Quantitative evaluation of palatal bone thickness to guide miniscrews insertion : a CBCT assessment

Introduction: The use of mini-implants has become common in orthodontic practice as it has increased the possibility of skeletal anchorage. The palate constitutes a site of choice for the insertion of miniscrews purposes because it is a site with relatively safety with appropriate bone thickness and less suitability for inflammation. Aim: To quantitatively evaluate the thickness of the palatal bone for miniscrews insertion. Material and Methods: Forty-seven sets of cone beam computed tomographic (CBCT) images were selected. The sample consisted of cone beam computed tomography from 47 patients (20 male, 27 female; mean age 22.4 years old/± 3.01 years). Palatal bone thickness (PBT) was measured in millimeters (mm) with 5 regions of interest (ROIs) which were determined used the coronal reconstructions of the patatal area: 4, 6, 8 and 10 mm posterior to the incisive foramen were evaluated. A total of 940 ROIs were evaluated. Results: Significant differences were observed for PBT between various palatal sections (p<.01). The thickest area (6.31-7.03 mm) was found in the anterior part of the palate. The mean bone thicknesses in the 6, 8 and 10 mm sections were significantly less than those observed at 4 mm from the incisive foramen. Conclusions: The thickness of the palatal bone is progressively thinner from the palatine foramen to the posterior region. Transversally, the bone was thicker in the palatine suture than in paramedian areas, mainly in the coronal reconstructions located more laterally.


INTRODUCTION
The use of miniscrews has become widespread and common in orthodontic practice. Their application has increased the possibility of skeletal anchorage due to their suitability for positioning in many areas of the alveolar bone. The introduction of immediately loadable miniscrew has further expanded its therapeutic potential as an effective alternative to intra and extraoral conventional anchorage. [1][2][3][4][5] Although miniscrew stability allows its clinical use, they can loosen during treatment. 6 The stability of miniscrews supported by bone tissue and simple mechanical retention is reported to be lower when compared with osteointegrated implants. [7][8] Factors associated to miniscrew clinical success and stability such as age, gender, screw features, surgical procedure, inflammation, sites of insertion, and bone quality have been related on the literature. 7,[9][10][11][12] The palate constitutes a site of choice for the insertion of miniscrews for orthodontic purposes because it is a site with relatively safety with appropriate bone thickness and less suitability for inflammation. 13,14 Both bone quality and quantity play important roles in the success of miniscrews. 10 Therefore, the knowledge of bone conditions in the area of interest will allow clinicians to decide more surely regarding the miniscrew. 13 A few studies have used conventional radiography CT for bone quantification before implant placement for orthodontic anchorage. In addition to being a reliable imaging method mainly in for linear measures, 15-23 cone beam computed tomography (CBCT) has the advantage of producing less amount of radiation to the patient. 24 In recognition of the need for objective determinations of bone thickness to guide miniscrews placement, the aim of the present study was to evaluate variations in the bone thickness in palatal regions potentially used for miniscrew placement.  Verification of the normality and homogeneity of variables was respectively performed by the Shapiro-Wilk and Levene tests, being the distribution of the sample considered normal. There were no differences between men and women (Student t-test for independent samples) so the combined sample was used.

MATERIAL AND METHODS
The one-way analysis of variance (ANOVA) was performed to analyze differences of palatal bone thickness at different areas and the Tukey multiple range test was used for multiple comparisons. The Pearson correlation test was also used to verify the relationship between patient age and the measurements of interest.

Statistical analysis was performed using Statistical
Package for the Social Sciences 23.0 (SPSS Inc., Chicago, USA) and significance level was established at p<0.05.

RESULTS
The intraclass correlation coefficient (ICC=.984) showed that reproducibility was excellent.

DISCUSSION
The stability of miniscrews is closely related to the quantity and quality of the cortical bone. The success of miniscrews can also be affected by bone density and thickness, just as the success of dental implants is influenced by bone quality. 20 Several studies evaluated bone quantity  (thickness) through conventional Computed Tomography (CT). 25,26 Quantitative computed tomography is the modality of choice to determine bone mineral density (BMD), but the X-ray dose absorbed by the patient during conventional CT scanning may limit the use of this modality for routine diagnosis in orthodontics. 25,27 According to a previous study, 26    are similar to those reported by Gracco et al. 1 In this study, the palatal bone thickness tends to progressively decrease from the foramen toward

CONCLUSION
The thickness of the palatal bone is progressively thinner from the palatine foramen to the posterior region and from the palatine suture to the paramedian areas.
Bone thickness was greater than 4.8 mm in all regions until 6 mm posterior to the palatine foramen.