Abstract
Objective
This retrospective clinical study aims to evaluate the clinical incidence of open gingival embrasures (OGEs) in the anterior maxillary region following sequential distal movement of the maxillary dentition with clear aligner therapy. It also investigates associated factors for OGE to provide a theoretical basis for establishing a preoperative risk assessment system for orthodontic treatment.
Methods
This retrospective study included 75 patients in the permanent dentition stage who underwent sequential maxillary molar distalization (2–4 mm per side) with clear aligners and had complete pre- and post-treatment records. Patients were divided into non-OGE and OGE groups based on post-treatment interdental papilla filling. Intergroup comparisons were performed using the Chi-square test, independent-samples t-test, Welch’s t-test, or Mann-Whitney U test as appropriate. Univariate and multivariate logistic regression analyses were used to identify associated factors, with odds ratios (ORs) and 95% confidence intervals (95% CIs) reported.
Results
A total of 75 patients (mean age, 25.32 ± 9.44 years) were enrolled from an initial cohort of 630. An incidence of 17.3% for OGE was observed following sequential distal movement of the maxillary dentition using clear aligners. A significant difference in age was found between the non-OGE and OGE groups (P < 0.05). Furthermore, regression analysis demonstrated that age, anteroposterior overlap, ANB angle, and U1-NA angle were statistically associated with OGE (P < 0.05).
Conclusion
The results indicate an incidence of 17.3% for OGEs in patients undergoing sequential maxillary molar distalization with clear aligners. Age, anteroposterior overlap, ANB angle, and U1-NA angle were found to be significantly associated with OGE occurrence.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12903-026-08532-y.
Keywords: Open gingival embrasures, Clear aligners, Maxillary molar distalization, Associated factors
Background
Open gingival embrasures (OGEs) refer to pathological triangular spaces between adjacent teeth that occur when the interdental papilla fails to completely fill the gingival embrasure [1]. Previous studies have reported that the incidence of OGEs in patients treated with fixed appliances ranges from 22% to 58%, most commonly occurring in the anterior maxillary and mandibular regions [2–4]. Jung et al. reported that the incidence of OGEs was 51% in the maxillary incisor region and 64% in the mandibular incisor region after orthodontic treatment [5]. Burke et al. found that among 500 adolescent orthodontic patients with maxillary central incisor crowding, the post-treatment incidence of OGEs was 41.9% [6]. Kurth et al. reported that the prevalence of OGEs after orthodontic treatment in adults was 38% [7]. The incidence of OGEs is relatively high, particularly in the anterior regions of both the maxilla and mandible. Several studies have also found a strong association between the occurrence of OGEs and the amount of tooth movement. An et al. demonstrated that for every 1 mm lingual movement of the maxillary central incisors, the risk of OGEs increased by 1.18 times [4]. Kandasamy et al. suggested that labial movement may also elevate the risk of OGE due to thinning of the gingival margin and increased susceptibility to recession [3]. OGEs pose challenges that extend beyond oral health, including difficulties in maintaining oral hygiene and an increased risk of interproximal caries, periodontitis, and root hypersensitivity. Moreover, OGE may have detrimental psychological effects, as the compromised esthetics can reduce self-confidence and negatively influence both oral and mental well-being.
With rising public demand for oral health and superior esthetics, clear aligner therapy has gained widespread acceptance owing to its aesthetic appeal and non-invasive nature. With the increasing popularity of this orthodontic technique, the clinical scenario of OGE in the anterior region following maxillary molar distalization with clear aligners has gained growing attention. Yang et al. demonstrated that the incidence of OGEs in maxillary and mandibular incisors was significantly higher in clear aligner patients (35.0% and 38.0%) than in fixed appliance patients (18.0% and 24.0%) [8]. Wang et al. reported that in non-extraction cases treated with clear aligners, the incidence of OGE was 25.7% in the maxillary incisors and 40.3% in the mandibular incisors [3].
Molar distalization, as a key treatment technique in clear aligner therapy, is commonly applied in non-extraction cases to provide space for mild dental crowding. It is particularly suitable for establishing a Class I molar relationship in cases where the molars are initially in a distal position [9]. A key treatment strategy involves sequential molar distalization, which is typically initiated at the bilateral second molars and progresses mesially in a characteristic V-shaped pattern [10, 11]. Although this approach has been shown to achieve high distalization efficiency, it also presents significant clinical challenges, which is it poses a challenge for anchorage control [12]. Inadequate anchorage may result in undesirable mesial movement of the anterior teeth, thereby elevating the risk of OGEs [13].
In summary, OGEs are a common complication following orthodontic treatment, with their onset associated with a variety of mechanical and biological factors. However, several gaps remain in the existing research:
Most studies fail to distinguish between different types of orthodontic appliances or mix different treatment modalities.
There is a lack of dedicated research investigating whether the unique biomechanical pattern of sequential distalization with clear aligners confers a distinct risk of OGEs in the anterior dentition.
Few studies have specifically analyzed the risk factors for OGEs in the patient population scheduled for maxillary molar distalization.
Through a retrospective analysis, this study aimed to determine the incidence of OGEs following clear aligner‑mediated maxillary molar distalization and to analyze the association between pre‑treatment biological factors and OGE development. Based on the existing literature gap and the clinical characteristics of this treatment modality, we proposed the following hypothesis: in patients undergoing sequential maxillary molar distalization with clear aligners, age, gingival biotype, initial malocclusion characteristics (including ANB angle, U1‑NA angle, and anteroposterior overlap), and anterior teeth retraction are associated with the occurrence of OGE. Ultimately, we intend to develop a risk prediction model tailored to this specific clinical scenario, thereby providing evidence‑based guidance for pre‑treatment risk assessment and treatment planning.
Materials and methods
Ethics statements
Access to and analysis of patient records for this study was approved by the Ethical Committee Department of Tianjin Stomatological Hospital (Protocol number: YPH2024-G-003).
Study design and population
A retrospective single-center cohort study was conducted. Independent case screening was conducted by two researchers, with disagreements resolved through discussion with a senior investigator to avoid single-observer subjective bias. Clinical records of 75 Chinese patients were retrospectively selected from a pool of individuals who had completed orthodontic treatment at the Department of Orthodontics, Tianjin Stomatological Hospital, between June 2014 and September 2024. All patients had complete medical histories and orthodontic records, including digital casts, lateral cephalograms, and intraoral and extraoral digital photographs. Each patient was treated by a designated orthodontist with over 10 years of professional experience.
The sample inclusion criteria were as follows: (1) Patients in the permanent dentition stage; (2) Those treated with sequential maxillary molar distalization using Invisalign clear aligners; (3) Bilateral distal movement of maxillary molars limited to 2–4 mm per side; (4) Availability of complete clinical records, including pre- and post-treatment intraoral photographs, digital dental models, and cephalometric radiographs. The exclusion criteria were as follows: (1) Presence of pre-treatment diastema or OGE between maxillary anterior region; (2) History of maxillary anterior tooth extraction or congenital absence of one or more maxillary anterior teeth; (3) Poor oral hygiene with gingivitis or periodontitis in the maxillary anterior region; (4) Existing restorations (fillings or crowns) in the maxillary anterior teeth; (5) History of interproximal reduction in the maxillary anterior teeth during orthodontic treatment; (6) Patients undergoing secondary orthodontic treatment; (7) Inability to attend regular follow-up appointments as scheduled; (8) Presence of systemic diseases (e.g., diabetes). The flowchart of this experiment is shown in Fig. 1.
Fig. 1.
Experimental flowchart
Data collection
Data from three categories (sociodemographic characteristics, malocclusion characteristics, and orthodontic treatment characteristics) were collected by a single operator under strict single-blinded conditions. All pre- and post-treatment frontal intraoral photographs were obtained via a standardized protocol to ensure consistency. An independent researcher de-identified all assessment materials by removing patient names, medical record numbers, and other identifying information, retaining only anonymous serial numbers. The operator responsible for measurements was blinded to all clinical patient data and subsequent group allocation (OGE/non-OGE) throughout the assessment. Intra-observer reliability was evaluated by having the same operator re-measure 15% of the samples after a one-month interval. Intraclass correlation coefficients (ICC) were calculated for continuous variables, while Cohen’s Kappa coefficients were used for categorical variables; values of ICC ≥ 0.885 and Kappa ≥ 0.824 indicated excellent reliability. Based on previous studies, 17 potential factors associated with OGE were investigated, and these factors are presented in Supplementary Table 1. The data sources are described below.
Sociodemographic characteristics: patients’ medical records.
Malocclusion characteristics: cephalometric radiograph measurements obtained with Dolphin Imaging Software (Dolphin Imaging, version 11.95; Dolphin Imaging & Management Solutions) and digital model measurements obtained with OrthoCAD software (OrthoCAD, version 5.9.1.50; Cadent Inc.) (Fig. 2).
Orthodontic treatment characteristics: clear aligner treatment plan.
Fig. 2.
Model measurements in OrthoCAD software. A Anteroposterior overlap. B Transverse overlap. C Rotation
All pre-treatment intraoral photographs were taken within one week before orthodontic treatment initiation, while post-treatment photographs were taken on the day of active treatment completion. Based on pre- and post-treatment frontal intraoral photographs, the Jemt index [14] was used to evaluate interdental papilla filling between the maxillary central incisors, with scores ranging from 0 to 4.
0: Absent papilla with a flat gingival curve of adjacent teeth;
1: Actual papilla height < 1/2 of the ideal papilla height;
2: Actual papilla height ≥ 1/2 of the ideal papilla height but not reaching the interproximal contact point;
3: Papilla reaching the interproximal contact point with a harmonious gingival curve of adjacent teeth;
4: Papillary hyperplasia extending beyond the interdental space with an irregular gingival curve.
Patients were subsequently classified into an OGE group (scores 0, 1, or 2) and a non-OGE group (scores 3 or 4) according to the index results.
Based on the concept of gingival biotype proposed by Seibert J and Lindhe (1998) [15], the gingival tissue was further classified into the thin biotype and the thick biotype according to its clinical characteristics and anatomical morphology.
Statistical analysis
Descriptive statistics were used to summarize the relative and absolute frequencies of the predictor variables and the outcome of OGE. The Chi-square test was applied to assess associations between categorical variables. Continuous variables were tested for normality using the Kolmogorov-Smirnov or Shapiro-Wilk tests. Homogeneity of variance was assessed using Levene’s test. Normally distributed data with equal variances were analyzed using an independent-samples t-test, whereas data with unequal variances were analyzed using a Welch’ s t-test. For non-normally distributed data, the Mann-Whitney U test (Wilcoxon rank-sum test) was used. Categorical variables are summarized as frequencies and percentages. Continuous variables are presented as mean ± standard deviation for normally distributed data, or as median and interquartile range for non-normally distributed data.
Univariate logistic regression was performed to identify factors associated with OGE. Variables with a P-value < 0.1 were then entered into a multivariate logistic regression model to determine their independent effects. Data were analysed using SPSS software (version 21.0; IBM Corp.).
Multicollinearity among variables included in the multivariate logistic regression model was assessed using the variance inflation factors (VIF) and tolerance values, with a VIF < 5 and a tolerance > 0.2 indicating no significant multicollinearity. Model fit was evaluated using the Hosmer‑Lemeshow goodness‑of‑fit test.
Results
General characteristics
Among the 75 patients included in this study, 12 were male (16.0%) and 63 were female (84.0%), with a mean age of 25.32 ± 9.44 years. Regarding gingival biotype, 34 patients (37.3%) were classified as thick biotype and 41 patients (62.7%) as thin biotype. After completion of clear aligner therapy, 62 patients exhibited normal gingiva between the maxillary central incisors, while 13 patients presented with OGE, corresponding to an incidence of 17.3%.
Comparison between groups
Comparison of categorical baseline variables by chi-square analysis demonstrated no significant association between OGE occurrence and either gender or gingival biotype (P > 0.05) (Table 1). For continuous baseline variables, independent-sample t-tests and Mann-Whitney U tests were applied. The analysis showed no significant intergroup differences in overjet, overbite, crowding level, anteroposterior overlap, transverse overlap, rotation, anterior teeth retraction, ANB, MP-SN, MP-FH, U1-SN, U1-NA, or U1-L1 (P > 0.05), indicating that these factors were not significantly associated with the development of OGE. In contrast, age demonstrated a statistically significant difference between the non-OGEs and OGEs groups (P < 0.05) (Table 2).
Table 1.
Categorical characteristics of non-OGE and OGE groups
| Categorical Variable | Non-occurrence (n = 62) |
Occurrence (n = 13) |
P |
|---|---|---|---|
| Gender | |||
| Male | 10 (16.1) | 2 (15.4) | 0.974 |
| Female | 52 (83.9) | 11 (84.6) | |
| Gingival biotype | |||
| Thin gingival biotype | 25 (40.3) | 3 (23.1) | 0.242 |
| Thick gingival biotype | 37 (59.7) | 10 (76.9) | |
Table 2.
Quantitative characteristics of non-OGE and OGE groups
| Continuous Variable | Non-occurrence (n = 62) |
Occurrence (n = 13) |
P |
|---|---|---|---|
| Age (years)a | 21.00 (17.00 ~ 29.00) | 34.00 (23.00 ~ 40.50) | 0.005** |
| Overjet (mm)a | 3.95 (2.80 ~ 5.33) | 3.90 (3.55 ~ 5.40) | 0.845 |
| Overbite (mm)a | 3.00 (1.98 ~ 4.70) | 2.80 (1.75 ~ 3.90) | 0.649 |
| Crowding Level (mm)a | 3.38 (1.86 ~ 6.91) | 4.26 (2.74 ~ 6.62) | 0.576 |
| Anteroposterior overlap (mm)a | 0.65 (0.30 ~ 1.03) | 0.50 (0.25 ~ 0.80) | 0.428 |
| Transverse overlap (mm)a | 1.38 (0.99 ~ 1.97) | 1.20 (0.83 ~ 1.65) | 0.358 |
| Rotation (°)a | 167.45 (155.60 ~ 173.38) | 166.40 (156.70 ~ 171.85) | 0.758 |
| Anterior teeth retraction (mm)a | 2.00 (0.95 ~ 3.10) | 2.45 (1.13 ~ 3.65) | 0.366 |
| ANB (°)a | 3.22 (1.70 ~ 4.63) | 5.10 (2.90 ~ 5.85) | 0.073 |
| MP-SN (°) | 35.59 ± 5.65 | 36.35 ± 5.49 | 0.656 |
| MP-FH (°) | 24.31 ± 4.74 | 24.35 ± 4.94 | 0.977 |
| U1-SN (°)a | 106.50 (102.30 ~ 111.55) | 108.70 (103.75 ~ 113.90) | 0.345 |
| U1-NA (°) | 25.50 ± 8.45 | 25.92 ± 6.90 | 0.868 |
| U1-NA (mm) | 6.18 ± 3.37 | 6.65 ± 2.63 | 0.640 |
| U1-L1 (°) | 122.60 ± 12.33 | 117.94 ± 7.84 | 0.197 |
aNon-normally distributed variables are expressed as median (25%, 75%)
**P value < 0.01
Logistic regression analysis
Univariate logistic regression analysis revealed that age (OR = 1.26, 95% CI: 1.09–1.46), overjet (OR = 0.35, 95% CI: 0.12–1.02), anteroposterior overlap (OR = 2.97, 95% CI: 0.89–9.91), ANB angle (OR = 3.65, 95% CI: 1.22–10.91), U1-NA angle (OR = 1.52, 95% CI: 0.98–2.35), and U1-L1 angle (OR = 1.41, 95% CI: 1.05–1.89) were significantly associated with OGE occurrence (P < 0.1). After adjustment for confounders in multivariate logistic regression, age (OR = 1.17, 95% CI: 1.07–1.30), anteroposterior overlap (OR = 2.41, 95% CI: 1.05–5.54), ANB angle (OR = 2.31, 95% CI: 1.08–4.94), and U1-NA angle (OR = 1.39, 95% CI: 1.03–1.89) remained independently associated with OGE (P < 0.05) (Table 3). Receiver operating characteristic (ROC) curve analysis demonstrated an area under the curve (AUC) of 0.839 (95% CI: 0.72–0.96) for predicting OGE between the maxillary central incisors, indicating good discriminatory power (Fig. 3).
Table 3.
Association of risk factors with OGE: univariate and multivariate analyses
| Variables | Univariate analysis | Multivariate analysis | ||
|---|---|---|---|---|
| P | OR (95% CI) | P | OR (95% CI) | |
| Gender | ||||
| Male | 0.620 | 2.07 (0.12–36.82) | ||
| Female | ||||
| Age (years) | 0.002** | 1.26 (1.09–1.46) | 0.001** | 1.17 (1.07–1.30) |
| Gingival biotype | ||||
| Thin gingival biotype | 0.124 | 0.13 (0.01–1.74) | ||
| Thick gingival biotype | ||||
| Overjet (mm) | 0.055 | 0.35 (0.12–1.02) | ||
| Overbite (mm) | 0.565 | 1.22 (0.62–2.38) | ||
| Crowding level (mm) | 0.581 | 1.11 (0.77–1.59) | ||
| Anteroposterior overlap (mm) | 0.077 | 2.97 (0.89–9.91) | 0.038* | 2.41 (1.05–5.54) |
| Transverse overlap (mm) | 0.352 | 0.46 (0.09–2.39) | ||
| Rotation (°) | 0.423 | 0.96 (0.85–1.07) | ||
| Anterior teeth retraction (mm) | 0.126 | 2.94 (0.74–11.67) | ||
| ANB (°) | 0.021* | 3.65 (1.22–10.91) | 0.031* | 2.31 (1.08–4.94) |
| MP-SN (°) | 0.120 | 1.32 (0.93–1.87) | ||
| MP-FH (°) | 0.268 | 0.80 (0.54–1.19) | ||
| U1-SN (°) | 0.308 | 1.09 (0.92–1.29) | ||
| U1-NA (°) | 0.063 | 1.52 (0.98–2.35) | 0.032* | 1.39 (1.03–1.89) |
| U1-NA (mm) | 0.249 | 1.57 (0.73–3.35) | ||
| U1-L1 (°) | 0.022* | 1.41 (1.05–1.89) | ||
Abbreviations: CI Confidence interval, OR Odds ratio
*P value < 0.05, **P value < 0.01
Fig. 3.
The ROC curve of predictive model
Discussion
The incidence of OGEs following orthodontic treatment is relatively high, and multiple risk factors have been identified [7]. Accurate diagnosis not only facilitates the design of appropriate treatment plans to prevent OGE but also enhances anterior esthetics. To identify factors associated with OGE after sequential molar distalization with clear aligners, we analyzed data from 75 non‑extraction patients. We hypothesized that age, gingival biotype, initial malocclusion characteristics (e.g., ANB angle, U1‑NA angle, anteroposterior overlap), and anterior teeth retraction are independent risk factors for OGE in this population. The results partially supported this hypothesis: age, ANB angle, U1‑NA angle, and anteroposterior overlap were identified as significant independent factors, consistent with our initial speculation. However, gingival biotype and the amount of anterior teeth retraction showed no statistically significant association with OGE, a finding that deviates from our original assumption and warrants further investigation.
We performed a preliminary analysis to explore potential explanations for these findings. Age was significantly associated with OGE occurrence. In this study, the mean age was 23.69 ± 8.30 years in the non‑OGE group and 33.08 ± 10.96 years in the OGE group, representing a difference of approximately nine years. Multivariate analysis showed that age had an OR of 1.17 (95% CI: 1.07–1.30), indicating that each one‑year increase in age was associated with a 17% increase in the odds of developing OGE. We speculate that with advancing age, the efficiency of tooth movement gradually declines, increasing the demand for anchorage during molar distalization. When anchorage control is insufficient, reactive forces may induce mesial movement of the anterior teeth, resulting in back‑and‑forth movement during the final retraction stage. Previous studies have shown [16–18] that alveolar bone density decreases with age, while the activity of osteoblasts and osteoclasts declines, leading to a marked reduction in bone metabolism. Since molar distalization depends on continuous orthodontic forces to induce alveolar bone remodeling, older patients with slower bone turnover may require stronger anchorage to achieve effective molar distalization. In addition to reduced remodeling capacity of hard tissues, aging also causes degenerative changes in gingival papillae, leading to reduced height and a higher likelihood of OGE occurrence [19]. Some studies have reached results consistent with ours, identifying age as a contributing factor to OGE [5, 20–22]. However, other reports found no significant correlation between age and OGE [23, 24]. These discrepancies likely stem from variations in inclusion and exclusion criteria across studies. In the present study, patients with periodontitis were excluded, which may have led to an underestimation of the effect of age.
In addition, a strong association was observed between increased anteroposterior overlap and OGE occurrence. Multivariate logistic regression analysis demonstrated an OR of 2.41 (95% CI: 1.05–5.54), indicating that each 1-mm increase in anteroposterior overlap was associated with a 141% increase in OGE risk. Excessive anteroposterior overlap reflects localized sagittal crowding in the anterior region. Prolonged mechanical stress may disrupt alveolar bone continuity, resulting in reduced horizontal alveolar crest height compared with normally aligned teeth and consequently insufficient osseous support for the interdental papilla. Such defects often become apparent after alignment. Following alignment, the interproximal contact point (ICP) shifts coronally, further increasing the distance between the ICP and the alveolar bone crest (ABC). When this distance exceeds the compensatory capacity of the alveolar crest and interdental papilla, adequate papillary filling may not be achieved, thereby increasing OGE risk. This finding is consistent with the study by Mahasneh et al. In their study, the incidence of OGEs was reported to be 2% when the distance from ICP to ABC was ≤ 5 mm, and the incidence increased as this distance increased [25]. Therefore, periapical parallel radiographs should be incorporated in future studies to further investigate whether the ICP–ABC distance is associated with differences in OGE incidence.
Regarding whether the type of orthodontic tooth movement affects OGE formation, no definitive conclusion has been reached [26–28]. An et al. reported that every 1 mm of lingual movement of maxillary central incisors increased the risk of OGE by 1.18 times [4]. The results of this study demonstrated that the ANB angle and the U1-NA angle were significant factors associated with the occurrence of OGE. Relevant studies have indeed confirmed that the ANB angle and U1-NA angle not only affect the positional relationship between tooth roots and alveolar bone but also serve as key parameters determining smile aesthetics [29]. ANB angle and U1-NA angle Logistic regression analysis revealed that the ANB angle had an OR of 2.31 (95% CI: 1.08–4.94), meaning each 1° increase in the ANB angle was associated with a 2.31-fold higher odds of OGE development. Similarly, the U1-NA angle had an OR of 1.39 (95% CI: 1.03–1.89), with each 1° increase correlating with a 1.39-fold increased odds of OGE occurrence. In the present study, all included patients were treated with sequential molar distalization to create space, relieve crowding, and retract the anterior teeth. A larger ANB angle typically reflects a skeletal Class II tendency. During clear aligner therapy involving molar distalization, compensatory retraction and lingual movement of the anterior teeth are often required to camouflage the underlying skeletal discrepancy. Compared with patients with smaller ANB angles, those with larger ANB angles tend to undergo greater lingual displacement of the anterior teeth, thereby increasing the risk of OGE. Similarly, patients with a larger initial U1-NA angle generally experience greater changes in incisor inclination during treatment. The increased magnitude of anterior tooth inclination adjustment may further aggravate lingual movement of the incisors, consequently elevating the risk of OGE. This finding appears to contrast with the observation by Kandasamy et al. [30] that labial tooth movement also increases OGE risk due to gingival margin thinning and recession. We reasonably hypothesize that both labial and lingual movements may pose a threat to gingival integrity. As the magnitude of tooth movement increases, the accompanying alveolar bone remodeling may outpace the adaptive capacity of the soft tissue, ultimately leading to gingival recession and OGE.
From a biomechanical perspective, the independent correlation between these risk factors and OGE may be explained by the unique characteristics of clear aligner‑mediated sequential molar distalization, which differ significantly from those of fixed appliance therapy. During sequential distalization, distal movement occurs step by step starting from the most distal molar. In the initial stage, the anterior teeth act as anchorage. Inappropriate control of reactive forces may result in uncontrolled labial inclination of the anterior segment. Subsequently, as the extraction space is dissipated anteriorly, the anterior teeth are retracted to close the space. These unintended positional changes, especially the reciprocating movement of the tooth roots, may compress the interradicular alveolar bone crest and induce alveolar bone resorption, thereby widening the distance between the interproximal contact points and the alveolar bone crest. This directly elevates the risk that the interdental papilla fails to fill the gingival embrasure. However, the above interpretations remain speculative and cannot be directly confirmed. Future longitudinal studies based on imaging data are needed to directly evaluate changes in alveolar bone and root position during treatment, thereby providing more direct evidence to validate our findings and to further elucidate the proposed biological mechanism. Additionally, clear aligners exert forces by wrapping around the teeth and engaging attachments. The force application points are located at the middle and upper thirds of the clinical crown, above the center of resistance. The moment-to-force (M/F) ratio is generally lower than that of fixed appliances [29]. This limits root control and increases the risk of anterior anchorage loss, potentially leading to discrepancies between actual outcomes and pre-planned treatment design.
Based on the above biomechanical mechanisms and clinical factor analysis, the discriminative ability of the identified risk factors for predicting OGE was further evaluated using ROC curve analysis. The AUC was 0.839 (95% CI: 0.72–0.96), indicating good discriminatory power. This suggests that the combination of age, anteroposterior overlap, ANB angle, and U1-NA angle can effectively distinguish patients at higher risk of developing OGE following sequential maxillary molar distalization with clear aligners. Although our model requires external validation in larger, independent cohorts, these preliminary findings suggest the potential utility of integrating these factors into a preoperative risk assessment tool tailored to this specific clinical scenario. Future studies with larger sample sizes, more outcome events, and external validation in independent multicenter cohorts are warranted to confirm its generalizability and clinical utility.
In contrast, our analysis showed no significant association between OGE formation and the following variables: gender, gingival biotype, overbite, crowding level, transverse overlap, rotation, anterior teeth retraction, MP-SN, MP-FH, U1-SN, and U1-NA. Consistent with many previous studies, gender was not a significant risk factor [5, 20, 21]. However, regarding gingival biotype, most research suggests that thin biotype predisposes patients to gingival recession regardless of the type of tooth movement. Al-Zarea et al. [31] clearly identified thin gingival biotype as a primary cause of OGE. Gul et al. [32]. reported that patients with a thin gingival biotype had a 7.742-fold higher risk of periodontal soft tissue changes after orthodontic treatment, which may compromise papilla integrity and promote OGE development. However, several considerations should be noted. First, there are notable differences between clear aligner therapy in this study and MBT™ fixed orthodontic treatment. Second, gingival biotype was assessed visually in that study, with certain discrepancies compared to the standard periodontal assessment methods. Such differences may explain why gingival biotype did not emerge as a significant factor for OGE in our study.
As for the relationship between crowding and OGE, different studies have reported conflicting results. Some found no significant association [4, 5, 20], while others reported a positive correlation. Cui et al. [21]. noted that anterior crowding may increase OGE risk by stretching gingival fibers and altering the morphology of the gingival embrasure. After alignment, the contact point may shift coronally, preventing the papilla from fully filling the embrasure space. However, crowding may interact with other factors—such as alveolar bone height and papilla fullness—which may explain why it did not emerge as an independent risk factor for OGE in the present study.
This study has several limitations. First, the sample size is relatively small according to the Events Per Variable (EPV) criterion. Nevertheless, this research holds significant clinical value. Future studies should expand the sample size and adopt a multi-center design to enhance generalizability and robustness. Second, the extent of sequential distalization varied among patients, and the use of auxiliary anchorage (e.g., mini‑implants) was not categorized, which may have influenced the results. Third, OGE was evaluated using 2D intraoral photographs and the Jemt index, which may cause slight measurement errors related to image capture. Future studies should combine 3D imaging or periapical radiographs to enhance the objectivity and accuracy of OGE diagnosis. Fourth, gingival biotype assessment relied solely on visual inspection, which is operator-dependent and may lead to misclassification. Future studies should combine visual evaluation with objective methods such as probe transparency and periodontal probing to obtain more objective and reproducible measurements. To minimize bias, stricter inclusion and exclusion criteria were applied. Furthermore, the study sample was limited to patients undergoing sequential molar distalization with clear aligners and consisted exclusively of Chinese individuals, with females accounting for 84% of all participants; therefore, caution is warranted when generalizing these findings to other treatment modalities or populations.
To our knowledge, this is the first study to specifically investigate risk factors for OGE in patients undergoing sequential maxillary molar distalization with clear aligners, a clinically relevant yet underexplored population. Additionally, the inclusion of comprehensive pre‑treatment variables and the use of standardized assessment protocols strengthen the reliability of our findings. These results provide a foundation for developing a preoperative risk assessment system tailored to this specific clinical scenario.
Conclusion
The results of this study indicate a relatively high incidence (17.3%) of OGEs in the maxillary incisor region among patients undergoing sequential maxillary molar distalization with clear aligners. Multivariate regression analysis confirmed that age, anteroposterior overlap, ANB angle, and U1-NA angle were significantly associated with the development of OGE. Therefore, considering these factors comprehensively is essential to minimize OGE while preserving oral health and smile aesthetics.
Supplementary Information
Acknowledgements
The authors would like to express their gratitude to all individuals who have contributed to this research.
Abbreviations
- OGE
Open gingival embrasures
- ORs
Odds ratios
- ANB
Angle between A-point, Nasion, B-point
- MP-SN
Angle between Mandibular Plane and Sella-Nasion Plane
- MP-FH
Angle between Mandibular Plane and Frankfort Horizontal Plane
- U1-SN
Angle between Upper Incisor and Sella-Nasion Plane
- U1-NA
Angle between Upper Incisor and NA Line
- U1-L1
Angle between Upper Incisor and Lower Incisor
- ICP
Interproximal contact point
- ABC
Alveolar bone crest
Authors’ contributions
XMS and XND contributed to the study design, data collection, data analysis and manuscript writing. CL and YPZ contributed to the data collation and manuscript revision. JLC, JYY and ZYL contributed to study design, manuscript editing and providing content expertise.
Funding
This work is funded by Tianjin Natural Science Foundation of China [25JCQNJC01420]. This work was also sponsored by Tianjin Health Research Project [Grand No. TJWJ2025QN075] and Scientific Research Project of Tianjin Education Commission (Natural Science) [2024ZX016].
Data availability
The data generated and analyzed in this study can be obtained from the corresponding author upon reasonable request.
Declarations
Ethics approval and consent to participate
This study was performed in line with the principles of the Declaration of Helsinki. Access to and analysis of patient records for this study was approved by the Ethics Committee of Tianjin Stomatological Hospital (Protocol number: YPH2024-G-003). Written informed consent was obtained from all participants in this study. For minors (i.e., participants under 16 years of age), informed consent was obtained from both the participant and their parents or legal guardians.
Consent for publication
Not applicable.
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.
Xueming Shi, Xiaoning Duan and Chen Lin contributed equally to this work.
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Data Availability Statement
The data generated and analyzed in this study can be obtained from the corresponding author upon reasonable request.