Introduction
To investigate the anatomy of the posterior palatal alveolar process, which is often used for placement of the orthodontic mini-implant (OMI), and to suggest simple guidelines for safe placement of OMI.
Methods
Cone-beam computed tomography (CBCT) scans of 60 patients (30 men, 30 women; age range, 18-39 years; average age, 25.8 years) was used to measure the palatal interradicular distance, the palatal bone thickness, and the palatal soft-tissue thickness. Measurements were performed on the area from the maxillary canine to the maxillary second molar based on the vertical distance apical from the cementoenamel junction. The CBCT data were analyzed by Bonferroni correction for multiple testing and the multivariable mixed linear model.
Results
The palatal interradicular distance was the widest between the second premolar and the first molar and the narrowest between the first and second premolars. The palatal bone thickness at interdental sites was the thickest between the first and second premolars and the thinnest between the first and second molars. The interdental palatal soft-tissue thickness from the canine to the second premolar was thicker than any other area. There were minor measurement differences between genders and positive correlations between vertical distance from the cementoenamel junction plane and all of the parameters.
Conclusion
In this study, we evaluated the anatomy of the posterior palatal area using CBCT scans of adult patients. The data will provide guidelines to the clinicians before OMI placement in the posterior palatal alveolar process.
Graphical abstract
Highlights
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Anatomy of the palatal alveolar process was assessed for mini-implant placement.
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Palatal interradicular distance was widest between the second premolar and first molar.
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Palatal bone thickness was greatest between the first and second premolars.
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Interdental palatal soft tissue was thickest from canine to the second premolar.
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Anatomic mapping using cone-beam computed tomography may provide guidelines for placing mini-implants.
Orthodontic mini-implants (OMIs) play an important role in contemporary orthodontics. The OMIs provide effective anchorage control during space closure, nonsurgical maxillary expansion, intrusion, or distalization of the entire dentition. The OMIs reduced dental side effects and contributed to long-term stability. , However, the problem encountered when applying OMIs is the loosening or failure of OMIs. OMI stability is affected by various factors, including age, sex, craniofacial skeletal pattern, loading protocol, OMI type, and anatomic factors of the implantation site. Among anatomic factors, osseous conditions are primarily responsible for the anchorage capacity of OMIs. Better primary stability is achieved with more osseous contact. OMI-root contact could cause the failure of the OMI. Thin, soft tissue is more advantageous because the likelihood of inflammation is lower. To choose the proper site for OMI placement, the clinician will consider both the hard tissue and the soft tissue.
The palate is a reliable site for OMI placement because the bone quality and quantity are favorable. Because of its importance, the anatomy of the palate has been thoroughly studied. Baumgaertel investigated cortical bone thickness and bone depth for the posterior palatal alveolar process, and the measurements were performed at sites that were 4, 8, and 12 mm away from the alveolar crest. Yang et al investigated the interradicular spaces between maxillary teeth, including the palatal alveolar process, and measured the bone thickness based on the apical distance from the cementoenamel junction (CEJ).
Few studies have assessed the soft tissue anatomy of the posterior palatal alveolar process. Vu et al measured the palatal soft-tissue thickness (PST) using cone-beam computed tomography (CBCT) scans with intersecting reference lines around the midpalatal suture and observed that the lateral area presented thicker soft tissue than the sutural area. Barriviera et al evaluated the palatal mucosal thickness using a plastic lip retractor and wooden spatulas to retract soft tissues away from the teeth and gingiva and reported the average palatal mucosal thickness from the canine to the second molar.
This retrospective study aimed to measure the palatal interradicular distance (PID), the palatal bone thickness (PBT), and the PST in the posterior palatal alveolar process using CBCT scans. In addition, this study evaluated the gender and positional differences in this parameter.
Material and methods
Our samples consisted of CBCT data from 60 adult patients (30 men, 30 women; aged 18-39 years; mean age 25.8 years) who visited the orthodontic department of Kyung Hee University Dental Hospital from 2014 to 2018 ( Table I ). All subjects in the sample had intact maxillary jaws, permanent dentition, and no pathology on the radiology report. The scans were selected on the basis of the following exclusion criteria: (1) radiographic signs of periodontal disease; (2) moderate-to-severe overlapping of crowns or roots of adjacent teeth; (3) maxillofacial trauma; (4) missing teeth except for third molars; (5) history of systemic conditions, pathologies, or therapies that may significantly affect alveolar bone properties; (6) orthodontic treatment history; and (7) contact between the tongue and the palatal gingiva ( Fig 1 ). The study design was reviewed and approved by the Institutional Review Board of Kyung Hee University Dental Hospital (IRB no. KH-DT19003).
| Vertical level | Interdental sites | |||||||
|---|---|---|---|---|---|---|---|---|
| 3 − 4 | 4 − 5 | 5 − 6 | 6 − 7 | |||||
| Mean | SD | Mean | SD | Mean | SD | Mean | SD | |
| 4 | 2.65 | 0.59 | 2.42 | 0.57 | 5.29 | 1.02 | 3.21 | 0.83 |
| 5 | 2.64 | 0.62 | 2.43 | 0.61 | 5.56 | 1.06 | 3.24 | 0.85 |
| 6 | 2.65 | 0.62 | 2.49 | 0.63 | 5.82 | 1.09 | 3.27 | 0.86 |
| 7 | 2.60 | 0.58 | 2.53 | 0.68 | 6.19 | 1.17 | 3.40 | 0.97 |
| 8 | 2.60 | 0.60 | 2.52 | 0.67 | 6.52 | 1.27 | 3.60 | 1.14 |
The images were acquired on CBCT (Alphard vega-3030; Asahi Roentgen, Kyoto, Japan) at 80 kVp, 10 mA, 0.39-mm 3 voxel size, scan time of 17 seconds, and field of view of 16 × 13 cm, in the C-mode. CBCT data were saved as digital imaging and communications in medicine files in a picture archiving and communication system (Infinit Technology Solutions, Seoul, South Korea).
InVivo Dental (version 5.1.11; Anatomage, San Jose, Calif), a volumetric imaging software, was used for reorientation and measurements (window width, 5000 Hounsfield units; window level, 1500 Hounsfield units). The images were oriented as follows. In the sagittal view, the vertical observation line was parallel to the long axis of the roots. In the axial view, the vertical observation line passed through the alveolar crest from the canine to the second molar on the measurement side. In the coronal view, the vertical observation line was parallel to the long axis of the roots on the measurement side, and the horizontal observation line was parallel to the CEJ plane ( Fig 2 ).
Five dental and 4 interdental sites from the maxillary canine to the maxillary second molar on each side were surveyed, and the measurements were taken at 5 vertical levels, which were 4, 5, 6, 7, and 8 mm apical to the CEJ plane, resulting in 45 measurement sites ( Fig 3 ).
To perform measurements, we constructed each axial view sequentially at the vertical levels apical to the CEJ plane. The PID was defined as the narrowest interradicular distance between neighboring roots. For the area between the first and second molars, the narrowest distance between the palatal roots was measured considering the multi-root nature of the maxillary molars. The PBT at each dental site was defined as the narrowest distance from the lingual surface of the tooth to the cortical bone surface. The PBT at the interdental site was defined as the narrowest distance from the narrowest interradicular space to the palatal cortical bone surface. The PST at the dental and interdental sites was also measured ( Fig 4 ).
Statistical analysis
To test the intraexaminer reliability, we randomly selected 5 scans from each group, which were assessed 2 weeks later by the same person (J.L.). The descriptive statistics were presented as means ± standard deviation. Intraclass correlations evaluated the reliability of the measurements. Two-way analysis of variance was performed to evaluate the difference according to the vertical depth and tooth position, and Bonferroni post hoc analysis was further performed for positional difference. The multivariable mixed linear model was used to evaluate gender and positional differences. P <0.05 was considered statistically significant. The data were analyzed using the SPSS (version 9.4; SAS, Cary, NC).
Results
The 5 replicate measurements taken at the 45 sites showed high reliability for CBCT scans. The interclass correlations coefficient of the measurements was 1.00.
There was a significant difference in interradicular distance between the site ( Table I , P <0.001). The greatest area was between the second premolar and the first molar (5ˆ6, 5.36-6.59 mm), followed by between the first and second molars (6ˆ7, 3.21-3.60 mm), between the canine and the first premolar (3ˆ4, 2.60-2.65 mm), and between the first and second premolars (4ˆ5, 2.42-2.53 mm). There was no significant gender difference in the PID ( Table II ). There was a positive correlation between vertical distance from the CEJ and the PID except between the canine and the first premolar distance ( Table III , P <0.01 at 4 − 5; P <0.001, both at 5ˆ6, and 6ˆ7).
| Tooth | Palatal bone thickness | Soft-tissue thickness | Interradicular distances | |||
|---|---|---|---|---|---|---|
| β estimate † | P value ‡ | β estimate | P value | β estimate † | P value | |
| 3 | −0.01 | 0.963 | 0.50 | 0.011∗ | ||
| 3 − 4 | 0.29 | 0.038∗ | 0.54 | 0.009∗∗ | 0.07 | 0.556 |
| 4 | −0.03 | 0.755 | 0.56 | 0.009∗∗ | ||
| 4 − 5 | 0.26 | 0.042∗ | 0.46 | 0.018∗ | −0.18 | 0.155 |
| 5 | −0.07 | 0.497 | 0.50 | 0.041∗ | ||
| 5 − 6 | 0.21 | 0.308 | 0.23 | 0.382 | −0.14 | 0.536 |
| 6 | 0.12 | 0.232 | 0.11 | 0.684 | ||
| 6 − 7 | 0.38 | 0.043∗ | −0.23 | 0.439 | 0.22 | 0.209 |
| 7 | 0.20 | 0.168 | −0.33 | 0.330 | ||
† β estimate is positive when 2 variables have a positive relationship.
| Tooth | Palatal bone thickness | Soft-tissue thickness | Interradicular distances | |||
|---|---|---|---|---|---|---|
| β estimate † | P value ‡ | β estimate | P value | β estimate † | P value | |
| 3 | 0.28 | <0.001∗∗∗ | 0.06 | 0.026∗ | ||
| 3 − 4 | 0.21 | <0.001∗∗∗ | 0.11 | <0.001∗∗∗ | −0.02 | 0.2033 |
| 4 | 0.26 | <0.001∗∗∗ | 0.23 | <0.001∗∗∗ | ||
| 4 − 5 | 0.14 | <0.001∗∗∗ | 0.27 | <0.001∗∗∗ | 0.03 | 0.008∗∗ |
| 5 | 0.24 | <0.001∗∗∗ | 0.33 | <0.001∗∗∗ | ||
| 5 − 6 | 0.06 | <0.001∗∗∗ | 0.31 | <0.001∗∗∗ | 0.31 | <0.001∗∗∗ |
| 6 | 0.03 | 0.004∗∗ | 0.25 | <0.001∗∗∗ | ||
| 6 − 7 | −0.04 | 0.005∗∗ | 0.28 | <0.001∗∗∗ | 0.09 | <0.001∗∗∗ |
| 7 | 0.02 | 0.1618 | 0.44 | <0.001∗∗∗ | ||
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