Abstract
This study evaluated the association between planned anterior tooth movements, attachment use, and the need for refinement in clear aligner (CA) therapy using logistic regression. A retrospective analysis was conducted on 116 patients and 696 anterior teeth treated with Invisalign®. For each tooth, planned movement magnitudes (extrusion, intrusion, rotation, angulation, inclination) and the presence of attachments were recorded. Refinement was defined as a binary outcome. Multivariable logistic regression and independent t-tests were used to assess associations and compare movement magnitudes between refined and non-refined teeth. In tooth 11, greater planned rotation was associated with a modest but statistically significant reduction in refinement likelihood (OR = 0.92, p < .05). Attachment use emerged as a significant factor for teeth 12 and 21, decreasing refinement risk in tooth 21 (OR = 0.05, p < .01), yet increasing it in tooth 12 (OR = 4.95, p < .05). For tooth 22, increased planned extrusion reduced the probability of refinement (OR = 0.34, p < .05), whereas higher degrees of rotation and inclination were associated with increased refinement rates (OR = 1.13 and OR = 1.52, respectively; p < .05). No statistically significant predictors were identified for canines (teeth 13 and 23). These findings underscore the heterogeneity in biomechanical response among maxillary anterior teeth during CA. Specifically, lateral incisors demonstrated a greater susceptibility to refinement needs in the presence of complex movements such as rotation and inclination. These results support the adoption of individualized, tooth-specific treatment planning strategies to enhance predictability and minimize the need for refinement in CA therapy.
Keywords: Clear aligners, Orthodontic refinement, Tooth-specific movement, Anterior teeth, Logistic regression analysis, Invisalign®
Subject terms: Anatomy, Health care, Medical research
Introduction
CA therapy has become a widely accepted alternative to conventional fixed appliances, particularly among adult patients who prioritize aesthetics and convenience. The increasing use of aligners, such as Invisalign®, has been driven by technological advances in digital treatment planning and aligner manufacturing. However, a significant clinical challenge remains: high rates of refinement—defined as the need for additional aligners to achieve desired outcomes—are frequently observed and can compromise treatment efficiency and patient satisfaction1,2.
Several studies have reported that over 70% of CA cases require at least one refinement stage, with many undergoing two or more3,4. These additional phases not only increase treatment time but also place additional demands on clinician time and patient compliance5. Understanding the causes behind these refinements is therefore essential for optimizing treatment outcomes.
Refinements are often associated with limitations in force delivery and aligner fit, particularly for complex tooth movements such as rotation, extrusion, and torque control—movements that remain biomechanically challenging for plastic appliances6,7. Other contributing factors include patient-related variables (e.g., inconsistent wear), treatment planning limitations, and attachment loss or misplacement8,9.
While these studies have provided valuable insights, most analyses are conducted at the patient level or case level, without accounting for variations between individual teeth. In clinical practice, however, refinement needs are often localized to specific teeth—particularly anterior teeth—where rotational and angular corrections are frequently unpredictable10. Despite this, there is a notable gap in the literature regarding tooth-level predictive models for refinement, and few studies have examined how individual planned movements and attachment presence correlate with the likelihood of refinement on a per-tooth basis.
A tooth-specific approach is essential to enhance the predictability of aligner therapy. Identifying which planned movements—and in which teeth—are linked to refinement can guide clinical decisions and patient communication. This study addresses this gap using a logistic regression model to assess associations between planned anterior movements, attachment use, and refinement needs, aiming to improve treatment planning and reduce inefficiencies.
Materials and methods
The study was approved by the Non-Interventional Health Research Ethics Committee of Istanbul Medeniyet University (Approval No: 25/517, Date: May 30, 2025). In accordance with the committee’s requirements, written informed consent was obtained from all participants following the development of the study protocol and prior to the initiation of data collection. All methods were carried out in accordance with relevant guidelines and regulations. The study included only adult patients, with a mean age of 24.63 ± 4.38 years. Digital records of patients who underwent Invisalign treatment between 2020 and 2024 at a private orthodontic clinic were analyzed. Patients were selected in accordance with the inclusion and exclusion criteria summarized in Table 1. The analysis focused on the maxillary anterior region—specifically teeth 11, 12, 13, 21, 22, and 23—due to their clinical relevance and biomechanical sensitivity in CA therapy. Only the initial ClinCheck® treatment plans were analyzed, as the study aimed to evaluate predictors of refinement requirements based on the planned tooth movements outlined in the original treatment setup. For each anterior tooth specific planned movements—extrusion, intrusion, rotation, angulation, and inclination—were recorded, along with the presence or absence of attachments. The need for refinement, defined as requiring additional aligners after completing the initial planned movement, was noted as a binary yes/no variable. Each tooth was assessed individually based on movement type, attachment presence, and refinement requirement. Accordingly, cases were classified into two groups: the refinement group (RT) (teeth that required additional aligners after the initial treatment plan) and the control group (CT) (teeth that achieved the planned movement without refinement).
Table 1.
Inclusion and exclusion Criteria.
| Inclusion criteria | Exclusion criteria |
|---|---|
| Patients aged 18 years or older (adults) | Patients under the age of 18 (growing individuals) |
| Complete eruption of all permanent teeth | Partially erupted or missing permanent teeth |
| Patients who have undergone clear aligner therapy | Patients who discontinued treatment or experienced serious complications |
| Planned orthodontic movement inall anterior teeth (incisors andcanines) | Patients with no planned movementin one or more anterior teeth |
| Availability of complete initial | Missing or inaccessible initial |
| ClinCheck® treatment plans | ClinCheck® treatment plans |
| No systemic health conditions contraindicating orthodontic treatment | Presence of systemic disease or medication interfering with orthodontic movement |
Power analysis
An a priori power analysis was conducted using G*Power version 3.1 with a two-tailed test, a significance level of 5% and a target power of 80%. The dependent variable was refinement (yes/no), and the independent variables were the types of planned tooth movements. The model was assumed to explain approximately 10% of the variance (R² = 0.10), and the expected prevalence of refinement was estimated at 30%. Under the assumption of independent observations, a minimum of 58 patients was required. However, since six anterior teeth from each patient were included in the tooth-level analyses, a design effect of approximately 2 was calculated based on an assumed intraclass correlation coefficient (ICC) of 0.20. Accordingly, the required sample size was adjusted to 116 patients.
Statistical analysis
To handle missing data and avoid bias, multiple imputation (MI) using the fully conditional specification (FCS) method was performed with SPSS version 25. Multiple imputation was applied to the extrusion and intrusion variables with missing data to avoid the bias and variance underestimation associated with single imputation methods, as it generates multiple completed datasets under the assumption of randomness, thereby providing more reliable and unbiased statistical estimates11. A total of five imputed datasets were created, each based on ten iterations, and analyses were conducted on pooled results. Logistic regression was performed separately for each of the six anterior teeth to identify factors associated with refinement need (yes/no), with planned tooth movements and attachment use as predictors. Model significance was assessed via β coefficients, p-values, and odds ratios. Additionally, independent samples t-tests compared mean planned movements between patients who required refinement and those who did not. Prior to interpretation, assumptions of normality (Kolmogorov–Smirnov test) and homogeneity of variances (Levene’s test) were checked. Effect sizes were calculated using Cohen’s d. All analyses and interpretations were based on the pooled imputed data.
Results
Logistic regression findings
Logistic regression results and model validity indicators are summarized in Table 2. Key findings by tooth are as follows:
Table 2.
Logistic regression results for predicting refinement requirement.
| Variables | B | SE | p | Exp(β)/OR | %95 CI | ||
|---|---|---|---|---|---|---|---|
| Lower | Upper | ||||||
| Extrusion | 0.35 | 0.39 | 0.375 | 1.42 | 0.64 | 3.14 | |
| Intrusion | 1.51 | 0.82 | 0.094 | 4.54 | 0.73 | 28.10 | |
| Rotation | -0.05 | 0.04 | 0.159 | 0.95 | 0.87 | 1.02 | |
| 23 | Angulation | -0.06 | 0.07 | 0.379 | 0.94 | 0.82 | 1.08 |
| Inclination | -0.16 | 0.12 | 0.196 | 0.86 | 0.67 | 1.08 | |
| Attachment | -0.06 | 1.05 | 0.953 | 0.94 | 0.12 | 7.52 | |
| Constant | 0.15 | 1.27 | 0.905 | 1.16 | 0.09 | 14.58 | |
| Extrusion | -1.07 | 0.34 | 0.002 | 0.34 | 0.18 | 0.67 | |
| Intrusion | -0.77 | 0.79 | 0.345 | 0.46 | 0.08 | 2.56 | |
| Rotation | 0.12 | 0.04 | 0.007 | 1.13 | 1.03 | 1.23 | |
| 22 | Angulation | 0.01 | 0.09 | 0.920 | 1.01 | 0.84 | 1.22 |
| Inclination | 0.42 | 0.11 | 0.000 | 1.52 | 1.23 | 1.87 | |
| Attachment | -0.54 | 1.09 | 0.618 | 0.58 | 0.07 | 4.94 | |
| Constant | -1.80 | 1.18 | 0.128 | 0.17 | 0.02 | 1.69 | |
| Extrusion | 0.43 | 0.71 | 0.571 | 1.53 | 0.26 | 9.04 | |
| Intrusion | 0.59 | 0.51 | 0.245 | 1.81 | 0.66 | 4.94 | |
| Rotation | 0.00 | 0.07 | 0.976 | 1.00 | 0.86 | 1.16 | |
| 21 | Angulation | 0.20 | 0.19 | 0.315 | 1.22 | 0.81 | 1.83 |
| Inclination | 0.19 | 0.22 | 0.425 | 1.21 | 0.70 | 2.06 | |
| Attachment | -2.94 | 0.76 | 0.000 | 0.05 | 0.01 | 0.24 | |
| Constant | 0.20 | 1.44 | 0.890 | 1.22 | 0.05 | 27.74 | |
| Extrusion | 0.68 | 0.57 | 0.249 | 1.98 | 0.58 | 6.75 | |
| Intrusion | -0.04 | 0.49 | 0.942 | 0.96 | 0.33 | 2.79 | |
| 13 | Rotation | 0.00 | 0.03 | 0.956 | 1.00 | 0.94 | 1.07 |
| Angulation | 0.19 | 0.14 | 0.165 | 1.21 | 0.92 | 1.59 | |
| Inclination | 0.01 | 0.12 | 0.920 | 1.01 | 0.79 | 1.29 | |
| Constant | -1.04 | 0.59 | 0.090 | 0.35 | 0.11 | 1.19 | |
| Extrusion | -0.46 | 0.64 | 0.490 | 0.63 | 0.15 | 2.70 | |
| Intrusion | 0.03 | 0.78 | 0.968 | 1.03 | 0.16 | 6.49 | |
| Rotation | -0.02 | 0.04 | 0.712 | 0.98 | 0.90 | 1.08 | |
| 12 | Angulation | 0.00 | 0.06 | 0.976 | 1.00 | 0.89 | 1.11 |
| Inclination | 0.00 | 0.10 | 0.989 | 1.00 | 0.79 | 1.27 | |
| Attachment | 1.60 | 0.70 | 0.025 | 4.95 | 1.23 | 19.93 | |
| Constant | -0.95 | 0.92 | 0.308 | 0.39 | 0.06 | 2.51 | |
| Extrusion | 0.10 | 0.46 | 0.825 | 1.11 | 0.43 | 2.83 | |
| Intrusion | -0.33 | 0.29 | 0.245 | 0.72 | 0.40 | 1.26 | |
| Rotation | -0.08 | 0.04 | 0.024 | 0.92 | 0.86 | 0.99 | |
| 11 | Angulation | 0.02 | 0.09 | 0.861 | 1.02 | 0.86 | 1.20 |
| Inclination | 0.07 | 0.05 | 0.133 | 1.07 | 0.98 | 1.18 | |
| Attachment | -0.55 | 0.41 | 0.179 | 0.57 | 0.26 | 1.29 | |
| Constant | 0.49 | 0.57 | 0.391 | 1.63 | 0.53 | 4.99 | |
RT refinement group, CT control group, B regression cofficient, SE standard error; Exp(β)/OR: Odds ratio.
-Tooth 22: Extrusion significantly decreased the likelihood of requiring refinement (OR = 0.34). In contrast, rotation (OR = 1.13) and inclination (OR = 1.52) were associated with increased refinement probability.
-Tooth 21: The use of attachments was the only significant variable, markedly reducing the likelihood of refinement (OR = 0.05).
-Tooth 12: Unlike other teeth, attachment presence was linked to a higher refinement likelihood (OR = 4.95), suggesting a different underlying mechanism.
-Tooth 11: Rotation was the only significant predictor and was associated with a lower refinement risk (OR = 0.92).
-Teeth 23 and 13: No variables showed statistically significant associations with refinement outcomes.
Intergroup comparisons of planned tooth movements
Statistical comparisons of planned movement magnitudes between the RT and CT groups are presented in Table 3. Significant differences with corresponding effect sizes were identified as follows:
Table 3.
Comparison of the magnitudes of planned tooth movements between the RT and CT groups.
| Tooth | Variables | CT | RT | p value | d value | ||
|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | ||||
| 23 | Extrusion | 0.68 | 0.77 | 0.80 | 0.88 | 0.455 | -- |
| Intrusion | 0.40 | 0.45 | 0.73 | 0.61 | 0.086 | -- | |
| Rotation | 10.91 | 8.59 | 9.14 | 8.73 | 0.276 | -- | |
| Angulation | 3.94 | 2.32 | 3.75 | 3.61 | 0.742 | -- | |
| Inclination | 2.76 | 2.05 | 2.58 | 1.98 | 0.641 | -- | |
| 22 | Extrusion | 0.69 | 0.88 | 0.60 | 1.42 | 0.708 | -- |
| Intrusion | 0.62 | 0.48 | 0.46 | 0.57 | 0.139 | -- | |
| Rotation | 9.97 | 8.79 | 13.96 | 9.26 | 0.019 | 0.44 | |
| Angulation | 4.25 | 2.56 | 5.30 | 3.39 | 0.063 | -- | |
| Inclination | 3.06 | 2.47 | 7.22 | 5.41 | 0.000 | 0.99 | |
| 21 | Extrusion | − 0.61 | 4.13 | 0.70 | 6.00 | 0.657 | -- |
| Intrusion | 0.92 | 0.46 | 0.89 | 0.75 | 0.800 | -- | |
| Rotation | 5.17 | 4.88 | 7.60 | 6.86 | 0.030 | 0.41 | |
| Angulation | 2.94 | 2.09 | 3.90 | 2.86 | 0.040 | 0.39 | |
| Inclination | 4.84 | 3.80 | 5.71 | 5.44 | 0.321 | -- | |
| 13 | Extrusion | 0.35 | 0.54 | 0.65 | 0.71 | 0.074 | -- |
| Intrusion | 0.67 | 0.79 | 0.79 | 0.63 | 0.483 | -- | |
| Rotation | 8.74 | 6.54 | 9.53 | 7.24 | 0.542 | -- | |
| Angulation | 2.96 | 1.64 | 4.03 | 2.74 | 0.011 | 0.48 | |
| Inclination | 2.21 | 2.28 | 3.18 | 2.44 | 0.029 | 0.41 | |
| 12 | Extrusion | 0.52 | 0.71 | 0.35 | 0.87 | 0.460 | -- |
| Intrusion | 0.74 | 0.58 | 0.74 | 0.69 | 0.994 | -- | |
| Rotation | 13.85 | 9.91 | 12.14 | 7.96 | 0.311 | -- | |
| Angulation | 4.39 | 3.44 | 4.60 | 4.37 | 0.772 | -- | |
| Inclination | 4.83 | 5.58 | 4.49 | 4.38 | 0.717 | -- | |
| 11 | Extrusion | 0.82 | 0.56 | 0.85 | 0.59 | 0.871 | -- |
| Intrusion | 0.92 | 0.68 | 0.79 | 1.07 | 0.521 | -- | |
| Rotation | 8.59 | 6.68 | 6.04 | 5.48 | 0.026 | 0.42 | |
| Angulation | 2.95 | 2.67 | 2.69 | 2.32 | 0.584 | -- | |
| Inclination | 5.78 | 3.64 | 6.68 | 6.09 | 0.338 | -- | |
RT refinement group, CT control group, SD standard deviation.
-Tooth 22: Inclination was significantly greater in the RT group (M = 7.22, SD = 5.41) than in the CT group (M = 3.06, SD = 2.47), with a large effect size (p < .001, d = 0.99). Rotation also differed significantly, showing a moderate effect (p = .019, d = 0.44).
-Tooth 21: The RT group exhibited significantly higher rotation (p = .030, d = 0.41) and angulation values (p = .040, d = 0.39), both indicating moderate effects.
-Tooth 13: Significantly higher values were observed in the RT group for both angulation (p = .011, d = 0.48) and inclination (p = .029, d = 0.41).
-Tooth 11: A significant difference was found in rotation (p = .026, d = 0.42), with a moderate effect.
No statistically significant differences (p > .05) or meaningful effect sizes were observed for teeth 12 and 23 or movement types.
A detailed summary of these comparisons is provided in Table 3, while differences in attachment presence between groups are reported in Table 4.
Table 4.
Comparison of attachment presence between the RT and CT groups.
| Tooth | Attachment | CT | RT | p | φ | ||
|---|---|---|---|---|---|---|---|
| f | % | f | % | ||||
| 23 | 0 | 3 | 5.2 | 7 | 12.3 | 0.176 | -- |
| 1 | 55 | 94.8 | 50 | 87.7 | |||
| 22 | 0 | 3 | 5.2 | 3 | 5.3 | 0.983 | -- |
| 1 | 55 | 94.8 | 54 | 94.7 | |||
| 21 | 0 | 3 | 5.2 | 28 | 49.1 | 0.000 | 0.50 |
| 1 | 55 | 94.8 | 29 | 50.9 | |||
| 13 | 0 | 3 | 5.2 | 0 | 0.0 | 0.082 | -- |
| 1 | 55 | 94.8 | 57 | 100.0 | |||
| 12 | 0 | 14 | 24.1 | 4 | 7.0 | 0.012 | 0.24 |
| 1 | 44 | 75.9 | 53 | 93.0 | |||
| 11 | 0 | 33 | 56.9 | 39 | 68.4 | 0.202 | -- |
| 1 | 25 | 43.1 | 18 | 31.6 | |||
RT refinement group, CT control group: 0: Absence; 1: Presence.
Discussion
This study investigated factors influencing the need for refinement in CA therapy by performing tooth-level multivariable logistic regression analyses. A key strength lies in modeling each anterior tooth separately, capturing anatomical and biomechanical heterogeneity that aggregated analyses often miss12,13. By conducting separate regressions per tooth, we isolated movement effects within distinct clinical contexts, accounting for differences in movement difficulty and predictability. Independent t-tests comparing planned movement magnitudes between refined and non-refined teeth further clarified whether variation in refinement rates was driven by movement complexity. This approach allowed us to disentangle the contributions of movement amount and tooth type, strengthening the reliability of our conclusions.
For tooth 11, despite significantly greater planned rotational movements in refined cases, regression showed a marginal protective effect of rotation on refinement risk (OR = 0.92). This paradox may reflect effective clinical management of complex movements rather than a true risk reduction. No other movement or attachment variable was significantly associated with refinement, consistent with findings that rotational movements pose specific biomechanical challenges14,15.
Tooth 21 demonstrated a strong protective association of attachments against refinement (OR = 0.05), underscoring the importance of auxiliary mechanics in controlling challenging movements such as rotation and angulation, which were more prominent in refined cases15,16. This highlights attachments’ role in mitigating the need for additional refinements despite increased movement complexity.
Tooth 22 showed increased refinement risk linked to greater rotational and inclination movements, while extrusion inversely correlated with refinement, suggesting extrusion may be more predictable in aligner therapy17. High and consistent attachment use across groups indicates standard application rather than a direct influence on refinement, reflecting routine biomechanical strategies12.
For tooth 12, no significant movement-related predictors of refinement emerged, but attachments were more frequent in refined cases. This likely represents a “marker of complexity,” where attachments are placed more often in difficult cases rather than causing refinements per se—an example of confounding by indication common in multifactorial clinical studies18.
For teeth 13 and 23, neither movement magnitude nor attachment presence significantly predicted refinement, suggesting other factors such as individual anatomical variations, periodontal conditions, or patient compliance may play greater roles15,16.
The observed discrepancies in refinement rates between contralateral teeth—particularly between teeth 12 and 22—despite similar planned movement types, highlight the potential contribution of anatomical asymmetries. Root morphology, including curvature or taper, can influence the effectiveness of complex movements like rotation and inclination18,19. Similarly, variations in alveolar bone width or density between the right and left maxillary segments may impact the transmission of aligner forces and attachment engagement20,21. Given that CA therapy relies heavily on precise biomechanical control, even subtle asymmetries may affect treatment predictability. These observations support the need for more individualized digital planning that considers patient-specific three-dimensional anatomical differences rather than relying solely on crown morphology.
Our findings highlight that refinement in CA therapy is multifactorial, arising from complex interactions between movement type and magnitude, tooth-specific biomechanics, and adjunctive mechanics like attachments. The absence of consistent statistical significance across variables does not mean these factors lack effect, but rather underscores the importance of individualized treatment planning and careful clinical judgement—especially for angular movements in anterior teeth. These results support calls for larger, prospective research to better clarify the roles of attachment design and staging protocols in optimizing treatment outcomes.
Limitations
This study has several limitations. First, the data were collected from a single clinical center, which may restrict the generalizability of the findings. Second, attachments were evaluated only based on their presence or absence, without accounting for variations in shape, size, or position—factors that can significantly influence biomechanical control. Additionally, the retrospective nature of the study limits control over confounding variables and patient selection. The analysis relied on planned tooth movements rather than the actual achieved outcomes, which may not fully reflect clinical performance. Moreover, longitudinal validation of treatment results and refinement necessity was not conducted, thereby limiting conclusions about long-term effectiveness. Lastly, as the analyses were conducted at the individual tooth level, intra-patient correlations were not explicitly controlled for, which may have influenced the statistical estimates and their precision.
Acknowledgements
The author would like to thank Dr. Özge Özaydin for providing access to the patient data used in this study, and Dr. Mehmet Şata for statistical guidance.
Author contributions
T.H.O. designed the study, performed the data collection and analysis, interpreted the results, and wrote the manuscript.
Data availability
The datasets generated and analyzed during the current study are available from the corresponding author (T.H.O.) on reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets generated and analyzed during the current study are available from the corresponding author (T.H.O.) on reasonable request.