This retrospective clinical study aimed to evaluate the efficacy of different types of incisor movements with clear aligners in the sagittal plane.
Pretreatment and posttreatment cone-beam computed tomography (CBCT) scans were collected from 69 patients who underwent nonextraction treatment with clear aligners (Invisalign; Align Technology, San Jose, Calif). Integrated 3-dimensional models of the virtual incisor position (ClinCheck; Align Technology) and the posttreatment incisor position (from posttreatment CBCT scans) were superimposed over the pretreatment position (from pretreatment CBCT scans) using Mimics software (Materialise, Leuven, Belgium). On the basis of the location of the rotation center, incisors showing pure tipping (>10°), controlled tipping (>10°), translation (>1 mm), or torque (>10°) movements were selected. Efficacy was determined by comparing the predicted and achieved incisor movement, and differences with efficacy were analyzed using Kruskal-Wallis and Shapiro-Wilk tests (α = 0.05).
In measurements for 231 incisors, the mean efficacy of incisor movements in the sagittal plane was 55.58%. The most and least predictable movements were pure tipping (72.48%) and torque (35.21%), respectively. Labial root movement was significantly more predictable than lingual root movement, and labial movement of the mandibular incisors was significantly easier than that of the maxillary incisors. The type of tooth movement achieved was different from the type designed.
The efficacy of incisor movement in the sagittal plane using clear aligners varies with designed movement type, and labial root movement appears to be more accurate than the lingual root movement. The biomechanics of clear aligners remains to be further elucidated to achieve more predictable treatment results.
The efficacy of incisor movements with clear aligners in the sagittal plane was evaluated.
The most predictable movement was pure tipping, and the least predictable was torque.
Labial root movement was significantly more accurate than the lingual root movement.
The labial movement was significantly easier for the mandibular than maxillary incisors.
The type of movement achieved was likely to be different from the type designed.
The clear aligner system, which contains a series of removable polyurethane appliances, has been widely used in clinical practice as an esthetic and more comfortable alternative to fixed orthodontic appliances. For example, Invisalign was used to treat over 300,000 orthodontic patients with a variety of malocclusions in its first decade, and over 6 million as of today.
Although the consumer demand and the professional use of clear aligners continue to grow, questions regarding the efficacy of this system still remain. According to the manufacturer, clear aligners could effectively achieve major tooth movement, including premolar derotation up to 50° and root movement of the maxillary central incisors up to 4 mm. Despite the advocated efficiency of the treatment, its clinical potency still remains debatable; opponents pointed out its significant limitations when treating complex malocclusions, whereas advocates remained convinced by the patients with successful outcomes.
To achieve the desired goals, patients treated with clear aligners require either a midcourse correction or case refinement. Align Technology reported that 20%-30% of patients might require additional aligners, whereas many clinical orthodontists reported a proportion of 70%-80%. Accurately assessed efficacy of tooth movement could help design the amount of movement before treatment and reduce the frequency of refinements.
A substantial body of studies has concentrated on the efficacy of tooth movement on clear aligners. Kravitz et al evaluated the accuracy of anterior tooth movement and concluded that maxillary incisor lingual crown tipping (53.1%) was significantly more accurate than labial crown tipping (37.6%). Castroflorio et al examined 12 maxillary incisors in Invisalign patients needing lingual root torque and found that when a torque correction of about 10° was required, torque loss was negligible. Simon et al reported the mean accuracy for maxillary incisor torque was 42% and concluded that no differences were observed if the torque (>10°) was supported with a horizontal ellipsoid attachment or with a power ridge. These results varied considerably from each other, and the reason might be related to the absence of root information and unclear definitions of tooth movements.
Incisor movements in the sagittal direction can be classified into 4 types according to the position of the rotation center: pure tipping, controlled tipping, translation, and torque. These 4 types of movements are supported by completely different extents of root control. However, limited published data have assessed the differences and evaluated the efficacy of these 4 types of incisor movements.
Consequently, this retrospective study aimed to evaluate the efficacy of these 4 types of incisor movements by clear aligner treatment with additional root information from integrated 3-dimensional (3D) digital models and determine whether any inputs could be derived from the differences between efficacy. The predicted amount of tooth movement was compared with the achieved amount after treatment, and the amount was calculated by measuring the area of the root movements.
Material and methods
Patients treated with Invisalign (Align Technology, San Jose, Calif) between January 2016 and December 2018 at the Department of Orthodontics, Shanghai Ninth People’s Hospital were selected. The following inclusion criteria were used: (1) age ≥20 years; (2) the presence of crowding that could be harmonized using conservative space-gaining measures such as protrusion, proclination, expansion, and interproximal enamel reduction; (3) completed treatment with the whole active stages of the first serial of aligners. Availability of 1 CBCT scan each from before and after the treatment; (4) no auxiliary device such as segmental wire and elastics was used on incisors; and (5) CBCT voxel size ranging from 0.20 mm to 0.30 mm.
The following parameters represented exclusion criteria: (1) unclear CBCT images of teeth and jaws and (2) presence of alveolar cleft or other bone defects.
Both CBCT scans (pretreatment and posttreatment) were collected and evaluated. The use of the data was approved by the ethics committee of the Shanghai Ninth People’s Hospital (SH9H-2018-T63-1).
Three-dimensional models of jaws (from both pre- and post-CBCT scans) and each incisor (from the post-CBCT scan) were reconstructed using Mimics software (version 19.0; Materialise, Leuven, Belgium) and imported into 3-Matic software (Materialise) to perform surface-based superimposition. The 3D models of the initial dentition (pre-D) and the virtual setup dentition were obtained from ClinCheck (Invisalign; Align Technology) and imported into the 3-Matic program.
Surface characteristic-based automated registration of pre-CBCT jaws and pre-D was performed using the crowns as areas of optimal overlap. Subsequently, each individually segmented incisors were registered to corresponding pre-D and virtual setup dentition crowns. Finally, the pretreatment integrated model and virtual-treatment model were completed, which included accurate crowns determined by ClinCheck models and roots and pre-CBCT jaws positions derived from CBCT imaging. The posttreatment integrated models, including post-CBCT jaws and incisors derived by post-CBCT scans, did not require any superimposition ( Fig 1 ).
To analyze incisor positions in the same space coordinates, we superimposed both integrated models (pretreatment integrated model and posttreatment integrated models) on the basis of skeletal marks (eg, anterior nasal ridge), and then global registration was performed with an iterative closest point algorithm. Because the virtual and the pretreatment incisor positions were automatically superimposed in ClinCheck, the 3 incisor positions were finally matched to the same spatial coordinates.
The alveolar bone height (ABH) was represented by the distance between the midpoint of the mesiodistal alveolar ridge crest (pre-C) and apex (pre-R), as the root length (RL) was represented between the apex (pre-R) and the midpoint of the mesiodistal cementoenamel junction ( Fig 2 , A ). To ensure the universality of the results, teeth with short roots and those in which the alveolar bone had absorbed more than a third of RL were excluded.
A reference coordinate system was set up using 3-Matic software, and the following reference points were used: pre represents the pretreatment position, vir represents the virtual position, post represents the posttreatment position ( Fig 2 , B ): (1) pre/vir/post-R: the root apex point; (2) pre-M: the most prominent point of the lingual protuberance; (3) pre/vir/post-I: midpoint of the incisal edge; (4) pre/vir/post-RI: the line from point-R to point-I, which was defined as the axis of the incisor; (5) pre/vir/post-C: the point of the alveolar ridge crest that was on line-RI and was of the same length as ABH from point-R; (6) pre/vir/post-Cs: center of resistance, the point on line-RI that was half the length of the ABH from point-R ; (7) sagittal plane: the plane covering the points pre-R, pre-M, and pretreatment midpoint of the incisal edge; (8) vir/post-RI’: the projection of the incisor axis (vir/post-RI) in the sagittal plane; (9) vir/post-RC: the line from point-R to point-C with length the same as ABH; (10) vir/post-RC’: the projection of vir/post-RC in the sagittal plane, of which the length was usually shorter than ABH; (11) center of rotation (Ctr): the intersection point of pre-RI and vir-RI.’ In a right-handed coordinate system, pre-R was defined as the coordinate origin, pre-RI was defined as the y-axis, and the line in the sagittal plane perpendicular to the y-axis from the origin was defined as the x-axis.
The angle (α) formed by vir-RI′ and y-axis and the distance (d) between pre-Cs and virtual center of resistance (vir-Cs) was recorded. To ensure that measurements were limited to the sagittal direction, the length of vir/post-RC′ was corrected as follows: on the basis of the vir/post-Cs, the points vir/post-C′ and vir/post-R′ were marked on vir/post-RI’ with the length between these 2 points the same as ABH.
On the basis of the location of Ctr, 4 types of tooth movement were identified as follows ( Fig 3 , A ): (1) pure tipping: α >10°, a movement in which Ctr was within 2 mm from pre-Cs; (2) controlled tipping: α >10°, a movement in which Ctr was within 2 mm from pre-R; (3) translation: d >1 mm, a movement in which Ctr was more than 6 times the length of the ABH from pre-Cs; and (4) torque: α >10°, a movement in which Ctr was within 2 mm of the midpoint of the incisal edge.
Considering the relationship between pre-Cs and vir-Cs, we defined the direction of root movement as follows ( Fig 3 , B ): if vir-Cs was on the labial side of pre-Cs, the direction was recorded as labial ; otherwise, the direction was recorded as lingual .
Tooth movement efficacy was calculated by measuring the area of the root movement in the sagittal plane. The accuracy of each tooth movement was determined by the following equation: percentage of accuracy = post-S/vir-S × 100% ( post-S , the area composed of the pre-root and post-root; vir-S , the area composed the pre-root and virtual root) ( Fig 4 ).
All statistical analyses were performed with SAS software (version 8.02; SAS Institute Inc, Cary, NC). The differences between the 4 types of tooth movement were compared using the Wilcoxon test. The differences between labial vs lingual and maxilla vs mandible were analyzed by the Shapiro-Wilk test. The significance level was set at P = 0.05.
Sixty-nine patients (44 female and 25 male) aged between 20 and 41 years (mean age, 28.5 ± 5.7 years) were collected. The mean number of maxillary and mandibular aligners per treatment was 45.0 ± 7.5 and 41.9 ± 9.3, respectively. According to our criteria, 231 teeth out of 552 maxillary and mandibular incisors were included in the present study. The average RL and ABH of incisors are listed in Table I . The average predicted amount of change for each type of movement was as follows: 13.28° for pure tipping, 13.75° for controlled tipping, 12.91° for torque, and 1.69 mm for translation.