Evaluation of the palatal bone in different facial patterns for orthodontic mini-implants insertion: A cone-beam computed tomography study

ABSTRACT Objective: Evaluate the height, thickness and cortical density of the palatal bone of adults with different vertical facial patterns using Cone-Beam Computed Tomography (CBCT). Methods: This study analyzed 75 CBCTs of patients between 18 and 35 years old (45 men and 30 women). The CBCTs were classified into three groups based on their facial pattern: normodivergent, hypodivergent and hyperdivergent as determined from lateral cephalograms synthesized from the CBCTs. The height, cortical thickness and cortical density of the palatal bone were measured at 4, 8, 12, 16 and 20mm posterior to the incisive foramen, and at 3, 6 and 9mm lateral to the midpalatal suture. ANOVA with Tukey post-hoc tests were used for analysis of the data, at significance level of p< 0.05. Results: The hypodivergent pattern had a significant difference and the greatest height and cortical thickness of the palatal bone, followed by the hyperdivergent and the normodivergent patterns. No significant differences were found in minimum and maximum values of cortical density. Conclusion: The palatal bone is a favorable anatomical area to install different orthodontic temporary anchorage devices (TADs), where individuals with the hypodivergent vertical facial pattern have a higher height and cortical thickness of the palatal bone, followed by the hyperdivergent pattern and finally the normodivergent pattern. No significant differences in the cortical density of the palatal bone in the three facial patterns were found.


INTRODUCTION
During orthodontic treatment, teeth are exposed to forces and moments generated by the appliances used. The applied forces generate reciprocal forces of the same magnitude in the opposite direction. Thus, one of the most difficult clinical challenges is to minimize these reciprocal forces. Successful treatment generally depends on meticulous planning of the anchorage. 1 A reliable method is to use temporary anchorage devices (TADs).
The palatal region is very important for the installation of TADs as an aid in the orthodontic treatment, showing a high clinical versatility, with more precise and predictable tooth movement regardless of patient cooperation. 2,3 One factor determining the success of TADs placement is the quantity of the surrounding bone. 4 The insertion on the palate depends on the structural characteristics of the palatine bone, such as height, cortical thickness and cortical density. It has been reported that a suitable bone thickness of the palate should be greater than 4 mm. 5 Bone characteristics can be evaluated through the cone-beam computed tomography (CBCT), which provides highly accurate and detailed information. 6,7 The skeletal morphology in the craniofacial region is primarily controlled by genetic factors. However, the functional demands can have a significant effect on the growth and craniofacial development. Vidalón JA, Liñan C, Tay LY, Meneses A, Lagravère M -Evaluation of the palatal bone in different facial patterns for orthodontic mini-implants insertion: A cone-beam computed tomography study 5 Each of the facial patterns in the vertical dimension (hyperdivergent, normodivergent and hypodivergent) present differences in the muscle load during function, due to skeletal compensation.
This muscle load can alter the height and thickness of the cortical bone and the density of the palatal bone, not only in muscle attachment sites but also in other skeletal sections. 8 It could be said that there is a significant relationship between the facial type and the morphological characteristics of the jaws. [9][10][11] Sadek et al. 10 reported that patients with a hyperdivergent pattern have a narrow alveolus, compared to the normal and hypodivergent patients.
The aim of this study was to determine the height, thickness and cortical density characteristics of the palatal bone in the different vertical facial patterns using CBCTs. This information would give background or guidelines in terms of possible TAD placement sites in the palatal bone dependent on the patient growth pattern. Vidalón JA, Liñan C, Tay LY, Meneses A, Lagravère M -Evaluation of the palatal bone in different facial patterns for orthodontic mini-implants insertion: A cone-beam computed tomography study 6 based on their vertical facial pattern and with no sagittal malocclusion, as determined from lateral cephalograms synthesized from the CBCTs. These facial patterns were determined by the angle formed using the following cephalometric measurements: 1) Mandibular plane -the angle between the anterior cranial base (sella to nasion) and Mandibular plane (gonion to menton) -patients between 29 to 36 degrees were classified as normodivergent; patients with more than 36 degrees, as hyperdivergent; and less than 29 degrees, as hypodivergent; 12 2) Face height index -the ratio of posterior face height to anterior face height, using the measurements of distance from sella (S) to gonion (Go) divided by the distance of nasion (N) to menton (Me) -ratios of < 61%, 61% to 69%, and > 69% indicated hyperdivergent, normodivergent and hypodivergent patterns, respectively 13 (Fig 1 and Table 1).

MATERIAL AND METHODS
Subjects had to fit into a single facial pattern category for both measurements, in order to be included in the study. Vidalón JA, Liñan C, Tay LY, Meneses A, Lagravère M -Evaluation of the palatal bone in different facial patterns for orthodontic mini-implants insertion: A cone-beam computed tomography study   All subsequent measurements were made perpendicular to this reference line 9-15 (Fig 2). and P20, respectively. Measurements taken at 3, 6 and 9mm lateral to midpalatal suture were designated as D3, D6 and D9, respectively. A total of 15 measurements were performed for each patient (Fig 3). Several studies used these measurements to evaluate the palatal bone before the installation of TADs. 9,11,12  To evaluate the reliability of the method, the same examiner measured ten randomly selected subjects for all points, with a two week interval between trials. Intraclass correlation coefficient (ICC) was 0.92, showing an acceptable intraobserver agreement of repeated measurements. The inter-examiner reliability was evaluated between a dental radiologist and the principal examiner. Results showed a high correlation of 0.89.
The results were evaluated at the significance level p < 0.05, with a 95% confidence interval.
The measurement error for the height and thickness of cortical     (Table 3).

CORTICAL DENSITY (PALATAL BONE)
No statistically significant differences were found in any of the locations indicated in the data collection sheet (Table 4 and Table 5).    In the present study, by measuring the height of the palatal bone, statistically significant differences were found between the facial patterns. However, the hypodivergent sample had a greater palatal bone height, followed by hyperdivergent and normodivergent patterns. Sadek et al. 10 found different results, where the dentoalveolar process in the upper and lower jaw is influenced not only by genetic factors, but also by the dentoalveolar adaptation process against different loads of oral and perioral muscle strength. 10,[16][17][18][19] For example, the tongue activity pattern during the swallowing and breathing can affect the morphological development of the palatal bone. 19 During the process of growth and development, the palatal bone in normal situations suffers a process of remodelling, with respect to its height, due to the resorption in the nasal chambers and bone-apposition on the buccal side of the

CORTICAL DENSITY (PALATAL BONE)
No statistically significant differences were found in any of the vertical facial patterns.
Han et al. 9 reported a higher density of cortical and trabecular bones in adults, compared to teenagers, in CBCTs. These measurements were presented in Hounsfield units (HU), which differed from the present study, which used attenuation coefficients.
Moon et al. 11 and Han et al. 9 found a higher density in women. posterior sectors were found in the present study. Thus when comparing with Moon et al., 11 it can be mentioned that data obtained from CT scanners cannot be extrapolated to CBCTs.
Similarly, Ozdemir et al. 8 found no significant differences in the cortical density of the palatine bone between the right and left sides between the dentoalveolar buccal and palatal areas.
According to different published studies, most of these use CBCT and express the cortical density in HU, using the correct term: attenuation coefficient -the unit indicated to express the cortical density. 29,30 Based on the results of the present study, the following clinical recommendations can be made: In patients with hypodivergent pattern, it is suggested to install TADs in the area between 4 and 12mm posterior to the incisal foramen and 3 to 9mm lateral to the middle palatal suture. This area has dimensions of maximum height and cortical thickness of 11.81mm/2.99mm respectively (canine distal approx.) and minimum cortical height and thickness of the palatal bone of 6.18mm, 1.87mm respectively (second premolar distal approx.), as seen in Figure 4.
No statistically significant differences were found in patients with normodivergent and hyperdivergent patterns, being sug-  Investigations comparing the dimensions of the palatal bone and vertical facial patterns, gender and age group, are suggested as a complement to this investigation.

CONCLUSION
The palatal bone is a favorable anatomical area to install dif-