Three-dimensional analysis of the tooth movement and arch dimension changes in Class I malocclusions treated with first premolar extractions: A guideline for virtual treatment planning

Introduction

Our objective was to analyze patterns of tooth movement and changes of arch dimension by superimposing 3-dimensional (3D) virtual models.

Methods

The sample consisted of 24 Korean adults with Class I malocclusion and minimal crowding, treated by first premolar extractions, sliding mechanics (0.022-in MBT brackets [3M Unitek, Monrovia, Calif] with 0.019 × 0.025-in stainless steel wire) and moderate anchorage. The 3D virtual maxillary casts at pretreatment and posttreatment were superimposed with the best-fit method. Linear and angular variables were measured with 3Txer program (Orapix, Seoul, Korea). Wilcoxon signed rank and Mann-Whitney tests were used for statistical analysis.

Results

There was no significant difference in the individual tooth movement between the right and left sides ( P >0.05). For the movement of each tooth, the maxillary central incisors (U1), lateral incisors (U2), and canines (U3) were significantly inclined lingually, extruded, and moved posteriorly and laterally. The maxillary second premolar (U5), first molar (U6), and second molar (U7) had significant mesial inward rotation, anterior movement, and contracted toward the midsagittal plane. The ratio of anteroposterior movement between the maxillary anterior and posterior teeth was 5:1. The amounts of contraction in U5, U6, and U7 were 1.4, 1.3, and 1.2 mm, respectively. When the amount of change between the adjacent teeth were compared, the linguoversion in U1 was significantly greater than that of U2. U3 and U5 showed significant opposite movements in all variables. There were differences only in angulation and vertical displacement between U6 and U7.

Conclusions

Superimposition of 3D virtual models could be a guideline for precise virtual treatment planning.

To analyze patterns of individual tooth movement and changes of arch dimensions in the 3-dimensional (3D) spaces is a prerequisite to making a practical treatment plan. Recently, 3D virtual model analysis has been applied to measure tooth size, and width and length of the dental arch, and to analyze tooth movement by mathematical superimposition of pretreatment and posttreatment models.

Three-dimensional scanning is a type of reverse engineering, the original purposes of which were to reduce the period of development and cost, to establish standardization, and to increase the accuracy of products. Now it is applied in the medical engineering area. The basic concepts of the 3D dental scanner (Orapix, Seoul, Korea; accuracy level: ± 0.02 mm/10 mm; resolution: 1024 × 768 pixels) are to shoot the laser light to the target material surface, detect reflection, and calculate its 3D coordinate system by using the trigonometric function. According to DeLong et al, the accuracy of measurement of 10 stone casts of a “dental” standard with known dimensions using 3D computer models was almost equal to that of a conventional contact digitizer.

The median palatal raphe, the palatal rugae, and the hard palate around the maxillary right and left first molars 4 have been suggested as reference landmarks and areas for superimposition of the maxillary models. Cha et al reported no significant difference in the horizontal and vertical movements of the maxillary central incisor and first molar between superimpositions of the lateral cephalogram and superimpositions of 3D virtual models. However, a few 3D studies have analyzed the angular and linear changes of individual tooth positions in terms of the facial axis (FA) point, which was used as the reference point for bracket bonding. Moreover, some 3D virtual model studies about tooth movement seem to be insufficient to assess 3D tooth movement. Chong et al evaluated the 3D virtual models for only rotational movements of the maxillary posterior teeth in patients whose maxillary first premolars were extracted. Cha et al measured only changes of the maxillary central incisor and first molar in the 3D virtual models, not in the whole dentition.

Therefore, the purposes of this study were to analyze patterns of individual tooth movement and changes of the arch dimensions in patients having first premolar extractions by using superimposition of 3D virtual models obtained from pretreatment (T0) and posttreatment (T1) and to present a guideline for precise virtual treatment planning.

Material and methods

The sample consisted of 24 young Korean adults (4 men, 20 women; average age, 24 years 8 months ± 4 years 11 months; SNA, 83.29° ± 3.98°; ANB, 3.10° ± 1.52°; U1-SN, 115.22° ± 5.47°; FMA, 26.48° ± 2.71°; lower lip to E-line, 3.80 ± 2.02 mm; crowding in the maxillary arch, 2.55 ± 1.08 mm; and average treatment period, 21.87 ± 4.14 months).

Since the patterns of orthodontic tooth movement and the changes in arch dimensions could be various according to the conditions of the malocclusion and the diverse treatment procedures, the sample was limited according to age, skeletal pattern, soft-tissue profile, dentition, and treatment methods: (1) young adults over 18 years old; (2) Class I malocclusion, normodivergent pattern, less than 4 mm of crowding in each arch, and ovoid and symmetric arch form; (3) lip protrusion (lower lip to Ricketts’ esthetic line >2 mm); (4) treatment including extraction of maxillary and mandibular first premolars with sliding mechanics (0.022-in slot, MBT brackets, 3M Unitek, Monrovia, Calif) and 0.019 × 0.025-in stainless steel wire, maxillary and mandibular second molars banded or bonded in the fixed treatment, and no headgear, transpalatal arches, or orthodontic miniscrews (temporary anchorage devices) used; (5) finished with Class I canine and molar relationships with normal overbite and overjet (>2 and <4 mm, respectively); and (6) after the brackets were removed carefully with debonding pliers, the remnant resin was removed by using an 8-bladed tungsten carbide bur (Fressima, FIT, Turin, Italy), an ideal cutting tool for ductile substrates such as resin materials, with little effect on the enamel structure.

The maxillary casts at T0 and T1 were scanned by using the 3D dental scanner and virtually constructed with the 3Txer program (Orapix).

In the anterior area of the hard palate, the palatal rugae has been used as a stable landmark. Especially, the medial and lateral points of the third palatal rugae seem to be fairly stable for the estimation of changes in spite of vertical growth. In the posterior area of the hard palate, the midpalate area between the maxillary right and left first and second molars seems to show the least change during treatment, and deformation of the palatal mucosa in this area during impression taking could be minimal because of unmovable soft tissues. Therefore, we chose these areas as the stable landmarks for superimposition of the 3D virtual maxillary models at T0 and T1. Three-dimensional surface-to-surface matching (best-fit method, Fig 1 ) was used with a mean least squares technique by using polygons created by point data and a function of the Rapidform 2006 (INUS Technology, Seoul, Korea).

Fig 1
Superimposition of the 3D virtual models between T0 (gray) and T1 (green). Color-coding system for the degree of surface matching: blue , perfect fit; red , insufficient fit (mismatch level, 1.65 mm).

To prevent errors in landmark positioning, superimposition of the 3D virtual maxillary models was confirmed by superimposition of the lateral cephalometric radiographs as follows.

  • 1.

    After the occlusal planes of the 3D virtual models at T0 and T1 were established, the angular differences between T0 and T1 were measured ( Fig 2 ).

    Fig 2
    The occlusal plane on the 3D virtual model was established by using the midpoint of the maxillary right central incisor edge and the mesiobuccal cusp tip of the maxillary right and left first molars. The angular difference in the occlusal plane between T0 and T1 was measured.
  • 2.

    After the angles between the Frankfort horizontal plane and the occlusal plane of the lateral cephalograms at T0 and T1 were measured, the differences were evaluated ( Fig 3 ).

    Fig 3
    The angle between the Frankfort horizontal ( FH ) plane and the occlusal plane on the lateral cephalograms was established by using the maxillary central incisor edge and the mesiobuccal cusp tip of the maxillary first molar ( dotted line , parallel displacement of Frankfort horizontal plane at T0). The angular difference between T0 ( dotted line ) and T1 ( solid line ) stages was measured.
  • 3.

    If the difference between the 3D virtual models and the lateral cephalograms was greater than 5°, superimposition of the 3D virtual models was adjusted according to the amount of occlusal plane change in the lateral cephalograms.

Definitions of the reference planes and landmarks are given in Figures 4 and 5 . Since the occlusal third including the incisal edge or cusp tip showed greater anatomic variations than the gingival third and could be changed during treatment because of attrition or fracture, it might not be a proper reference point to measure tooth movement. However, the FA point is easily recognized and used as a relatively stable guide for bracket positioning. In addition, the incisal edge and cusp tip can have greater changes due to the longer distance of the center of rotation than the FA point.

Fig 4
Definitions of the reference planes and origin: horizontal plane, to which the high-fit region in the midpalatal area was extended; coronal plane, which connects the FA points between the upper right ( #17 ) and left second molars ( #27 ) at T0 and is perpendicular to the horizontal plane; midsagittal plane, which passes through a midpoint between the FA points of #17 and #27 and is perpendicular to the horizontal and coronal planes; the origin was set at the intersection of the 3 reference planes.

Fig 5
Definitions of the landmarks and reference planes: 1 , gingival point: the lowest and most concave point in the gingival margin; 2 , occlusal point: the midpoint of the edge of the incisors, the cusp tip of the canine and the second premolar, and the most concave point between the mesiobuccal and distobuccal cusps of the molars; 3 , mesial point: the most mesial point created by the intersection between a parallel line with the Andrews plane and the facial axis of the clinical crown (FACC); 4 , distal point: the most distal point created by the intersection between a parallel line with the FACC and the Andrews plane; the FA point ( 5 ) and the Andrews plane ( 6 ) bisect the clinical crown into gingival and occlusal parts in the FACC ( 7 ).

Angular and linear variables of the maxillary dentition (central incisor, U1; lateral incisor, U2; canine, U3; second premolar, U5; first molar, U6; and second molar, U7) were measured with the 3Txer program. The coordinate system for angular measurement of each tooth and the definitions of inclination, angulation, and rotation are described in Figure 6 . Reference lines and landmarks for the linear measurements and the definitions of vertical, anteroposterior, and lateral displacement are described in Figure 7 .

Fig 6
A, Example of the coordinate system established at FA, the origin point. X-axis, horizontal axis; y-axis, vertical axis, perpendicular to x-axis; z-axis, sagittal axis, perpendicular to the x-axis and the y-axis. B , Inclination (°): the labiolingual or buccolingual slope of the clinical crown to the occlusal plane. The difference between T0 and T1 (T0 – T1): positive means labioversion or buccoversion; negative means linguoversion. C , angulation (°): the mesiodistal slope of the clinical crown to the occlusal plane. T0 – T1: positive means distal tipping; negative means mesial tipping. D , Rotation (°): the angle made with the x-axis and the midsagittal plane at the occlusal view. T0 – T1: positive means mesial inward rotation; negative means distal inward rotation.

Fig 7
A , Vertical displacement of FA point from the horizontal plane (mm). T0 – T1: positive means intrusion; negative means extrusion. B , Anteroposterior displacement of FA point from the coronal plane (mm). T0-T1: positive means posterior movement; negative means anterior movement. C , Lateral displacement of FA point from the midsagittal plane (mm). T0-T1: positive means contraction; negative means distraction.

The reference points were digitized 3 times with a 2-week interval by 1 examiner (M.Y.C.). Intraclass correlation coefficients (ICC) for reference point identification were computed to assess intraexaminer reliability (repeatability). Since the assessment of the intraexaminer reliability of reference point identification showed excellent ICC values ( Table I ), the first digitized data were used.

Table I
Intraclass correlation coefficients of intraexaminer reliability
Variable Intraexaminer reliability P value
#11
x 0.998 0.000
y 0.998 0.000
z 0.950 0.001
#12
x 0.960 0.001
y 0.993 0.000
z 0.993 0.000
#13
x 0.838 0.012
y 0.992 0.000
z 0.988 0.000
#15
x 0.999 0.000
y 0.997 0.000
z 0.891 0.005
#16
x 0.995 0.000
y 0.995 0.000
z 0.948 0.001
#17
x 0.910 0.004
y 0.997 0.000
z 0.985 0.000
#21
x 0.893 0.005
y 0.994 0.000
z 0.910 0.004
#22
x 0.978 0.000
y 0.993 0.000
z 0.997 0.000
#23
x 0.912 0.003
y 0.989 0.000
z 0.993 0.000
#25
x 0.958 0.001
y 0.998 0.000
z 0.968 0.000
#26
x 0.931 0.002
y 0.997 0.000
z 0.991 0.000
#27
x 0.987 0.000
y 0.995 0.000
z 0.998 0.000

Reference points were digitized 3 times with a 2-week interval. Intraexaminer reliability was obtained with the 1-way random effects model. ICC values were significantly different from 0: P <0.05; P <0.01; P <0.001.
#11, the upper right central incisor; #12, the upper right lateral incisor; #13, the upper right canine; #15, the upper right second premolar; #16, the upper right first molar; #17, the upper right second molar; #21, the upper left central incisor; #22, the upper left lateral incisor; #23, the upper left canine; #25, the upper left second premolar; #26, the upper left first molar; #27, the upper left second molar.

Because there was no significant difference in the individual tooth movements between the right and left sides between T0 and T1 ( P >0.05, Table II ), the data from the 2 sides were merged. Arch dimensions were measured with the 3Txer program. The definitions of arch width and depth are described in Figure 8 .

Table II
Comparison of data between the right and left sides
Variable T0 T0 – T1
Right side Left side P value Right side Left side P value
Mean SD Mean SD Mean SD Mean SD
Inclination (°)
U1 67.87 5.05 68.86 6.01 0.5471 −11.68 5.17 −12.83 4.93 0.4450
U2 76.02 5.97 78.63 5.00 0.1157 −6.42 5.66 −5.22 5.99 0.4889
U3 83.69 4.85 83.21 4.92 0.7373 2.17 5.97 1.91 6.62 0.8925
U5 79.97 6.26 81.37 6.42 0.4564 −0.57 6.17 −0.17 5.50 0.8177
U6 80.22 6.65 79.96 5.51 0.8851 2.05 5.04 1.22 4.88 0.5737
U7 83.86 4.49 84.03 3.51 0.8837 2.56 3.76 2.47 4.03 0.9343
Angulation (°)
U1 90.92 0.71 91.55 1.34 0.0533 0.00 1.03 0.33 2.01 0.4845
U2 91.30 1.34 91.56 1.09 0.4703 0.16 1.56 −0.43 1.63 0.2126
U3 91.87 1.65 92.03 1.61 0.7382 −0.50 1.77 −0.55 1.93 0.9199
U5 91.35 1.02 91.78 1.10 0.1769 0.00 1.02 0.21 1.84 0.6447
U6 93.80 2.01 93.96 2.04 0.7859 0.22 2.10 0.11 1.91 0.8603
U7 93.23 1.80 93.66 2.06 0.4505 −1.43 2.50 −1.52 2.97 0.9154
Rotation (°)
U1 96.31 4.80 96.32 3.29 0.9929 −5.45 5.62 −4.65 2.86 0.5472
U2 114.47 10.85 115.36 7.54 0.7482 −6.96 10.95 −6.49 5.57 0.8560
U3 145.57 6.59 146.78 7.63 0.5674 −0.53 6.56 0.12 7.46 0.7553
U5 160.05 6.31 162.21 4.25 0.1812 5.21 5.73 6.14 4.36 0.5406
U6 169.62 2.71 168.42 3.84 0.2282 4.36 2.95 3.75 3.01 0.4947
U7 173.08 4.00 171.86 3.69 0.2906 4.41 2.48 3.50 2.53 0.2254
Vertical displacement (mm)
U1 31.31 5.30 31.43 5.08 0.9383 −2.93 1.68 −2.94 1.57 0.9785
U2 30.39 5.16 30.27 4.94 0.9361 −2.28 1.64 −2.25 1.48 0.9491
U3 29.24 5.00 29.42 4.62 0.9026 −1.72 1.50 −1.71 1.48 0.9843
U5 27.45 4.90 27.22 4.48 0.8657 0.75 0.91 1.12 1.22 0.2595
U6 26.02 5.05 25.48 5.01 0.7135 0.15 0.81 0.19 1.11 0.8757
U7 25.23 5.77 24.78 5.90 0.7950 −0.43 0.83 −0.38 1.17 0.8509
Anteroposterior
displacement (mm)
U1 40.96 4.13 42.57 3.61 0.1673 5.48 1.23 5.35 1.12 0.7174
U2 37.80 3.40 38.47 3.08 0.4865 5.33 1.39 5.04 1.45 0.4828
U3 33.18 2.59 33.16 2.41 0.9762 5.64 1.06 5.40 1.42 0.5323
U5 19.09 1.53 18.90 1.23 0.6439 −1.10 0.94 −1.59 1.23 0.1367
U6 10.12 0.91 10.29 1.08 0.5656 −1.16 0.92 −1.52 1.85 0.4066
U7 0.00 0.00 0.00 0.00 NA −1.04 0.97 −1.26 0.89 0.4366
Lateral displacement (mm)
U1 3.37 1.40 2.97 1.59 0.3663 −0.76 0.84 −0.55 0.39 0.2913
U2 11.00 2.11 9.99 2.71 0.1634 −1.75 1.18 −1.57 1.04 0.5780
U3 17.80 2.11 16.69 3.11 0.1611 −2.90 1.38 −2.25 1.22 0.0967
U5 23.33 2.34 22.65 2.75 0.3725 1.40 2.02 1.34 0.86 0.8874
U6 26.06 2.07 26.01 2.36 0.9390 1.14 0.97 1.49 1.11 0.2727
U7 29.30 1.85 29.19 2.54 0.8740 1.16 1.14 1.28 1.18 0.7241

NA, Not applicable; U1, the upper central incisor; U2, the upper lateral incisor; U3, the upper canine; U5, the upper second premolar; U6, the upper first molar; U7, the upper second molar.

Fig 8
Arch dimensions: A , arch width; B , arch depth. ICW , intercanine width (distance between the cusp tip of the right and left canines); IP2W, intersecond premolar width (distance between the cusp tip of the right and left second premolars); IM1W , interfirst molar width (distance between the mesiobuccal cusp tip of right and left first molars); IM2W , intersecond molar width (distance between the mesiobuccal cusp tip of the right and left second molars); CD , canine depth (shortest distance from a line connecting the cusp tip of the right and left canines to the contact point between the right and left central incisors); MD , molar depth (shortest distance from a line connecting the mesiobuccal cusp tip of the right and left first molars to the contact point between the right and left central incisors).

To compare the amounts of change in each tooth between T0 and T1, the movement patterns in the adjacent teeth (U1 vs U2, U3 vs U5, and U6 vs U7) and the arch dimensions, the Wilcoxon signed rank and Mann-Whitney tests were used.

Results

We compared the changes in each tooth between T0 and T1. U1 and U2 showed a similar tendency of change as follows: significantly inclined lingually (12.3°, P <0.001; 5.8°, P <0.001, Table III ), rotated distally (5.1°, P <0.001; 6.7°, P <0.001, Table III ), extruded (2.9 mm, P <0.001; 2.3 mm, P <0.001, Table IV ), moved backward (5.4 mm, P <0.001; 5.2 mm, P <0.001, Table IV ), and moved laterally (0.7 mm, P <0.001; 1.7 mm, P <0.001, Table IV ). There was no significant change in angulation of U1 and U2 ( Table III ).

Table III
Comparison of angular variables between T0 and T1 in each tooth
Angular variable (°) T0 T1 P value T0 – T1 Power
(1-β)
Mean SD Mean SD Mean SD
Inclination
U1 68.37 5.51 80.63 5.39 0.0000 −12.26 5.03 0.999
U2 77.33 5.61 83.15 4.16 0.0000 −5.82 5.79 0.999
U3 83.45 4.84 81.41 4.81 0.0160 2.04 6.23 0.683
U5 80.67 6.31 81.04 6.29 0.8698 −0.37 5.78 0.136
U6 80.09 6.04 78.46 6.34 0.0706 1.63 4.93 0.412
U7 83.94 3.98 81.43 4.33 0.0002 2.51 3.85 0.844
Angulation
U1 91.24 1.11 91.07 1.13 0.3488 0.17 1.59 0.048
U2 91.43 1.21 91.56 1.27 0.6780 −0.13 1.61 0.072
U3 91.95 1.61 92.47 1.34 0.0595 −0.52 1.83 0.282
U5 91.57 1.07 91.46 1.22 0.7764 0.10 1.47 0.049
U6 93.88 2.01 93.72 1.89 0.6038 0.17 1.99 0.059
U7 93.45 1.93 94.92 2.92 0.0010 −1.47 2.71 0.048
Rotation
U1 96.32 4.07 101.37 3.68 0.0000 −5.05 4.43 0.998
U2 114.92 9.25 121.65 4.31 0.0000 −6.73 8.59 0.801
U3 146.17 7.08 146.38 9.16 0.8058 −0.21 6.96 0.07
U5 161.13 5.43 155.46 6.06 0.0000 5.67 5.06 0.993
U6 169.02 3.34 164.97 3.85 0.0000 4.05 2.97 0.12
U7 172.47 3.85 168.51 4.06 0.0000 3.96 2.52 0.292
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Apr 13, 2017 | Posted by in Orthodontics | Comments Off on Three-dimensional analysis of the tooth movement and arch dimension changes in Class I malocclusions treated with first premolar extractions: A guideline for virtual treatment planning
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