ABSTRACT
Background
This retrospective study aimed to evaluate the skeletal and dentoalveolar effects of maxillary expansion using the Invisalign First system in the mixed dentition.
Methods
The study was conducted in the orthodontic department of the Fourth Affiliated Hospital of Zhejiang University School of Medicine between 2021 and 2024. Inclusion criteria were mixed dentition patients with maxillary transverse deficiency, fully erupted first molars, arch width discrepancy ≤ 5 mm, mild to moderate crowding and pre‐peak growth status (CS2). Exclusion criteria were Angle's Class III malocclusion, previous orthodontic treatment, congenitally missing teeth, TMJ disorders, cleft lip/palate, or use of additional appliances. All patients were treated with a standardised digital protocol using the Invisalign First system with optimised expansion support attachments. No additional buccal root torque was programmed, and Class II elastics were not used. Pre‐ and post‐treatment intraoral scan digital models and cone‐beam computed tomography data were obtained. Measured parameters included arch width, nasal width, apical base width, alveolar width, palatal depth and first molar inclination. Statistical analyses were performed using paired t‐tests for normally distributed data and Wilcoxon signed‐rank tests for non‐normally distributed data, with a significance level set at p < 0.05.
Results
The study included 45 patients (mean age 8.84 ± 1.01 years; mean treatment duration 18.26 ± 0.95 months). Significant transverse increases were observed in all maxillary arch widths, particularly in the canine and deciduous molar regions (canine dental width: 4.17 ± 1.91 mm, p < 0.001; first deciduous molar width: 3.86 ± 1.93 mm, p < 0.001; second deciduous molar width: 4.38 ± 1.78 mm, p < 0.001). Corresponding significant expansion was noted at the alveolar bone level (anterior alveolar process width: 4.04 ± 3.37 mm, p < 0.001; posterior alveolar process width: 2.51 ± 1.71 mm, p < 0.001). The maxillary first molars showed controlled buccal inclination of 1.6° ± 4.09° (p < 0.01). The upper arch perimeter increased by 3.68 ± 2.95 mm (p < 0.001) with crowding reduction of 3.83 ± 3.00 mm (p < 0.001). No significant palatal depth changes occurred (0.42 ± 3.36 mm, p = 0.41).
Conclusion
In the mixed dentition, the Invisalign First system can effectively expand the maxillary dental‐alveolar width, maintain molar inclination and improve the transverse deficiencies and arch form in the canine–premolar region.
Keywords: expansion, Invisalign First system, mixed dentition
1. Background
Maxillary transverse deficiency is one of the most common skeletal malocclusion types in orthodontics, with a prevalence of 21% in mixed dentition patients [1, 2, 3]. It is often associated with unilateral or bilateral posterior crossbite and anterior crowding, leading to functional mandibular displacement and mandibular asymmetry, which have long‐term impacts on craniofacial structure and function development [4]. Traditional treatment can be classified into slow maxillary expansion (SME) and rapid maxillary expansion (RME) [5]. These appliances work by opening the midpalatal suture, appropriately increasing maxillary width, addressing posterior crossbite and maxillary underdevelopment, thereby reducing the severity of future dental crowding in growing children [6, 7]. Numerous studies have validated the effectiveness of various SME and RME appliances [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. As age increases, the maturation of the midpalatal suture advances, which may reduce the potential for skeletal expansion and prolong the bone remodelling period after active treatment [21]. Therefore, early preventive treatment is meaningful [22, 23].
In recent years, Align Technology (Santa Clara, CA, USA) has introduced the Invisalign First system, designed for patients aged 6–10 years, to address problems such as arch constriction and dental crowding [2, 24, 25, 26, 27, 28]. Compared to traditional methods, this appliance combines alveolar expansion, tooth alignment and restoration of correct arch form while being more comfortable, aesthetically pleasing and conducive to maintaining oral hygiene. In 2019, Blevins et al. [27] first reported three cases using the Invisalign First system for treating dental crowding, Class II malocclusion, deep bite and anterior crossbite. In 2020, Staderini et al. [26] published a further case report, including two cases using the Invisalign First system to treat anterior crossbite in 8‐year‐old children. In 2021, Levrini et al. [2] evaluate maxillary arch changes in mixed dentition patients with the Invisalign First system, finding significant increases in arch width‐related measurements while arch depth and molar inclination significantly decreased. Lione et al. [28] discovered that the greatest width increase induced by the Invisalign First system in early mixed dentition children occurred at the maxillary first deciduous molar level, followed by the second deciduous molar and deciduous canine. In 2022, Lione et al. [29] further evaluated the modifications of gingival margins following Invisalign First treatment, reporting significant improvements in gingival contour and aesthetics, particularly in the canine region. In the same year, Lombardo et al. [30] compared the morphological changes of the upper arch during treatment with the Invisalign First system and traditional RME appliances, finding statistically significant morphological changes in the posterior arch area but no significant changes in the anterior arch area with RME. The Invisalign First system increased width at the canine and first deciduous molar levels, and due to anterior tooth alignment, the anterior arch shape also changed. In 2023, Lu et al. [31] conducted a prospective cohort study including a blank control group to exclude growth effects, comparing dental‐alveolar changes by the Invisalign First system and RME. Both groups showed significant increases in post‐treatment width indices, with the RME group exhibiting a significantly superior transverse expansion effect in most measurements compared to the Invisalign First group. Therefore, the researchers believed that in mixed dentition, both the Invisalign First system and RME can expand the maxillary arch. For mild to moderate maxillary transverse deficiency, the Invisalign First system might be a reasonable choice, while RME demonstrated superior arch expansion effects for severe maxillary transverse deficiency patients. Most recently, in 2024, Gazzani et al. [32] demonstrated the efficacy of clear aligners in managing severe lower arch crowding and midline deviation secondary to premature canine resorption, highlighting the role of the Invisalign First in interceptive treatment for arch development and occlusal guidance. However, it is noteworthy that the sample sizes in these studies were limited and mainly based on pre‐ and post‐treatment plaster models or digital models, often including only dental‐alveolar indices.
In previous studies on maxillary expansion, the effectiveness of RME and SME on dental and skeletal structures could be determined through dental models, panoramic radiographs, posteroanterior cephalograms and zygomatic arch axial cephalograms, but two‐dimensional imaging has certain limitations such as image distortion, magnification errors and difficulty distinguishing structures due to overlap. In the late 1990s, cone‐beam computed tomography (CBCT) was introduced into the dental field and has been increasingly used for orthodontic diagnosis, treatment planning and clinical research [33]. With the application of CBCT, the entire maxillofacial complex can be visualised without any magnification or superimposition, and linear and angular measurements of teeth and bones can be achieved with minimal image distortion, ensuring high precision and accuracy [34, 35, 36].
Therefore, this study aims to explore the skeletal and dental‐alveolar expansion changes after Invisalign First system treatment in mixed dentition patients, by combining pre‐ and post‐treatment digital model data and CBCT data, providing a reference for the clinical application of the Invisalign First system.
2. Materials and Methods
The retrospective study was approved by the Ethics Committee of the Fourth Affiliated Hospital Zhejiang University School of Medicine (Approval No. K2023180). All patients and their parents provided signed informed consent. From January 2021 to December 2024, patients who visited the Fourth Affiliated Hospital of Zhejiang University School of Medicine and completed treatment with the Invisalign First system were screened according to the following inclusion and exclusion criteria. Pre‐ and post‐treatment CBCT scans, clinically indicated for orthodontic diagnosis and outcome assessment, were utilised to accurately evaluate three‐dimensional skeletal and dentoalveolar changes. All scans were obtained using a low‐dose, limited field‐of‐view protocol in strict adherence to the ALARA (As Low As Reasonably Achievable) principle.
Inclusion criteria: (1) completion of standardised Invisalign First treatment with comprehensive records, (2) mixed dentition with fully erupted first molars, (3) the presence of maxillary transverse deficiency, (4) maxillary arch width difference from the mandibular arch width ≤ 5 mm, (5) mild to moderate crowding and (6) pre‐peak growth stage (cervical vertebral maturation stages CS2).
Exclusion criteria: (1) Angle's Class III malocclusion, (2) previous orthodontic treatment, (3) congenitally missing teeth, (4) temporomandibular joint disorders, (5) cleft lip and/or palate and (6) use of additional orthodontic appliances.
The ClinCheck treatment plan for all included patients followed a standardised digital protocol for maxillary expansion. The expansion was sequenced with molars moving first, followed by the simultaneous movement of all posterior deciduous teeth and canines. The amount of expansion at each stage was 0.15 mm. Optimised expansion support attachments were automatically placed by the Invisalign software on the posterior teeth to enhance force delivery and retention. No additional buccal root torque was programmed for the molars or other teeth in the expansion protocol. Class II elastics were not used in any patient as part of the treatment regimen.
Patients were instructed to wear the aligners full‐time (20–22 h per day), removing them only for meals and oral hygiene. Aligners were replaced every 7 days. Clinical monitoring was performed every 8 weeks (or every 4 aligner stages) to verify aligner fit, check the integrity of attachments and assess compliance. In the event of premature loss of deciduous teeth or eruption of permanent teeth, a re‐scan was performed to fabricate new aligners that would continue the treatment according to the original prescription and final treatment goals.
ClinCheck plan provided pre‐treatment (T1) and post‐treatment (T2) arch width tables. Each width measurement represents the linear distance between occlusal surface points (intersection of the tooth's long axis and occlusal surface) of the corresponding teeth pairs, as shown in Table 1.
TABLE 1.
Definition of ClinCheck plan arch width variables.
| Measurement | Definition |
|---|---|
| Maxillary (or mandibular) | |
| Canine dental width (53–63 or 73–83) | Linear distance between occlusal surface points of the maxillary (or mandibular) deciduous canines. |
| First deciduous molar dental width (54–64 or 74–84) | Linear distance between occlusal surface points of the maxillary (or mandibular) first deciduous molar dental. |
| Second deciduous molar dental width (55–65 or 75–85) | Linear distance between occlusal surface points of the maxillary (or mandibular) second deciduous molar dental width. |
| First permanent molar dental width (16–26 or 36–46) | Linear distance between occlusal surface points of the maxillary (or mandibular) first permanent molar dental width. |
3D Slicer 5.0.3 software was used to analyse the CBCT data at T1 and T2. After importing the CBCT DICOM data into the software, the orientation was adjusted [19]:the sagittal plane was perpendicular to the palatal plane, the transverse plane passed through the root tips of the bilateral maxillary first molars, and the coronal plane was perpendicular to the hard palate. The measurement indicators [19, 37, 38, 39] are shown in Table 2 and Figure 1.
TABLE 2.
Definition of CBCT variables.
| Measurement | Definition |
|---|---|
| Skeletal variables | |
| Anterior nasal width (A‐NW) | Distance between the widest points of the nasal cavity parallel to the hard palate at the level of the permanent canines in the coronal plane. |
| Posterior nasal width (P‐NW) | Distance between the widest points of the nasal cavity parallel to the hard palate at the level of the first permanent molars in the coronal plane. |
| Maxillary width (MW) | Distance between the uppermost points of the zygomaticomaxillary sutures at the level of the first permanent molars in the coronal plane. |
| Anterior apical base width (A‐ABW) | Distance between the buccal outer edges of the maxilla at the same transverse plane level as the base of the nasal cavity at the level of the permanent canines in the coronal plane. |
| Posterior apical base width (P‐ABW) | Distance between the buccal outer edges of the maxilla at the same transverse plane level as the base of the nasal cavity at the level of the first permanent molars in the coronal plane. |
| Maxillary mid‐alveolar width (MMW) | Distance between the right and left S points (points halfway vertically between the buccal alveolar crest and the maxillary first permanent molar buccal root apex) on the mid‐alveolar bone level. |
| Palatal depth (PD) | Vertical distance between the highest point of the palate and the midpoint of the width between the first permanent molars. |
| Dental‐Alveolar variables | |
| Anterior alveolar process width (AAPW) | Distance between the alveolar ridges on the left and right sides at the level of the permanent canines in the coronal plane. |
| Posterior alveolar process width (PAPW) | Distance between the alveolar ridges on the left and right sides at the level of the first molars in the coronal plane. |
| Upper intermolar width (UIMW) | Distance between the mesiopalatal cusps of the left and right first molars. |
| Upper second intermolar width (USIMW) | Distance between the mesiopalatal cusps of the left and right second permanent molars. |
| Lower intermolar width (LIMW) | Distance between the central fossae of the left and right first molars. |
| Upper intercanine width (UICW) | Distance between the cusps of the permanent canines on the left and right sides. |
| Lower intercanine width (LICW) | Distance between the cusps of the permanent canines on the left and right sides. |
| First maxillary molar axial angle (UMAA) | Angle between the long axis of the maxillary first molar and the functional occlusal plane. |
| Upper arch perimeter (UAP) | Length of the arch between the left and right first molars. |
| Lower arch perimeter (LAP) | Length of the arch between the left and right first molars. |
| Upper arch crowding (UAC) | Difference between the length of the upper arch and the amount of the upper tooth size. |
| Lower arch crowding (LAC) | Difference between the length of the lower arch and the amount of the lower tooth size. |
| Periodontal variables | |
| Buccal alveolar bone thickness (BABT) | Distance between the buccal root surface and the outer wall of the alveolar bone at the furcation level of the first molar. |
| Palatal alveolar bone thickness (PABT) | Distance between the palatal root surface and the inner wall of the alveolar bone at the furcation level of the first molar. |
| Buccal alveolar bone height (BABH) | Distance between the mesiobuccal cusp of the first molar and the buccal alveolar crest. |
| Palatal alveolar bone height (PAPH) | Distance between the palatal cusp of the first molar and the palatal alveolar crest. |
FIGURE 1.
Measurement methods for some CBCT variables. (A) Measurements of A‐NW, A‐ABW, UICW and AAPW. (B) Measurements of P‐NW, P‐ABW, MMW, PAPW, UIMW and PD. (C) Measurements of BABT, PABT, BABH, PABH and UMAA. A‐ABW, anterior apical base width; AAPW, anterior alveolar process width; A‐NW, anterior nasal width; MMW, maxillary mid‐alveolar width; P‐ABW, posterior apical base width; PAPW, posterior alveolar process width; PD, palatal depth; P‐NW, posterior nasal width; UICW, upper intercanine width; UIMW, upper intermolar width.
3. Data Analysis
Based on previous studies [31], the study outcome variable was set to be the change in maxillary fist intermolar width. The null hypothesis was that there was no significant difference in maxillary fist intermolar width before and after treatment. The increase in maxillary fist intermolar width after treatment was 2.43 mm, with a standard deviation of 1.42. The minimum sample size was calculated to be 7, using PASS (15.0.5) software, with a significance level (alpha) of 0.05 using a two‐sided paired t‐test.
Statistical analysis was performed using SPSS 27.0 software (IBM Corp., Armonk, NY, USA) with a significance level of p < 0.05. The normality of the data distributions was examined using the Shapiro–Wilk test. The homogeneity of variance was assessed using the Levene test. Paired‐sample t‐tests (for normally distributed data) or Wilcoxon signed‐rank tests (for non‐normally distributed data) were used to compare changes between T1 and T2. On 20% of the sample, measurements were taken twice by the same operator after 7 days. Intraclass correlation coefficient (ICC) was used to assess reliability.
4. Results
The ICC was used to assess the reliability of the measurements, with all variables in this study showing an ICC of ≥ 0.80. A total of 45 patients were included in this study, comprising 24 males and 21 females, with an average age of 8.84 ± 1.01 years and an average treatment duration of 18.26 ± 0.95 months. The SNA angle was 79.03° ± 3.32°, the SNB angle was 75.27° ± 3.44° and the MP‐SN angle was 37.05° ± 5.43°. All patients were in the CS2 stage of cervical vertebral maturation (Table 3).
TABLE 3.
Characteristics of subjects.
| Gender | Male:female (24:21) |
| Age (y) | 8.84 ± 1.01 |
| Treatment duration (m) | 18.26 ± 0.95 |
| SNA (degree) | 79.03 ± 3.32 |
| SNB (degree) | 75.27 ± 3.44 |
| MP‐SN (degree) | 37.05 ± 5.43 |
| Cervical vertebral maturation stage | CS2 |
Note: Values are presented as mean ± standard deviation.
After expansion, the maxillary canine dental width (53–63), the maxillary first deciduous molar dental width (54–64), the maxillary second deciduous molar dental width (55–65), the maxillary first permanent molar dental width (16–26). Corresponding mandibular indicators also showed significant increases (Table 4).
TABLE 4.
ClinCheck plan arch width variables assessment.
| Variables | T1 | T2 | T2 − T1 | p | T2 − T1/T1 (%) |
|---|---|---|---|---|---|
| 53–63 | 32.99 ± 2.22 | 37.15 ± 1.78 | 4.17 ± 1.91 | < 0.001 | 12.64 |
| 54–64 | 35.75 ± 1.84 | 39.61 ± 2.24 | 3.86 ± 1.93 | < 0.001 | 10.80 |
| 55–65 | 40.84 ± 1.66 | 45.22 ± 2.12 | 4.38 ± 1.78 | < 0.001 | 10.72 |
| 16–26 | 46.67 ± 1.85 | 49.91 ± 2.2 | 3.24 ± 1.58 | < 0.001 | 6.94 |
| 73–83 | 25.24 ± 2.07 | 28.24 ± 2.22 | 3.01 ± 2.63 | < 0.001 | 11.93 |
| 74–84 | 29.94 ± 1.98 | 34.08 ± 1.77 | 4.14 ± 2.08 | < 0.001 | 13.83 |
| 75–85 | 36.57 ± 1.43 | 40.27 ± 1.69 | 3.69 ± 1.51 | < 0.001 | 10.09 |
| 36–46 | 42.26 ± 1.7 | 45.75 ± 1.81 | 3.49 ± 1.63 | < 0.001 | 8.26 |
Note: Values are presented as mean ± standard deviation.
CBCT variables showed significant increases in dental‐alveolar widths of both maxilla and mandible after expansion. The first maxillary molar axial angle decreased (Figure 2). The lengths of both the maxillary and mandibular arches significantly increased, corresponding to significant reductions in the crowding of both arches (Table 5). Periodontal indicators of the maxillary first molar showed a significant reduction in BABT, while PABT and BABH significantly increased, and PABH did not significantly change (Figure 2; Table 6). The basal bone widths at different levels of the maxilla and mandible significantly increased, with no significant difference in PD (Table 7).
FIGURE 2.
Schematic diagram of changes in inclination and periodontal indicators of the maxillary first molar. (A) Measurements of M, D and P indicate the cross‐sections of the mesiobuccal root, distobuccal root and palatal root of the maxillary first molar respectively. The solid black line represents the alveolar bone contour at T1, and the dashed grey line represents the alveolar bone contour at T2. (B) The solid black line represents the first molar and alveolar bone contour at T1, and the dashed grey line represents the first molar and alveolar bone contour at T2.
TABLE 5.
CBCT dental‐alveolar variables assessment.
| Variables | T1 | T2 | T2 − T1 | p | T2 − T1/T1 (%) |
|---|---|---|---|---|---|
| AAPW | 37.37 ± 4.15 | 41.41 ± 3.72 | 4.04 ± 3.37 | < 0.001 | 10.81 |
| PAPW | 57.84 ± 2.83 | 60.34 ± 3.19 | 2.51 ± 1.71 | < 0.001 | 4.34 |
| UIMW | 40.06 ± 2.01 | 43.27 ± 2.42 | 3.21 ± 2.06 | < 0.001 | 8.01 |
| USIMW | 42.64 ± 4.08 | 44.56 ± 3 | 1.93 ± 3.6 | < 0.001 | 4.53 |
| LIMW | 42.45 ± 2.07 | 45.44 ± 2.29 | 2.99 ± 2.23 | < 0.001 | 7.04 |
| UICW | 28.51 ± 3.59 | 33.74 ± 4.36 | 5.23 ± 4.66 | < 0.001 | 18.34 |
| LICW | 23.19 ± 3.32 | 26.92 ± 3.32 | 3.74 ± 3.27 | < 0.001 | 16.13 |
| UMAA | 81.12 ± 5.21 | 79.52 ± 4.08 | −1.6 ± 4.09 | < 0.01 | −1.97 |
| UAP | 95.16 ± 4.89 | 98.84 ± 4.92 | 3.68 ± 2.95 | < 0.001 | 3.87 |
| LAP | 90.64 ± 4.25 | 92.9 ± 4.03 | 2.26 ± 3.27 | < 0.001 | 2.49 |
| UAC | 6.17 ± 3.93 | 2.35 ± 2.96 | −3.83 ± 3.00 | < 0.001 | −62.07 |
| LAC | 3.8 ± 4.85 | 1.37 ± 2.76 | −2.43 ± 3.32 | < 0.001 | −63.95 |
Note: Values are presented as mean ± standard deviation.
Abbreviations: AAPW, anterior alveolar process width; LAC, lower arch crowding; LAP, lower arch perimeter; LICW, lower intercanine width; LIMW, lower intermolar width; PAPW, posterior alveolar process width; UAC, upper arch crowding; UAP, upper arch perimeter; UICW, upper intercanine width; UIMW, upper intermolar width; UMAA, first maxillary molar axial angle; USIMW, upper second intermolar width.
TABLE 6.
CBCT periodontal variables assessment of the maxillary first molar.
| Variables | T1 | T2 | T2 − T1 | p | T2 − T1/T1 (%) |
|---|---|---|---|---|---|
| BABT | 3.69 ± 0.92 | 3.41 ± 0.98 | −0.28 ± 0.81 | < 0.001 | −7.59 |
| PABT | 1.73 ± 0.76 | 2.58 ± 0.86 | 0.85 ± 0.75 | < 0.001 | 49.13 |
| BABH | 7.68 ± 1.32 | 8.21 ± 0.91 | 0.53 ± 1.44 | 0.173 | / |
| PABH | 7.49 ± 1.00 | 7.77 ± 0.91 | 0.28 ± 1.12 | < 0.05 | 3.74 |
Note: Values are presented as mean ± standard deviation.
Abbreviations: BABH, buccal alveolar bone height; BABT, buccal alveolar bone thickness; PABT, palatal alveolar bone thickness; PAPH, palatal alveolar bone height.
TABLE 7.
CBCT skeletal variables assessment.
| Variables | T1 | T2 | T2 − T1 | p | T2 − T1/T1 (%) |
|---|---|---|---|---|---|
| A‐NW | 22.49 ± 2.04 | 23.93 ± 2.27 | 1.44 ± 1.12 | < 0.001 | 6.40 |
| P‐NW | 27.38 ± 2.04 | 28.53 ± 2.01 | 1.14 ± 0.79 | < 0.001 | 4.16 |
| MW | 85.11 ± 3.91 | 87.09 ± 4.25 | 1.98 ± 1.86 | < 0.001 | 2.33 |
| A‐ABW | 39.34 ± 3.56 | 40.79 ± 3.84 | 1.45 ± 1.96 | < 0.01 | 3.69 |
| P‐ABW | 62.85 ± 3.52 | 65.02 ± 3.93 | 2.17 ± 1.3 | < 0.001 | 3.45 |
| MMW | 28.05 ± 2.21 | 28.76 ± 2.22 | 0.71 ± 1.12 | < 0.001 | 2.53 |
| PD | 16.46 ± 4.00 | 16.87 ± 2.07 | 0.42 ± 3.36 | 0.41 | / |
Note: Values are presented as mean ± standard deviation.
Abbreviations: A‐ABW, anterior apical base width; A‐NW, anterior nasal width; MMW, maxillary mid‐alveolar width; MW, maxillary width; P‐ABW, posterior apical base width; PD, palatal depth; P‐NW, posterior nasal width.
5. Discussion
The expansion effects of the Invisalign system in adult patients have been validated in several studies. In 2020, Morales et al. [40] explored the effects of the Invisalign system in adults with non‐skeletal maxillary alveolar constriction, finding better effects in the premolar region and less effective in the canine and second molar regions, with expansion amounts ranging from 0.45 to 3.45 mm. In 2023, Tien et al. [41] similarly found that expansion predictability was better in the premolar region than in the molar region in adult patients. Houle et al. [42] and Maria‐Luisa et al. [43] found that expansion predictability was better in the mandible compared to the maxilla and at the crown level compared to the gingival level. In 2020, Zhou and Guo et al. [44] found that the expansion amounts at the crown and root levels were 1.06 and 0.29 mm, respectively, with an efficiency of bodily expansion movement of 36.35%. Thus, in the permanent dentition, the increase in arch width with aligners is mainly achieved through tipping movements, with predictability related to tooth position, pre‐treatment torque and the preset expansion amount [44].
Research on the Invisalign First system in mixed dentition patients is relatively scarce. In 2021, Levrini et al. [2] conducted a retrospective study involving 20 patients with an average age of 8.9 years, evaluating maxillary arch changes after 8 months of treatment with the Invisalign First system. At the occlusal level, the mean increases in width between canines, first deciduous molars, second deciduous molars and first molars were 2.8, 3.28, 3.72 and 3.05 mm, respectively. At the gingival level, the mean increases in width between canines, first deciduous molars, second deciduous molars and first molars were 2.01, 2.24, 2.59 and 2 mm, respectively. Through superimposition of pre‐ and post‐treatment digital models, the changes in the alveolar ridge levels of maxillary deciduous canines, first deciduous molars, second deciduous molars and first molars were 1.88, 1.6, 1.4 and 1.16 mm (p < 0.05). Lione et al. [28] conducted a prospective study involving 23 patients with an average age of 9.4 years, reporting mean increases in width at the canine level, first deciduous molar level, second deciduous molar level and first molar level (palatal) of 2.6, 3.7, 3.4 and 1.2 mm, respectively, also noting buccal tipping and rotation of the first molars during treatment. More recently, Bruni et al. [45, 46] conducted both RCT and prospective studies evaluating Invisalign First in mixed dentition. Their randomised trial showed that while both RME and clear aligners increased palatal volume and transverse dimensions, RME produced greater increases (532 mm3 vs. 244 mm3, though not statistically significant) and significantly greater intermolar width at the gingival level, suggesting more buccal tipping with aligners. Their predictability study found expansion accuracy decreased posteriorly: canines (87.7% at cusp, 82.7% at gingival), deciduous molars (84.9% cusp, 80.5% gingival), permanent molars (77.8% cusp, 67.9% gingival), indicating that expansion was achieved primarily through crown tipping rather than bodily movement.
Similar to the above studies, in this study, intraoral scan data also showed significant increases in all arch width indices, with more pronounced expansion amounts and rates at the deciduous molar and canine levels compared to the molar levels. The expansion amounts at the maxillary deciduous canine, first deciduous molar, second deciduous molar and first molar levels were 4.17, 3.86, 4.38 and 3.24 mm, respectively. At the mandibular deciduous canine, first deciduous molar, second deciduous molar and first molar levels, the expansion amounts were 3.01, 4.14, 3.69 and 3.49 mm, respectively. The expansion rates at the deciduous canine and deciduous molar levels were higher than those at the first molar level. Clearly, the expansion amounts in this study were higher than those reported in previous studies, possibly due to our relatively longer treatment duration. In previous studies on the Invisalign First system, ‘alveolar ridge level’ was often based on digital models obtained from intraoral scan, while in this study, the changes in alveolar ridge width were measured using CBCT data, with higher expansion amounts and rates in the anterior and posterior alveolar ridge width, 4.04 mm (10.81%) and 2.51 mm (4.34%), respectively.
Traditional early treatments for maxillary constriction include rapid or slow maxillary expansion protocols using fixed or removable expanders. Lombardo et al. [30] found that compared to rapid maxillary expansion, the Invisalign First system effectively improved the maxillary arch shape. To further determine the efficacy of the Invisalign First system, Lu et al. [31] included a rapid maxillary expansion group and a control group. Compared to the control group, both the Invisalign First system and the rapid maxillary expansion group showed greater increases in dental‐alveolar width indices. However, compared to the rapid maxillary expansion group, the Invisalign First system showed smaller increases in dental‐alveolar width indices, leading the authors to recommend using the Invisalign First system for mild to moderate maxillary constriction. Similarly, Wang et al. [47] included a slow maxillary expansion group and a control group, finding that increases in canine arch width in the Invisalign First system group, slow maxillary expansion group and control group were 3.10, 4.77 and 0.54 mm, respectively. The increases in molar arch width were 1.95, 4.76 and 0.54 mm, respectively. Additionally, the increase in the anterior and middle one‐third palatal surface area was significantly higher in the Invisalign First system group than in the control group, while the increase in the posterior one‐third palatal surface area was not significantly different from the control group. The middle one‐third palatal surface area increase was comparable to the slow maxillary expansion group. This further verifies the findings of previous studies and this study that the Invisalign First system has better expansion effects in the anterior and middle segments of the arch compared to the posterior segment. This is attributed to the characteristics of the Invisalign First aligners: (1) The aligners are made of elastic materials and the ends are most flexible, resulting in less effective and predictable expansion at the arch ends in both permanent and mixed dentitions; (2) The aligners cover the clinical crowns of all teeth, effectively causing alveolar bone response in the anterior and middle segments of the arch.
Notably, both studies [31, 47] highlighted the good control of buccal inclination of the maxillary first molars in the Invisalign First system group, with buccal inclination increases of 4.49° and 0.08°–0.24°, respectively, compared to 1.6° in this study, possibly due to the pre‐designed buccal root torque of the first molars. This study also preliminarily explored other periodontal effects of the Invisalign First system on the first molars, finding that the buccal alveolar bone thickness decreased by 0.28 mm post‐treatment, while the palatal alveolar bone thickness increased by 0.85 mm, and the palatal alveolar bone height increased by 0.28 mm, possibly related to the buccal displacement and tipping of the first molars [19].
As none of the above studies included CBCT data, no studies have yet explored the skeletal expansion effects of the Invisalign First system. Researchers generally believe that compared to rapid maxillary expansion, clear aligners mainly induce dental‐alveolar changes rather than skeletal change [30, 47]. Referring to studies on rapid or slow maxillary expansion in mixed dentition patients [39], the increases in anterior nasal width, posterior nasal width, anterior apical base width and posterior apical base width in rapid maxillary expansion were 2.65, 2.38, 3.47 and 2.72 mm, respectively, while the skeletal effects of slow expansion were less than those of rapid expansion. In this study, the increases in anterior nasal width, posterior nasal width, anterior apical base width and posterior apical base width were 1.44, 1.14, 1.45 and 2.17 mm, respectively. Given the differences in specific measurement methods and treatment duration between this study and that study, direct comparison of specific values is not meaningful. Although the increases in skeletal width indices in this study were statistically significant, we cannot determine whether there are skeletal effects in the expansion of the Invisalign First system due to the lack of exclusion of growth factors. According to Hesby et al. [48], in early and mid‐mixed dentition children (mean age 7.6–10.3 years) without treatment, the increases in maxillary width, maxillary mid‐alveolar width and posterior alveolar ridge width were 3.02, 2.33 and 1.48 mm, respectively. In this study, the increases in maxillary width, maxillary mid‐alveolar width and posterior alveolar ridge width over an 18‐month treatment duration were 1.98, 0.71 and 2.51 mm, respectively. Thus, we can preliminarily speculate that the Invisalign First system can expand maxillary alveolar arch width and maxillary arch width but not maxillary basal bone width and nasal cavity width. Recently, a three‐dimensional finite element analysis by Kurnaz and Dayan [49] compared the biomechanical effects of rapid palatal expanders (RPE) and clear aligners with buccal or palatal attachments in mixed dentition. Their findings indicated that RPE induced greater stress on the palatal and buccal bones around primary and permanent molars, with significant stress concentration in the midpalatal suture region, whereas clear aligners produced more homogeneous force distribution with minimal skeletal effect. These results support the notion that clear aligners primarily induce dentoalveolar rather than skeletal expansion, which aligns with the CBCT findings of the present study showing minimal changes in basal bone width.
The main limitation of this study is the lack of a control group. Due to ethical considerations, it is controversial to observe patients with confirmed maxillary arch constriction without treatment for up to 18 months. According to Bishara et al. [50], although canine width and molar width significantly increase between the ages of 8 and 13, the natural increase in maxillary width in children of this age group within an 18‐month treatment interval should be less than 1 mm, with even less increase in the mandible. Related studies have shown that the increase in maxillary arch width in untreated control groups did not exceed 0.6 mm and often showed no significant change [31, 47]. Additionally, as the dental‐alveolar structures are in a growth and development stage during the mixed dentition period, the measurement of each indicator should set a relatively stable coordinate system, and some indicators' definitions and measurement methods need optimised.
6. Conclusion
Within the limitations of this study, the following conclusions can be drawn: In the mixed dentition period, the Invisalign First system can effectively expand maxillary dental‐alveolar width, maintain molar inclination, improve the narrowness in the canine‐premolar region and optimise arch form.
Author Contributions
Q.W. and C.W. contributed equally to this article. Categories of the authors' contribution are as follows: concept/design (Q.W., C.W., Y.C. and J.S.), data collection (Q.W., C.W., Y.C. and Y.F.), data analysis/interpretation (Q.W., C.W. and Y.C.), drafting of the article (C.W.), critical revision of the article (Q.W., C.W., Y.C. and J.S.) and approval of the article (Q.W., C.W., Y.C., Y.F. and J.S.). All authors have read and approved the manuscript.
Funding
The authors have nothing to report.
Ethics Statement
The retrospective study was approved by the Ethics Committee of the Fourth Affiliated Hospital Zhejiang University School of Medicine (Approval No. K2023180).
Consent
All patients and their parents provided signed informed consent.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgements
The authors have nothing to report.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.