Background
Posterior open bite (POB) is a common and challenging complication during clear aligner therapy, primarily because of the biomechanical susceptibility of posterior teeth to intrusion, occlusal block effect, and excessive expansion. Although posterior vertical elastic traction has been proposed as a potential intervention, its clinical efficacy and the factors influencing treatment outcomes remain poorly understood. This prospective study aimed to evaluate the effectiveness of posterior settling elastics in correcting POB and identify key predictors of treatment success.
Methods
Fifty-two patients treated with Invisalign (Align Technology, Santa Clara, Calif) who exhibited POB, defined as a vertical distance >0 mm for mesiobuccal (MB), distobuccal (DB), or palatal (PL) cusps of maxillary and mandibular molars in centric occlusion after the refinement of the aligner, were enrolled. A total of 172 molar sites were assessed, of which 95 received elastic traction (vertical or triangular), and 77 were treated with aligners alone. Digital models at pretreatment (T0), predicted (T1), and posttreatment (T2) stages were acquired via iTero Element scanning (Align Technology, San Jose, Calif) and registered to a standardized coordinate system using Geomagic Studio 2014 (3D Systems, Rock Hill, SC). Actual cusp extrusion distances relative to the reference xy-plane were measured, and occlusal contact areas were quantified using Boolean superposition. Univariate and mixed-effects multivariate linear regression analyses were conducted to identify predictors of cusp extrusion.
Results
Multivariate analysis revealed that MB cusp extrusion was positively associated with elastic traction ( P <0.001), baseline MB cusp distance ( P <0.001), predicted extrusion ( P <0.001), and female sex ( P <0.001) and was negatively associated with pretreatment anterior overjet and overbite. Similar patterns were observed for the DB cusp extrusion. PL cusp extrusion was primarily predicted by its baseline distance ( P <0.05) and predicted extrusion ( P <0.05), and was negatively influenced by the pretreatment DB cusp distance.
Conclusions
Posterior vertical settling elastics are effective in correcting clear aligner–induced POB, particularly by improving maxillary buccal cusp occlusal contacts. Optimal correction requires substantial overcorrection in the planned posterior extrusion and careful evaluation of baseline POB severity, anterior overjet and overbite, and patient sex to maximize treatment predictability.
Highlights
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Vertical settling elastics effectively resolve posterior open bite (POB) during clear aligner therapy.
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Overtreatment in predicted posterior extrusion is suggested to achieve the desired POB correction.
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The baseline POB severity positively correlates with the magnitude of achieved extrusion.
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Pretreatment of deep overbite and large overjet negatively impacts POB correction.
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Female patients demonstrate greater buccal cusp extrusion compared with male patients.
With the rapid advancement of clear aligner technology, an increasing number of innovations have been integrated into these systems. Such technological progress has expanded the indications and broadened the scope of clear aligner therapy, enabling the effective management of complex cases that were previously considered unsuitable for this approach. , At present, clear aligners are increasingly favored by both clinicians and patients because of their advantages in comfort, esthetics, ease of oral hygiene maintenance, and their growing efficacy in addressing challenging orthodontic cases. ,
Despite these advancements, achieving adequate occlusal contact at the completion of orthodontic treatment remains critical for functional efficiency, postoperative stability, and minimizing the risk of relapse. , However, multiple studies have consistently reported that clear aligner therapy is associated with a greater reduction in posterior occlusal contact area compared with conventional fixed appliances. ,,,, This limitation highlights that, although clear aligners offer significant clinical benefits, careful consideration is required when managing patients in whom occlusal stability is a primary concern.
From a biomechanical perspective, the undesirable occlusal contacts observed after clear aligner therapy can be attributed to several factors, including the material properties of the aligners, aligner-specific biomechanics, the occlusal-block effect, and limitations in transverse expansion control. The thermoplastic nature of clear aligners results in an elastic modulus considerably lower than that of stainless steel archwires, rendering them less resistant to forces. Specifically, posterior teeth are biomechanically prone to intrusion during a variety of orthodontic movements, such as anterior retraction and molar distalization. ,, In addition, because of the occlusal-block effect, posterior teeth are susceptible to intrusion under masticatory forces. Another contributing factor is the limited efficiency of arch expansion with aligners. In most patients, expansion primarily occurs through buccal tipping of the crowns rather than bodily movement, with the efficiency of true bodily expansion reported at 36.4% ± 29.3%. ,
Furthermore, several treatment- and patient-related variables influence the extent of posterior occlusal contact reduction. First, patients with extraction tend to experience more pronounced reductions in posterior contacts. The substantial space closure and anterior retraction required after premolar extraction may increase posterior intrusion and transient open bite, as demonstrated in randomized controlled trials comparing aligners with fixed appliances. Second, the patient’s vertical skeletal pattern plays a critical role. Patients with a hypodivergent facial pattern typically exhibit stronger masticatory forces. , Although recent longitudinal data suggest these patients may retain larger total occlusal contact areas, this finding is likely driven by increased anterior contacts associated with their deepbite tendency, rather than superior posterior settling. In fact, the heavy musculature in patients with hypodivergent facial patterns generates powerful intrusive forces. These forces not only exacerbate the bite-block effect but also create a biomechanically unfavorable environment for posterior extrusion, potentially making the correction of posterior open bite (POB) more resistant to elastic traction compared with patients with a hyperdivergent facial pattern and weaker musculature. , Future studies with larger sample sizes are needed to further isolate the specific impact of skeletal patterns on posterior vs anterior contact distribution.
Collectively, these findings indicate that changes in occlusal contact area with clear aligners are multifactorial, influenced not only by the appliance system but also by extraction biomechanics and the patient’s vertical skeletal pattern. Although several clinical strategies have been proposed to mitigate posterior contact loss—such as overcorrection of maxillary incisor torque, strategic mandibular interproximal reduction, and avoidance of thermoplastic retainers immediately after therapy—effective and validated approaches remain limited. , For instance, Tang et al recommended the use of buccal attachments to enhance torque control during arch expansion to avoid premature palatal cusp contact. Also, additional aligners at night posttreatment could increase posterior occlusal contacts. However, this improvement reflects natural occlusal settling occurring primarily within the first 3 months and is therefore insufficient for addressing POB, which requires active biomechanical refinement through aligner therapy. Moreover, sectioning of the aligners in the posterior region to allow settling is proposed as an effective way to manage POB.
Notably, buccal-to-buccal vertical settling elastics represent a theoretically promising approach to enhance posterior intercuspation by applying controlled extrusive forces on the molars. However, there is a lack of quantitative evidence regarding their effectiveness during clear aligner treatment, and previous studies have not distinguished whether such elastics promote true vertical eruption or merely induce unintended torque changes of the molar cusps. This absence of empirical data represents a critical clinical gap, particularly given the growing use of clear aligners for the management of complex malocclusions.
From a biomechanical perspective, the application of vertical settling elastics in the posterior region may help address POB. Nevertheless, this strategy has not yet been clinically validated for the correction of posterior occlusal noncontacts or open bites, and the factors influencing its effectiveness remain largely unknown. Therefore, we conducted a prospective clinical study to evaluate the efficacy of posterior settling elastics in managing posterior occlusal noncontacts and open bites, as well as to investigate the variables that may affect their clinical performance.
Material and methods
This single-center prospective clinical study was approved by the ethics committee of West China Hospital of Stomatology, Sichuan University. Consecutive patients treated with Invisalign (Align Technology, Santa Clara, Calif) by the same experienced orthodontist (H.L.) in the Department of Orthodontics between January 2020 and September 2024 were included. The inclusion criteria were as follows: (1) patients presenting with POB in centric occlusion after the completion of ≥1 set of clear aligners (refinement stage), rather than patients who had completed the entire orthodontic treatment; (2) POB was defined as a distance >0 mm between the corresponding mesiobuccal, distobuccal, or palatal cusps (MB, DB, or PL, respectively) of the maxillary and mandibular molars; (3) permanent dentition without eruption of the third molars and nonextraction treatment in both arches; and (4) availability of complete prerefinement and postrefinement dentition data. All clinical measurements and digital models were obtained immediately after the completion of the refinement stage and before the retention phase to minimize potential confounding effects from spontaneous posttreatment occlusal changes. Exclusion criteria included the following: (1) developmental anomalies of the posterior teeth or the presence of dental implants or prosthetic restorations affecting occlusal anatomy; (2) systemic disorders affecting bone metabolism; (3) severe transverse or sagittal skeletal discrepancies requiring orthognathic surgery, defined as ANB >8° or <–4°; and (4) poor tracking because of noncompliance. All participants were confirmed to have good oral hygiene and no active dental pathology before treatment.
As this study is the first to investigate the effectiveness of adjunctive auxiliaries in molar extrusion with clear aligners, an a priori sample size calculation was performed based on data from a pilot sample (10 molars in each group: elastics vs nonelastics). In the pilot analysis, the mean achieved extrusion was 0.23 ± 0.29 mm in the nonelastics group and 1.65 ± 0.86 mm in the elastics group. Based on these effect sizes, a total sample size of 62 was calculated to achieve 95.8% power with a significance level of 0.05, using a 2-sided 2-sample unequal-variance t test in PASS 15.0 (NCSS, LLC, Kaysville, Utah). To account for a potential 20% dropout rate or exclusion because of tracking issues, a minimum total sample size of 78 teeth was set as the required sample size. Consequently, our final sample of 172 teeth well exceeded this requirement, ensuring adequate statistical power.
The refinement treatment protocol was designed to correct POB based on the ClinCheck (Align Technology, Santa Clara, Calif) setup. Posterior teeth extrusion was specifically planned to establish buccal interdigitation and was intentionally overcorrected to achieve heavy occlusal contacts. In vertical settling elastics cases, vertical elastics (1/8, 3.5 oz) were applied from the cutouts on the maxillary and mandibular molars to enhance the predictability of posterior extrusion. The cutouts were positioned at the middle and gingival third of the buccal surfaces of the crown, and a button was bonded to the exposed tooth surface, through which the posterior buccal-to-buccal settling elastics were attached during treatment. Patients were instructed to wear the aligners for at least 22 hours per day and to change them every 7 days.
The baseline demographic data, including age and sex, as well as tooth types, attachment designs, and the use of elastics, were recorded. Clinical parameters evaluated before refinement, as predicted by ClinCheck, and after refinement included the distances of the MB, DB, and PL cusps, anterior and posterior overbite and overjet, and overall occlusal contact area. A total of 52 patients (mean age: 30.44 ± 8.50 years; 25% males and 75% females) were included in the study.
Pretreatment (T0), predicted (T1), and posttreatment (T2) dentition models were digitized using the iTero Element intraoral scanner (Align Technology, San Jose, Calif) and uploaded to Clincheck software. Subsequently, these models were exported as stereolithography files and imported into Geomagic Studio 2014 (3D Systems, Rock Hill, SC) for further analysis. To quantify vertical movements, all T0, T1, and T2 models were oriented to a standardized reference plane defined by the maxillary bilateral first molar PL cusp tips and maxillary incisal edge. Using a single, consistent reference plane ensured internal comparability across subjects and timepoints.
To ensure consistent and reproducible measurements, a global coordinate system was established based on the pretreatment (T0) model, with the z-axis perpendicular to the xy-plane defined by stable maxillary landmarks. This approach aligns conceptually with previously validated 3-dimensional methodologies involving coordinate registration and landmark-based tracking. All subsequent time points (T1 and T2) were registered to this reference frame, allowing isolation of the actual vertical displacement of each cusp. Specifically, the x-axis connected the PL cusps of the bilateral maxillary first molars, the y-axis extended from the incisal edge to the midpoint of the x-axis, and the z-axis was perpendicular to the xy-plane ( Fig 1 , A ). All dentition models were registered and aligned to this coordinate system. Anatomic landmarks, including the MB, DB, and PL cusps, as well as the central fossa of the first and second molars, were marked, and their corresponding coordinates were recorded ( Fig 1 , B – E ). The xy-plane served as the reference plane, with the discrepancy in z-coordinates between maxillary and mandibular buccal cusps representing the actual extrusion of the buccal cusps, and the discrepancy between maxillary PL cusps and mandibular central fossa representing the extrusion of the PL cusps.
Model superimposition and measurements: A, Establishment of the coordinate system; B and C, Landmark identification on maxillary posterior teeth (MB, DB, and PL cusps); D and E, Landmark identification on mandibular posterior teeth (MB, DB, and central fossa); F, Measurement of occlusal contact area.
Occlusal contact areas were measured by superimposing maxillary and mandibular arches using Boolean operations, extracting the intersecting triangular mesh areas to quantify occlusal contacts ( Fig 1 , F ).
To assess intrarater reliability, 20% of the data were randomly remeasured by the same examiner (L.X.) after a 2-week interval, and intraclass correlation coefficients were calculated.
Statistical analysis
Data normality was assessed using the Shapiro-Wilk test. Potential predictors were first screened using univariate regression with a selection threshold of P <0.20. To appropriately account for the hierarchical structure of the dataset—in which multiple teeth (up to 6 per patient) and bilateral measurements were nested within the same patient—a linear mixed-effects multivariate regression model was employed. Patient identification was specified as a random intercept to account for intrapatient correlations arising from repeated measurements across multiple teeth and both sides of the arch. ,, Actual cusp extrusion served as the dependent variable. Fixed effects included sex, anterior overjet, anterior overbite, use of intermaxillary elastics, and other relevant clinical parameters. The mixed-effects framework is particularly suitable for datasets in which fixed-effect predictors may be correlated. Even when clinically related variables exhibit partial interdependence, the model provides valid and interpretable regression coefficients by estimating the unique contribution of each covariate while treating them independently.
Analyses were conducted in SPSS Statistics 26.0 (IBM Corp, Armonk, NY) and GraphPad Prism 10 (GraphPad Software Inc, San Diego, Calif). Statistical significance was set at P <0.05.
Results
A total of 172 posterior teeth from 52 patients were included after the comprehensive assessment of POB severity in the ClinCheck system. Baseline demographic and clinical characteristics are presented in Table I .
Table I
Characteristics of the study population
| Characteristics | Value/percentage (%) | |
|---|---|---|
| Age (y) | 30.44 ± 8.50 | |
| Adult | 46 (88.46) | |
| Teenager | 6 (11.54) | |
| Sex | Males | 13 (25.00) |
| Females | 39 (75.00) | |
| Tooth type | First molar | 93 (54.07) |
| Second molar | 79 (45.93) | |
| Elastic | 95 (55.23) | |
| Attachments | Target tooth | 133 (77.33) |
| Adjacent tooth | 152 (88.37) |
Note. Values are presented as mean ± standard deviation or number (percentage).
High measurement reliability was confirmed, with intraclass correlation coefficients of 0.941 for MB cusp extrusion, 0.942 for DB cusp extrusion, and 0.950 for PL cusp extrusion, indicating excellent consistency in 3-dimensional model registration and landmark quantification.
Univariate linear regression identified 10 clinical factors associated with actual MB cusp extrusion ( P <0.2), spanning both categorical variables (eg, sex and elastic use) and continuous variables (eg, pretreatment MB, DB and PL cusp distances, and anterior overjet and overbite) ( Fig 2 , A and B , and Fig 3 , A-H ; Table II ). In the mixed-effect multivariate model, 6 predictors remained statistically significant ( P <0.05). Achieved MB cusp extrusion demonstrated positive associations with female sex (B = 0.309, P <0.001), use of vertical elastics (B = 0.307, P <0.001), greater baseline MB cusp distance (B = 0.466, P <0.001), and larger predicted extrusion (B = 0.172, P <0.001) ( Table III ).
A, Effect of sex on MB cusp extrusion; B, Influence of elastic traction on MB cusp extrusion. ns, not significant; ∗ P <0.05; ∗∗∗∗ P <0.0001.
Impact of key variables on MB cusp extrusion: A, Pretreatment MB cusp distance; B, Pretreatment DB cusp distance; C, Pretreatment PL cusp distance; D, Pretreatment posterior overjet; E, Pretreatment anterior overjet; F, Pretreatment anterior overbite; G, Predicted MB cusp extrusion; H, Pretreatment occlusal contact area.
Table II
Univariate linear regression analysis for the extrusion of the MB cusp of molars
| Variables | B (95% CI) | P value | |
|---|---|---|---|
| Sex | Females | 0.227 (–0.022, 0.476) | 0.074 |
| Males | Reference | ||
| Tooth type | First molar | 0.083 (–0.133, 0.300) | 0.449 |
| Second molar | Reference | ||
| Target tooth attachments | Yes | –0.046 (–0.304, 0.212) | 0.726 |
| No | Reference | ||
| Adjacent tooth attachments | Yes | –0.174 (–0.510, 0.162) | 0.309 |
| No | Reference | ||
| Bite ramp | Yes | 0.025 (–0.193, 0.244) | 0.819 |
| No | Reference | ||
| Elastic | Yes | 0.429 (0.222, 0.637) | <0.001 |
| No | Reference | ||
| Age | –0.003 (–0.017, 0.011) | 0.69 | |
| Pretreatment | MB cusp distance | 0.540 (0.441, 0.640) | <0.001 |
| DB cusp distance | 0.357 (0.259, 0.454) | <0.001 | |
| PL cusp distance | 0.057 (0.005, 0.109) | 0.033 | |
| Posterior overjet | 0.064 (–0.027, 0.155) | 0.168 | |
| Anterior overjet | –0.193 (–0.313,–0.072) | 0.002 | |
| Anterior overbite | 0.068 (–0.033, 0.169) | 0.185 | |
| Occlusal contact area | –0.071 (–0.118,–0.025) | 0.003 | |
| Predicted | MB extrusion | 0.397 (0.305, 0.488) | <0.001 |
| Occlusal contact area changes | –0.002 (–0.006, 0.002) | 0.421 | |
CI, confidence interval.
Table III
Mixed-effect multivariate linear regression analysis for the extrusion of the MB cusp of molars
| Variables | B (95% CI) | P value | |
|---|---|---|---|
| Sex | Females | 0.309 (0.141, 0.476) | <0.001 |
| Males | Reference | ||
| Elastic | Yes | 0.307 (0.150, 0.464) | <0.001 |
| No | Reference | ||
| Pretreatment | MB cusp distance | 0.466 (0.338, 0.595) | <0.001 |
| DB cusp distance | 0.029 (–0.083, 0.141) | 0.611 | |
| PL cusp distance | 0.020 (–0.014, 0.054) | 0.239 | |
| Posterior overjet | 0.053 (–0.009, 0.114) | 0.095 | |
| Anterior overjet | –0.169 (–0.271,–0.066) | 0.001 | |
| Anterior overbite | –0.151 (–0.240,–0.062) | 0.001 | |
| Occlusal contact area | 0.0139 (–0.019, 0.046) | 0.402 | |
| Predicted MB cusp extrusion | 0.172 (0.085, 0.260) | <0.001 |
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