Comparison of achieved and predicted crown movement in adults after 4 first premolar extraction treatment with Invisalign

Introduction

In this study, we compared achieved and predicted crown movements of maxillary and mandibular first molars, canines, and central incisors in adults after 4 first premolar extraction treatment with Invisalign.

Methods

Seventeen adult patients who received 4 first premolar extraction treatment with Invisalign and completed the first series of aligners were included. Superimposition of pretreatment and actual posttreatment dental models was acquired using registrations of pretreatment and posttreatment craniofacial models on the basis of bone surfaces and registrations of craniofacial and dental models on the basis of dental crown surfaces, respectively. Superimposition of pretreatment and predicted posttreatment models was acquired from ClinCheck software. Achieved and predicted 3-dimensional crown movements of maxillary and mandibular first molars, canines, and central incisors were then compared using the paired t test.

Results

Relative to predicted changes, first molars achieved greater mesial displacement, mesial tipping, and buccal inclination in both the maxilla and mandible, greater intrusion in the maxilla, and greater mesial-lingual rotation and less constriction in the mandible. Canines achieved greater distal tipping in both the maxilla and mandible, less retraction in the maxilla, and greater lingual inclination and extrusion in the mandible. Central incisors achieved greater distal tipping and lingual inclination and extrusion in both the maxilla and mandible and less retraction in the maxilla.

Conclusions

Tooth crown movements were not fully achieved as predicted following Invisalign treatment. Differences focused on mesial tipping, buccal inclination, mesial displacement, and intrusion of the first molars, as well as distal tipping, lingual inclination, insufficient retraction, and intrusion of the canines and central incisors.

Highlights

  • Tooth crown movements were not fully achieved as predicted.

  • First molars showed greater anchorage loss, buccal inclination, and intrusion.

  • Canines and central incisors showed greater distal tipping and lingual inclination.

  • Canines and central incisors showed insufficient retraction and intrusion.

  • Achieved-predicted differences varied between the maxilla and mandible.

Clear aligners have become increasingly popular in clinical orthodontics, both for nonextraction and extraction patients. At the start of treatment, the final 3-dimensional (3D) tooth crown position is predicted on the basis of digital dental models, the realization of which is then met using a series of aligners. However, differences have been reported between predicted and achieved tooth crown movement in nonextraction and extraction treatment. To date, the majority of studies concerning the efficacy of tooth crown movement with clear aligners have examined nonextraction treatment or extraction of a front tooth, with few studies documenting premolar extraction treatment. For example, an early study by Baldwin et al showed inadequate control of tooth movement using aligners in premolar extraction patients, whereby teeth adjacent to the extraction sites showed significant tipping in the maxilla and mandible. As a result, no subject completed the initial series of aligners, and only 1 ultimately completed treatment using aligners, the remainder switching to fixed appliances. However, a later study by Li et al showed good root angulation results in extraction patients treated with Invisalign. The authors claimed this was due to the use of correct attachments. Nevertheless, occlusal contacts and buccolingual inclination were not as good as with braces.

Development of the Invisalign system, which uses SmartTrack material, SmartForce, and SmartStage, , has resulted in an increase in the number of successful extraction patients. , A recent study by Dai et al reported improved control of tooth crown movement in maxillary first premolar extraction patients compared with Baldwin et al, with 52% of subjects completing the initial series of aligners and showing only mild mesial tipping in the maxillary first molars. Nevertheless, crown movements were not achieved as predicted in the anteroposterior displacement of maxillary first molars and central incisors, mesial tipping of first molars, and lingual inclination of central incisors.

Understanding the efficacy of tooth movement is essential in determining final treatment goals and calculating treatment times and costs. Although Dai et al examined tooth crown movement efficacy of maxillary first molars and central incisors in maxillary premolar extraction patients, little is known about the movement efficacy of maxillary canines and mandibular teeth. Therefore, this retrospective study aimed to compare predicted and achieved crown movements of maxillary and mandibular teeth in 4 first premolar extraction patients treated with Invisalign, providing more evidence for virtual design during clear aligner treatment.

Material and methods

From August 2015 to August 2017, 33 adult patients started orthodontic treatment at Second Clinical Division, Peking University School and Hospital of Stomatology using Invisalign clear aligners following extraction of 4 first premolars. Of these, 17 patients (2 men, 15 women; mean age: 25.4 ± 5.0 years) finished the first series of aligners and were included in this study. Inclusion criteria were (1) no missing permanent maxillary teeth before treatment (except for third molars), (2) completion of the first series of aligners without midcourse correction, (3) no combined treatment with fixed or other auxiliary appliances, and (4) complete records of pretreatment and posttreatment (refers to posttreatment of the first series of aligners throughout this article) cone-beam computed tomography (CBCT) images and digital dental models. Of the 17 patients, 12 presented with a Class I occlusion, 4 presented with cusp-to-cusp Class II occlusions, and 1 presented with a cusp-to-cusp Class III molar occlusion. Before treatment, the average overbite was 2.1 ± 1.1 mm, with overjet of 2.8 ± 1.2 mm, maxillary crowding of 3.5 ± 2.0 mm, and mandibular crowding of 4.2 ± 1.7 mm. All patients changed aligners every 2 weeks after the manufacturer’s protocol. The average treatment time for the first series of aligners was 21.0 ± 4.2 months. One patient used bilateral Class II elastics, and 2 patients used unilateral Class II elastics. The attachments and power ridges used on the teeth are described in Table I . The protocol for this retrospective study was approved by the institutional review board.

Table I
Attachments and power ridges used on maxillary and mandibular teeth
Teeth Attachment/Power ridge
First molar G6 optimized attachment 5-mm HR attachment 4-mm HR attachment 3-mm HR attachment 3-mm VR attachment Total
Maxilla 13 9 6 2 4 34
Mandible 9 5 10 4 6 34
Second premolar G6 optimized attachment 4-mm HR attachment 3-mm HR attachment 3-mm VR attachment Total
Maxilla 15 10 2 7 34
Mandible 17 10 1 6 34
Canine G6 optimized attachment 4-mm VR attachment 3-mm VR attachment Total
Maxilla 27 7 0 34
Mandible 24 9 1 34
Central incisor Power ridge Optimized attachment None Total
Maxilla 7 2 25 34
Mandible 5 0 29 34

HR , horizontal rectangular; VR , vertical rectangular.

CBCT images were taken with the patient in the intercuspal position, with CBCT scanner (DCT Pro; Vatech Co, Yongin-Si, South Korea) settings as follows: a 20 × 19 cm or 16 × 19 cm field of view, 90 kV tube voltage, 7 mA tube current and 0.3 mm pixel size. The images were imported into Mimics software (version 10.01; Materialise, Leuven, Belgium), then 3D models of the craniofacial hard tissue, including maxilla, mandible, and teeth, were reconstructed using the threshold segmentation method. The maximum segmentation threshold was set at 3071 HU, whereas the minimum was kept within a range of 600-1200 HU depending on the patient to acquire complete and clear teeth. Maxillary and mandibular registrations of pretreatment and posttreatment craniofacial models were conducted using Rapidform 2006 (Inus Technology, Seoul, South Korea). The maxillary registration region included the zygoma, zygomatic arch, palate, and maxilla above the root apices of the teeth ( Fig 1 , A ), whereas the mandibular registration region included bilateral ramus and the body of the mandible below the root apices of the teeth ( Fig 1 , B ).

Fig 1
A, The maxillary registration region including the zygoma, zygomatic arch, palate, and maxilla above the root apices of the teeth ( blue ); B, the mandibular registration region including bilateral ramus and the body of the mandible below the root apices of the teeth ( blue ).

Treatment plans were based on digital dental models acquired using the iTero intraoral scanner (Align Technology Inc, San Jose, Calif) and uploaded to ClinCheck Pro software (Align Techonology Inc). Pretreatment and predicted posttreatment dental models (maxillary and mandibular dental arches separated but oriented with occlusion) were exported from ClinCheck (the first ClinCheck plan). Actual posttreatment dental models, corresponding to the pretreatment models of the next refinement strategy (the second ClinCheck plan), were also exported from ClinCheck. All 3 models were then imported into Rapidform 2006. Pretreatment dental models were superimposed with corresponding pretreatment craniofacial models, whereas actual posttreatment dental models were superimposed with corresponding posttreatment craniofacial models, using the crown surfaces of all teeth as the registration region ( Figs 2 , A and B ). Superimposition of pretreatment and actual posttreatment maxillary dental models based on maxillary registration ( Fig 2 , C and E ) and superimposition of mandibular dental models based on mandibular registration ( Fig 2 , D and F ) were then acquired. Ultimately, pretreatment, predicted posttreatment, and actual posttreatment dental models were superimposed on the same coordinate system ( Figs 2 , I and J ), and predicted and achieved tooth crown movements were measured.

Fig 2
A, Superimposition of a pretreatment dental model ( blue ) and craniofacial model ( gray ) based on crown surfaces of all teeth; B, superimposition of an actual posttreatment dental model ( red ) and craniofacial model ( green ); C, superimposition of pretreatment and posttreatment craniofacial models based on the maxillary registration region; D, superimposition of pretreatment and posttreatment craniofacial models based on the mandibular registration region; E, superimposition of pretreatment and actual posttreatment maxillary dental models based on the maxillary bone registration; F, superimposition of pretreatment and actual posttreatment mandibular dental models based on mandibular bone registration; G, pretreatment and predicted posttreatment ( yellow ) maxillary dental models exported from ClinCheck; H, pretreatment and predicted posttreatment mandibular dental models exported from ClinCheck; I, final superimposition of pretreatment, predicted posttreatment and actual posttreatment maxillary dental models; J, final superimposition of pretreatment, predicted posttreatment and actual posttreatment mandibular dental models.

The coordinate system for crown movement measurement was generated on the basis of the pretreatment dental models using the method described by Dai et al, whereby the x-axis indicates the anteroposterior direction, the y-axis the occlusal-gingival direction, and the z-axis the medial-lateral direction. Three-dimensional displacements and angular changes (crown angulation, inclination, and rotation) were measured for each maxillary and mandibular first molar, canine and central incisor. For displacement measurements, the mesial buccal cusp of the first molar, cusp of the canine, and midpoint of the incisor edge of the central incisor were located on the pretreatment model, then the 3D position of the corresponding reference points was recorded. Angular changes were measured as described by Dai et al and Chen et al Taking the canine as an example, the most mesial and distal points along the cusp and their projected points on the transverse plane (xz plane) were used to determine the mesiodistal plane of the tooth, whereas the plane perpendicular to both the transverse and mesiodistal plane was used to determine the labiolingual plane. The long axis of the canine crown, generated by connecting the cusp point and the most gingival point along the midline of the labial surface, was then projected onto the mesiodistal and labiolingual planes, respectively. The angle between the projected line on the mesiodistal plane and the vertical line perpendicular to the transverse plane was defined as angulation ( Fig 3 , A ), whereas the angle between the projected line on the labiolingual plane and vertical line perpendicular to the transverse plane was defined as an inclination ( Fig 3 , B ). The angle between the intersecting line of the mesiodistal plane and transverse plane and x-axis was defined as rotation ( Fig 3 , C ). In addition, reference points on the pretreatment model were transferred onto the actual and predicted posttreatment models through tooth crown surface superimposition then 3D positions and angles were measured again. Subsequently, achieved and predicted 3D displacements and angular changes were calculated.

Dec 24, 2021 | Posted by in Orthodontics | Comments Off on Comparison of achieved and predicted crown movement in adults after 4 first premolar extraction treatment with Invisalign
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