INTRODUCTION

Maxillary expansion is a widely used treatment modality to address maxillary constriction and arch length deficiency during the mixed dentition.^1,2 Various expansion devices, such as the Quad helix, fixed palatal expander with a jackscrew, Haas expander, and a split removable plate, are utilized in mixed dentition patients.^1,2

In 2018, Invisalign First (Align Technology Inc., Tempe, AZ, USA) was introduced as a phase I orthodontic treatment option. It has gained popularity as an esthetic and comfortable appliance for children. However, due to its relatively recent launch compared to clear aligner therapy in adults and adolescents, there are limited studies investigating the efficiency and treatment outcomes of the Invisalign First System (IFS).^3–6

There have been extensive studies on the effects of rapid palatal expansion (RPE) on the facial skeleton and dentition in the literature.^7–11 Previous studies have shown that RPE treatment tends to increase the vertical dimension due to the backward rotation of the mandible, a consequence of the downward movement of the maxillary arch during midpalatal suture opening. Additionally, dental tipping and extrusion of lingual cusps of maxillary posterior teeth commonly follow RPE.^7–9 Recent studies on clear aligner therapy in adults suggest its effectiveness in controlling the vertical dimension.^12,13 However, there have been no studies assessing the changes in skeletal vertical dimensions in mixed dentition patients undergoing treatment with the IFS.

The primary purpose of this study was to evaluate the vertical and transverse changes between different treatment groups using maxillary Hyrax expanders with fixed appliances (Hyrax) versus the Invisalign First System (IFS) in mixed dentition patients, compared to a control group. Two specific hypotheses were tested:

1. No significant differences exist in vertical dimension changes among the Hyrax, IFS, and control groups.

2. No significant differences are observed in dental transverse dimension or arch width changes among the Hyrax, IFS, and control groups.

Additionally, the secondary purpose of this study was to evaluate the treatment efficiency and the attainable magnitude of maxillary arch expansion using the IFS.

MATERIALS AND METHODS

Approval for this study was obtained from the Institutional Review Board at the University of the Pacific in San Francisco, California (#20-51). It was conducted as a retrospective cohort study. The treatment group consisted of patients in the mixed dentition collected from two private practitioners (M. B. and S. T.), both of whom hold American Board of Orthodontics certification and Diamond and Diamond Plus Invisalign provider status.

To minimize selection bias, the private practitioners were not involved in the sampling process. For the Hyrax group, a list of patients who underwent Phase I treatment in the mixed dentition stage between October 2015 and February 2022 was obtained. For the IFS group, a list of patients who were treated in the mixed dentition stage with the IFS between September 2018 and November 2022 was collected from the clinician’s Invisalign account. Figure 1 displays sample flow charts and exclusion criteria for treatment groups. The main inclusion criteria for both treatment and control groups comprised mixed dentition, eruption of the permanent first maxillary molars, and absence of anterior crossbite and bilateral posterior crossbite; both clinicians employ rapid palatal expansion for bilateral posterior crossbites to obtain skeletal maxillary expansion. After applying exclusion criteria, the Hyrax and IFS sample consisted of 40 mixed dentition patients (Figure 1). Forty control subjects were collected from the American Association of Orthodontists Foundation Craniofacial Growth Legacy Collection and matched based on sex, age, and Angle classification.

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Sample size calculations were performed to ensure study power. To detect significant differences of 1°, an effect size of 0.7 was chosen based on a prior study.¹⁰ With a power of 80% and a significance level of 0.05, a sample size of 34 subjects was estimated.

In the Hyrax group, Hyrax expanders were anchored to the permanent maxillary first molars. The magnitude of expansion was determined based on narrow arch widths as assessed by the clinician. The expansion protocol involved one turn per day (0.25 mm) for 4 weeks, followed by three turns per week as needed, considered a semirapid or slow expansion (less than 2 mm per week) according to Proffit.² After completing arch expansion in 2 to 3 months, Hyrax expanders were retained for an additional 3 months. Subsequently, the Hyrax expanders were removed, and fixed appliances were placed on the maxillary arch (2 × 4 fixed appliances). For the mandibular arch, lower lingual arches were placed at the beginning of treatment for most of the patients.

In the IFS group, patients had ClinCheck plans with sequential staging expansion protocols that allowed molars to expand first, followed by the simultaneous expansion of primary molars and canines. The expansion amount at each stage was 0.25 mm. Patients were instructed to wear their aligners only after school and at night, with a 7-day aligner change schedule.

Pretreatment (T1) and posttreatment (T2) lateral cephalograms and study casts were collected for both treatment groups, and two matched time point records were collected for the control group.

Cephalometric tracing was performed for the three groups using Dolphin Imaging software (Version 11.7, Chatsworth, Calif, USA) by a single examiner (P. M.). Then, a faculty member (H. O.) checked the accuracy of landmark identification. To ensure intra-examiner reliability, a subset of 20 lateral cephalograms was randomly chosen and retraced at an interval of at least 4 weeks.

To assess the comparability of the three groups, 15 cephalometric measurements at T1 were examined (Table 1, Figure 2). The following vertical skeletal and dental measurements were used to test the first hypothesis: SNMPA, FMA, PPMPA, OB, U1PPd, U6PPd, L1MPd, L6MPd. Additionally, the hyperdivergent subjects with SNMPA > 38 in each group were further analyzed for vertical skeletal measurements.

Table 1. Cephalometric Measurements and Definition

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To test the second hypothesis, digital casts were imported into Ortho Analyzer software (3Shape Inc, Copenhagen, Denmark). Three calibrated dentists identified dental landmarks, specifically the cusp tips of primary canines and central fossae of primary and permanent molars. Eight transverse measurements were taken at each time point for all three groups (Figure 3). The average values of the three judges’ estimates were used as final values, and the changes between different time points were calculated.

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RESULTS

The study comprised three groups: Control (n = 40, 15 males: 25 females), Hyrax (n = 40, 14 males:26 females), IFS (n = 40, 19 males:21 females). The distribution of Class I percentages among the groups was 65%, 60%, and 62.5%, respectively. No statistically significant differences were observed in Angle class and sex distribution among the three groups. Within the Hyrax group, 55% of patients received lower lingual arches, 30% were treated with lower 2x4 appliances, and 15% had no mandibular arch treatment.

Table 2 presents descriptive statistics for T1. The mean initial ages were 8.88 ± 1.25 years, 9.07 ± 1.08 years, and 8.91 ± 1.2 years for the Control, Hyrax, and IFS groups respectively. There were no statistically significant differences in the initial age among these groups. When comparing cephalometric measurements, all initial skeletal and dental characteristics showed no statistically significant differences among groups except the IMPA, with the IFS group displaying more retroclined mandibular incisors relative to the Hyrax group.

Table 2. Age and Cephalometric Measurements at T1^a,b

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Table 3 shows initial arch width measurements. IFS showed increased arch dimensions between the primary first and second maxillary molars compared to the control. There were no statistically significant differences in the mandibular arch widths among the three groups at T1. Initial characteristics of cephalometric and study casts were comparable for the three groups.

Table 3. Arch Width Measurements at T1^a

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Table 4 presents the results pertaining to the primary objective of this study and displays cephalometric changes in the vertical dimension from T1 to T2 for all three groups. The mean time interval from T1 to T2 was 1.32 ± 0.51 years, 1.22 ± 0.46 years, and 1.19 ± 0.45 years for the Control, Hyrax, and IFS groups, respectively. There were no statistically significant differences in the time between T1 and T2 among the three groups. Regarding the Control group, all skeletal and dental vertical measurements showed statistically significant changes characterized by an increase in vertical linear measurement and a decrease in mandibular plane angle measured by SNMPA and FMA as a result of normal growth. The IFS group showed similar results, with decreases in SNMPA and FMA of –0.73 ± 0.51° and –0.88 ± 0.25°, respectively, while no significant changes were observed for PPMPA, OB, and U1PPD. In contrast, for the Hyrax group, SNMPA, FMA, PPMPA, OB, and U1PPD showed no changes during treatment, indicating potential treatment effects. Comparing the three groups, the only statistical difference was found in OB, specifically between the Control and IFS groups (Table 4). No other statistically significant differences were found in the vertical skeletal and dental measurements among the three groups. However, it is worth noting that there was an observable trend in the vertical skeletal dimension, with SNMPA, FMA, and PPMPA showing a slightly lesser reduction in the Hyrax group compared to the IFS and Control groups, with P values of .062, .054, and .056, respectively.

Table 4. Vertical Cephalometric Measurement Changes From T1-T2^a

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When comparing a subgroup of hyperdivergent patients (MPSN > 38) among the three groups, there were no statistically significant differences observed in the vertical skeletal dimension, as shown in Table 5.

Table 5. Skeletal Vertical Changes From T1-T2 in Hyperdivergent Patients (MPSNA > 38°)^a

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Table 6 presents the arch width changes from T1 to T2 for the three groups. The two treatment groups (Hyrax and IFS) exhibited significantly greater increases in all arch widths compared to the control group, which only displayed statistically significant changes in UE-E and U6-6 through normal growth and arch development, measuring 0.45 ± 0.8 mm and 0.53 ± 0.7 mm, respectively. None of the mandibular arch widths showed statistically significant changes in the Control group. Focusing specifically on the two treatment groups (Hyrax and IFS), the Hyrax group showed greater increases in the UE-E and U6-6 arch widths by 1.53 ± 1.61 mm and 2.41 ± 1.41 mm, respectively. On the other hand, in the IFS group, the LC-C, LD-D, and LE-E arch widths showed a greater increase of 2.20 ± 1.62 mm, 2.83 ± 1.94 mm, and 1.62 ± 1.67 mm, respectively (Table 6). These differences were highly significant.

Table 6. Arch Width Changes From T1-T2^a

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Table 7 addresses the secondary objective of this study by comparing the expansion goals determined on Invisalign ClinCheck vs the actual transverse changes from T1 to T2 measured on the study casts in the IFS group. In the maxilla, the efficiency of Invisalign expansion, as measured against the ClinCheck goal, ranged from 52.3% to 76.87%, with the most successful expansion occurring in the anterior region between the primary canines.

Table 7. Efficiency of IFS (Actual Changes Achieved Vs Expansion Goals Planned)

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Similarly, in the mandible, the efficiency of IFS expansion ranged from 64.25% to 79.95% and the highest efficiency of expansion was also observed in the anterior region between the primary canines. On the other hand, the lowest efficiency of expansion was observed in the maxillary first molar width. On average, the expansion magnitude of the maxillary and mandibular molar width appeared to be approximately 2.5 mm.

DISCUSSION

The utilization of clear aligner therapy in the mixed dentition has been steadily increasing, appealing to both patients and parents seeking more comfortable and esthetically pleasing solutions for orthodontic treatment in children. However, when a new tool becomes available, it is essential to assess its efficacy and effectiveness in comparison to the established conventional treatments. In this study, the vertical and transverse arch width changes using IFS compared to both the control and Hyrax groups was evaluated, shedding light on the effects of clear aligners in mixed dentition.

Clear aligner therapy has gained recognition for its effectiveness in managing the vertical dimension, particularly in correcting mild open bites and addressing various malocclusions in patients with hyperdivergent facial types.^12,13 While this study did not find statistically significant differences in skeletal and dental vertical dimension changes among the Control, Hyrax, and IFS groups, there was a noticeable trend suggesting greater mandibular plane angle reduction in the IFS group compared to the Hyrax group. The IFS group displayed a similar reduction in mandibular plane angle compared to the control group, while no significant changes were observed in the Hyrax group. However, it is worth noting that the differences between the two treatment groups for both SNMPA and FMA were less than 1°, which may not be clinically significant. No other studies have investigated the vertical dimension changes in growing patients undergoing mixed dentition treatment with IFS compared to a Hyrax expander with fixed appliances.

When examining the hyperdivergent subgroup, no statistically significant differences in the vertical dimension between the treatment and control groups were observed. Although the sample size of the hyperdivergent subgroup was small, this was consistent with findings from another study by Rozzi et al.⁸ They reported that there was a transient increase in the vertical dimension immediately after maxillary expansion, but this increase was reduced over one year and was ultimately insignificant in treatment outcome between three different skeletal vertical patterns.⁸

In the present study, IFS was employed primarily to address dentoalveolar arch width concerns, to correct a narrow maxillary arch width, and to increase arch length to alleviate crowding during the mixed dentition. It is important to note that the IFS provider in this study utilizes Hyrax-type maxillary expanders to correct posterior crossbites in the mixed dentition. Therefore, it was decided to exclude cases with bilateral posterior crossbites from both the Hyrax and IFS groups to enhance the comparability of the samples at T1.

Considering the actual transverse changes observed in the IFS group compared to the ClinCheck goals, it appears that clinicians may need to consider additional expansion, especially in the maxillary posterior region, since the U6-6 and UE-E widths had the least amount of goal achievement, at 52.3% and 62.23%, respectively. Goncalves et al. reported an effectiveness of 61.1% for U6-6 expansion relative to the ClinCheck goal, which was slightly higher than the rate observed in the current study.³ However, the mean expansion achieved was similar in both studies, with 2.8 mm and 2.5 mm, respectively.

Arch width changes can vary depending on treatment modality, as observed in previous studies.⁴ The current study found that the Hyrax group showed greater maxillary expansion in the posterior region, while IFS resulted in greater anterior expansion. These findings align with the results reported by Wang et al.⁶ These findings were also in agreement with a recent study highlighting that IFS tends to induce more significant alterations in arch shape compared to fixed appliances.⁶ It appears that IFS can be a viable option for addressing mild maxillary arch width deficiencies, with a predictable increase in intermolar and intercanine widths of approximately 2.5 mm and 3.7 mm, respectively. The efficiency rate of intermolar width increase was approximately 50%–60%, which suggests that clinicians should consider nearly twice the amount of expansion for the intermolar width to achieve the desired results when planning IFS treatment. On the other hand, IFS has demonstrated the potential to achieve greater interprimary canine and interprimary molar width increases of approximately 3–4 mm, with a higher efficiency rate ranging from 60% to 77%. It remains uncertain whether the gained 3–4 mm intercanine and interpremolar width can be maintained as the permanent successor teeth erupt. Therefore, further research on the long-term stability of these changes is warranted.

While efforts were made to minimize bias, it is essential to acknowledge the inherent limitations in this study, including the presence of missing records, a common challenge in retrospective research. Further studies are warranted to assess the long-term stability of the study findings, evaluating whether the maxillary and mandibular arch expansion with IFS is sustained in the permanent dentition and whether the observed trends in the vertical dimension are maintained or changed over time.

CONCLUSIONS

• There were no statistically significant changes in the skeletal and dental vertical dimensions among the Control, Hyrax, and IFS groups during the mixed dentition stage. However, there was a trend, with the IFS group showing a greater reduction in the mandibular plane angle compared to the Hyrax group.

• The Hyrax group showed significantly greater expansion in maxillary intermolar width than the IFS group: 4.4 mm vs 2.5 mm, respectively. This suggests that IFS may not be considered a viable option when greater than 2.5 mm of intermolar expansion is required.

• In the IFS group, the mean efficiency rates of the maxillary and mandibular intermolar width increases were approximately 52% and 68%, respectively. Additionally, higher expansion efficiency rates were observed in the anterior regions of both the maxillary and mandibular arches compared to the posterior regions, with a greater extent in the maxillary arch, in the range of approximately 3.6–3.7 mm.