INTRODUCTION

A narrower maxillary arch and crowding can be observed with dental Class II malocclusion.^1,2 For nonextraction treatment in patients exhibiting these characteristics with a skeletal Class I pattern, treatment modalities such as maxillary expansion and molar distalization can be performed sequentially.

Rapid palatal expanders (RPEs) can induce expansion of the maxillary arch by separating the midpalatal suture and opening the circummaxillary sutures.³ However, with age, RPEs tend to yield greater dentoalveolar effects than skeletal effects due to increased resistance of these sutures.⁴ Therefore, a patient’s skeletal maturation and status of the midpalatal suture should be carefully assessed before RPE use.^5,6 A miniscrew-assisted RPE (MARPE), a type of tooth and bone-borne appliance, can be efficiently used to enhance skeletal expansion and reduce dentoalveolar effects by applying a transverse force directly to the basal bone in adolescents or young adults.^7–9 However, if further distalization of the molars is required, the RPE or MARPE should be removed after approximately 3 months to 4 months of retention to allow for sutural ossification.

For maxillary molar distalization, a pendulum appliance has been reported, which distalizes the first molars but produces more distal tipping.^10–12 Also, using a distal jet with telescopic rigid arms acting in close proximity to the center of resistance of the first molar has been shown to achieve more bodily distal movement of the first molars.^13,14

These tooth-borne distalization appliances can inevitably result in protrusion and mesial movement of anchor teeth positioned anterior to the maxillary molars.^11,13,14 Therefore, miniscrew-supported pendulum or distal jet appliances have achieved greater distalization of the maxillary first molars and spontaneous distal drift of first premolars.^15–20 Unfortunately, these distalization appliances are unsuitable for expanding the maxillary arch.

Authors of few cohort studies have assessed sequential molar distalization after maxillary arch expansion using MARPE or RPE. Therefore, in this study, we aimed to compare treatment effects between the modified MARPE and RPE with distalizers in skeletal Class I patients with Class II malocclusion and a narrow maxillary arch.

MATERIALS AND METHODS

This retrospective cohort study was reviewed and approved by the Institutional Review Board at Seoul National University Bundang Hospital (B-2403-889-103).

Twenty-eight subjects were selected retrospectively from patients who underwent arch expansion and molar distalization in the maxillary dentition using a modified MARPE with distalizer (MMD) or a modified RPE with distalizer (MRD) at Seoul National University Bundang Hospital between January 2016 and February 2024, according to the following inclusion criteria: (1) adolescents, (2) Class II molar relationship (from one-quarter Class II to full Class II), (3) skeletal Class I (0° < ANB <4°), (4) maxillary crowding of >4 mm, (5) maxillomandibular transverse differential index <5 mm, (6) treated with either MMD or MRD for palatal expansion followed by distalization, (7) nonextraction treatment, (8) no previous orthodontic treatment, (9) no missing permanent teeth, and (10) the absence of craniofacial syndromes.

The samples were divided into two groups: Group 1 treated with MMD (14; age 12.2 ± 1.5 years) and Group 2 treated with MRD (14; age 10.7 ± 1.0 years; Table 1).

Table 1. Demographic Characteristics of MMD and MRD Groups^a

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MMD/MRD Appliances

Working models were acquired by taking impressions of the patients with bands placed on their maxillary first premolars and molars to facilitate the fabrication of an appliance. The two anterior and two posterior arms of a MARPE (A⁺ expander, Myungsung C&M, Gyeonggi, South Korea) or RPE (Hyrax Click, Dentaurum, Ispringen, Germany) were adapted onto the models. Palatal tubes 1 mm in diameter with extended wire (Myungsung C and M) were positioned on molar bands after being oriented 5 mm apical to the palatal gingival margin of the first molars. A 0.9 mm stainless-steel wire was affixed between the anterior arms of the MARPE/RPE and palatal tube positioned on the molar bands, which was oriented to the occlusal plane. An open-coil spring (0.010 × 0.036-in, Ortho Technology, West Columbia, SC) was placed on the stainless-steel wire to provide a molar distalization force of 250 gm.

After fabrication, the MMD or MRD appliance was cemented intraorally (Figure 1). The MMD was installed monocortically using two miniscrews (1.8 mm diameter and 10 mm length; ORTHO MI System, OSTEONIC, Seoul, South Korea), based on a cone beam computed tomography evaluation.

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The expansion screw was activated by a quarter turn per day (one activation, 0.2 mm) until the desired palatal expansion was achieved, using the same protocol for both the MMD and MRD groups. During the retention period for sutural ossification after expansion, the posterior arms were cut from both appliances to distalize the molars sequentially until a Class I relationship occurred, and then the anterior arms were cut to allow distal movement of the first premolars.

Alignment was performed using 0.022-in fixed orthodontic appliances to alleviate crowding from the right first molar to the left first molar. All procedures were performed by an experienced orthodontic specialist (N.-K.L.).

Cephalometric and Dental Cast Measurements

Lateral cephalograms and dental casts were taken at pretreatment (T1) and immediately after expansion, molar distalization, and at the initiation of alignment (T2) in patients using either MMD or MRD. All tracing and analysis of cephalograms were conducted by the same orthodontist (S.-Y.K.) using V-ceph software (version 6.0; Osstem, Seoul, South Korea). The cephalometric landmarks, reference planes and measurements, and maxillary cast measurements are shown in Figures 2 through 4.

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To evaluate intraexaminer reliability, cephalograms and dental casts of eight randomly selected patients were retraced, remeasured, and reanalyzed 2 weeks apart by the same examiner. Intraexaminer reliability was evaluated for all measurements by the intraclass correlation coefficient (ICC), which showed them to be reliable (ICC > 0.90).

RESULTS

Before treatment (T1), no significant differences were found in maxillary arch length discrepancy (7.5 mm vs 7.6 mm), cephalometric variables, and dental cast variables except for age in the MMD and MRD groups (Tables 1 and 2).

Table 2. Comparison of Cephalometric and Cast Variables Between MMD and MRD Groups at Pretreatment (T1)^a

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During treatment (T2-T1), the maxillary first molars exhibited a mean distalization of 3.0 mm and 2.1 mm at crown level (P < .001) and 2.4 mm and 1.4 mm at root level (P < .001) in the MMD and MRD groups, respectively (Table 3 and Figure 5) with no significant difference between the groups. The MRD group showed distal tipping of 2.8° at the first molars, which was significantly greater than that in the MMD group (P < .001).

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Table 3. Changes in Cephalometric and Cast Variables in Each Group and Between Groups During Treatment (from T1 to T2)^a

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The maxillary incisors showed significant extrusion of 1.0 mm (P < .002) in the MRD group and no change in the MMD group, which was significantly different between groups. However, no significant difference was found in the skeletal changes between groups.

During treatment, the maxillary arch width increased between the first premolars (6.2 mm vs 5.5 mm) and between the first molars (3.7 mm vs 3.3 mm) in the MMD and MRD groups, respectively, but no significant difference was found between groups. The MRD group exhibited mesiobuccal rotation of the first molars (4.2°, P = .002) but not significantly different than that in the MMD group (2.5°; Figure 5).

DISCUSSION

Maxillary arch expansion and molar distalization may be necessary for successful nonextraction treatment in skeletal Class I patients presenting with a Class II malocclusion and a narrow maxillary arch. Traditionally, RPE or MARPE has been used to expand the maxillary dental and basal arches, and their transverse effects have been reported. However, authors of few studies have evaluated the distalization of posterior teeth by modifying these appliances. Therefore, the purpose of this study was to evaluate the effects of MMD on molar distalization compared with the effects of MRD.

Authors of previous studies on miniscrew-supported distalization appliances have reported mean values of maxillary molar distalization ranging from 1.3 mm to 5.4 mm, depending on the number and location of miniscrews and the design of the distalization appliances.^22–24 In addition, authors of previous studies have demonstrated distinct distal tipping ranging from 2.8° to 11.3° during distalization.^15,22

In the present study, the maxillary first molars showed significant distalization of 3.0 mm at the crown level, 2.4 mm at the root level, with distal tipping of 0.03° in the MMD group, and 2.1 mm, 1.4 mm, and tipping of 2.82° in the MRD group. Similarly, authors of some studies have shown 2.6 mm to 2.9 mm of posterior movement of the first molars using palatal miniscrew-supported distalization systems or buccal miniscrews.^18,19,25 The miniscrew-supported appliances showed greater distalization of maxillary first molars than distalization with conventional appliances.^16,26

The MMD group showed distalization at the crown and root level and significantly less distal tipping of the maxillary molars than that in the MRD group. These findings indicated nearly bodily distal movement of the molars in the MMD group. This means that, although the distalization force essentially passed through the center of resistance of the maxillary first molar in both groups, the MMD demonstrated greater efficacy in transmitting force in the appropriate direction for bodily translation of the molars. This might have been due to greater rigidity of the MMD appliance, which is supported by miniscrews, than that of the MRD.

In agreement with these results, a meta-analysis demonstrated that nonrigid palatal appliances produced greater distal tipping than rigid appliances.²³ Gelgor et al.¹⁷ and Sar et al.¹⁸ reported minimal distal tipping ranging from 0.75° to 1.65° during distalization of the first molars, which was the result of the direct force being applied through the apex level of the maxillary first molars under relatively rigid anterior screw-supported anchorage. In contrast, authors of studies on the pendulum and miniscrew-assisted pendulum have generally indicated that these appliances induced increased distal molar tipping, primarily because of inherently greater flexibility and nonrigidity, particularly in the distalization arms, and their application of force at the coronal level.^11,16,23

In this study, both MRD and MMD were connected to the first premolars and molars; the first premolars would need to be aligned and distalized with fixed orthodontic appliances following molar distalization. Therefore, no difference was found in the amount of distalization of the first premolars between the two groups in the current study.

Authors of many studies have reported incisor extrusion ranging from 0.3 mm to 0.8 mm during molar distalization or total maxillary arch distalization.^12,25–27 In this study, only the MRD group displayed significant extrusive movement of 1.0 mm. Also, no significant anteroposterior change was found in the maxillary incisors between the two groups during treatment. This means that proclination of the incisors did not occur during alignment using fixed appliances after molar distalization followed by subsequent distalization of the first premolars.

Both expansion groups showed significant increases in arch width due to the palatal expansion performed, but despite the difference in their anchorage, no intergroup difference was found in the increases recorded. In addition, although only the MRD group exhibited statistically significant mesiobuccal rotation of the first molars, this rotation was not significantly different from that of the MMD group.

Based on the results in this study, either an MMD or an MRD appliance can be clinically chosen according to the patient’s age and skeletal maturation, the amount and type of molar distalization necessary, and the initial molar angulation. MMD might be preferable in patients who need maxillary arch expansion as well as maximum molar distalization while maintaining lip profile and incisor position.

The limitations of this study included a small sample size, early adolescence, age difference between the groups, and no estimation of the maturational stage of the midpalatal suture. In addition, the two-dimensional lateral cephalogram did not allow for evaluation of transverse skeletal changes.²⁸ Therefore, authors of future studies should include three-dimensional evaluation of treatment outcomes during expansion and molar distalization in a larger, late adolescent sample including erupted second molars, along with a midpalatal suture assessment.

CONCLUSIONS

• MMD and MRD groups exhibited molar distalization effects without proclination of the maxillary incisors.

• No statistically significant difference was found between the amount of maxillary molar distalization achieved between the MMD and MRD groups.

• The MMD group had no distal tipping of molars, while the MRD group had significant distal tipping.

• MMD can be recommended as one of the nonextraction treatment modalities for maxillary expansion and molar distalization with bodily translation.