The purpose of this study was to use cone-beam computed tomography (CBCT) to determine bone thickness in the mandibular buccal shelf (MBS) and the infrazygomatic crest (IC) in individuals with different vertical facial heights for ultimate placement of miniscrews.
The sample consisted of 100 individuals aged at least 16 years, of whom 58 were women, and 42 were men. The mean age was 19.18 years (± 5.5 standard deviation). The patients’ facial height was determined by the gonial angle. Cross-sectional slices of the MBS and IC were obtained with CBCT to evaluate bone thickness for the insertion of miniscrews in these extra-alveolar sites. Spearman’s nonparametric test was used to correlate the gonial angle with MBS and IC thickness. The level of significance was 5%.
The gonial angle ranged from 102.4° to 143.2°. Bone thickness in the MBS increased posteriorly, whereas bone thickness in the IC decreased posteriorly. There was an inversely proportional correlation between the gonial angle and the MBS. There was no correlation between the IC and the gonial angle.
Short-faced individuals had higher bone thickness values in the MBS than long-faced ones. There was no correlation between the patients’ vertical face height and the bone width in the IC. The best site to install miniscrews in the MBS is buccal to the second molar distal root, whereas in the IC, it is buccal to the first molar mesiobuccal root. CBCT may be necessary to install extra-alveolar miniscrews correctly, especially in the IC.
Mandibular buccal shelf thickness increased posteriorly.
Infrazygomatic crest (IC) thickness decreased posteriorly.
The best site to install miniscrews in the mandibular buccal shelf is buccal to second molars’ distal root.
The best site to install miniscrews in the IC is buccal to first molars’ mesiobuccal root.
Cone-beam computed tomography may be needed to place extra-alveolar miniscrews correctly, especially in the IC.
Skeletal anchorage with miniscrews has been widely used with great efficiency in recent years. However, care should be taken when planning the insertion of these appliances, particularly in regard to the thickness and height of the attached gingiva, and the proximity of anatomic structures. In addition, primary stability is the most important aspect to achieve successful placement of miniscrews, because they provide no osseointegration. Although interradicular areas are routinely selected for miniscrews, these sites should be carefully evaluated, because they must have a minimum width of 4 mm. , As an alternative, different anatomic structures have already been used for the installation of miniscrews, including the hard palate, the mandibular buccal shelf, and the infrazygomatic crest (IC).
Extra-alveolar sites are distant from dental roots, , and usually have higher bone density, thus tending to increase the primary stability of miniscrews. One of the most commonly used extra-alveolar regions is the mandibular buccal shelf (MBS). Located in the posterior part of the mandibular body, buccal to the roots of the mandibular molars, it is a widely recommended anatomic site for the installation of miniscrews. However, this region is questionable, especially regarding anatomic and bone-width variability. , In contrast, the extra-alveolar area in the maxilla most suitable for miniscrew insertion is the IC, located at the base of the eminence of the zygomatic crest, buccal to the roots of the maxillary molars. According to a previous study, miniscrews placed in this area could invade the maxillary sinus.
It is known that the patients’ vertical facial dimension affects the alveolar bone width. Previous studies have already analyzed the MBS and IC morphology , , ; however, they have not evaluated if the patients’ vertical facial dimension affects the extra-alveolar bone width in those areas, apart from Ozdemir et al, who only made 1 measurement in the MBS and the IC. Understanding the influence of the vertical facial dimension in these sites can be useful when planning the use of extra-alveolar miniscrews in patients with different vertical profiles.
The use of cone-beam computed tomography (CBCT) has been available for over a decade. CBCT provides 3-dimensional imaging with minimum distortion, but higher radiation doses are usually necessary than in conventional dental radiography. , Therefore, it becomes necessary to justify the use of CBCT. Although extra-alveolar miniscrews are commonly used, there are currently different image exams that can be used to guide the miniscrews’ installation, and having a selection criterion can provide the orthodontist with useful information within which to work.
The purpose of this study was to use CBCT to determine the bone thickness in the MBS and the IC in individuals with different vertical facial heights for the ultimate placement of miniscrews. Although the literature shows that the patients’ vertical facial pattern affects the alveolar bone width, the null hypothesis is that the cortical bone width in the mandibular buccal shelf and the IC sites is not affected by the patients’ vertical facial dimension.
Material and methods
This research was approved by the Research Ethics Committee of the Clementino Fraga Filho University Hospital of the Federal University of Rio de Janeiro (protocol no. 2.981.865). It was a descriptive, retrospective, observational study in which the MBS was evaluated in both the transverse and vertical directions, and the IC, in the vertical direction. The sample size was calculated using the formula described by Pandis, based on a study power of 95% (α = 0.05), and considering the standard deviation described by Nucera.
The sample consisted of 100 individuals who were aged at least 16 years, of whom 58 were women, and 42 were men, who were selected after their skeletal maturity was complete. The mean age was 19.18 years (± 5.5 standard deviation). Inclusion criteria consisted of individuals who were at least 16 years old with their growth already fulfilled, with Angle’s Class I, II, or III malocclusion, and who came to the Department of Orthodontics of the Universidade Federal do Rio de Janeiro seeking orthodontic treatment. Skeletal maturity was assessed by the cervical vertebral maturation method. The department’s diagnostic protocol before any orthodontic treatment is based on CBCT exams because scanning provides an entire 3-dimensional view of the craniofacial anatomy and contributes to better diagnosis and treatment planning, justifying their use in this study. However, care should be taken when prescribing CBCT exams because of the high radiation doses, in order not to use them indiscriminately. , Exclusion criteria consisted of syndromic individuals, periodontal disease, absence of any tooth other than the third molar, individuals who had undergone orthognathic surgery, and those who had evident asymmetries. Patients were considered asymmetric when the deviation from the menton point to the midsagittal plane was higher than 2 mm.
The difference between the patients’ facial height was determined by the gonial angle (angle composed by the ramus line and the mandibular line, where the first is tangent to the posterior border of the mandible and the last is tangent to the lower border of the mandible through the gnathion), which was used as a reference measurement. This criterion was selected because it can be evaluated clinically, quickly, and simply. The greater the aperture of this angle, the greater the vertical facial height; conversely, the smaller the angle, the smaller the vertical facial height.
The tomographic exams were obtained by the Kodak K9500 scanner (Carestream Health, Rochester, NY) with 0.3-mm voxel size, scan time 10.8 seconds, and a field of view of 18 cm × 20.6 cm. The tomographic exams were manipulated and analyzed in digital imaging and communications in medicine format using Dolphin Imaging software 11.5.
The same operator (EV) performed all the measurements and evaluations. First, the cephalometric analysis of the individuals was performed only with the gonial angle as a variable. The following procedure was done on the whole sample to obtain the proper visualization for correct quantitative and qualitative analysis of the MBS. Three multiplanar views were used as guidance: green line for the coronal plane, red line for the sagittal plane, and blue line for the axial plane. Initially, the axial plane was oriented so that the root furcations of the mandibular left molars became collinear ( Fig 1 , A ). Then, the sagittal plane was oriented so that the blue line passed through the long axis of the molar’s root, which was going to be assessed at the moment ( Fig 1 , B ). Finally, the coronal plane was reoriented so that the red line followed the long axis of the same root ( Fig 1 , C ). The measurements were performed on the 4 roots of the mandibular left first and second molars. All roots were evaluated transverse and vertically. Evaluation of the transverse buccal bone thickness was made apically at 6 mm and 11 mm from the cementoenamel junction (CEJ) ( Figs 2 , A and B ) because 6 mm represents the minimum standard miniscrew length and extra-alveolar miniscrews are usually longer than 10 mm. , Evaluation of the vertical buccal bone thickness was made at 4 mm and 5 mm from the CEJ ( Figs 2 , C and D ).
The following procedure was done on the whole sample to obtain proper visualization for the correct quantitative and qualitative analysis of the IC. The multiplanar views were oriented in order to standardize the images for the measurements performed in the maxillary molars. Initially, the images from the CBCTs were oriented so that the occlusal plane was parallel to the lower border of the display window. Then, the green line was positioned so as to pass through the molar’s root that was going to be assessed at the moment. In the same plane, the blue line was oriented at the same root’s tip ( Fig 3 , A ). In the coronal plane, the red line was placed tangent to the maxillary buccal cortical bone ( Fig 3 , B ), because it guarantees safety to the tooth’s root from injury during the miniscrew insertion. At the site where both red and blue lines met, the bone thickness of the IC was evaluated at 70° and 65° angulations in the sagittal plane, following a previous study ( Fig 3 , C ). This evaluation was performed in the buccal roots of the maxillary left first molar and the region between the first and second molars on the left side. The upper limit for measuring bone thickness between the molars (blue line) was the highest apex between the distobuccal root of the maxillary first molar and the mesiobuccal root of the second molar. The root tip was selected as the upper limit for the maxillary first molar roots’ area because this way, it becomes more difficult to injure the roots during the extra-alveolar miniscrew placement. Regarding the interradicular area, the highest apex must be selected in order to avoid the same problem, as mentioned before.
The method was validated, and the level of intraobserver and interobserver error was assessed by performing a pilot study with 20 tomographic exams in which the MBS was evaluated. All the measurements were remeasured after 14 days. The average intraclass correlation coefficient was 99.75%, whereas the interclass correlation coefficient was 98%.
Statistical analysis was performed with the SPSS software (version 22; SPSS Inc, Chicago, Ill). Normality was verified by means of the Kolmogorov-Smirnov test. The descriptive analysis was performed, and the non-normal distribution of the data was verified. Thus, Spearman’s nonparametric test was used to correlate the gonial angle with the MBS and the IC thicknesses. The level of significance was 5%.
The gonial angle ranged from 102.4° to 143.2°. Descriptive statistics for the MBS and IC thickness values are shown in the Table . The box plot diagram showed that the MBS thickness increased posteriorly ( Fig 4 ). In contrast, bone thickness in the IC decreased posteriorly ( Fig 5 ). In the MBS, buccal to the first molar, nearly all the samples had less than 4 mm of transverse bone thickness. However, buccal to the distal root of the second molar, nearly 75% of the sample had more than 4 mm of transverse bone thickness. In contrast, in the IC, more than 50% of the entire sample had less than 4 mm bone thickness, and in the proximal area, it got to more than 75%. Spearman’s nonparametric test showed that there was an inversely proportional statistic correlation with the gonial angle. However, there was no correlation between the IC and the gonial angle ( Table ).