment procedure and appliance limitations. Traditionally, unwanted forward movement of molars, expressed as posterior anchorage loss, occurs in many cases, and in the Invisalign situation, clinicians should expect it to occur even though it is not mentioned in the treatment plan. To investigate posterior anchorage loss precisely, the final molar position immediately after distalization (T1) has been compared with the final treatment model after anterior tooth alignment before refinement (T2). The results showed that the average percentage of posterior anchorage loss was 34.02%, where the maxillary 1st molar showed slightly less anchorage loss (32.27%) than the maxillary 2nd molar (35.77%). Additionally, there was a statistically significant correlation between the amount of maxillary molar distal movement and the amount of posterior anchorage loss (0.76, P < 0.001) for the 1st molar and (0.74, P < 0.001) for the 2nd molar. The use of Class II elastics can offset or reduce the labial inclination of anterior teeth in T0-T1 stage, thus indirectly controlling the anchorage of anterior teeth in the distal movement stage of molars. The elastics force can also reduce the inevitable mesial movement of molars in the anterior teeth retraction stage. Therefore, both stages are conducive to achieving preset tooth movement more effectively. The mesial movement of molars is not pre-designed, but inevitably occurs, which is consistent with the findings of Miao et al.¹⁹. These findings underscore the importance of considering anchorage loss of posterior teeth in treatment planning, as it significantly affects the potential spatial utilization of anterior teeth arrangement. Therefore, the lack of the exact amount of space gained through molar distalization can be considered a factor explaining the poor accuracy of lingual movement of anterior teeth (34.12%).

Current research also found that the overall mean accuracy of anterior tooth alignment (54.79%) is similar to results reported in other studies. Nguyen⁷ and Haouili⁹ reported that a mean anterior tooth movement accuracy of 50–56% can be achieved using Invisalign.

Results also showed no statistically significant difference in labial movement of the lateral incisor (P = 0.056) and the canine (P = 0.167); even for the central incisors, the difference was minor (P = 0.039). The labial movement achieved the highest accuracy (68%), while lingual movement achieved the lowest (34.12%). The labial movement of the central incisor (44.8%) was less accurate than that of the lateral incisors (82.95%) and the labial movement of canines (76.85%). Also, the labial inclination movement of anterior teeth (64.76%) was more accurate than lingual inclination. The labial inclination movement of the central incisor (57.07%), lateral incisor (62.65%), and canine (74.57%). This is true because, at the end of the molar distalization process, the anterior teeth attain a more labial position, as evidenced by anterior anchorage loss and anterior teeth flaring even before they start to move, as predicted^2,12. Patterson et al.²⁰ found that no improvement in anteroposterior correction was observed, with overbite correction of 29% and 39% of the predicted values in patients with Class I and Class II malocclusion, respectively. A finite element analysis conducted by Liu et al.¹¹ also reported that during maxillary molar distalization, upper anterior teeth were labially and mesially proclined. Our study results also showed that mesial linear movement of central incisors (63.06%) and lateral incisors (44.37%) was more frequently predicted than distal movement, which also agreed with Grunhied et al.‘s study, which showed that all anterior teeth were positioned more mesially than predicted⁸. Additionally, Krieger et al., in their study on anterior tooth movement using Invisalign, concluded that applying incisor protrusion for dental alignment appears easy to predict and implement²¹.

Concerning anterior teeth vertical movement, the accuracy of intrusion was 41.52%, and the accuracy of extrusion movement was 42.31%. The intrusion movement accuracy of central incisors (26.68%) and lateral incisors (30.49%) was shown to be less accurate than predicted. Grünheid et al. reported that when using the Invisalign appliance, the anterior teeth attain a more occlusal position⁸. Kravitz et al. also reported poor accuracy in vertical anterior tooth movement, with only 41% of the predicted intrusion movement achieved¹⁴. Haouili and Sachdev also concluded that maxillary incisor intrusion without auxiliaries is still challenging^9,10. Also, Al-Balaa et al.‘s results were not far off, with a mean precision of 51.19% for anterior tooth intrusion. Therefore, the correction of a deep overbite with Invisalign appears less predictable. Hence, overtreatment can achieve incisor intrusion, and the effect can be enhanced by adding attachments²².

Our study also showed that mesial rotation accuracy of anterior teeth was 65.21%, slightly higher than distal rotation (63.34%). The mesial rotation of central incisors (75.2%) and canines (62.02%) was significantly more accurate than distal rotation (62.5%) and (54.07%). These results agreed with those of Haouili⁹, in which mesial rotation of central incisors (61%) and maxillary canines (51.5%) was more accurate than distal rotation.

Practitioners must use Clincheck not only to visualize anticipated treatment results but also to design the biomechanics involved. Understanding the aspects where the actual tooth position differs more from the predicted position enables them to prioritise difficult movements and incorporate deliberate overcorrections into their virtual treatment plans. This approach enhances the treatment efficiency and facilitates superior treatment outcomes.

While Invisalign has demonstrated considerable efficacy in maxillary molar distalization and acceptable outcomes for anterior tooth alignment, a comprehensive understanding of the challenges, limitations, and patient-related factors is crucial for achieving successful, stable treatment results. The ongoing exploration of these factors through further research is essential to refine treatment protocols and enhance the overall success of orthodontic interventions with Invisalign.

It’s common to switch to the next aligner every 7, 10, or 14 days. Drake et al. indicated that there was no significant difference in orthodontic tooth movement between patients who changed aligners every 14 days and those who changed them after 1 week²³. Al-Nadawi et al. suggested that a 14-day wear protocol may be beneficial in patients who require more complex angular and posterior tooth movements (i.e., tip, torque, and rotations)²⁴. In this study, the aligners were changed every 10 days because a slightly longer interval allows for better adaptation to the new aligners, reducing discomfort and improving compliance.

This study has several limitations, including a clinical sample characterized by limited molar distalization, with few movements exceeding 3 mm. Therefore, future investigations with cases requiring greater distalization would strengthen these findings. Furthermore, both the treatment delivery and the source of predicted tooth movement data (ClinCheck software) involved proprietary products from the same manufacturer (Align Technology). While this is inherent to evaluating the system’s integrated workflow, it represents a single-source reference for the predicted values used as the comparison standard. Additionally, the assessment of accuracy was confined to the initial treatment phase before refinement, meaning that movements with poor initial accuracy may have been corrected with subsequent aligners, and the potential impact of overcorrection prescriptions was not evaluated. Finally, as patient compliance is a critical factor in clear aligner therapy, a key direction for future research is the development of objective methods to improve patient adherence and enhance our understanding of treatment outcomes.

Conclusions

• The Invisalign appliance was reliable for achieving the desired outcomes of maxillary molar distalization for non-growing adult patients. However, orthodontists must consider the potential for posterior anchorage loss and the subsequent challenges in achieving precise lingual movement of anterior teeth.

• Anterior teeth vertical movements highlight the need for careful consideration and potential overcorrections in the treatment planning phase.

• Additional refinements for anterior teeth alignment and increased treatment duration would be required to achieve ideal results.

• Clinicians must design appropriate biomechanics to account for potential discrepancies between the planned and actual tooth positions, which ultimately leads to improved treatment efficiency and more favorable treatment outcomes.

Author contributions

BSS: Data curation, Methodology, Software, Visualization, Formal Analysis, Writing – Original Draft. YT: Software, Visualization, Formal Analysis. WB: Software, Formal Analysis. YL: Methodology, Formal Analysis. LJ: Methodology, Validation. SD: Methodology, Validation. FP: Conceptualization, Formal Analysis, Resources. YG: Conceptualization, Supervision, Writing – Review & Editing, Project Administration, Funding Acquisition.

Funding

This work was supported by the National Natural Science Foundation of China (No. 81701869) and the China Postdoctoral Science Foundation (No. 2019M653664).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

This prospective study was ethically approved by the Stomatological Hospital of Xi’an Jiaotong University Medical Ethics Committee (No.45).

Informed Consent

Written informed consent was obtained from all study participants.