Introduction
This study aimed to establish maxillary basal arch forms using the root apices and to determine the differences in the basal arch forms in adult women with different sagittal skeletal patterns.
Methods
This retrospective study included 91 adult women, with either a Class I (n = 24), Class II Division 1 (n = 22), Class II Division 2 (n = 23), or Class III (n = 22) malocclusion, who underwent cone-beam computed tomography. Three-dimensional coordinates of the root apices were determined using the multiplanar reformation mode of OnDemand3D software (Cybermed Inc, Seoul, South Korea). Two-dimensional coordinates were converted from acquired 3-dimensional coordinates via projection on the palatal plane, and the Procrustes superimposition method was used to build the basal arch form. Finally, interroot width measurements were performed for basal arch form comparisons.
Results
There were significant differences among the 4 groups ( P <0.05) with respect to the intercanine width. The intercanine width of Class II Division 1 group was significantly narrower than that of the other groups. The Class II Division 1 and Class II Division 2 groups tended to have tapered arch forms and squared arch forms, respectively.
Conclusions
We established maxillary basal arch forms using the root apices. The Class II Division 1 group had a significantly narrower intercanine distance. The use of the root apex to depict the basal arch form seems reasonable.
Highlights
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We established maxillary basal arch forms using the root apices.
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There are differences in the arch forms according to sagittal skeletal patterns.
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The Class II Division 2 group tended to have squared arch form.
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The Class II Division 1 group tended to have tapered arch form.
A primary focus of orthodontics is posttreatment stability. Many components attribute to posttreatment stability, and change in the arch form is one of them. Cruz et al showed that greater changes in the arch form after orthodontic treatment resulted in a higher risk of relapse. Therefore, to reduce the risk of relapse, conserving pretreatment arch forms was suggested.
Recently, increased use of straight wire appliances has allowed clinicians to use preformed wires. To use proper preformed wires, it is necessary to select types of wires most similar to pretreatment arch forms of patients. Arch forms differ by not only ethnicity and sex but also dentofacial skeletal patterns. For example, Korean individuals have wider arch forms than North American individuals, and men have wider arch forms than women. Patients with a skeletal Class III pattern have a larger mandibular arch form than those with a skeletal Class II pattern.
Arch forms include the dental arch form, which is made of a crown, and the basal arch form, which is made of the basal or alveolar bone. Because excessive arch expansion or movement of teeth beyond the basal bone could cause periodontal problems such as dehiscence and gingival recession, the basal arch form is as important as the dental arch form.
Until now, most studies have been focused on dental arch forms. Few previous studies on basal arch forms used points 8-10 mm from the gingival margin on the plaster model or points near the mucogingival junction on the virtual model to create a basal arch form. These assumed basal points were indirectly measured due to the limitations of measurements.
In 1925, Lundström defined basal arch forms as follows: “In normal cases the apical base will in the horizontal plane coincide with the region in which the apices of the roots are located.” In other words, Lundström argued that the position of the root apex should be used to depict basal arch forms. A recent study showed that even when the crowding of teeth was severe, the location of the root apex remained clinically unchanged. This finding suggests that it is appropriate to make basal arch forms using the position of the root apex.
To the best of our knowledge, no studies have been conducted on the maxillary basal arch form using the root apex in adult women with different skeletal patterns. This study aimed to establish maxillary basal arch forms using the root apex and to determine the differences in the basal arch forms in adult women with different sagittal skeletal patterns. This study was made possible by the advent of cone-beam computed tomography (CBCT).
Material and methods
This retrospective study was designed as a pilot project to propose a new method to depict maxillary basal arch forms. In this study, we evaluated randomly selected Korean adult women who visited the Pusan National University Dental Hospital from January 2012 to May 2018 and underwent CBCT (Pax-Zenith3D; VATECH Co, Ltd, Seoul, South Korea), which was performed in centric relation for other reasons. Only female subjects were included as there was a noticeable difference in arch width between men and women. Pretreatment cephalograms were generated from the CBCT scans to analyze skeletal and dental evaluations. ANB angle and Wits appraisal followed by the first molar relationships were used to classify malocclusion into skeletal Class I (ANB, 0°-4°; Wits appraisal, −3 mm to 1 mm), Class II (ANB, >4°; Wits appraisal, >1 mm), and Class III (ANB, <0°; Wits appraisal, <−3 mm). , Only those showing consistency between the ANB angle and the Wits appraisal were included. Moreover, the Class II Division 1 group was composed of subjects with protrusive maxillary incisors and excessive overjet (>5 mm), and the Class II Division 2 group was composed of subjects with retroclination of 2 or more maxillary incisors and overbite deeper than 3 mm. The exclusion criteria were as follows: any history of trauma, impacted tooth, missing tooth, dilacerated roots, previous orthodontic treatment, posterior crossbite, any history of temporomandibular joint dysfunction, or other developmental syndromes. A total of 24 adult women with skeletal Class I malocclusion (mean age, 24.1 ± 3.0 years; range, 19-27 years), 22 adult women with skeletal Class II Division 1 malocclusion (mean age, 24.7 ± 4.5 years; range, 18-30 years), 23 adult women with skeletal Class II Division 2 malocclusion (mean age, 23.5 ± 3.2 years; range, 18-29 years), and 22 adult women with skeletal Class III malocclusion (mean age, 21.0 ± 3.6 years; range, 18-30 years) were included. This study was reviewed and approved by the Institutional Review Board of Pusan National University Dental Hospital (PNUDH-2014019).
The CBCT (Pax-Zenith3D) settings were as follows: 4 mA, 90 kVp, voxel size of 0.3 mm, and scanning area of 20 × 19 cm. CBCT data were converted to DICOM files, and 3-dimensional (3D) coordinates to locate root apices were obtained in the multiplanar reformation mode of OnDemand3D software (Cybermed Inc, Seoul, South Korea) ( Fig 1 ). For maxillary premolars or molars, the number of roots can vary from 2 to 3. In these subjects, 3D coordinates were obtained to locate each root apex, and mean coordinates of each root apex were used as a landmark point.
Two-dimensional coordinates were converted from acquired 3D coordinates of root apices via projection on the palatal plane using the MATLAB software (MathWorks, Natick, Mass) ( Fig 2 ). Thereafter, the Procrustes superimposition method was used to derive the basal arch form; the centroids of each sample were translated, rotated, and then superimposed.
The polynomial function was used for best curve fitting of the basal arch form. For the basal arch form comparison, we superimposed mean shapes of basal arch forms that were constructed from average locations of the root apices. That is, we calculated the average x and y coordinates of the root apex of each tooth in the group and used these coordinates for average locations of the root apices. We used the midpoint of maxillary central incisors for the reference of superimposition. Moreover, the following linear interroot width measurements were obtained: intercentral incisor width, interlateral incisor width, intercanine width, interfirst premolar width, intersecond premolar width, and interfirst molar width.
Statistical analysis
Each measurement was performed by 1 investigator (S.-H.K.). To evaluate intraexaminer reliability, we reobtained data for 30 randomly selected patients after 1 month. The intraexaminer agreement was evaluated using the intraclass correlation coefficient. The intraexaminer agreement was high, with an average intraclass correlation coefficient value of 0.867. The 1-way analysis of variance test was used to analyze differences in the interroot width ( P <0.05). Scheffé method was used for post-hoc analysis. Statistical analysis was performed using SPSS software (version 23.0; IBM, Armonk, NY).
Results
The basal arch forms for each group are shown in Figure 3 . Each group’s distribution resulted in the following polynomial function:
- (1)
Class I group: y = 0.4073 + 0.018 x 2 + 2.9357 × 10 −5 x 4
- (2)
Class II Division 1 group: y = 0.1342 + 0.037 x 2 + 2.9918 × 10 −5 x 4
- (3)
Class II Division 2 group: y = 0.7625 + 3.405 × 10 −3 x 2 + 1.099 × 10 −5 x 4 + 9.454 × 10 −8 x 6
- (4)
Class III group: y = −0.4295 + 0.031 x 2 + 4.4071 × 10 −6 x 4
The mean shapes of basal arch forms for each group are shown in Figure 4 . The standard deviations of x and y coordinates of the root apices from the average locations are presented by group in Table I . The Class II Division 1 group tended to have tapered arch forms, and the Class II Division 2 group tended to have squared arch forms. Class I and Class III groups had similar ovoid arch forms.
| Tooth no. | Class I | Class II Division 1 | Class II Division 2 | Class III | ||||
|---|---|---|---|---|---|---|---|---|
| SD_X | SD_Y | SD_X | SD_Y | SD_X | SD_Y | SD_X | SD_Y | |
| 11 | 0.743 | 0.934 | 0.593 | 1.223 | 0.510 | 1.549 | 0.826 | 1.012 |
| 12 | 0.691 | 1.287 | 0.807 | 1.043 | 1.023 | 1.256 | 0.762 | 0.823 |
| 13 | 1.239 | 1.038 | 1.356 | 1.223 | 1.836 | 1.485 | 0.812 | 1.425 |
| 14 | 1.539 | 0.912 | 1.323 | 0.915 | 1.655 | 0.739 | 0.903 | 0.611 |
| 15 | 1.512 | 0.695 | 1.435 | 1.061 | 1.924 | 1.067 | 1.056 | 0.993 |
| 16 | 1.469 | 1.109 | 2.405 | 1.849 | 2.073 | 1.723 | 1.123 | 1.076 |
| 21 | 0.811 | 0.847 | 1.112 | 1.264 | 0.598 | 1.313 | 0.235 | 1.023 |
| 22 | 1.237 | 1.236 | 0.854 | 1.077 | 1.036 | 0.805 | 0.567 | 1.132 |
| 23 | 1.736 | 0.948 | 1.515 | 1.103 | 1.723 | 0.923 | 1.134 | 1.216 |
| 24 | 1.412 | 0.950 | 1.073 | 1.135 | 1.410 | 0.773 | 0.967 | 0.996 |
| 25 | 1.632 | 0.712 | 0.918 | 0.927 | 1.943 | 1.245 | 1.823 | 1.331 |
| 26 | 1.065 | 0.878 | 1.328 | 1.053 | 2.137 | 1.123 | 1.356 | 1.333 |
| Mean | 1.257 | 0.962 | 1.227 | 1.156 | 1.489 | 1.167 | 0.963 | 1.080 |
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