INTRODUCTION

Mandibular setback surgery (MSS) is the primary choice of treatment to correct skeletal Class III malocclusion with mandibular prognathism. However, camouflage treatment can be considered for patients with mild-to-moderate Class III malocclusion who are reluctant to undergo surgical-orthodontic treatment. Several treatment alternatives such as extraction, Class III elastics, multiloop edgewise archwires (MEAW), mandibular lip bumper, and cervical headgear have been used.^1–7 These modalities are associated with innate problems such as tipping rather than bodily tooth movement, unfavorable movement of anchorage teeth, and a dependence on patient compliance.

Traditionally, posterior movement of the mandibular molar in adults has been considered to be the most difficult among the various types of orthodontic tooth movement. However, the incorporation of temporary skeletal anchorage devices (TSADs) into orthodontic treatment has enabled predictable molar distalization in the mandibular dentition with minimal compliance, such as with maintaining good oral hygiene. Therefore, total mandibular arch distalization (TMAD) using interdental, buccal shelf or retromolar miniscrews,^8–14 or miniplates,^15,16 has been proposed as a camouflage treatment modality in Class III malocclusion. However, when interradicular mini-implants are used between the mandibular second premolar and first molar, potential root damage and the need for relocation to obtain additional distal movement need to be considered.^8,9

In terms of stability after mandibular molar distalization using TSADs, Sugawara et al¹⁵ reported that the average relapse was 0.3 mm at both the crown and root apex of the mandibular first molars. Chung et al¹⁷ reported that, despite improvement of overjet, overbite, and facial balance after molar distalization in a Class III patient, there was a slight decrease of overbite and relapse to Class III molar and canine relationships after 8 months of retention. In contrast, some studies have reported favorable outcomes with good occlusion at retention using a combination of miniscrews inserted in the intermolar space and sliding jigs with MEAW and Class III elastics.^9,10,18 In addition, regarding the stability after MSS, long-term studies reported that the mandible remained generally stable despite a slight forward relapse.^19,20

Few cohort studies have evaluated changes in the dentoskeletal structure during retention after TMAD using mini-implants and compared the results to those after orthognathic surgery. Therefore, the purpose of this study was to compare posttreatment stability between TMAD with buccal interradicular mini-implants and MSS in skeletal Class III patients.

MATERIALS AND METHODS

This retrospective cohort study was reviewed and approved by the Institutional Review Board at Seoul National University Bundang Hospital (B-1809-492-105).

Forty subjects were selected from skeletal Class III patients treated by orthodontic or surgical-orthodontic modalities at Seoul National University Bundang Hospital between March 2011 and August 2018 according to the following inclusion criteria: (1) −4° < ANB < 1°, (2) Wits appraisal less than −2 mm, (3) Angle Class III malocclusion, (4) nonextraction treatment, (5) crowding less than 2 mm in the mandibular dentition, (6) treatment modality of either TMAD using mini-implants or MSS, and (7) a minimum of 1-year posttreatment follow up. Patients were excluded if they had missing teeth, craniofacial syndromes, maxillary advancement surgery, or two-jaw surgery.

The samples were divided into two groups according to the type of treatment: group 1 (TMAD group; n = 20; mean age, 23.8 ± 1.9 years) and group 2 (MSS group; n = 20; mean age, 21.0 ± 3.5 years) (Table 1).

Table 1. Characteristics of Subjects in the Total Mandibular Arch Distalization (TMAD) and Mandibular Setback Surgery (MSS) Groups^a

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In the TMAD group, buccal mini-implants (diameter, 1.5 mm; length, 7 mm; BioMaterials Inc., Seoul, South Korea) were placed between the mandibular second premolar and first molar bilaterally by an experienced orthodontist (Dr Lee). Elastomeric chains were engaged with a force of approximately 250 g per side between mini-implants and hooks positioned between the canine and lateral incisor on 0.019 × 0.025-inch stainless steel archwire (Figure 1A). In the MSS group, bilateral sagittal split ramus osteotomy was conducted by one surgeon. After the mandible was fixated with metal plates and monocortical screws, the interdental mini-implants placed at surgery were used with a surgical stent for intermaxillary fixation for 2 weeks (Figure 1B). Functional jaw exercises were followed for the next 2 weeks.

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Lateral cephalograms were obtained before (T1) and after treatment (T2) and at least 1 year after posttreatment (T3). All cephalograms were traced and analyzed by an orthodontist (Dr Kim) using V-ceph software (version 6.0; Osstem, Seoul, South Korea). To evaluate intraexaminer reliability, lateral cephalograms of 10 randomly selected cases were retraced and remeasured after 2 weeks by the same examiner. The intraclass correlation coefficient for all cephalometric variables ranged between 0.99 and 0.91. The cephalometric landmarks, reference planes, and measurements are shown in Figure 2.

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RESULTS

The mean treatment periods in the TMAD and MSS group were 26.6 ± 9.7 months and 23.5 ± 7.0 months, respectively. The mean retention periods were 34.4 ± 22.8 months and 33.1 ± 21.2 months, respectively (Table 1).

The two groups showed similar cephalometric characteristics at baseline (T1). However, the MSS group had a lower (more negative) ANB (P < .05) and Class III Wits appraisal (P < .001) than the TMAD group did (Table 2), confirming that the MSS group had a more severe skeletal Class III relationship than did the TMAD group.

Table 2. Comparison of Changes in Cephalometric Variables at T1 and After Treatment (T1–T2) in the TMAD and MSS Groups and Between Groups^a

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The TMAD group showed significant distal tipping of 4.7° and intrusion of 1.1 mm of the mandibular first molars (P < .001) after treatment (T1–T2). Distalization of the first molar crown and root were 1.9 mm (P < .001) and 1.4 mm (P < .01), respectively (Table 2). The mandibular incisors showed a lingual inclination of 4.2° (P < .01). Distalization of the incisor crown and root were 2.3 mm and 1.7 mm, respectively (P < .001). Overjet increased by 2.8 mm (P < .001), and the lower lip was retracted by 1.4 mm (P < .01). In addition, the Wits appraisal increased by 1.2 mm (P < .01) and occlusal plane angle decreased by 1.3° (P < .05), respectively.

The MSS group showed a significant decrease in SNB of 2.6°, an increase in ANB of 2.6°, and an increase in the Wits appraisal of 5.9 mm (P < .001) after treatment. The overjet and overbite increased by 4.4 mm and 2.0 mm (P < .001), respectively. In addition, the upper lip to E-line increased by 1.0 mm and lower lip to E-line decreased by 1.5 mm (P < .05).

Comparing the two groups, the MSS group exhibited a greater decrease in SNB and an increase in ANB and the Wits appraisal (P < .001) and a greater increase in overbite (P < .01), overjet, and upper lip to E-line (P < .05) compared with the TMAD group.

During the retention period (T2–T3), the TMAD group showed mesial tipping and movement of the mandibular first molars by 1.4° (P < .01) and 0.4 mm (P < .05), respectively, and extrusion of 0.6 mm (P < .05; Table 3). The incisors showed labial inclination and movement of 1.9° (P < .001) and 0.8 mm (P < .01), respectively. The overjet showed a decrease of 0.5 mm (P < .001).

Table 3. Comparison of Changes in Cephalometric Variables at T2 and During Retention (T2–T3) in Each TMAD and MSS Group and Between Groups^a

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In the MSS group, the mandibular incisors moved labially at the crown by 0.6 mm (P < .05) and at the root by 0.5 mm (P < .01) during retention.

The comparison between the two groups showed a significant difference in ANB, PFH/AFH, L1C to MP, and overbite during retention. However, there was no significant changes in these variables within the groups.

DISCUSSION

MSS is commonly used to enhance facial esthetics and improve occlusion in skeletal Class III cases with mandibular prognathism. However, camouflage treatment can be considered in patients with mild-to-moderate Class III malocclusion who refuse surgical-orthodontic treatment, but the procedure is difficult and challenging.^18,23 Since the introduction of miniscrews and miniplates, at least six studies have been published reporting the treatment results when these anchorage devices were used for TMAD in Class III malocclusion,^10–17 but only three case reports have documented posttreatment stability.^9,11,17 Therefore, this study was designed to compare posttreatment stability between TMAD with buccal interradicular mini-implants and MSS in skeletal Class III patients.

Dental changes after treatment (T1–T2) in the TMAD group showed significant changes of 1.9 mm posterior movement and 1.1 mm intrusion of the mandibular first molars and 2.3 mm of retraction and 4.2° of lingual tipping of the incisors. In contrast, there were no significant dental changes in the MSS group. In agreement with the results observed in the TMAD group, Yu et al.¹⁶ reported that the first molars at the crown were distalized by 2.1 mm. However, previous studies showed greater molar distalization ranging from 2.5 mm to 4.9 mm and lingual movement of 2.6 mm to 3.4 mm with 2.2 mm to 2.4 mm extrusion of mandibular incisors.^{12,15,18,23–25} Variations among study results might be attributed to multiple factors such as the placement location and types of TSADs used, the force vectors, application of Class III elastics, and severity of the Class III patients.^{15,16,23–27}

In the TMAD group, there was a counterclockwise rotation of the occlusal plane, which might have occurred because the line of force between the mini-implant and anterior retraction hook on the archwire passed above the center of resistance of the mandibular dentition.^26,27 In comparison, there was no change in the occlusal plane angle in the MSS group, which indicated that the setback movement did not affect the occlusal plane.

With regard to the skeletal change after treatment, there was a significant sagittal change in the MSS group due to the posterior movement of the mandible, whereas no changes occurred in the TMAD group. Therefore, the MSS group had a greater improvement in overjet, overbite, and Wits appraisal compared with the TMAD group (Table 2).

In addition, similar to changes in the lower lips reported in previous studies,^11,12,16,18 there was retraction of the lower lip (1.4 mm) in the TMAD group without any vertical or sagittal changes in the upper lips and teeth. This indicated that the lip posture was improved via distal movement of the mandibular dentition without mesial movement of the maxillary incisors and molars. Both lips were changed due to the setback of the mandible in the MSS group, and there was a significant intergroup difference in the upper lip (P < .05).

During the 34-month average duration of retention, the TMAD group showed mesial tipping of the mandibular incisors and molars, resulting in a decrease in overjet (Table 3). Regarding this posttreatment relapse, He et al.¹⁸ reported that the mandibular first molars moved 0.2 mm and tipped 0.3° mesially, while the incisors moved 0.1 mm labially due to MEAW treatment and modified Class III elastics from the maxillary miniscrews. Interestingly, there were a couple of reports of cases that showed good occlusion and facial esthetics at the 2-year follow-up after miniscrew-assisted mandibular arch distalization with sliding jigs or with second molar extractions.^9,11 It should be noted that Chen et al.¹¹ and Chung et al.¹⁷ stated the importance of preventing tipping of the mandibular molars for better stability after distalization. Despite mesial tipping and extrusion of the mandibular first molars during the retention period in the TMAD group, the counterclockwise rotation of the occlusal plane was well maintained.

As previously stated, the MSS group experienced no significant posttreatment skeletal or dental changes except for positional change of the mandibular incisors over an average of 33 months of retention. Similarly, Eggensperger et al.¹⁹ reported that the mandible remained stable over the long term after mandibular setback in Class III patients. Some studies, however, showed anterior relapse of 0.24 mm or 26% of the initial setback over long-term follow-up after mandibular surgery.^20,28

In comparing the TMAD and MSS groups during retention (Table 3), there were no differences in stability of the mandibular dentition between the two groups, although the TMAD group did show some dental relapse such as slight flaring of the mandibular incisors and mesial tipping of the molars and decreased overjet. This suggested that the results after TMAD showed posttreatment stability over the course of this study (Figure 3).

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This study had some limitations including a small sample size, short follow-up, sagittal discrepancy at baseline between the groups, the lack of an untreated Class III control group and a three-dimensional radiographic evaluation.²⁹ A prospective comparative randomized study would be needed to evaluate the efficacy and long-term stability of various mini-implant- or miniplate-assisted TMAD techniques compared with bimaxillary surgery in similar Class III malocclusions.

Clinicians must carefully evaluate the dental and skeletal characteristics of each patient and also consider treatment stability when planning TMAD using skeletal anchorage as an alternative treatment for mild-to-moderate Class III patients.

CONCLUSIONS

• The MSS group showed a significant skeletal change of the mandible, whereas the TMAD group did not exhibit a skeletal improvement at the end of treatment.

• The TMAD group presented slightly decreased overjet with flaring of the mandibular incisors and mesial tipping of the first molars during retention.

• The MSS group showed no significant skeletal or dental changes except for labial movement of the mandibular incisors during retention.

• Posttreatment stability of the mandibular dentition was not significantly different between the TMAD and MSS group.