Introduction
The purposes of this research were to identify the buccolingual inclinations of the mandibular teeth and the mandibular symphysis remodeling that result from the orthodontic decompensation movement.
Methods
The sample consisted of 30 adults with Class III dentofacial deformity, who had presurgical orthodontic treatment. Three-dimensional images were generated by cone-beam computed tomography scans at 2 different times (initial and before orthognathic surgery). Three-dimensional virtual models were obtained and superimposed using automated voxel-based registration at the mandible to evaluate B-point displacement, mandibular molar and incisor decompensation movement, and symphysis inclination and thickness. The 3-dimensional displacements of landmarks at the symphysis were quantified and visualized with color-coded maps using 3D Slicer (version 4.0; www.slicer.org ) software.
Results
The measurements showed high reproducibility. The patients presented mandibular incisor proclination, which was consistent with the movement of tooth decompensation caused by the presurgical orthodontic treatment. Statistically significant correlations were found between the inclination of the mandibular incisors, symphysis inclination, and B-point displacement. Regarding the thickness of the symphysis and the inclination of the incisors, no statistically significant correlation was found.
Conclusions
The buccolingual orthodontic movement of the mandibular incisors with presurgical leveling is correlated with the inclination of the mandibular symphysis and repositioning of the B-point but not correlated to the thickness of the symphysis.
Highlights
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We compared mandibular incisor inclination and mandibular symphysis remodeling at 2 times.
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Incisor proclination was observed, consistent with presurgical orthodontic treatment.
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Incisor inclination, symphysis inclination, and B-point displacement were correlated.
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The thickness of the symphysis and inclination of the incisors were not correlated.
Class III dentofacial deformities are characterized by a maxillomandibular discrepancy, which can be treated by orthognathic surgery or, in less severe cases, by orthodontic compensation. The skeletal etiology of these conditions may include retruded position and/or deficient size of the maxillary jaw, protruded position and/or large size of the mandibular jaw, or a combination of both. The dentoalveolar characteristics of these conditions often present variable degrees of compensation that maintain occlusal function and mask the underlying skeletal discrepancy. Typically, the maxillary incisors are proclined, and the mandibular incisors are retroinclined. ,
The conventional orthodontic-surgical treatment of Class III dentofacial deformities consists of 3 stages: presurgical orthodontics, surgery, and postsurgical orthodontics. Preoperative orthodontics in patients with Class III dentofacial deformities aim to decompensate the inclination of the maxillary and mandibular incisors, obtaining adequate dental inclinations in their respective bone bases. The presurgical orthodontic phase influences the magnitude of the movements obtained at surgery because the occlusion is used as a surgical guide. Therefore, the decompensation of the incisors is one of the main contributory factors for the overall esthetic and functional result.
The envelope of tooth movement for achieving adequate decompensation is often complicated by neuromuscular function, occlusion, periodontal health, and thickness of the mandibular symphysis. The morphology of the mandibular symphysis is a complex phenotype that results from the interaction of different genetic, adaptive, and environmental factors. The size and shape of the mandibular symphysis are important in the assessment of orthodontic patients. With a wider symphysis, greater protrusion of the incisors is acceptable. However, a more elongated and narrow symphysis is generally associated with protrusion of the chin and increased lower anterior facial height. , Patients with Class III malocclusions and increased vertical dimensions have predominantly narrow mandibular symphysis, with less alveolar bone at vestibular and lingual cortices of the mandibular incisors. , In these patients, pronounced sagittal movement of the incisors is a critical factor for progressive buccal and lingual bone loss. , , The symphysis morphology serves as the primary reference for facial profile esthetics, as it determines the planning of the mandibular incisor position during orthodontic preparation for orthognathic surgery. , Limiting the orthodontic movement of the incisor within the bone structure is essential for obtaining stable results and periodontal health.
Before the introduction of cone-beam computed tomography (CBCT) in dentistry, buccolingual inclinations of the incisors were measured by lateral cephalograms. , , Because these exams show a 2-dimensional image of 3-dimensional (3D) areas, measurements in the symphysial region are susceptible to intrinsic errors. Errors in 2-dimensional measurements are due to overlap of anatomic structures, difficulties in identifying landmarks, and magnification errors caused by divergence of the radiation beam. With the advent of CBCT, precise evaluations of the dental inclinations and symphysis remodeling can guide the amount of dental decompensation possible in the orthodontic-surgical treatment of patients with Class III dentofacial deformities without possible deleterious effects to the periodontium. , , The 3D superimposition methodology of virtual models for the evaluation of results and treatment stability in Class III patients has been described in the orthodontic literature. The superimposition of stable mandibular structures can be used for growth, treatment, and stability assessment. Three-dimensional images can be superimposed or registered using thousands of points, shapes, or volumes allowing the evaluation based on the differences obtained directly in these images.
The superimposition of 3D models and measurements of the distances between surfaces at different times can identify and quantify the values and the direction of changes.
The envelope of the limits of planned orthodontic movement is an important factor to be considered in all orthodontic treatments, particularly in cases of dental decompensation before orthognathic surgery. To date, there are no 3D studies describing the alteration on symphysis remodeling after mandibular incisor decompensation in preparation for orthognathic surgery using 3D superimposition of virtual models. This study’s objective was to evaluate the 3D presurgical orthodontic changes in the mandibular incisors’ inclination and its relation to the symphysis remodeling. The null hypothesis was that dentoalveolar decompensation changes with presurgical orthodontic leveling are not correlated to the mandibular symphysis remodeling.
Material and methods
The study sample consisted of 30 adult patients (mean age: 23 years and 4 months) with skeletal Class III malocclusion submitted to presurgical orthodontic treatment. The project was approved by the institutional review board under protocol 1.121.847, and informed consent was obtained from each patient before treatment. The inclusion criteria were skeletal Class III malocclusion characterized by an anterior crossbite or incisor edge-to-edge relationship, Class III molar relationship, and a concave facial profile. All patients had complete permanent dentition with periodontal health (absence of bleeding on probing and probing depths <3 mm), minimal to moderate crowding in the mandibular arch as stated by Little (≤6 mm), indication for orthodontic-surgical treatment, skeletal maturity, no previous orthodontic treatment, no extractions in the mandibular arch, and no local or general contraindications for surgery. Exclusion criteria were cleft lip or palate, missing teeth, previous orthodontic treatment, patients with severe crowding (>6 mm), and patients with both severe deepbite or overclosure with overeruption of mandibular incisors as well as open bite patients so that vertical correction would not lead to heterogeneity of leveling goals.
For sample size calculation, we considered a minimum correlation coefficient of 0.5, with a level of significance of 5% and statistical power of 80%. With these parameters, we reached a minimum sample size of 29 participants.
The patients’ average cephalometric features at baseline were as follows: ANB = −3.78° (±4.07), SNA = 82.18° (±3.44), SNB = 85.92° (±3.72), and IMPA = 81.92° (±8.5). All patients were treated orthodontically using active self-ligating straight-wire bracket system (GAC In-Ovation R, Dentsply GAC, NY) ensuing the following arch sequence: 0.012-in nickel-titanium (NiTi) thermo-activated, 0.016-in NiTi thermo-activated, 0.016 × 0.022-in NiTi thermo-activated, 0.019 × 0.025-in NiTi thermo-activated, and 0.019 × 0.025-in stainless steel. The wire sequence protocol, as well as the change periodicity, was well defined. The preestablished wire change at intervals of 2 months was performed if mechanical targets were obtained. The archwire change protocol took into account the residual deflection and the possibility of introducing the subsequent wire without great difficulties so that the forces of leveling and alignment were light.
CBCT scans were obtained before the beginning of the treatment (T1) and before surgery (T2). Both scans were acquired using a ProMax 3D machine (Planmeca, Helsinki, Finland) with a12.52-second scan time and a 23 × 26-cm field of view, with a voxel dimension of 0.4 mm. The data from each CBCT scan were saved as digital imaging and communications in medicine files. Segmentations of the CBCT volumes were performed using open-source software, ITK-SNAP, version 2.4.0 ( www.itksnap.org ). The initial (T1) and presurgical (T2) 3D models were created, oriented to obtain a common coordinate system, approximated having as reference the best fit of the contours of the mandibular body in multiplanar cross-sections, and superimposed using the automated voxel-based registration on the mandible of the 3D SlicerCMF (version 4.0) software ( www.slicer.org ). These image analysis procedures made possible the evaluation of the changes in symphysis inclination and thickness and the mandibular incisor decompensation movements that resulted from the presurgical orthodontic treatment.
Qualitative assessments of treatment response were visualized using color-coded maps ( Fig 1 ). Distances of corresponding surfaces were graphically displayed by the magnitude of the distance coded by color. Distance maps provided the magnitude of changes between 2 corresponding models ( Fig 1 ).
Quantitative assessments were calculated using and point-to-point landmark identification. Landmarks selected for this study were the following: first molars crown and root, incisal edge and root apex, B-point, pogonion, menton, gonion, pogonion at lingual cortical plate of the symphysis, and B-point at lingual cortical plate of the symphysis ( Supplementary Fig 1 ) ( Table I ). Landmarks were prelabeled at T1 and T2 registered scans and the landmarks were then identified in 3D surface models using the 3D Slicer Q3DC (Quantification of 3D Components) tool (3D Slicer version 4.0). The following linear and angular measurements were calculated: (1) B-point displacement (distance between B-point at T1 and T2 in mm), (2) distance between first molar crowns at T1 and T2 (mm), (3) incisor inclination (pitch)(°), (4) molar inclination (roll)(°), (5) symphysis inclination (angle formed by B-point, pogonion, menton, and midpoint between the gonion) and (6) symphysis thickness (measured at B-point and pogonion in millimeters) ( Supplementary Figs 2 and 3 ).
| Landmarks | Description |
|---|---|
| Right first molar crown | Point located at the tip of the mesiobuccal cusp of the first molar |
| Left first molar crown | |
| Right first molar root | Point located at the apex of the root of the first molar |
| Left first molar root | |
| Right central incisor crown | Point located at the tip of the incisal edge of the central incisor |
| Left central incisor crown | |
| Right central incisor root | Point located at the apex of the root of the central incisor |
| Left central incisor root | |
| B-point | Deepest point of the anterior alveolar process of the mandible |
| Pogonion | Most anterior point of the contour of the mandibular symphysis |
| Menton | Lowest point of the contour of the mandibular symphysis |
| Right gonion | Point determined by the bisector of the angle formed by the mandibular plane and the tangent to the posterior border of the ascending ramus of the mandible |
| Left gonion | |
| Pogonion at lingual cortical | Most posterior point located in the external lingual cortical of mandibular symphysis |
| B-point at lingual cortical | Point corresponding to the B-point demarcated at the lingual cortex of the symphysis |
| Midpoint between the gonion | Point demarcated by the software as the midpoint between the right and left gonion |
The point-to-point measurements are reported as 3D distances and their lateral (x), anteroposterior (y), and vertical (z) components. The 3D landmark point-to-point changes were decomposed into the 3 axes to provide more precise information regarding the number of changes in each direction. For the y-axis, positive values indicated anterior displacement, and negative values indicated posterior displacement. For the z-axis, positive values indicated superior displacement, and negative values indicated inferior displacement.
Statistical analysis
As the Anderson-Darling test determined the normal distribution of the data, parametric tests were used. Descriptive analysis was used to describe the means, standard deviations, and ranges values at T1 and T2 for the following measurements: B-point displacement, incisor and molar inclination, molar crown distances, and symphysis inclination and thickness.
Pearson correlation tests were performed to evaluate the correlations between mandibular incisor inclination and symphysis inclination and thickness, and the correlations between mandibular incisor inclination and B-point displacement. The intraclass correlation coefficients were calculated with the respective 95% confidence intervals to evaluate the systematic error. To compare the measures of thickness of the symphysis between T1 and T2, the paired Student t test was used. The level of significance was set at 5%, and MedCalc was used for statistical analysis (version 18.6; MedCalc Software, Ostend, Belgium).
Results
The descriptive statistics are summarized in Table II .
| Variable | n | Mean | SD | 95% Confidence interval | Minimum | Maximum |
|---|---|---|---|---|---|---|
| B-point displacement (3D) total | 30 | 1.2399 | 0.9771 | 0.8750 to 1.6047 | 0.2530 | 4.8120 |
| B-point displacement (AP) <0 | 22 | −0.6257 | 0.5485 | −0.8689 to −0.3825 | −1.8270 | −0.0150 |
| B-point displacement (AP) ≥0 | 8 | 0.3359 | 0.3048 | 0.08105 to 0.5907 | 0.0000 | 0.8940 |
| B-point displacement (AP) total | 30 | −0.3693 | 0.6537 | −0.6134 to −0.1252 | −1.8270 | 0.8940 |
| B-point displacement (SI) <0 | 8 | −0.4724 | 0.2456 | −0.6777 to −0.2670 | −0.7400 | −0.0580 |
| B-point displacement (SI) ≥0 | 22 | 0.9957 | 1.0564 | 0.5273 to 1.4641 | 0.0380 | 4.3150 |
| B-point displacement (SI) total | 30 | 0.6042 | 1.1219 | 0.1853 to 1.0231 | −0.7400 | 4.3150 |
| Right central incisor pitch <0 | 26 | −8.6980 | 6.4232 | −11.2924 to −6.1036 | −26.9150 | −1.3510 |
| Right central incisor pitch ≥0 | 4 | 2.6862 | 1.7659 | −0.1237 to 5.4962 | 0.9830 | 4.2960 |
| Right central incisor pitch total | 30 | −7.1801 | 7.1681 | −9.8567 to −4.5034 | −26.9150 | 4.2960 |
| Left central incisor pitch <0 | 25 | −10.2263 | 6.9698 | −13.1033 to −7.3493 | −24.6350 | −0.6320 |
| Left central incisor pitch ≥0 | 5 | 2.0058 | 1.1671 | 0.5567 to 3.4549 | 0.2560 | 3.2360 |
| Left central incisor pitch total | 30 | −8.1876 | 7.8669 | −11.1252 to −5.2501 | −24.6350 | 3.2360 |
| Right first molar roll <0 | 19 | 7.1663 | 4.1393 | −9.1614 to −5.1712 | −17.8110 | −1.4310 |
| Right first molar roll ≥0 | 11 | 3.5631 | 1.6176 | 2.4764 to 4.6498 | 0.6880 | 5.8710 |
| Right first molar roll total | 30 | 3.2322 | 6.2604 | −5.5699 to −0.8945 | −17.8110 | 5.8710 |
| Left first molar roll <0 | 9 | 3.0900 | 2.9722 | −5.3746 to −0.8054 | −8.8320 | −0.3250 |
| Left first molar roll ≥0 | 21 | 5.6302 | 3.8870 | 3.8609 to 7.3996 | 0.0510 | 13.2580 |
| Left first molar roll total | 30 | 3.0142 | 5.4200 | 0.9903 to 5.0380 | −8.8320 | 13.2580 |
| Right first molar crown distances (3D) total | 30 | 2.0164 | 1.2936 | 1.5334 to 2.4995 | 0.2180 | 5.0570 |
| Right first molar crown distances (SI) <0 | 8 | −0.4350 | 0.3768 | −0.7500 to −0.1200 | −0.8690 | −0.0270 |
| Right first molar crown distances (SI) ≥0 | 22 | 0.8592 | 0.7237 | 0.5383 to 1.1800 | 0.0020 | 2.7620 |
| Right first molar crown distances (SI) total | 30 | 0.5141 | 0.8674 | 0.1902 to 0.8380 | −0.8690 | 2.7620 |
| Left first molar crown distances (3D) total | 30 | 1.8720 | 1.0377 | 1.4845 to 2.2595 | 0.2950 | 4.5080 |
| Left first molar crown distances (SI) <0 | 10 | −0.6798 | 0.4306 | −0.9878 to −0.3718 | −1.6390 | −0.2980 |
| Left first molar crown distances (SI) ≥0 | 20 | 0.8942 | 0.8706 | 0.4867 to 1.3016 | 0.0690 | 2.9740 |
| Left first molar crown distances (SI) total | 30 | 0.3695 | 1.0600 | −0.02632 to 0.7653 | −1.6390 | 2.9740 |
| Symphysis inclination (T1) <0 | 30 | 126.6890 | 7.0461 | −129.3201 to −124.0580 | −138.8280 | −111.6480 |
| Symphysis inclination (T2) <0 | 30 | 126.1229 | 7.6512 | −128.9799 to −123.2658 | −138.9520 | −106.9970 |
| Symphysis thickness (Pog) (3D) total | 30 | 13.8732 | 2.0126 | 13.1217 to 14.6247 | 9.5890 | 18.3340 |
| Symphysis thickness (Pog) (AP) total | 30 | −13.8622 | 2.0140 | −14.6143 to −13.1102 | −18.3290 | −9.5860 |
| Symphysis thickness T1 (B) (3D) total | 30 | 7.1186 | 1.8659 | 6.4219 to 7.8153 | 4.4190 | 10.9980 |
| Symphysis thickness T1 (B) (AP) total | 30 | −7.1006 | 1.8760 | −7.8011 to −6.4001 | −10.9950 | −4.3020 |
| Symphysis thickness T2 (B) (3D) total | 30 | 6.8608 | 2.2632 | 6.0157 to 7.7059 | 3.6180 | 12.2220 |
| Symphysis thickness T2 (B) (AP) total | 30 | −6.8406 | 2.2698 | −7.6881 to −5.9930 | −12.2200 | −3.5910 |
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