Introduction
The objective of this study was to evaluate the dental periodontal and skeletal response to ≥5 mm of expansion width achieved by C-expander treatment with posterior miniscrews placed between the first and second molars in adults.
Methods
A total of 28 patients aged 21.91 ± 3.20 years with maxillary transverse deficiency underwent C-expander treatment. Anterior miniscrews were positioned between the first and second premolars, whereas posterior miniscrews were positioned between the first and second molars. Cone-beam computed tomography records were obtained before expansion and 3 months after expansion. The dental periodontal and skeletal changes for all patients were recorded.
Results
The C-expander treatment expanded the palatal suture with slight buccal alveolar bone inclination. An increase in the nasal cavity width and a greater increase in the maxillary base bone width were observed after maxillary expansion. The expansion at the posterior nasal spine (3.78 mm) was approximately 85.7% of that at the anterior nasal spine (4.41 mm). No significant buccal dehiscence occurred after expansion, whereas the mesiobuccal alveolar bone thickness of the first molars was decreased at the 8 mm level with respect to the cementoenamel junction. The first molar showed decreased inclination (right, −0.45°; left, −0.38°, P >0.05), whereas the expansion at the apical level was less than that at the crown level. Age and the skeletal/dental expansion ratio had no discernible relationship.
Conclusions
Miniscrew-assisted C-expander treatment can be effective for adults with maxillary transverse deficiency. Rearward placement of the miniscrews may create an approximately parallel expansion. Most maxillary expansion was derived from skeletal expansion with slight alveolar bone buccal inclination.
Highlights
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In adult patients, C-expanders can achieve significant palatal expansion.
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Miniscrews (1.4 × 8.0 mm) can provide enough anchorage for C-expander application.
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The rearward placement of the miniscrews may create an approximately parallel expansion.
Maxillary transverse deficiency is regarded as a common deformity that can affect the appearance and function of the maxillary dentition. If ignored, this condition can lead to various abnormalities, including dentition crowding, maxillary incisor protrusion, and posterior crossbite.
Rapid maxillary expansion (RME) and slow maxillary expansion are widely used to correct transverse insufficiency. However, expansion always occurs at the dentoalveolar level rather than at the skeletal level in adults because of the closed midpalatal suture. To achieve more midpalatal expansion, surgically assisted RME (SARME) is used as an alternative treatment for adult patients, which involves performing an osteotomy to release the lateral walls of the maxilla. Previous studies have also reported the high efficiency of SARME in expanding the maxillary region; however, this procedure is an invasive operation and is not acceptable for some patients. ,
Along with the application of miniscrews, miniscrew-assisted rapid palatal expansion (MARPE) has been widely used in maxillary expansion, especially in adults. An et al and Choi et al have suggested that MARPE with a tooth-bone-borne appliance can increase the parallelism and skeletal expansion of the maxilla without affecting its vertical dimension or gingival recession in adults, although it increases molar inclination. To minimize adverse reactions, bone-borne appliances have been suggested, including the maxillary skeletal expander (MSE) and the C-expander. Lee et al compared the stress distribution and displacement of the maxilla between the MSE and C-expander through finite element analysis and found that the C-expander caused the least stress concentration around the miniscrews and less buccal inclination of the molars while expanding the palatal suture in a parallel manner. Because of these advantages, the C-expander has been suggested for use in patients with poor periodontal support.
Although it allows successful expansion, the C-expander produces less dental expansion than does the MSE. The expansion amount may be a major factor that clinicians consider when choosing a maxillary expander. The C-expander is applied in the palatal region and supported with miniscrews, which are placed in front of the first molars (eg, between the canine and the first premolar, the first and second premolars, or the second premolar and the first molar). Because the midpalatal and pterygomaxillary regions show greater resistance to expansion forces, the position of the device may affect the amount of maxillary expansion. Yoon et al compared the position and hook lengths of a bone-borne RME appliance with finite element analysis and found an increase in stress in the second molar region when the expander was positioned in a more posterior region, as well as when the hook length was increased by 3 mm. Although the effects of the expander with miniscrews were demonstrated, they were not sufficient to support the conventional wisdom regarding the benefits of the procedure because they were examined only after a 1-turn expansion. Moreover, few studies have been conducted on the skeletal and dental effects of C-expander treatment with miniscrews placed between the first and second molars.
We analyzed the dental and skeletal changes after expansion to investigate the effects of C-expander treatment in adult patients with posterior miniscrews placed between the first and second molars.
Material and methods
We set the α at 0.05 and β at 0.1. The sample size was calculated to detect a mean difference of 2.38 mm with a standard deviation of 1.35 in the changes in the width of the palatal after RME. The results suggested that a minimum of 18 patients were needed.
This retrospective study included 28 adult patients with maxillary transverse deficiency who visited the Department of Orthodontics at Xiangya Hospital between January 2020 and December 2022. The inclusion criteria were as follows: (1) aged >18 years, (2) cone-beam computed tomography (CBCT) records obtained before and 3 months after maxillary expansion (T1), (4) the presence of maxillary transverse deficiency, and (4) an expansion width >5 mm. The exclusion criteria were the presence of cleft lip and palate, craniofacial anomalies, or syndromic conditions. Patients underwent orthodontic treatment with maxillary expansion as the first step.
The C-expander comprised 2 resin pieces and an expansion screw ( Fig 1 ). To fix the C-expander, 4 miniscrews (1.4 × 8.0 mm, Ormco, Glendora, Calif) were placed in the palatal region. Two miniscrews were placed between the first and second premolars, whereas 2 miniscrews were placed between the first and second molars ( Fig 1 , A ). All the steps were performed by the same clinician. The expansion screw was rotated by a quarter turn every 2 days until the required expansion was attained. After expansion, the C-expander was kept in place for 3 months.
CBCT records were obtained at the initial evaluation (T0) and T1. The CBCT scans were performed with KaVo equipment (KaVo 3D eXam, KaVo Dental, Biberach, Germany) in 120 kV and 5 mA pulse mode. For all scans, the field of view was 17 × 23 cm, and the scan time was 26.9 seconds with a voxel size of 0.20 mm. The images were exported in digital imaging and communication in medicine format using eXam Vision software, and the digital imaging and communication in medicine files were imported into Dolphin Imaging software (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, Calif) for analysis.
Initially, 3-dimensional images were reconstructed in Dolphin. The models were reoriented in 3 planes to minimize the measurement errors at T0 and T1, including the sagittal plane (through the nasion and the anterior nasal spine [ANS] points), the horizontal plane (parallel to the Frankfort plane [FH]), and the coronal plane (connecting through the bilateral Porion points) ( Fig 2 ).
A list of measurements used in the model is shown in Table I , including the measurements of the skeletal and dental periodontal changes ( Figs 3-6 ).
| Measurement | Definition |
|---|---|
| NW | The widest distance of the nasal cavity parallel to the FH plane |
| MBBW | Maxillary width tangent to the nasal floor at its most inferior level |
| MABW | The widest distance between the maxillary buccal alveolar processes |
| AMS | Midpalatal suture width in the anterior region (same CBCT coronal view of the posterior maxilla distance) |
| PMS | Midpalatal suture width in the posterior region |
| Alveolar inclination | The angle between the tangent to the palatal side of the alveolar bone and the FH plane |
| Molar inclination | The angle formed by the dental axis (line passing through the palatal cusp and palatal root apex) and FH plane |
| SNA (°) | The angle between A point and sella at nasion |
| MP-FH (°) | The angle between the mandibular plane and FH plane (the mandibular plane was assessed by using gonion-gnathion as reference points) |
| U1-SN (°) | The angle between the long axis of the maxillary central incisor and the SN plane |
| PP-FH (°) | The angle between the palatal plane and the FH plane |
| IAW | The linear distance between palatal root apices of the first molars |
| ICW | The linear distance between palatal CEJ of the first molars |
| IFW | The linear distance between the central fossa of the first molars |
| Buccal alveolar bone height | The distance from the CEJ to the alveolar crest on the buccal side |
| Alveolar bone thickness | The distance from the roots at 3 mm and 5 mm level beneath the CEJ to the outermost point of the bones |
| h1 | The vertical distance from the frontonasal suture to the nasal floor |
| h2 | The vertical distance from the frontonasal suture to the central fossa level of the first molars |
| h3 | The vertical distance from the palatal cusp to the apex |
The total maxillary expansion encompassed the nasomaxillary complex (NMC) external rotation, alveolar bone bending, and dental inclination. A previous study showed that the frontonasal suture serves as the rotational center during the displacement of the dentomaxillary complex. The base of the triangle was half of the skeletal expansion at the nasal floor level, and the height of the triangle was the vertical distance from the frontonasal suture to the nasal floor. We used the tangent function to measure the NMC rotation (ө) as follows: tan ө = opposite (half of the skeletal expansion at the nasal floor plane)/adjacent nasal floor plane) ( Fig 7 , A and C ). Pure alveolar bone bending was determined by calculating the change in alveolar inclination − the angle ө. Pure skeletal expansion at the fossa level was obtained by calculating tan ө × the vertical distance from the frontonasal suture to the central fossa level of the first molars (h2) ( Fig 7 , B and C ).
To establish the random error of the method, measurements were assessed by an examiner at 3 distinct time points, with an interval of 2 weeks between each assessment. The intraclass correlation coefficient was used to determine the reliability of the method.
Statistical analysis
Statistical analysis was performed using SPSS software (version 22.0; IBM, Armonk, NY) at a significance level of P <0.05. The normality of the data distributions was examined using the Shapiro-Wilk test. The homogeneity of variance was checked by the Levene test. The results of the numerous variables were described as the means and standard deviations. To evaluate the differences in measurements between T0 and T1, a paired t test was carried out. The Wilcoxon rank sum test was used to analyze the nonnormally distributed data. A Spearman correlation analysis was used to analyze the associations between the initial age and the skeletal/dentoalveolar expansion ratio and the midpalatal suture width in the posterior/anterior region (PMS/AMS) expansion ratio.
Results
The intraclass correlation coefficient, used to evaluate measurement reliability, was ≥0.80 for all cephalometric and cast variables examined in this study. The variances of the data were equal. The patients’ average age was 21.91 years (range, 18-27 years). The range of expansion screw activation was 6-9 mm, with an average activation of 7.04 ± 0.74 mm.
After expansion, significant increases were found in the nasal cavity width (NW) and the maxillary width. The NMC split transversely in a pyramidal pattern on the coronal plane because of less NW expansion than in the maxillary alveolar bone width ( Fig 8 , A ). The midpalatal suture was opened in a nearly parallel manner, as the expansions for the AMS and PMS were 4.41 ± 1.50 mm and 3.78 ± 1.25 mm, respectively ( Fig 8 , B ).
Three variables (SNA, PP-FH, and MP-FH) were used to evaluate sagittal changes. There was no significant change in the SNA (0.31° ± 0.55°) ( P >0.05). The PP-FH was significantly increased (0.27° ± 0.48°) ( P <0.05), indicating clockwise rotation of the palatal plane. The mandibular plane showed clockwise rotation, as verified by the changes in the MP-FH (1.99° ± 1.86°) ( P <0.05) ( Table II ).
| Measurements | T0 | T1 | T1−T0 | P value |
|---|---|---|---|---|
| Skeletal changes | ||||
| NW | 34.33 ± 2.40 | 36.81 ± 2.28 | 2.49 ± 1.44 | <0.001 |
| MBBW | 65.21 ± 1.93 | 68.82 ± 3.91 | 3.61 ± 1.34 | <0.001 |
| MAW | 59.98 ± 2.32 | 64.42 ± 1.93 | 4.43 ± 1.67 | <0.001 |
| AMS | 0 ± 0 | 4.41 ± 1.50 | 4.41 ± 1.50 | <0.001 |
| PMS | 0 ± 0 | 3.78 ± 1.25 | 3.78 ± 1.25 | <0.001 |
| SNA (°) | 79.90 ± 3.01 | 80.21 ± 2.94 | 0.31 ± 0.55 | 0.851 |
| PP-FH (°) | −0.46 ± 3.59 | −0.19 ± 3.65 | 0.27 ± 0.48 | 0.047 |
| MP-FH (°) | 29.67 ± 7.39 | 31.66 ± 6.72 | 1.99 ± 1.86 | 0.042 |
| h1 | 52.72 ± 1.82 | |||
| h2 | 73.39 ± 2.13 | |||
| Dental-alveolar changes | ||||
| IAW | 35.01 ± 2.53 | 39.82 ± 2.20 | 4.81 ± 1.35 | <0.001 |
| ICW | 33.88 ± 2.25 | 38.92 ± 2.31 | 5.04 ± 1.38 | <0.001 |
| IFW | 45.32 ± 3.22 | 50.48 ± 2.99 | 5.22 ± 1.62 | <0.001 |
| Alveolar inclination-R | 99.13 ± 3.78 | 101.39 ± 5.81 | 2.26 ± 3.74 | 0.042 |
| Alveolar inclination-L | 103.24 ± 4.04 | 106.11 ± 4.23 | 2.87 ± 2.41 | 0.011 |
| 6-molar inclination-R | 97.22 ± 5.19 | 96.77 ± 4.76 | −0.45 ± 4.89 | 0.627 |
| 6-molar inclination-L | 97.37 ± 4.01 | 97.0 ± 4.40 | −0.38 ± 4.78 | 0.705 |
| SN-U1 (°) | 109.18 ± 7.96 | 107.44 ± 10.8 | −1.74 ± 4.27 | 0.535 |
| Ratio | ||||
| MBBW/IFW | 69.2 | |||
| PMS/AMS | 85.7 |
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