INTRODUCTION

The use of temporary anchorage devices (TADs) has broadened the scope of nonsurgical orthodontic treatment by enabling three-dimensional movement of the entire dentition.¹ Total arch distalization with TADs has been reported to address Class II or III malocclusion effectively, while improving vertical and transverse dimensions,^2–5 soft tissue esthetics^6–8 and long-term stability.^9,10 Total arch distalization ranging from 1.29 to 3.5 mm can be successfully achieved using bilateral TADs in the posterior interradicular space of the mandible to improve Class II,^2,11 and Class III malocclusions.^3,12,13

Clinically, treatment outcome evaluation of total arch distalization has been relatively simplified, mostly by analyzing central incisor and first molar movement using serial lateral cephalometric radiographs.^10,11 Total arch distalization represents movement of the whole arch in the distal direction per se; nevertheless, it induces three-dimensional (3D) changes throughout the entire arch not restricted to the sagittal plane. Cone-beam computed tomography (CBCT) superimposition or best-fit digital model superimposition has suggested various regional extrusive, intrusive, and/or transverse changes affecting arch dimensions,^10,11 and orthopedic changes such as mandibular rotation, possibly inducing changes in the profile as well as smile line.^3,11

Given the complex dentofacial orthopedic changes induced in the sagittal, vertical, and transverse dimensions after total arch distalization, detailed and accurate 3D evaluation methods are required to fully understand the clinical outcome. However, unlike maxillary CBCT or cast superimposition, which are considered reliable and accurate methods that can be applied clinically,^14–16 mandibular CBCT or cast superimposition is not fully accepted due to various limitations, including lack of stable/reliable anatomical landmarks,¹⁷ late mandibular growth,¹⁸ tooth movement-induced rotation, and the possibility of temporomandibular joint (TMJ) displacement or condylar resorption.^19,20

Mandibular regional 3D superimposition using volumetric chin and symphysis areas with serial CBCTs has been suggested to address the problem.²¹ However, challenges such as separating the arches,²² and artifacts from metallic restorations or attachments, may hinder tooth identification.²³ Fusing the digital cast to CBCT addresses this issue, providing reliable, detailed tooth and arch surfaces.^24,25 Stepwise mandibular regional superimposition with digital cast fusion offers reliable reorientation and precise morphology of the dentition, including gingival margins, without interferences of the maxillary dentition, and mandibular positional or growth changes.

Therefore, the current study aimed to assess 3D mandibular tooth displacement, changes in arch dimensions and gingival margins after mandibular total arch distalization with interradicular TADs in Class III malocclusion by using a stepwise 3D mandibular CBCT superimposition followed by digital cast superimposition.

MATERIALS AND METHODS

Subjects

This study was approved by the institutional review board of Gangnam Severance Hospital (IRB No. 3-2023-0479).

Consecutive patients: (1) diagnosed with skeletal Class III malocclusion (ANB < 0) with full dentition excluding third molars; (2) who underwent mandibular total arch distalization with interradicular TADs along with fixed appliances; and (3) with full records, including pretreatment (T0) and post-treatment (T1) CBCTs, treated at the Department of Orthodontics, Gangnam Severance Hospital, were collected retrospectively. Patients with: (1) extraction of teeth other than third molars, (2) who underwent orthognathic surgery, (3) with severe asymmetry (menton deviation > 4 mm), and (4) poor CBCT or digital cast image quality were excluded.

Based on a previous report on total distalization using TADs,¹¹ the sample size was estimated as 16 to identify an effect size of 1.17 (α = 0.05, β = 0.01). Considering dropouts, 19 subjects (22.53 ± 7.8 years) were included for the study (Table 1). After levelling and alignment using Roth prescription self-ligating brackets, buccal interradicular TADs were inserted between the mandibular second premolar and first molar, or the first and second molars, bilaterally. Elastomeric chains were applied from the TADs to the crimpable long-hooks on the anterior segment for distalization. Cephalometric changes at T0 and T1 indicated mandibular total arch distalization (Table 2).

Table 1.Subject Characteristics

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Table 2.Cephalometric Analysis^a,b

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CBCT scans (Pax-zenith 3D; Vatech, Seoul, South Korea) were obtained at 120 kV and 10 mA, voxel size 0.3 nm, and rotation time 24 s. The patients were scanned in a natural head posture in maximum intercuspal position. T0 scans were taken for orthodontic diagnostic purposes. Given that the subjects underwent skeletal Class III camouflage treatment, T1 scans were taken for reasons including the evaluation of detailed 3D orthodontic treatment outcomes, periodontal conditions, assessment of airway, as well as TMJ position. Digital casts were produced using a digital model scanner (DOF Freedom HD; DOF, Seoul, Korea) or directly scanned using an intraoral iTero 5D Element scanner (Align Technology, Santa Clara, USA).

Stepwise Registration and Superimposition of the Mandible

Step 1. Registration and reorientation: Mandibular regional superimposition.

T0 and T1 CBCTs were superimposed using a voxel based regional mandibular superimposition method with ITK snap (Version 4.0.2; http://www.itksnap.org).²¹ Stable anatomical references including the inner cortical surface of the inferior border of the symphysis and the anterior surface of the chin above pogonion were set. 3D masks of the chin (Figure 1A, red) and symphysis (Figure 1a, blue) were created from the T0 CBCT. The T1 CBCT was then imported and superimposed onto T0 using the previously created 3D surface masks, reorienting the T1 to the T0 CBCT (Figure 1a, box).

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Step 2. Mandibular segmentation: 3D mandibular Models.

T0 and reoriented T1 CBCT volumes were segmented to generate 3D mandibular models (Figure 1B, green and red) using 3D Slicer 5.4.0 (http://www.slicer.org).

Step 3. Fusion of 3D mandibular models and digital casts.

The segmented mandibular models and the corresponding digital casts were fused using Slicer Craniomaxillofacial (CMF) registration, based on the Iterative Closest Point algorithm.²⁶ Regions of interest (ROIs) were defined by positioning four points: the cusp tips of both canines and central fossae of either the first or the second molars (Figure 1C, red), aligning the digital casts to the 3D mandibular models.²⁶

Step 4. Mandibular cast superimposition and landmark positioning.

T0 and T1 mandibular casts were superimposed and registered, sharing the same orientation. A representative landmark was placed on each tooth: L1 and L2, midpoints of the incisal edge; L3, canine cusp tip; L4 and L5, buccal cusp tip or highest point of the premolars; and L6 and L7, mesiobuccal cusp tip or highest point of the molars (Figure 1D).

Changes in the Occlusal Plane

The occlusal plane was defined as a plane passing through the left and right mesial buccal cusp tips of the lower first molars and the midpoint of the lower central incisor landmarks. Changes were measured as the angle between the T0 (Figure 2, green line) and T1 (Figure 2, red line) planes, and classified as clockwise rotation, counterclockwise rotation, intrusion and extrusion, or left and right rotation (Figure 2).

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RESULTS

3D Changes in the Mandibular Arch After Total Arch Distalization

The most prominent changes occurred in the sagittal plane (Δy), with significant distalization of 1.74–2.50 mm (P < .001). The first and second premolars and second molars showed significant lateral transverse (Δx) movement (P < .05). All teeth except the second molars exhibited significant extrusive vertical (Δz) movement (P < .001) (Table 3).

Table 3.3D Displacement of the Mandibular Teeth After Total Arch Distalization^a–c

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Changes in Arch Width After Total Arch Distalization

Interpremolar widths increased significantly (P < .05) (Table 4). Subclassification revealed inter-first and second premolar widths increased by 52.6 (10/19) to 57.9% (11/19) of the subjects, exceeding the 5.3 (1/19) to 15.8% (3/19) of subjects with decreased width. Intercanine, first and second molar widths were primarily maintained in 68.4% (13/19) and 42.1% (8/19), of subjects, respectively.

Table 4.Changes in Arch Width After Mandibular Total Arch Distalization^a,b

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Change in the Occlusal Plane After Total Arch Distalization

Extrusion of the occlusal plane was most frequently noted, in 42.1% of the subjects (8/19), followed by clockwise rotation in 21.1% (4/19) (Table 5).

Table 5.Changes in the Occlusal Plane After Mandibular Total Arch Distalization^a

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Changes In CCH and Gingival Recession After Total Arch Distalization

ΔCCH were limited to −.23 to 0.16 mm. CCH significantly increased in the first and second premolars, suggesting gingival recession, while it significantly decreased in the first molar, indicating gingival gain (P < .05) (Table 6).

Table 6.Changes in Clinical Crown Height After Mandibular Total Arch Distalization^a,b

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Overall, CCH was maintained in 72.5% (193/266) of the teeth evaluated. Mild to moderate recession was most prominently detected in the first (8/38) and second premolars (9/38). Gingival gain was noted in the central incisors (9/38) and the first (9/38) and second molars (8/28).

DISCUSSION

Stepwise superimposition revealed detailed treatment outcomes of mandibular total arch distalization in skeletal Class III malocclusion. Along with distal movement of the whole dentition, extrusion, occlusal plane changes, and increase in interpremolar widths highlighted complex 3D changes following treatment. Gingival margins were generally maintained, though mild-to-moderate recession was observed in around 20% of the premolars.

Voxel-based regional mandibular superimposition using serial CBCTs differentiated changes in the mandibular dentition by eliminating confounding factors affecting mandibular position, such as maxillary dental interferences, mandibular growth dynamics, rotational effects induced by tooth movement, and the potential occurrence of temporomandibular joint (TMJ) displacement or condylar resorption.^18–20 The stepwise registration of digital casts also compensated for the absence of reliable intraoral landmarks, enabling precise visualization of tooth surfaces and gingival margins.^17,24,25

Based on the results, the second molar showed the largest amount of distalization, while vertical and lateral displacements were most prominent in first and second premolars. Excessive buccal expansion or tipping may lead to gingival recession^29,30 and extrusion may increase CCH.³¹ The combined movement of the first and second premolars likely contributed to the increase in CCH and greater incidence of gingival recession compared to other teeth.²⁷ Overall, the average CCH changes were limited to 0.23 mm, consistent with a previous study.²⁷ Nevertheless, the results should be interpreted cautiously, as patients were relatively young and healthy with no history of periodontal issues. Gingival recession is multifactorial and influenced by host factors such as age, biotype, and oral hygiene, in addition to treatment effects.³² Thus, high-risk groups and long-term prognosis should be carefully monitored. Interestingly, CCH decreased in the first molars, and gingival gain was noted to some degree among the central incisors and the molars. This indicated the versatile response of the periodontal soft tissue after orthodontic tooth movement. The improvement of incisal position as well as buccal overjet may have resulted in the morphologic gain of the gingival level relative to the tooth crown.

Arch width increases were similar to previous reports,^10,11 with significant lateral displacement noted in the premolars and the second molar. Biomechanically, buccal tipping is well-accepted and expected when using buccal mechanics with buccal interradicular TADs.^11,33 The lack of buccal displacement of the first molar may reflect a focus on skeletal Class III treatment, often involving initial buccal crossbites and the continuous management of arch coordination to not increase buccal overjet specifically.^34,35 However, given that the results were based on the displacement pattern of crown landmarks, the increase or decrease of arch width may not represent pure changes at the root level. Further evaluation of the root displacement pattern would provide a more comprehensive understanding of the arch dimensions.

Vertically, occlusal plane changes were mainly observed as overall extrusion of the incisors and molars or clockwise rotation. These results were rather contradictory to previous findings indicating molar intrusion resulting in 1.3°–3.2° counterclockwise rotation after mandibular distalization.^3,12,36 This variation may be attributed to differences in 2-dimensional (2D) cephalometric, vs 3D imaging, methods, and evaluation techniques. Notably, this study was designed to eliminate the interference of mandibular rotation that may influence the interpretation of pure changes in the vertical dimension.

Due to its retrospective design and difficulties in acquiring interim CBCTs, assessments were based on pre-existing before-and-after records that represent the overall outcome of comprehensive orthodontic treatment encompassing all stages of tooth movement not solely limited to mandibular total arch distalization. For example, initial leveling of the Curve of Spee may have resulted in extrusion of the premolars while uprighting posterior teeth that were inclined lingually, may have affected interpremolar width, consequently changing the CCH and gingival margins. To overcome some of the limitations, efforts were made to ensure homogeneity by selecting skeletal Class III adults with a primary treatment objective focused on mandibular total arch distalization after a standardized protocol using interradicular TADs in the mandibular molar region.

Clinically, total arch distalization may inadvertently induce mandibular arch expansion, resulting in buccal crossbite given that patient with skeletal class III malocclusion are prone to have preexisting transverse discrepancies. Thorough pretreatment evaluation focusing on transverse dimensions as well as incorporating biomechanical compensatory strategies, such as optimization of force vectors, application of maxillary expansion or cross elastics and modifications in the archwire with toe-in bends or lingual torque, etc., can be strategically applied to mitigate the adverse effects. Because changes in the vertical dimension and occlusal plane can cause changes in the soft tissue profile, close monitoring of the overall dentofacial changes is also recommended.

Despite the retrospective nature of the study, stepwise registration using 3D digital imaging has contributed to the understanding of 3D individual tooth movement after total arch distalization in skeletal Class III malocclusion.

CONCLUSIONS

• Stepwise digital superimposition enabled visualization of 3D treatment outcomes after mandibular total arch distalization using interradicular TADs in skeletal Class III malocclusion.

• Together with distalization, extrusion and an increase of interpremolar width were noted.

• Although changes in the gingival margins were limited, a few instances of mild-to-moderate recession were noted in the first and second premolars.