Correlation between cortical bone thickness at mini-implant insertion sites and age of patient

ABSTRACT Introduction: Orthodontic mini-implants (MI) are a reliable alternative to provide temporary orthodontic anchorage. Prior to miniscrew insertion, the best approach would be to evaluate each possible insertion site and measure the cortical bone thickness, and verify whether it would provide adequate primary stability. Objective: This study aimed to evaluate the difference in cortical bone thickness in areas of mini-implants insertion in patients of different ages, by means of cone beam computed tomography (CBCT). Methods: The sample of this retrospective study was composed of 123 CBCT scans, which were used to measure cortical bone thickness in the buccal and palatal inter-radicular space in the mesial region of the first permanent molars. These measures were compared by using the Student’s t-test, ANOVA/Tukey tests, and Linear regression between male and female subjects, from 12 to 30 years old. Results: No significant difference was found in cortical bone thickness between sex, race and sagittal facial patterns. Significantly higher measurement values were observed in patients older than 12 years of age at all sites evaluated. The coefficient β at the adjusted linear regression analysis showed that at each increment in age, mean cortical thickness values increased by 0.06mm in the mandible, 0.03mm in the buccal region and 0.02mm in the palatal region of the maxilla. Conclusions: The increase in cortical bone thickness was positively associated with age; that is, the more advanced the patient’s age was, the less chance there was of failure due to primary stability.


INTRODUCTION
Orthodontic mini-implants (MI) are a reliable alternative to provide temporary orthodontic anchorage. 1 Approximately 80% of orthodontists use miniscrews, and according to approximately 78% of professionals, they provide better results in orthodontic treatments. 2 Nevertheless, failure rates range from 11% to 30%. 3 To stratify the risks of the procedure for inserting anchorage screws, it is necessary to know which site has the thickest cortical bone, at ''different ages, for the purpose of guaranteeing the primary stability of the mini-implant and making the outcome of the orthodontic treatment more predictable.
Primary stability of the miniscrews basically depends on the screw design, insertion technique, and quality and quantity of bone at the insertion site. [4][5][6][7] Among these factors, the cortical bone thickness of the insertion site is emphasized. 8,9 Prior to miniscrew insertion, the best approach would be to evaluate each possible insertion site by means of cone beam computed tomography (CBCT), to measure the cortical bone thickness, and to verify whether it would provide adequate primary stability. However, using this exam for this purpose is not indicated, according to the American Academy of Oral and Maxillofacial Radiology (AAOMR). 10 Thus, the aim of this study was to evaluate the cortical bone thickness at different mini-implant insertion sites in the maxilla and mandible, and correlate this with the age of the patient, since the choice of the best site and age has direct repercussion on the reduction of cost and treatment time.
Other variables related to the individuals, such as age, sex, skin color, vertical and sagittal facial patterns would be analyzed through a multivariate analysis, to verify its interaction on the main outcome (age). The hypothesis of the present study was that patients with a more advanced age would present a greater cortical bone thickness, and consequently mini-implants would present a higher degree of primary stability.

MATERIAL AND METHODS
To conduct this retrospective study, the clinical record charts of patients of the Orthodontic Specialization Course, treated at the Dental School of the Federal University of Rio Grande do Sul (Brazil), were reviewed with regard to the orthodontic documentation of patients that met the following inclusion criteria: (1) initial phase orthodontic patients; (2) mixed or permanent dentition; (3) orthodontic documentation containing data of CBCT performed at the same private radiology center (CBCT scanned in i-CAT ® , field of view 22x16cm, isotropic voxel of 0.4mm, images acquired with a rotation of 360º during 20s, 120 kVp, 36.90 mAs, and data saved in DICOM format). Patients with pathologies or radiolucency in the areas of measurement; periodontal disease; ectopic eruptions at sites of interest and important medical history were excluded.
Centeno ACT, Fensterseifer CK, Chami VO, Ferreira ES, Marquezan M, Ferrazzo VA Correlation between cortical bone thickness at mini-implant insertion sites and age of patient 6 The research protocol was submitted to and approved by the Ethics Committee of Federal University of Rio Grande do Sul (CAAE 83140118.4.0000.5347). The database search was performed between July 2017 and April 2018, and 800 orthodontic charts were reviewed. Of these, 123 were selected according to a sample calculation performed with data obtained from a pilot study using 57 individuals from the same database (study power of 80%, significance level of 0.05%, and correlation coefficient of 0.25). The cortical bone thickness was measured in cross-sections, generated after determining the arc curvature line (Fig 1).
The sites evaluated for cortical bone thickness were: inter-radicular space located in the mesial region of the maxillary and mandibular permanent first molars, where the maxilla was evaluated in the vestibular and palatine regions and the mandible, only in the vestibular region, since these are considered safe sites for mini-implant insertion. 13 The lingual cortical mandibular bone thickness was not measured, because it is not an area commonly used for miniscrew placement.
Measurements were performed at a distance of 5 mm from the alveolar bone crest (Fig 2), because there is usually an adequate amount of bone in this position for inserting miniscrews; 14,15 moreover, there is inserted gingiva that favors successful insertion of the device. 16 For each patient, six measurements   sis. In addition, the sex variable was also included as an adjustment variable. A 95% confidence interval and p-value < 0.05 represented a statistically significant relationship.

RESULTS
The median age of the 123 patients selected was 12.1 years (7.6 - 30.7). Table 1 shows the sample frequencies and distri-     (Table 3). More information about the patients of the sample (gender, race and age) can be found at the appendix A.  Appendix A (continuation): All patients gender, race and age.

DISCUSSION
The success of miniscrews is related to their primary stability that is defined by the absence of mobility in the bone after their insertion 17 and this depends on the adaptation and mechanical retention of these devices in the bone tissue. 18 The anatomy of the bony site, especially the cortical bone thickness, plays a fundamental role in this mechanical bracing, and consequently influences the success or failure of the device. A limitation of the present study was the 0.4 mm voxel size used in the CBCT acquisition protocol. According to Ballrick et al. 28 , the mean spatial resolution for voxel used in orthodontics is 0.7 mm. Thus, the accuracy of measurements smaller than 0.7 mm was not reliable, and should be observed with caution. However, considering that the advantage would be an increase in the precision and accuracy of the measurements obtained by means of CBCT, the disadvantage produced by the reduction in voxel size from 0.4mm to 0.25mm would be the increase in the dose of ionizing radiation to which the patients would be exposed. Moreover, the minority of thickness values obtained in this study were lower than 0.7 mm.
The main contribution of this study was the evaluation of cortical bone thickness with reference to different variables related to the individuals, such as age, sex, skin color and vertical and sagittal facial patterns, by performing the multivariate analysis, which made it possible to verify the influence of each individual variable and its interaction on the outcome.
It is important to point out that although young patients (<12 years) are not the individuals commonly eligible for the placement of miniscrews, their inclusion in this study allowed the variation in the pattern of cortical bone thickness to be according to age.

CONCLUSIONS
The increase of cortical bone thickness was positively associated with age. Adjusted linear regression analysis showed that at each increase in age, the mean cortical thickness values increased by 0.06mm in the mandible, 0.03mm in the vestibular region and 0.02mm in the palatal region of the maxilla.
Brachycephalic patients presented higher cortical bone thickness values. The variables sex, sagittal facial pattern and skin color of the patients did not influence the cortical bone thickness in the interradicular areas of miniscrews insertion.