Introduction
The objective of this study was to systematically evaluate cortical bone remodeling and mandibular incisor movement during presurgical orthodontic decompensation in patients with severe skeletal Class III malocclusion and explore more suitable indicators for determining the decompensation range.
Methods
Cone-beam computed tomography images of 46 patients with severe skeletal Class III malocclusion who underwent presurgical orthodontic treatment were analyzed. Vertical bone loss (sagittal view) was measured, and cortical bone remodeling and mandibular tooth movement (coronal view) were assessed at the crestal, midroot, and apical levels. Patients were grouped by their degree of alveolar bone deterioration. Mandibular incisor inclination angle (the angle between the long axis of the mandibular central incisor [MCI] and the long axis of the mandibular symphysis) and incisor mandibular plane angle (the angle between the long axis of the MCI and the mandibular plane) were compared between groups using t tests.
Results
MCIs exhibited tipping movement, with prominently increased lingual alveolar bone defects after decompensation. The mean mandibular incisor inclination angles were as follows: (1) pretreatment was 4.45° for group 1 (with better alveolar bone conditions) and 2.99° for group 2 (with worse alveolar bone conditions) ( P = 0.252), and (2) posttreatment (after decompensation) was 10.91° for group 1 and 19.31° for group 2 ( P <0.001). The mean incisor mandibular plane angles were as follows: (1) pretreatment was 77.63° for group 1 and 71.64° for group 2 ( P <0.05), and (2) posttreatment was 85.95° for group 1 and 84.84° for group 2 ( P = 0.624).
Conclusions
Instead of decompensating until the mandibular incisors are nearly upright relative to the mandibular plane, the relationship with the mandibular symphysis should be prioritized for healthier considerations.
Highlights
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•An indicator was introduced to supplement the incisor mandibular plane angle.
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•Cortical bone remodeling was evaluated around mandibular incisors.
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•The incidence of alveolar bone defects was prominently increased at the lingual surface.
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•A safe decompensation range for mandibular incisors was explored.
Class III malocclusion is a common type of orthodontic malocclusion that is often associated with dentomaxillofacial deformities as well as a concave profile. , Patients with severe skeletal Class III malocclusion are more willing to seek surgical orthodontic treatment because of the potential for esthetic improvement and social pressure. During presurgical orthodontic treatment, a decompensation procedure is meticulously performed. Specifically, the maxillary incisors are gently retracted, whereas the mandibular incisors are gradually proclined.
The incisor mandibular plane angle (IMPA) (the angle between the long axis of the mandibular central incisor [MCI] and mandibular plane) is traditionally used to guide preoperative mandibular incisor decompensation treatment. Previous studies have emphasized that sufficient presurgical mandibular incisor decompensation is conducive to tooth stability and postoperative maintenance of skeletal stability among patients with severe skeletal Class III malocclusion who undergo combined orthodontic and orthognathic treatment; more specifically, these studies suggest that the mandibular incisor should stand upright in the mandible. , However, the mandibular anterior alveolar bone is generally thinner in patients with severe skeletal Class III malocclusion than in patients with normal occlusion. ,, The mandibular anterior region in patients with skeletal Class III malocclusion often provides a very limited bone volume. , Complete decompensation of mandibular incisors based on the IMPA significantly increases the risk of fenestration and dehiscence, accompanied by gingival recession and the risk of other unacceptable side effects. ,,
The IMPA can reflect only the mandibular incisor inclination with respect to the mandibular plane. In previous studies, the concept of the mandibular incisor inclination angle (MIA) was defined as the angle between the long axis of the mandibular symphysis and the long axis of the MCI; this indicator reflects the mandibular incisor inclination with respect to the mandibular symphysis. , Compared with the IMPA, whether mandibular incisors stand upright in the anterior alveolar bone might be more crucial for stable and healthy treatment outcomes.
Indicators are necessary to guide the mandibular incisor decompensation range during presurgical orthodontic treatment, thus meeting the requirements of orthognathic surgery and ensuring adequate periodontal support for tooth conservation to achieve healthy orthodontic treatment. Previous studies have extensively investigated alveolar bone changes around mandibular anterior teeth and their movement patterns during orthodontic decompensation in patients with skeletal Class III malocclusion. Key factors influencing alveolar bone thickness and height included the mandibular plane angle, treatment duration, and inclination of the mandibular anterior teeth. However, the safe range for pretreatment decompensation remains inadequately defined, warranting further clarification in clinical practice. ,,
In this study, we investigated mandibular incisor movement and cortical bone remodeling (BR) around incisors by analyzing pre- (T0) and postdecompensation (presurgical) (T1) cone-beam computed tomography (CBCT) images in patients with severe skeletal Class III malocclusion. More importantly, a comparative analysis of MIA and IMPA measurements pre and posttreatment in different alveolar bone condition groups (superior vs inferior) was performed. The aim was to better determine the safe range of mandibular incisor decompensation based on the alveolar bone condition so that preemptive measures, such as bone grafting, could be planned to reduce the risk of periodontal health issues. The null hypothesis of this study was that no statistically significant differences in MIA occurred before and after decompensation treatment between groups with different alveolar bone conditions.
Material and methods
This retrospective study was approved by the institutional ethics committee (Approval No. 2022-Beijing Municipal Science-26).
The inclusion criteria were as follows: aged >18 years, permanent dentition, a severe skeletal and dental Class III malocclusion (ANB ≤–4°, Wits appraisals ≤–10 mm, Holdaway angle <10.3°, reverse overjet ≥3 mm, and the molar relationship was fully Class III or more severe at pretreatment), mild crowding (<4 mm) in the mandibular dental arch and crowding in the anterior teeth area <2 mm, initial rotation degree of the MCIs <15°, no extraction in the mandibular dental arch, and adequate quality CBCT images available to accurately define the cortical bone and root border. All patients were treated with a preadjusted edgewise appliance (0.022-inch slot size, McLaughlin Bennett Trevisi prescription) during presurgical orthodontic treatment. The mandibular arch was sequentially leveled and aligned with 0.014-in, 0.016-in, 0.018-in, and 0.018 × 0.025-in nickel-titanium wires, followed by a 0.018 × 0.025-in stainless steel wire. No additional torque was incorporated into the archwires during treatment.
The exclusion criteria were as follows: severe facial asymmetry (>3 mm of chin point deviation from the facial midline), poor oral hygiene and periodontal disease (plaque score >20%, sites with bleeding on probing ≥10%, and probing pocket depths >3 mm), missing or decayed teeth before treatment (except for the third molars), and a root length (the distance from the cementoenamel junction [CEJ] to apex) of <9 mm on CBCT images.
T0 and T1 CBCT images (acquired by a New Tom VG CBCT scanner [Aperio Services, Verona, Italy] with the same scanning parameters of 110 kV, 1.0-3.0 mA, 15 × 15 cm field of view, 3.6 second exposure time, and 0.30 mm voxels) of all patients were obtained with the same position and posture according to the guidelines of the manufacturer. In addition, all lateral cephalograms were obtained from the same cephalostat.
T1 CBCT data were imported into Dolphin Imaging (version 11.95; Dolphin Imaging & Management Solutions, Chatsworth, Calif) in digital imaging and communication in medical format. The head position was adjusted as follows: the horizontal plane was positioned passing through the gonion (Go) on both sides and gnathion (Gn) and the sagittal plane was positioned passing through the sella (S), nasion (N), and basion (Ba) ( Fig 1 , A ).
A-E , CBCT data processing using Dolphin imaging software: A, Head position; B, Results of the superimposition of the CBCT scans ( green , T0; white , T1); C-E, Voxel overlap: C, Sagittal slice; D, Coronal slice; E, Axial slice; F and G, Illustration of the measurement levels ( red , T0 images; green , T1 images) (T1 tooth axis, the line from the midpoint of the CEJ to the apical point; S1, S2, and S3: 3, 6, and 9 mm apical to the CEJ along the long axis of the T1 incisor). H-L, CBCT data processing using Procreate: H, T0 axial slice; I, T0 sketch: buccolingual border of the alveolar bone and the root contour; J, Merged image of the T0 sketch and T1 axial slice ( blue , the line passing through the T0 and T1 root center points [all the measurements are based on this line]); K, Illustration of TM: distance between the T0 and T1 root borders on the labial or lingual sides; L, Illustration of cortical BR: Distance between the T0 and T1 alveolar bone borders on the labial or lingual sides. If the alveolar bone covers the tooth root, the actual cortical bone distance is measured before and after treatment; if a bone defect is observed, the alveolar bone landmark is the intersection of the measurement reference line and the line passing through the highest point of the mesial and distal alveolar bone around the root ( black , the line passing through the highest point of the mesial and distal alveolar bone around the root). M-P, Example and illustrations of VBL measurements: M, Sagittal slice; N, Coronal slice; O, Axial slice; P, VBL-La: the vertical linear distance from the CEJ to the incisor alveolar bone crests parallel to the root length on the labial side, and VBL-L: the vertical linear distance from the CEJ to the incisor alveolar bone crest parallel to the root length on the lingual side.
CBCT images at T0 and T1 were superimposed using voxel overlap in the Dolphin Imaging software. Voxel overlap was performed with the chin, mandibular body, and basal bone of the mandibular incisors as the main overlapping areas, which were identified as stable areas in adults who underwent orthodontic treatment ( Fig 1 , B – E ). ,,, The MCIs on both sides were selected for measurement. In the superimposed sagittal view, T0 and T1 axial images were oriented along the long axis of the T1 MCIs at the crestal (S1), midroot (S2), and apical (S3) levels (3, 6, and 9 mm apical to the CEJ) ( Fig 1 , F – G ). Afterward, axial T0 and T1 images were exported with a scale ruler.
The axial T0 and T1 CBCT images were imported into Procreate (version 4.3; Savage Interactive, Hobart, Tasmania, Australia). In the T0 image, the buccolingual border of the alveolar bone and the root contour are depicted ( Fig 1 , H ). The T0 line ( Fig 1 , I ) overlapped with the T1 image ( Fig 1 , J ).
Cortical BR and root movement (TM) measurements at the S1, S2, and S3 levels were acquired from the superimposed images using ImageJ (version 1.48; National Institutes of Health and the Laboratory for Optical and Computational Instrumentation, University of Wisconsin, Madison, Wis) ( Fig 1 , K – L ). All measurements were performed along the reference line that passed through the centers of the incisors’ roots on T0 and T1 images. The definitions of the measured items are shown in Table I . The distances between the T0 and T1 cortical bone, as well as the root borders, were defined as the amount of BR and tooth movement, respectively. The BR/TM value represents cortical BR at different sites of the MCI region. When the direction of alveolar BR was opposite to the direction of tooth movement, the BR/TM value became negative.
Table I
Definitions of CBCT measurements
| Measurements | Definition | |
|---|---|---|
| Cortical BR (mm) | BR (S1) | Distance between T0 and T1 alveolar bone border on the labial or lingual sides at 3 mm apical from CEJ |
| BR (S2) | Distance between T0 and T1 alveolar bone border on the labial or lingual sides at 6 mm apical from CEJ | |
| BR (S3) | Distance between T0 and T1 alveolar bone border on the labial or lingual sides at 9 mm apical from CEJ | |
| TM (mm) | TM (S1) | Distance between T0 and T1 root border on the labial or lingual sides at 3 mm apical from CEJ |
| TM (S2) | Distance between T0 and T1 root border on the labial or lingual sides at 6 mm apical from CEJ | |
| TM (S3) | Distance between T0 and T1 root border on the labial or lingual sides at 9 mm apical from CEJ | |
| The ratio of cortical BR to TM | BR/TM value (S1) | BR (S1) divided by TM (S1) |
| BR/TM value (S2) | BR (S2) divided by TM (S2) | |
| BR/TM value (S3) | BR (S3) divided by TM (S3) | |
| VBL (mm) | VBL-La | VBL on the labial side of mandibular incisor |
| VBL-L | VBL on the lingual side of incisor |
The cortical BR was evaluated not only in the axial view but also in the sagittal view. We focused on vertical bone loss (VBL) after decompensation. Measurements were acquired from sagittal CBCT slices in which mandibular incisors were the widest labiolingually in the axial view, and the images were adjusted along the long axis of the root, as shown in Figure 1 , M – P . The vertical linear distance from the CEJ to the incisor alveolar bone crests parallel to the root length was measured as the VBL.
According to Sheng et al and Yagci et al, , the diagnostic criteria for alveolar bone defects in CBCT images were defined as the discontinuity or absence of cortical bone around the root in at least 3 consecutive axial and sagittal slices. Based on these criteria and the characteristics of fenestration and dehiscence on the superimposed axial CBCT images, we classified the patients into 2 groups according to the extent of new lingual bone defects detected after decompensation.
Group 1: The number of observed levels (S1, S2, and S3) with new lingual bone fenestration or dehiscence at T1 was ≤1. This group included patients whose root maintained adequate alveolar bone coverage at all 3 levels (S1, S2, and S3) before and after treatment, as well as patients who exhibited only 1 level of cortical bone discontinuity or absence (as per the diagnostic criteria) after treatment.
Group 2: The number of observed levels (S1, S2, and S3) with new lingual bone fenestration or dehiscence at T1 was ≥2, indicating that defects appeared at minimally 2 of the 3 levels after presurgical treatment.
Then, we measured the MIA ( Fig 2 ) in both groups using CBCT, namely, the angle between the long axis of the mandibular symphysis and the long axis of the MCI. The MIA reflects the inclination of the mandibular symphysis and the MCIs. The method used to measure the MIA has been described previously. , Tweed reported that the inclination of MCIs could be represented by the angle between the axis of the mandibular incisor and the mandibular plane (IMPA), , which is usually used to guide the range of mandibular incisor decompensation during presurgical orthodontic treatment. We performed comprehensive comparative studies of the MIA and IMPA before and after decompensation between groups 1 and 2 to evaluate whether the MIA was effective in determining a safe decompensation range.
Examples and illustrations of the MIA based on CBCT images. IA, long axis of the MCI; D, center point of the mandibular symphysis; dotted line , the line between the labial and lingual alveolar edge of the MCI; MA, the line passing through point D and the midpoint of the dotted line; MIA, the angle between the IA and the MA ( yellow , mandibular symphysis).
The minimum sample size was calculated based on the difference in the changes in the MIA and IMPA between T0 and T1, as determined by a power analysis (α = 0.05; power = 80%) using GPower (version 3.1.9.7; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). For the preliminary analysis, 10 patients were randomly selected from each group for measurement. The results revealed that the mean difference in the ΔMIA was 8.1° in group 1 and 13.8° in group 2, with a standard deviation of 6.1°. Similarly, the mean difference in ΔIMPA was 9.0° in group 1 and 13.1° in group 2, with a standard deviation of 4.1°. Based on these parameters, the power analysis indicated that a minimum of 18 patients per group would be needed to detect statistically significant differences between the groups.
T0 and T1 lateral cephalograms were imported into Dolphin Imaging software. The parameters included in the cephalometric measurement included SNA, SNB, ANB, Wits value, FH/NPo, NA/APo, U1/NA, U1-NA, L1/NB, L1-NB, U1-L1, U1-SN, MP-SN, MP-FH, L1-MP, y-axis, and Pog-NB.
Statistical analysis
All the statistical analyses were performed using SPSS (version 26.0; IBM, Armonk, NY). One investigator performed repeated measurements for 20 random samples at an interval of 2 weeks. Another investigator measured 30 samples at an interval of 2 weeks to perform interexaminer calibration. The intraclass correlation coefficient was calculated to determine the intra and interobserver reliability of the measurements. The examiners were blinded to both the timepoints (T0 and T1) and the group allocations during the measurements to prevent bias. All reported measurements exhibited intraclass correlation coefficients between 0.80-0.99 (excellent), confirming high consistency. The averages of these 2 measurements were used for statistical analyses.
The Shapiro-Wilk test was used to determine that the data were normally distributed. However, the variance in the S1 BR/TM values was not uniform and the Friedman test and pairwise comparisons were performed to compare the labial and lingual BR/TM (cortical BR/TM) values among the S1, S2, and S3 levels. The difference between the labial and lingual BR/TM values was analyzed with a paired-samples t test. Differences in the cephalometric variables and VBL between T0 and T1 were tested with paired-samples t tests. Independent-samples t tests were applied to compare MIA and IMPA between groups 1 and 2. Multivariate linear regression analysis was performed to assess the relationships between VBL and potentially related factors. For all tests, the significance level was set at P <0.05.
Results
Based on the comprehensive inclusion and exclusion criteria and a predefined sample size calculation, 46 patients with severe skeletal Class III malocclusion (22 men and 24 women; mean age, 23.07 ± 3.78 years) who underwent presurgical orthodontic treatment were included in this retrospective study, as detailed in the flow diagram ( Supplementary Fig ).
Paired-samples t tests revealed that L1-NB, L1/NB, and IMPA significantly increased and that U1-NA, U1/NA, and U1-SN significantly decreased, indicating that maxillary central incisors were lingually inclined and that MCIs were labially inclined during the presurgical decompensation procedure ( Supplementary Table I ).
MCIs moved in a tipping manner such that the incisal edge proclined and the apex retroclined for decompensation. As shown in Table II and Supplementary Table II , 76.7% of the MCIs exhibited lingual movement at the 3 levels. With increasing proximity to the root apex, MCIs exhibited greater lingual movement. The cortical bone was remodeled in the same direction as the MCIs moved. As shown in Table II , the mean BR/TM value was <1.00, especially on the lingual side, which indicated that the remodeling of the cortical bone was less than the amount of MCI movement. In most patients with root lingual movement during decompensation, the labial alveolar bone thickness increased, whereas the alveolar bone on the lingual side became thinner.
Table II
CBCT measurements of cortical BR and TM (mean ± standard deviation)
| Labial | Lingual | |||||
|---|---|---|---|---|---|---|
| BR (mm) | TM (mm) | BR/TM value | BR (mm) | TM (mm) | BR/TM value | |
| T31 (n = 46) | ||||||
| S1 | 0.96 ± 0.67 | 1.18 ± 0.91 | 0.95 ± 0.62 | 0.66 ± 0.73 | 1.14 ± 0.86 | 0.53 ± 0.48 |
| S2 | 1.23 ± 0.73 | 1.68 ± 0.95 | 0.77 ± 0.32 | 0.69 ± 0.69 | 1.66 ± 0.97 | 0.35 ± 0.28 |
| S3 | 1.05 ± 0.64 | 2.18 ± 1.05 | 0.49 ± 0.21 | 0.68 ± 0.60 | 2.16 ± 1.09 | 0.31 ± 0.22 |
| T41 (n = 46) | ||||||
| S1 | 0.94 ± 0.75 | 1.05 ± 0.77 | 0.83 ± 0.75 | 0.60 ± 0.77 | 1.05 ± 0.85 | 0.56 ± 0.58 |
| S2 | 1.15 ± 0.74 | 1.55 ± 0.96 | 0.79 ± 0.37 | 0.66 ± 0.62 | 1.60 ± 1.01 | 0.39 ± 0.28 |
| S3 | 0.99 ± 0.51 | 2.07 ± 1.01 | 0.53 ± 0.25 | 0.56 ± 0.59 | 2.01 ± 1.02 | 0.24 ± 0.23 |
T31, mandibular left central incisor; T41, mandibular right central incisor.
Table III
Comparison of BR/TM values among different levels of MCIs on the labial and lingual sides
| Labial | Lingual | |||
|---|---|---|---|---|
| Levels | P value | Levels | P value | |
| T31 | S1 vs S2 | 1.000 | S1 vs S2 | 0.077 |
| S1 vs S3 | 0.000 | S1 vs S3 | 0.000 | |
| S2 vs S3 | 0.000 | S2 vs S3 | 0.077 | |
| T41 | S1 vs S2 | 0.405 | S1 vs S2 | 0.298 |
| S1 vs S3 | 0.000 | S1 vs S3 | 0.000 | |
| S2 vs S3 | 0.011 | S2 vs S3 | 0.013 |
T31, mandibular left central incisor; T41, mandibular right central incisor.
The Friedman test revealed that the BR/TM value at the S1 level was significantly greater than that at the S3 level ( P <0.01). The root apical (S3) level showed the weakest BR ability with the movement of teeth, whereas BR was obviously more active at the root cervicogram (S1) level ( Table III ). The paired samples t test indicated that the labial BR/TM values were significantly greater than the lingual BR/TM values at the S2 and S3 levels ( P <0.05) ( Table IV ). The lingual cortical border showed weak BR capacity, indicating that it was less affected by TM. The measurement of VBL in the sagittal slices also revealed this characteristic. The VBL–labial and VBL–lingual values increased significantly ( P <0.001) from T0 to T1. The amount of bone loss was significantly greater at the lingual alveolar plate, especially after decompensation ( Table V ).
Table IV
Comparison of BR/TM values between the labial and lingual sides at different levels
| Levels | t value | P value | |
|---|---|---|---|
| T31 | S1 | 1.580 | 0.128 |
| S2 | 4.924 | 0.000 | |
| S3 | 2.465 | 0.027 | |
| T41 | S1 | 0.807 | 0.427 |
| S2 | 3.510 | 0.002 | |
| S3 | 3.921 | 0.001 |
T31, mandibular left central incisor; T41, mandibular right central incisor.
Table V
Vertical alveolar bone loss on the labial and lingual side from midsagittal slices of MCIs
| T31 | T41 | |||||
|---|---|---|---|---|---|---|
| T0 | T1 | P value | T0 | T1 | P value | |
| VBL-La (mm) | 2.19 ± 0.53 | 3.11 ± 1.09 | 0.000 | 2.12 ± 0.63 | 2.85 ± 0.99 | 0.000 |
| VBL-L (mm) | 2.51 ± 0.99 | 6.36 ± 2.63 | 0.000 | 2.64 ± 1.08 | 6.24 ± 2.65 | 0.000 |
| P value | 0.018 | 0.000 | 0.001 | 0.000 | ||
T31, mandibular left central incisor; T41, mandibular right central incisor; VBL-La, vertical alveolar bone loss on the labial side; VBL-L, vertical alveolar bone loss on the lingual side.
We evaluated bone defects on superimposed axial CBCT images and reported that the incidences of fenestration and dehiscence on the labial side did not increase substantially but did increase significantly on the lingual side after presurgical orthodontic treatment ( Supplementary Table III ). A considerable risk of alveolar bone deterioration was observed on the lingual side. The patients were classified into 2 groups according to the number of levels (S1, S2, and S3) in which new fenestration and dehiscence were detected after treatment based on the superimposed axial CBCT images. The bone defects were more severe in group 2. The differences in the MIA and IMPA before and after decompensation between groups 1 (n = 24) and 2 (n = 22) are shown in Table VI .
Table VI
Comparison of the MIA and IMPA before and after decompensation between groups 1 and 2
| The levels with new fenestration and dehiscence (L) | Group 1 | Group 2 | P value | |||
|---|---|---|---|---|---|---|
|
L = 0 (n = 15)
L = 1 (n = 9) |
95% CI |
L = 2 (n = 18)
L = 3 (n = 4) |
95% CI | |||
| MP-SN (°) | 35.18 ± 6.37 | 36.49 ± 7.64 | 0.526 | |||
| ANB (°) | –5.23 ± 1.93 | –5.71 ± 2.28 | 0.445 | |||
| Age | 23.75 ± 3.79 | 22.32 ± 3.71 | 0.203 | |||
| Sex (male/female) | 12/12 | 10/12 | 0.758 | |||
| T0 | MIA (°) | 4.45 ± 3.85 | 2.82-6.82 | 2.99 ± 4.71 | 0.90-5.08 | 0.252 |
| T1 | MIA (°) | 10.91 ± 5.63 | 8.53-13.28 | 19.31 ± 6.05 | 16.62-21.99 | 0.000 |
| ΔMIA (°) [T1MIA (°)- T0MIA (°)] | 6.45 ± 5.28 | 4.22-8.67 | 16.32 ± 5.48 | 13.89-18.75 | 0.000 | |
| T0 | IMPA (°) | 77.63 ± 6.87 | 74.72-80.53 | 71.64 ± 6.97 | 68.55-74.73 | 0.005 |
| T1 | IMPA (°) | 85.95 ± 7.87 | 82.62-89.27 | 84.84 ± 7.44 | 81.54-88.13 | 0.624 |
| ΔIMPA (°) [T1IMPA (°)- T0IMPA (°)] | 8.33 ± 3.90 | 6.67-9.97 | 13.19 ± 4.91 | 11.02-15.37 | 0.001 | |
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