A retrospective study of mandibular incisor intrusion with invisalign in adult deep-bite patients

Abstract

This retrospective study quantitatively evaluated mandibular incisor intrusion and associated vertical skeletal changes during deep-bite correction using Invisalign clear aligner therapy. Twenty-seven adult patients (19–54 years of age) with skeletal Class I or II malocclusion and deep overbite (> 3 mm) treated with Invisalign incorporating precision bite ramps were retrospectively analyzed. Lateral cephalometric radiographs obtained before (T0) and after treatment (T1) were superimposed on the cranial base and mandible. Mandibular incisor movement was assessed using centroid-based measurements to isolate pure intrusion and minimize the influence of tipping. Changes in the occlusal plane angle, mandibular plane angle (FMA), incisor mandibular plane angle (IMPA), and molar vertical position were also evaluated. Paired t-tests were performed for statistical comparisons. The mandibular incisor centroid demonstrated statistically significant pure intrusion of 1.63 mm (p < 0.001). Occlusal plane angle increased significantly (mean change: 2.38°, p = 0.001) and overbite decreased (mean change: -1.98 mm, p = 0.001), whereas changes in the FMA, IMPA, and mandibular molar extrusion were not statistically significant, suggesting limited vertical skeletal changes during treatment. These findings suggest that Invisalign therapy with precision bite ramps demonstrated mandibular incisor intrusion in adult deep-bite patients.

Introduction

Deep bite, which is characterized by excessive anterior extrusion, is a common clinical challenge observed in patients with both Class I and Class II skeletal patterns. This type of deep bite is not simply an esthetic concern but also presents considerable clinical challenges, including impaired masticatory function, increased stress on the temporomandibular joint, and excessive forces on the teeth and periodontal tissues. While Class II patients frequently exhibit severe deep bite due to compensatory mandibular incisor extrusion, the necessity for precise vertical control is equally important in Class I cases to ensure long-term stability and function¹. Therefore, resolving deep bite has long been considered one of the key components of successful orthodontic treatment.

In conventional orthodontic treatment, continuous archwires are commonly used to level the anterior segment. However, this approach may cause unwanted posterior extrusion, potentially increasing the lower facial height and altering occlusal relationships². To overcome these limitations, various biomechanical strategies have been introduced. Burstone³ proposed sectional mechanics to minimize unnecessary posterior extrusion, and mini-screws have further expanded the possibilities for controlled anterior intrusion⁴.

Recently, clear aligner therapy has become increasingly popular due to esthetic demands and advances in digital technology and aligner materials. Invisalign, one of the most widely used clear aligner systems, requires careful treatment planning to determine how accurately programmed tooth movements are expressed clinically. Because clear aligners cover the occlusal surfaces, occlusal forces may help control unwanted posterior tooth movement during anterior intrusion⁵. Anterior intrusion is therefore considered an important component of deep-bite correction with clear aligners⁶.

Rozzi et al.⁷ reported that Invisalign corrected the curve of Spee mainly through incisor intrusion with limited posterior extrusion and relatively minor changes in incisor inclination compared with continuous archwire mechanics. Goh et al.⁸ also evaluated curve of Spee leveling with Invisalign but focused mainly on premolar and molar extrusion rather than quantitative assessment of anterior intrusion. Al-Balaa et al.⁹ compared predicted and actual anterior intrusion using CBCT and reported limited accuracy, but pure mandibular incisor intrusion and associated vertical skeletal changes were not specifically evaluated.

The present study uses lateral cephalometric radiographs to quantitatively assess actual changes in mandibular anterior intrusion, with particular attention to pure intrusion and associated vertical skeletal changes. This analysis will serve as a reference for future individualized treatment planning and will provide clinically relevant guidance for appliance selection and force design in clear aligner therapy.

Materials and methods

Study sample size

Sample size estimation was performed using G*Power 3.1 (Heinrich Heine University, Düsseldorf, Germany). Under the conditions of effect size = 0.7, α = 0.05, and β = 0.9 for t-test of means, i.e., the difference between two dependent means (matched pairs), the minimum sample size was calculated as 24 subjects. Considering 10% drop out, a final sample size of 27 subjects was determined.

Among 128 patients who underwent orthodontic treatment using Invisalign at the Department of Orthodontics, AJOU University Dental Hospital, between January 1, 2018 and September 1, 2024, those who met the inclusion criteria and did not meet any of the exclusion criteria were selected. The final study sample consisted of 27 patients with a mean age of 31.4 ± 11.1 years (range, 19–54 years), comprising 6 males (22.2%) and 21 females (77.8%). Based on the skeletal classifications, 9 patients (33.3%) were categorized as Class I (ANB ≤ 4°), while 18 patients (66.7%) were Class II (ANB > 4°). To assess baseline comparability, the initial overbite was compared between the Class I and Class II groups. No significant difference was found between the Class I group (3.97 ± 0.60 mm) and the Class II group (4.47 ± 1.61 mm) (p = 0.455), indicating that the initial deep-bite severity was consistent across skeletal classifications. Detailed treatment parameters, including treatment duration, number of aligners, refinement stages, and the use of IPR and Class II elastics, are summarized in Tables 1.

Table 1.

Baseline characteristics of the study participants.

Variables	Value (N = 27)
Demographics
Sex, n (%)
Male	6 (22.2)
Female	21 (77.8)
Age, years	31.4 ± 11.1 (19–54)
Clinical Baseline
Angle classification, n (%)
Class I group (ANB ≤ 4°)	9 (33.3)
Class II group (ANB > 4°)	18 (66.7)
Initial overbite, mm
Class I group	3.97 ± 0.60
Class II group	4.47 ± 1.61
P-value	0.455
Treatment Parameters
Total treatment duration, months	15.83 ± 3.21 (9.6–22.0)
Number of aligners	46.52 ± 12.20 (28–77)
Number of refinement stages	1.07 ± 0.68 (0–2)
Clinical Interventions
IPR performed, n (%)	19 (70.4)
Amount of IPR, Upper, mm	2.30 ± 2.09 (0.0–5.4)
Amount of IPR, Lower, mm	0.93 ± 1.40 (0.0–2.5)
Class II elastics used, n (%)	21 (77.8)
Intervention Subgroups, n (%)
Both IPR and elastics	13 (48.1)
IPR only	6 (22.2)
Elastics only	8 (29.6)
Neither IPR nor elastics	0 (0.0)

Data are presented as n (%) for categorical variables or mean ± SD (range) for continuous variables. IPR, interproximal reduction; SD, standard deviation; ANB, A point, nasion, B point angle.

The inclusion criteria were as follows:

1. Patients diagnosed with skeletal Class I or II malocclusion (ANB > 0˚),

2. Adults aged 18 years or older,

3. Patients treated with Invisalign with precision bite ramps,

4. Presence of deep overbite (overbite > 3 mm).

The exclusion criteria were:

1. History of intraoral trauma,

2. Presence of congenital anomalies such as cleidocranial dysplasia or cleft palate,

3. Systemic diseases affecting bone metabolism,

4. Discontinuation of orthodontic treatment before completion,

5. Skeletal Class III malocclusion (ANB < 0˚),

6. Extraction of permanent teeth.

Patients with skeletal Class III patterns were excluded, as deep-bite correction in these individuals typically requires increasing the vertical dimension, introducing a significant confounding variable.

All human research procedures met the ethical standards of the committee responsible for human experimentation (institutional and national), and with the Helsinki Declaration of 1975, as revised in 2013. This study was reviewed and approved by the Institutional Review Board (IRB) of AJOU University Hospital (IRB No: AJOUIRB-DB-2024-229), who waived the requirement for patient-informed consent, as this was a retrospective study based on medical records and imaging data, and the waiver of consent posed minimal risk to the research participants.

All patients were treated by a single orthodontist (H.J.) with extensive Invisalign therapy experience. To achieve optimal clinical outcomes, the orthodontist modified the ClinCheck treatment plan as necessary and performed overcorrection and interproximal reduction, applying attachments as deemed appropriate. Precision bite ramps were prescribed for all patients. Patient data were collected retrospectively, and all identifiable patient information was anonymized for this study.

Lateral cephalometric radiographs were obtained with the patient in the natural head position. Cephalometric X-rays were taken using a CS800SC (Carestream, Atlanta, GA, USA). Pre-treatment (T0) and post-treatment (T1) images were used for analyses.

At both T0 and T1, cephalograms were manually traced by one examiner (H.H.C., Fig. 1, Table 2) Superimpositions were performed on the cranial base and mandibular structures¹⁰. To evaluate measurement precision, the cephalometric analyses were repeated for all of the patients at a 4-week interval. Measurement reproducibility was verified using the intraclass correlation coefficient, which exceeded 0.9, demonstrating excellent reliability. The Dahlberg error values were below 1.0 mm for linear measurements and below 1° for angular measurements, indicating clinically acceptable levels of random error. Therefore, the initial group of variables was utilized for the following statistical analyses.

Fig. 1.

Landmarks and reference lines used in cephalometric analysis. X-axis: FH plane line. Positive values represent labioversion of the maxillary and mandibular incisors. Y-axis: N-perpendicular line to the X-axis. Positive values represent intrusion of the maxillary teeth and extrusion of the mandibular teeth.

Table 2.

Nomenclature of cephalometric landmark and measurements.

		Explanation
Skeletal landmark
S (Sella)		The midpoint of the sella turcica
N (Nasion)		The most anterior point of the frontonasal suture
Po (Porion)		The most superior point of the external auditory meatus
Or (Orbitale)		The lowest point on the inferior margin of the orbit
A point (A)		The deepest point on the anterior contour of the maxillary alveolar process
B point (B)		The deepest point on the anterior contour of the mandibular alveolar process
Go (Gonion)		The most posterior-inferior point at the angle of the mandible
Pog (Pogonion)		The most anterior point on the bony chin
Me (Menton)		The most inferior point on the mandibular symphysis
Dental landmark
Upper incisor crown tip		The incisal edge of the maxillary central incisor
Upper incisor root tip		The apical end of the maxillary central incisor root
Lower incisor crown tip		The incisal edge of the mandibular central incisor
Lower incisor root tip		The apical end of the mandibular central incisor root
Lower incisor centroid		A point located at the one-third of the mandibular incisor root along the long axis of the tooth
um crown tip		The mesiobuccal cusp tip of the maxillary first molar
lm crown tip		The mesiobuccal cusp tip of the mandibular first molar
Skeletal measurements
SNA (°)		Angle of Sella-Nasion to Nasion-A point
SNB (°)		Angle of Sella-Nasion to Nasion-B point
ANB (°)		Angle of A point-Nasion to Nasion-B point
FMA (°)		Angle of Frankfort horizontal plane (FH) and the Mandibular plane
SN-GoMe (°) (Mandibular plane angle)		Angle of Sella-Nasion to Gonion-Menton
Dental measurements
IMPA(°)		Angle of Mandibular plane to L1 axis
Occlusal plane(°)		Angle of Sella-Nasion to line that bisects the vertical distance between the upper and lower incisal tips (U1 tip, L1 tip) and the upper and lower first molar occlusal surface

Cranial base superimposition was performed using the Frankfort horizontal (FH) plane as a reference. The FH line was set as the X-axis, and a line perpendicular to it passing through the nasion (N-perpendicular) was used as the Y-axis. The bisecting occlusal plane (BOP), defined as a line bisecting the molar and incisor overbites, was utilized to establish the horizontal reference framework. The selection of the BOP was justified based on established literature^11,12, as it effectively minimizes the structural skewing effects caused by severely flared or extruded individual incisors prior to orthodontic treatment. To verify the methodological stability and robustness of this primary framework, sensitivity analysis was performed. Ten subjects were randomly selected, and their treatment-induced alterations were re-measured using an alternative reference definition: the maxillary-mandibular bisector (MMB-Wits) occlusal plane. Comparative analysis yielded highly consistent outcomes between the two definitions, showing no statistically significant differences in any primary vertical or sagittal variables (p > 0.05). This high level of geometric agreement confirms that our primary occlusal plane findings are methodologically stable and free from definition-dependent bias.

For mandibular base superimposition (Fig. 1), the Gonion–Menton (Go–Mn) line was used as the reference¹³. The X-axis was defined by the Go–Mn line, and the Y-axis was established as a line perpendicular to the mandibular plane and passing through the pogonion to isolate tooth movement within the mandible, eliminating potential confounding effects from the maxilla or cranial base (Fig. 2).

Fig. 2.

Reference lines used for lower incisor movement measurements relative to the mandibular XY-coordinate system. X-axis: Gonion-Menton line. Positive values represent labioversion of the mandibular incisors. Y-axis: Line perpendicular to the X-axis passing through the pogonion. Positive values represent extrusion of the lower incisors.

In the cranial base superimposition, changes in the occlusal plane angle and mandibular plane angle were evaluated to determine whether vertical skeletal changes occurred during deep-bite correction. In addition, to assess labioversion of the mandibular incisors—a common phenomenon during bite correction—the incisor mandibular plane angle (IMPA) was measured before and after treatment (Table 3).

Table 3.

Difference Before(T0) and After(T1) Treatment (°).

Measurement	T0 Mean	T0 SD	T1 Mean	T1 SD	Δ Mean	Δ SD	95% CI of mean difference	Cohen’s d	p-value
A-B plane angle	-8.46	3.59	-8.63	2.93	-0.17	1.66	-0.83 to 0.49	0.102	0.605
ANB	5.01	2.48	5.21	2.14	0.20	1.08	-0.23 to 0.63	0.185	0.338
FMA	26.56	4.68	26.83	4.82	0.27	1.17	-0.19 to 0.73	0.231	0.249
IMPA	93.07	7.53	94.64	6.21	1.57	7.05	-1.22 to 4.36	0.223	0.257
SN-GoMe	34.23	5.69	34.61	5.61	0.38	1.16	-0.08 to 0.84	0.328	0.098
Occlusal Plane	18.42	5.13	20.80	4.17	2.38	4.72	0.51 to 4.25	0.504	0.001**
U1 to SN	99.34	9.41	94.60	7.44	-4.74	6.68	-7.38 to -2.10	0.710	0.001**
Overbite	4.29	1.54	2.31	0.81	-1.98	1.48	-2.57 to -1.39	1.338	0.001**

*Values are presented as mean ± standard deviation (SD). Δ = difference between T0 and T1.

*p < 0.05, **p < 0.01.

Intrusion was assessed by identifying coordinate points at the crown tip and root apex of the mandibular incisors at both T0 and T1. To quantify pure vertical intrusion while minimizing the confounding effects of crown tipping, centroid-based analysis was employed. The centroid was geometrically defined as the lower incisor center of resistance^13,14. The definition of this point involves the following steps: (1) establishing the long axis of the mandibular incisor connecting the incisal edge and the root apex; (2) identifying the level of the alveolar crest; and (3) marking the point on the long axis at one-third of the root length (measured from the alveolar crest level) apical to the crest. This reference point was specifically selected because its position remains relatively stable regardless of changes in axial inclination, ensuring that measured displacement represents true vertical tooth movement rather than labial flaring¹⁵. Molar extrusion was assessed by comparing the pre- and post-treatment positions of the mesiobuccal cusp tips of the maxillary and mandibular first molars.

The superimposed tracing images were saved as JPG files and imported into ImageJ (ImageJ software version 1.37, Bethesda, MD, USA). After calibration, the coordinates for each reference point were measured based on the predefined coordinate system. The differences between T1 and T0 values were calculated to determine the direction and magnitude of tooth movement.

Statistical analysis of the measurements was performed using IBM SPSS Statistics version 29.0 (IBM Corp., Armonk, NY, USA). The Shapiro-Wilk test was conducted to assess the normality of the data. Since all variables showed normal distributions, paired t-tests were used to evaluate changes in the measurements before and after treatment. A p value < 0.05 was considered statistically significant.

Invisalign treatment procedures for deep-bite correction

Deep-bite correction with Invisalign is achieved mainly through programmed lower-incisor intrusion, combined with controlled crown-labial tipping and minimal buccolingual root movement. This approach improves the predictability of intrusion and keeps the roots within the alveolar housing, which is especially important in skeletal Class II patients. Light and continuous intrusive forces are distributed across the mandibular incisors through natural bite forces and the support of SmartForce features, including precision bite ramps, pressure areas, and attachments on the lateral incisors (optimized intrusion), canines (supporting horizontal rectangular), and posterior teeth that provide additional anchorage for consistent intrusion.

An overcorrection setup for lower-incisor intrusion is also essential for achieving predictable results. This concept is well established in fixed-appliance therapy for vertical control of incisors, such as using a compensating curve in a stainless-steel archwire during extraction space closure or a reverse curve of Spee for leveling in deep-bite cases. In the general overcorrection strategy for deep bite correction, the canines were typically positioned 0.5–1.0 mm below the posterior occlusal plane, while the four incisors were set an additional 0.5–1.0 mm below the canine level. Accordingly, the final setup was usually designed to achieve an anterior overbite of approximately − 1 to − 2 mm, representing intentional overcorrection toward a slight anterior open bite.

In addition, the vertical position of the posterior teeth is preserved with an intrusive effect from natural occlusal forces. This helps maintain a stable vertical dimension during treatment, which is particularly important in Class II cases in which inter-arch elastics from the upper canines to the lower molars are used.

Results

Descriptive statistics for cephalometric measurements of the subjects included in this study are presented in Table 3. The mean ANB angle was 5.01° (SD = 2.48), indicating a general tendency toward a skeletal Class II relationship.

The A–B plane angle decreased from a mean of -8.46° (SD = 3.59) to -8.63° (SD = 2.93), with a mean change of -0.17° (SD = 1.66), with no statistical significance (p = 0.605). The ANB angle increased by 0.2°, but that change was not statistically significant either. The FMA increased slightly from 26.56° (SD = 4.68) at T0 to 26.83° (SD = 4.82) at T1, with a mean change of 0.27° (SD = 1.17); however, this change was not statistically significant (p = 0.249). The IMPA increased from 93.07° (SD = 7.53) to 94.64° (SD = 6.21), with a mean change of 1.57° (SD = 7.05), which was also not statistically significant (p = 0.257). The occlusal plane angle changed from 18.42° (SD = 5.13) to 20.80° (SD = 4.17), with a mean change of 2.38° (SD = 4.72), and that was statistically significant (p = 0.001) (Table 3). The inclination of the maxillary incisors, measured as U1 to SN, decreased from a pre-treatment mean of 99.34° (SD = 9.41) to a post-treatment mean of 94.60° (SD = 7.44), with a mean change of -4.74° (SD = 6.68), which was statistically significant (p = 0.001). The mean overbite decreased significantly from 4.29 mm (SD = 1.54) at T0 to 2.31 mm (SD = 0.81) at T1, demonstrating a mean net overbite reduction of 1.98 mm (SD = 1.48) (p < 0.001) (Table 3).

To clearly interpret the direction of these vertical tooth movements, positive values in the Y-axis coordinate system were defined as intrusion for the maxillary teeth and extrusion for the mandibular teeth, while negative values denoted the opposite. Under this directional framework, analysis of traced images of the maxillary incisors using ImageJ (Table 4) revealed a statistically significant decrease in the x-coordinate of the crown tip, from a pre-treatment mean of 6.72 mm to a post-treatment mean of 2.93 mm (p < 0.001). The root tip significantly intruded, with its y-coordinate increasing from − 61.85 mm to -60.71 mm (p = 0.003). In contrast, no statistically significant change was observed in the y-coordinate of the crown tip (p = 0.62) or the x-coordinate of the root tip (p > 0.05). Evaluation of the maxillary first molar position revealed that the crown tip (p = 0.03) exhibited slight extrusion. Although the changes in the x-coordinate were not statistically significant, those in the y-coordinate were significant (Table 4).

Table 4.

Changes in maxillary incisors & molars before and after treatment (cranial base superimposition) (mm).

Measurement	T0 Mean	T0 SD	T1 Mean	T1 SD	Δ Mean	Δ SD	95% CI of mean difference	Cohen’s d	p-value
crown tip(X)	6.72	9.75	2.93	9.20	-3.79	3.25	-5.08 to -2.50	1.166	< 0.001**
crown tip(y)	-99.85	26.58	-99.66	26.30	0.19	2.00	-0.60 to 0.98	0.095	0.621
root tip(x)	-5.30	6.78	-5.53	6.48	-0.24	1.93	-1.00 to 0.52	0.124	0.533
root tip(y)	-61.85	18.69	-60.71	18.17	1.13	1.82	0.41 to 1.85	0.621	0.003**
um-crown tip(x)	-57.99	15.46	-57.82	15.46	0.18	1.76	-0.52 to 0.88	0.102	0.6
um-crown tip(y)	-81.49	20.73	-82.27	21.30	-0.78	1.81	-1.50 to -0.06	0.431	0.03*

*Values are presented as mean ± standard deviation (SD). Δ = difference between T0 and T1.

*um-crown tip indicates the mesiobuccal cusp tip of the maxillary first molar.

*p < 0.05, **p < 0.01.

For the mandibular incisors, paired t-tests were utilized to analyze the positional changes of the crown tip, root tip, and centroid. The y-coordinate of the crown tip intruded significantly from − 91.09 mm to -95.47 mm (p < 0.001), and that of the root tip intruded from − 122.80 mm to -126.61 mm (p < 0.001). The y-coordinate of the centroid also intruded from − 107.76 mm to -112.00 mm, indicating significant downward movement in all Y-axis measurements (p < 0.001). No statistically significant changes were observed in the x-coordinates of the crown tip, root tip, or centroid (Table 5).

Table 5.

Changes in mandibular incisors before and after treatment (cranial base superimposition) (mm).

Measurement	T0 Mean	T0 SD	T1 Mean	T1 SD	Δ Mean	Δ SD	95% CI of mean difference	Cohen’s d	p-value
crown tip(x)	-0.81	12.16	0.11	11.34	0.92	2.65	-0.13 to 1.97	0.347	0.083
crown tip(y)	-91.09	22.61	-95.47	23.17	-4.37	3.43	-5.73 to -3.01	1.274	< 0.001**
root tip(x)	-18.93	11.11	-19.88	11.95	-0.95	2.42	-1.91 to 0.01	0.393	0.053
root tip(y)	-122.8	29.62	-126.61	30.37	-3.81	2.49	-4.80 to -2.82	1.530	< 0.001**
centroid(x)	-9.28	12.76	-9.32	12.67	-0.04	2.36	-0.97 to 0.89	0.017	0.932
centroid(y)	-107.76	26.22	-112	27.02	-4.25	2.4	-5.20 to -3.30	1.771	< 0.001**
lm-crown tip(x)	-61.44	32.94	-61.41	32.69	0.03	2.42	-0.93 to 0.99	0.012	0.947
lm-crown tip(y)	-92.87	37.92	-93.64	38.72	-0.77	1.99	-1.56 to 0.02	0.387	0.053

*Values are presented as mean ± standard deviation (SD). Δ = difference between T0 and T1.

*lm-crown tip indicates the mesiobuccal cusp tip of the mandibular first molar.

*p < 0.05, **p < 0.01.

To isolate localized dentoalveolar alterations from structural mandibular shifts, mandibular tooth movements were evaluated via mandibular plane superimposition. In this coordinate system based on the MP-POG reference, the x-coordinates of the incisor crown tip and root tip increased by 1.71 mm and 0.39 mm, respectively, indicating anterior movement of the teeth. The y-coordinates of the incisor crown and root tip were similarly intruded by 1.65 mm and 1.60 mm, respectively, and the centroid also showed a downward movement of 1.63 mm. All of these changes in the y-coordinates were statistically significant (p < 0.001). In the mandibular first molar, a minor extrusion of the crown tip was identified (p = 0.037). Although the x-coordinate changes were not significant, the y-coordinates demonstrated significant differences (Table 6).

Table 6.

Changes in mandibular incisors before and after treatment (mandibular plane superimposition) (mm).

Measurement	T0 Mean	T0 SD	T1 Mean	T1 SD	Δ Mean	Δ SD	95% CI of mean difference	Cohen’s d	p-value
crown tip(x)-MP	-11.02	3.72	-9.31	3.34	1.71	2.16	0.86 to 2.56	0.792	< 0.001**
crown tip(y)-MP	46.5	4.71	44.85	4.77	-1.65	1.48	-2.24 to -1.06	1.115	< 0.001**
root tip(x)-MP	-11.55	2	-11.16	2.57	0.39	1.36	-0.15 to 0.93	0.287	0.144
root tip(y)-MP	24.07	4.43	22.47	4.6	-1.6	1.52	-2.20 to -1.00	1.053	< 0.001**
centroid(x)-MP	-11.21	2.59	-10.21	2.72	0.99	1.18	0.52 to 1.46	0.839	< 0.001**
centroid(y)-MP	34.3	4.32	32.68	4.46	-1.63	1.52	-2.23 to -1.03	1.072	< 0.001**
lm-crown tip(x)	-37.19	7.55	-37.04	7.61	0.16	1.51	-0.44 to 0.76	0.106	0.588
lm-crown tip(y)	35.13	4.64	35.65	4.27	0.52	1.24	0.03 to 1.01	0.419	0.037*

*Values are presented as mean ± standard deviation (SD). Δ = difference between T0 and T1.

*lm-crown tip indicates the mesiobuccal cusp tip of the mandibular first molar.

*p < 0.05, **p < 0.01.

Discussion

The correction of a deep overbite must be approached with caution because unwanted extrusion of the posterior teeth can result in undesirable changes in facial appearance. Therefore, treatment strategies that minimize posterior extrusion are essential¹⁶. This study investigated how the planned intrusion of mandibular incisors was realized in actual clinical outcomes when using the Invisalign clear aligner system for deep-bite correction. Traditionally, fixed appliances and auxiliary devices have been used for deep-bite correction. The clinical significance of this study lies in its evaluation of outcomes achieved exclusively using clear aligners.

Many previous studies have either not quantified mandibular incisor intrusion or have reported only minimal amounts, as in the CBCT analysis by Al-Balaa et al.⁹, which found just 0.82 mm of actual intrusion. Goh et al.⁸ reported deep-bite improvement with Invisalign; however, the mechanism was described primarily in terms of first molar extrusion rather than mandibular incisor intrusion. More recently, studies have shown that clear aligners can effectively achieve anterior intrusion. Rozzi et al.⁷ reported an average mandibular incisor intrusion of 2.1 mm in the Invisalign group together with favorable vertical control. Kang et al.¹⁷ documented 0.93–1.14 mm of intrusion, and Kravitz et al.¹⁸ reported 0.9–1.7 mm, suggesting that meaningful intrusion can be achieved with appropriate staging and biomechanical setup.

However, previous studies that measured intrusion using cephalometric radiographs or CBCT generally relied on incisal–apical landmarks, so their assessments were based on changes in the incisal tip and root apex^7,9. Even in studies that used intraoral scan superimposition, mandibular incisor intrusion was assessed based on incisal-edge displacement¹⁷. Thus, previously reported values might have been over- or underestimated depending on the degree of tooth inclination, making it difficult to isolate pure mandibular intrusion. In contrast, this study used a centroid-based reference point recommended in a prior study¹³, thereby minimizing the influence of angular changes and allowing more accurate evaluation of pure intrusion. The observed 1.63 mm of mandibular incisor intrusion is clinically meaningful because it reflects pure intrusion assessed with a centroid-based measurement approach. Furthermore, it is greater than many previously reported values and provides additional evidence that clear aligners can achieve reproducible and quantitatively significant intrusion in adult deep-bite treatment.

The analysis of cranial base superimposition and mandibular plane superimposition data revealed significant downward movement in the crown tip, root tip, and centroid of the mandibular incisors, indicating that pure intrusion was successfully achieved. Notably, the distinct displacement of the centroid supports the presence of pure intrusion, and the comparable movement of the crown and root suggests minimal tipping, with vertical displacement being the dominant component (Table 6). Although slight extrusion of the maxillary and mandibular first-molar crown was observed (Tables 4, 5 and 6), the FMA did not change significantly, indicating preservation of the vertical skeletal dimension. This suggests that molar extrusion did not induce clockwise mandibular rotation and was likely counterbalanced by pure intrusion of the mandibular incisors. Accordingly, the stability of the FMA supports the notion that anterior intrusion, rather than posterior extrusion, was the primary mechanism of deep-bite correction in this study.

These findings are consistent with previous studies demonstrating that deep-bite correction with clear aligners can be achieved through selective anterior intrusion without increasing the vertical facial dimension^7,19. Significant changes in the occlusal-plane angle coupled with stable FMA further indicates that the observed vertical correction mainly resulted from dentoalveolar movement rather than mandibular-plane rotation. Similarly, recent studies have emphasized that appropriate staging and posterior anchorage with clear aligners can help maintain occlusal-plane stability even during anterior intrusion²⁰. A representative case after deep-bite correction is presented in Fig. 3. Pure intrusion of the lower incisors and occlusal plane steepening are shown, accompanied by only a slight change in the mandibular plane.

Fig. 3.

An example of cephalometric structural superimposition before and after treatment (black: before treatment, red: after treatment).

In the present study, significant vertical correction was achieved, with the mean overbite decreasing by 1.98 mm (p < 0.001). This clinically meaningful reduction was associated with pure mandibular incisor intrusion of 1.63 mm (centroid Y-axis displacement in Table 6). Although the FMA did not show significant changes in this study, this finding should be interpreted cautiously. The absence of statistical significance may suggest limited vertical skeletal change during treatment; however, it does not necessarily prove complete preservation of the vertical facial dimension. Alternative explanations, including the relatively small sample size, measurement variability inherent to two-dimensional cephalometric analysis, and insufficient statistical power to detect small skeletal changes should also be considered.

This study has several limitations. First, the study population was heterogeneous in terms of age range, skeletal classification, initial overbite severity, and treatment duration. Although all patients met the predefined inclusion criteria and were adult deep-bite patients treated with Invisalign and precision bite ramps, clinical variation may have influenced the treatment-associated changes observed in this study. Because of the relatively small sample size, subgroup analyses according to age, skeletal classification, and treatment duration were not performed. Future studies with larger sample sizes are needed to evaluate the influence of these factors. Further limitations include treatment performed by a single operator, the single-center study design, and the lack of complete treatment protocol standardization, which may limit the generalizability of the findings.

Considering age, Kravitz et al.¹⁸ noted that deep overbite correction via clear aligners can be affected by patient age, showing a tendency for lower absolute intrusion accuracy in adults compared to adolescents. Biomechanically, this difference is heavily attributed to age-related increases in alveolar bone density and a concomitant decline in cellular and molecular dynamics during orthodontic bone remodeling, as historically and molecularly documented by Reitan²¹, Krishnan and Davidovitch²².

Another limitation of this study is its retrospective approach and the reliance on two-dimensional cephalometric analysis, which inherently lacks the capacity to evaluate transverse changes or complex three-dimensional root movement. Although conventional lateral cephalograms are standard and reliable for sagittal and vertical linear measurements, further prospective research utilizing three-dimensional cone-beam computed tomography measurements would provide a more comprehensive assessment of tooth movement during deep-bite correction. Furthermore, while our sample demonstrated predictable overbite reduction, future studies with larger sample sizes and prospective control groups are needed to further validate these vertical control protocols. Finally, long-term follow-up clinical trials comparing conventional fixed appliances and clear aligner treatment are warranted to establish the definitive efficiency and post-treatment stability of aligner therapy for deep-bite correction.

Conclusion

In conclusion, this retrospective study observed mandibular incisor intrusion and overbite reduction in adult deep-bite patients treated with Invisalign and precision bite ramps. The occlusal plane angle increased significantly, whereas no significant change was observed in the FMA. However, because this study was retrospective and lacked a control group, the results should be interpreted as treatment-associated changes rather than evidence of comparative treatment effectiveness.

Author contributions

Hyunhee Choo : Data curation, Writing-Original draft, Visualization. Young Ho Kim: Conception, Formal analysis, Writing-Original draft. Kunyoung Chang: Drawing all the images, Methodology, Validation. Pureum Hong: Investigation, Writing-Review Editing.Minseo Kim: Analysis, Revision writing. Soo-Jin Kim: Resources, Conception, Jeong Won Shin : Investigation, Supervision. Hwa Sung Chae: Conception, Methodology, Writing-Review & Editing.

Data availability

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

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval and consent to participate

All human research procedures were followed in accordance with the ethical standards of the committee responsible for human experimentation (institutional and national), and with the Helsinki Declaration of 1975, as revised in 2013. This study was reviewed and approved by the Institutional Review Board (IRB) of AJOU University Hospital (IRB No: AJOUIRB-DB-2024-229). The IRB committees of Ajou University Hospital waived the requirement for patient-informed consent, as this was a retrospective study based on medical records and imaging data, and the waiver of consent posed minimal risk to the research participants.