Tomographic mapping of the hard palate and overlying mucosa

The aim of this study was to measure the thickness of the hard palate and its overlying mucosa using cone-beam computed tomography (CBCT), for purposes of miniscrew placement. The sample comprised 36 CBCT scans of patients aged 12 to 52 years from a database of the Orthodontics Department of the Federal University of Rio de Janeiro. Paracoronal views of the palatal region were reconstructed at 4, 8, 16 and 24  mm posterior to the incisive foramen. In each reconstruction measurements were taken at the suture, 3 mm and 6 mm bilaterally to it. Wilcoxon’s test verified the differences between the selected regions. Total bone height decreased from the anterior to the posterior region. In cross sections 4, 16 and 24, bone height decreased from the suture laterally to the 3 mm region and then increased in the 6 mm region. The cortical thickness does not seem to be a concern because it presented a mean thickness of at least 1 mm at all sites evaluated. The measurements of the mucosa thickness decreased from lateral to median and from anterior to posterior regions. The most suitable areas for miniscrew placement in the palate are located 4 mm posterior to the incisive foramen, in the median or paramedian regions 3 mm adjacent to the suture. Descriptors: Orthodontic Anchorage Procedures; Palate; Cone-Beam Computed Tomography. Introduction Dental implants were introduced as skeletal anchorage resources in Orthodontics in the mid-80s, but it was in 1997 that Kanomi1 introduced mini-implants, a variation of the surgical screws. Mini-implants are commonly inserted in the interseptal region, retromolar area and palate. The palate is probably the most favorable area for miniscrew placement because there is easy access, low risk of damaging important anatomical structures and there is attached keratinized tissue along its entire length.2-6 Moreover, mini-implants placed in these regions have the advantage of not interfering in the tooth movement.7 This area is chosen for placing mini-implants when forward or backward movements of posterior teeth are required,8 intrusion of maxillary teeth9,10 or traction of impacted maxillary teeth.11 In order to indicate the most appropriate mini-implant for each site, it is necessary to know which devices are available on the market and to have a detailed knowledge of the local anatomy. Cortical bone thickness at the site of mini-implant placement is a factor to consider for achieving Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript. Marquezan M, Nojima LI, Freitas AOA, Baratieri C, Alves Júnior M, Nojima MCG, Araújo MTS 37 Braz Oral Res. 2012 Jan-Feb;26(1):36-42 their primary stability.12,13 In addition to knowing the cortical bone thickness in this region, it is necessary to know the total bone thickness in order to choose the appropriate mini-implant length to avoid perforations in the nasal cavity floor.2-4,7,14-17 Knowledge of soft tissue thickness also helps in determining the overall implant length18 and implant collar height.19 Clinical examination, panoramic and cephalometric radiographs have limitations when assessing the amount of bone tissue in the palate.14 Recent studies have shown that computed tomography is a good method to evaluate the bone site most suitable for mini-implant placement2,4,15 since there is no distortion and superimposition of images. Cone-beam computed tomography (CBCT) is also a good tool to evaluate soft tissue thickness.20-22 The aim of this study was to evaluate the thickness of cortical bone, total bone and mucosa in the palatine suture and paramedian areas using CBCT to verify the most suitable areas for miniscrew placement. Methodology The sample consisted of 36 CBCT scans from the database of patients from the Orthodontics Department of the Federal University of Rio de Janeiro. Patients of both genders (8 male) who had tomography performed as part of their orthodontic exams were included. Their age range was 12 to 52 years (mean age 23.61  ±  11.89), so that the sample comprised growing patients and adults in order to evaluate the influence of age on palatal tissues. The exclusion criteria were: • presence of craniofacial malformations or syndromes, • previous history of trauma or surgery in the study area, and • CBCT scans in which the tongue was resting against the palate, hampering the measurement of the palatal mucosa. This study was approved by the Research Ethics Committee of the Institute of Public Health Studies, Federal University of Rio de Janeiro (no. 61/2010), and patients signed a document allowing the use of their tomographic exams for this study. Data were obtained using a 3D cone-beam volume scanner (i-CAT Cone Beam 3-D Imaging System, PA, USA). The following settings were used: • 22 cm field of view, • 120 kV, • 47mA, • exposure time 30 s and • slice thickness 0.4 mm. Multiplanar reconstruction of the head, orientation and measurements were performed with Dolphin Imaging Software (version 11, Chatsworth, CA, USA). Orientation was established by three reference planes: • the axial plane, passing through the right and left Orbitale points as well as the right Porion; • the coronal plane, passing through the left and right Porion perpendicular to the chosen axial plane; and • the sagittal plane, passing through the Nasion point, perpendicular to the chosen axial and coronal planes.23,24 Twenty points in each patient’s palate were chosen for measurements16 (Figure 1). The first step was to locate the incisive foramen. This was tracked by sagittal and axial views simultaneously (Figure 2). Figure 1 Occlusal view of a palate in CBCT. The horizontal lines represent the sites of coronal cross sections: 4, 8, 16 and 24 mm posterior to the incisive foramen. The vertical lines represent the median and paramedian regions evaluated (3 mm and 6 mm laterally to the suture). Their intersections represent the twenty points evaluated. Tomographic mapping of the hard palate and overlying mucosa 38 Braz Oral Res. 2012 Jan-Feb;26(1):36-42 Paracoronal views of the palatal region were reconstructed at 4, 8, 16 and 24 mm posterior to the distal wall of the incisive foramen. In each reconstruction, measurements were taken at the mid-palatal suture, 3  mm and 6  mm bilaterally to it (Figure 3). Measurements were taken in the vertical direction16 (Figure 3) and the three tissues (cortical, trabecular bone and overlying mucosa) were measured based on gray level differences. The cortical bone could not be evaluated separately from total bone in the median region because the suture imbrications make it hard to distinguish cortical and trabecular bone. Measurements were recorded in millimeters, tabulated and submitted to statistical analyses. They were performed by a single calibrated examiner twice with a 2-week interval. The reliability of measurements was satisfactory (for total bone measurement ICC = 0.968; for cortical thickness measurement ICC = 0.744; for mucosa ICC = 0.924). Analyses were performed using the Statistical Package for the Social Sciences (version 17, SPSS Inc., Chicago, IL, USA). Shapiro-Wilk and Levene tests were used to assess the normal distribution and homogeneity of variables. The nonparametric Wilcoxon test verified whether measurements varied according to the paracoronal view and the paramedian regions evaluated. The Spearman test was performed to verify whether age had any influence on Figure 3 Coronal slices reconstructed of 4 (a), 8(b), 16(c) and 24(d) mm posterior to the distal wall of the incisive foramen. These cross sections were used for palatal tissue measurements. The detail shows how measurements were taken in each reconstruction at the median suture, 3 and 6 mm laterally to it. Figure 2 Sagittal and axial slices generated simultaneously by Dolphin Imaging Software were used to determine the posterior wall of the incisive foramen, indicated by the white arrows. Marquezan M, Nojima LI, Freitas AOA, Baratieri C, Alves Júnior M, Nojima MCG, Araújo MTS 39 Braz Oral Res. 2012 Jan-Feb;26(1):36-42 the measurements. Results Descriptive statistics are presented in Tables 1 and 2. The total bone measurement decreased from the anterior to posterior regions up to section 16 (p ≤ 0.05). In cross sections 4, 16 and 24, measurements of the 6 mm paramedian region were similar to those of the suture, but differed from those in the 3 mm region, where values were lower (p ≤  0.05). Cortical thickness remained similar throughout the median and paramedian regions. However, this measurement was smaller in region 6 mm than those in the 3 mm sites in cross sections 16 and 24 (p  ≤  0.05). In paramedian region 3, the cortical thickness measurement showed no difference from the anterior to posterior regions. However, in paramedian region 6, cortical thickness measurements decreased from the anterior region (sections 4 and 8) to posterior region (sections 16 and 24) (p ≤ 0.05). When evaluating mucosa measurements, the values decreased from the lateral to median sites, and were smaller at the suture (p  ≤  0.05). A reduction was also observed from the anterior to posterior regions up to cross section 16 (p ≤ 0.05). No correlation was found between the patients’ age and palatal tissue measurements. Discussion Many studies have shown that the palate is a suitable area for miniscrew placement3,16 with emphasis on easy access2,3 and favorable distance from the roots, thus avoiding any damage to them2,3 or interference with tooth movement.7 This feature allows the insertion of large-diameter miniscrews (2 or 2.2  mm), providing improved stability.25 In the palate, the big challenge is the length of miniscrews. There must be sufficient bone thickness to receive the functional part of the miniscrew, ranging from 6 to 12 mm in length, without perforating the nasal cavity.2-4,7,14-17 The limited availability of palatal bone height was the 


Introduction
Dental implants were introduced as skeletal anchorage resources in Orthodontics in the mid-80s, but it was in 1997 that Kanomi 1 introduced mini-implants, a variation of the surgical screws.Mini-implants are commonly inserted in the interseptal region, retromolar area and palate.3][4][5][6] Moreover, mini-implants placed in these regions have the advantage of not interfering in the tooth movement. 7This area is chosen for placing mini-implants when forward or backward movements of posterior teeth are required, 8 intrusion of maxillary teeth 9,10 or traction of impacted maxillary teeth. 11n order to indicate the most appropriate mini-implant for each site, it is necessary to know which devices are available on the market and to have a detailed knowledge of the local anatomy.Cortical bone thickness at the site of mini-implant placement is a factor to consider for achieving Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.their primary stability. 12,1314][15][16][17] Knowledge of soft tissue thickness also helps in determining the overall implant length 18 and implant collar height. 19linical examination, panoramic and cephalometric radiographs have limitations when assessing the amount of bone tissue in the palate. 14Recent studies have shown that computed tomography is a good method to evaluate the bone site most suitable for mini-implant placement 2,4,15 since there is no distortion and superimposition of images.][22] The aim of this study was to evaluate the thickness of cortical bone, total bone and mucosa in the palatine suture and paramedian areas using CBCT to verify the most suitable areas for miniscrew placement.

Methodology
The sample consisted of 36 CBCT scans from the database of patients from the Orthodontics Department of the Federal University of Rio de Janeiro.Patients of both genders (8 male) who had tomography performed as part of their orthodontic exams were included.Their age range was 12 to 52 years (mean age 23.61 ± 11.89), so that the sample comprised growing patients and adults in order to evaluate the influence of age on palatal tissues.The exclusion criteria were: • presence of craniofacial malformations or syndromes, • previous history of trauma or surgery in the study area, and • CBCT scans in which the tongue was resting against the palate, hampering the measurement of the palatal mucosa.
This study was approved by the Research Ethics Committee of the Institute of Public Health Studies, Federal University of Rio de Janeiro (no.61/2010), and patients signed a document allowing the use of their tomographic exams for this study.
Data were obtained using a 3D cone-beam volume scanner (i-CAT Cone Beam 3-D Imaging System, PA, USA).The following settings were used: • 22  • the axial plane, passing through the right and left Orbitale points as well as the right Porion; • the coronal plane, passing through the left and right Porion perpendicular to the chosen axial plane; and • the sagittal plane, passing through the Nasion point, perpendicular to the chosen axial and coronal planes. 23,24enty points in each patient's palate were chosen for measurements 16 (Figure 1).The first step was to locate the incisive foramen.This was tracked by sagittal and axial views simultaneously (Figure 2).Paracoronal views of the palatal region were reconstructed at 4, 8, 16 and 24 mm posterior to the distal wall of the incisive foramen.In each reconstruction, measurements were taken at the mid-palatal suture, 3 mm and 6 mm bilaterally to it (Figure 3).Measurements were taken in the vertical direction 16 (Figure 3) and the three tissues (cortical, trabecular bone and overlying mucosa) were measured based on gray level differences.The cortical bone could not be evaluated separately from total bone in the median region because the suture imbrications make it hard to distinguish cortical and trabecular bone.Measurements were recorded in millimeters, tabulated and submitted to statistical analyses.They were performed by a single calibrated examiner twice with a 2-week interval.The reliability of measurements was satisfactory (for total bone measurement ICC = 0.968; for cortical thickness measurement ICC = 0.744; for mucosa ICC = 0.924).
Analyses were performed using the Statistical Package for the Social Sciences (version 17, SPSS Inc., Chicago, IL, USA).Shapiro-Wilk and Levene tests were used to assess the normal distribution and homogeneity of variables.The nonparametric Wilcoxon test verified whether measurements varied according to the paracoronal view and the paramedian regions evaluated.The Spearman test was performed to verify whether age had any influence on

Results
Descriptive statistics are presented in Tables 1  and 2. The total bone measurement decreased from the anterior to posterior regions up to section 16 (p ≤ 0.05).In cross sections 4, 16 and 24, measurements of the 6 mm paramedian region were similar to those of the suture, but differed from those in the 3 mm region, where values were lower (p ≤ 0.05).Cortical thickness remained similar throughout the median and paramedian regions.However, this measurement was smaller in region 6 mm than those in the 3 mm sites in cross sections 16 and 24 (p ≤ 0.05).In paramedian region 3, the cortical thickness measurement showed no difference from the anterior to posterior regions.However, in paramedian region 6, cortical thickness measurements decreased from the anterior region (sections 4 and 8) to posterior region (sections 16 and 24) (p ≤ 0.05).When evaluating mucosa measurements, the values decreased from the lateral to median sites, and were smaller at the suture (p ≤ 0.05).A reduction was also observed from the anterior to posterior regions up to cross section 16 (p ≤ 0.05).No correlation was found between the patients' age and palatal tissue measurements.

Discussion
Many studies have shown that the palate is a suitable area for miniscrew placement 3,16 with emphasis on easy access 2,3 and favorable distance from the roots, thus avoiding any damage to them 2,3 or interference with tooth movement. 7This feature allows the insertion of large-diameter miniscrews (2 or 2.2 mm), providing improved stability. 25In the palate, the big challenge is the length of miniscrews.14][15][16][17] The limited availability of palatal bone height was the reason for the development of  special short palatal implants for orthodontic anchorage (3 to 6 mm long). 14Nowadays conventional miniscrews can be used if the local anatomy is known.
A risky region for palatal implant placement is one with a height of less than 4 mm, 2,3 because the shortest endosseous part of this implant is 3 mm long and a safety margin of 1 mm is recommended. 4n view of this statement, cross section 4 is safe in both the suture and paramedian regions.In the other cross sections, only the midsagittal region presented bone height of over 4 mm.Nevertheless, based on the assertion that "the lower the angle between the plane of the anterior and posterior nasal spine and the implant axis, the wider the bone available", 7 we believe that the paramedian regions at cross section 8 could also be suitable when small implants are used and inserted with angulation.The paramedian regions at cross sections 16 and 24 are undoubtedly risky.6][17] The region of the midsagittal suture is an exception because additional height is provided by the nasal crest.In all of the cross sections, this region was shown to be at least 5 mm thick, in agreement with Kang 3 and Wehrbein. 14he safe and risky regions for miniscrew placement differ slightly from those for palatal implants.The endosseous part of palatal implants varies from 3 to 6 mm in length while the shortest miniscrew is 6 mm long.Nowadays, miniscrews are preferred to palatal implants because they are easier to insert, the possibility of immediate loading (it is not necessary to wait for osteointegration), and better healing after their removal.Thus, the total bone length is more critical.In this study, cross section 4 presented adequate total bone volume to receive a 6-mm long miniscrew.The suture was also a suitable place in all of the cross sections, considering the possibility of inserting the miniscrew with angulation.
As regards the lateral direction, a decrease in total bone thickness was found from the suture to paramedian region 3 mm and it increased from there on up to paramedian region 6 mm in sections 4, 16 and 24 (in section 8 the paramedian regions 3 and 6 mm presented no differences).Similar findings were observed by Gracco 16 in the anterior sections (4 and 8).Kang, 3 however, verified a decrease from the central to lateral regions.
If care is taken during the planning of miniscrew placement in palate, the risk of nasal floor perforation is reduced.However, if a slight bone perforation occurs, the thickness of the nasal mucosa can prevent an open connection between the nasal and mouth cavities. 14onsidering the cortical thickness, in paramedian regions 6 mm away from the suture, the cortical decreased from the anterior (cross sections 4 and 8) to posterior regions (16 and 24 sections), in agreement with Baumgaertel. 17In paramedian regions 3 mm away from the suture, however, no difference was observed.The cortical bone thickness in the palate does not seem to be a worrying aspect when choosing the ideal site for miniscrew placement because all of the sites evaluated in this study presented a thickness of over 1 mm, adequate for primary stability. 13,26The cortical bone was not evaluated separately from total bone at the suture because of the difficulty in distinguishing cortical from trabecular bone in this region.Nevertheless, the literature supports the concept that the median area of the palate contains high-quality cortical bone. 8henever possible, the mini-implant should be inserted through the attached gingiva 19 because it is believed that the risk of failure in nonkeratinized mucosa is higher than in keratinized mucosa 18 .Fortunately the entire area of the palate is lined with keratinized tissue, which is related to less inflammation. 3The length of the miniscrew should take into account the thickness of the mucosa, because a longer miniscrew should be selected to compensate for soft tissue thickness. 27Another option (and more appropriate, in the authors' opinion) is to choose a transmucosal neck compatible with the mucosa: the thicker the soft tissue, the longer the neck, as stated by Melsen. 19Another problem related to thick mucosa is miniscrew stability.The distance between the point of force application and the center of resistance of the screw will be greater than usual, thus generating a large moment when a force is applied, 19 so a very thick fibromucosa with marked folds can be a contraindication for miniscrew placement. 7o prevent big moments of force, the suture and the paramedian region 3 mm away from the suture are preferable to the 6 mm region, where the mucosa is thicker.Despite the importance of verifying the thickness of mucosal tissue prior to miniscrew placement, the majority of the studies that mapped the palate do not consider this issue.
Alternatively to using CBCT to measure soft tissue thickness, a periodontal probe with an endodontic file stopper can be used under local anesthesia.An ultrasonic device (ultrasonic gingival-thickness meter) can also be used. 27he most suitable areas for receiving miniscrews are located along the suture or in cross section 4; in the paramedian region 3 mm lateral to the suture, because the total bone thickness is adequate for receiving an angulated miniscrew 6 mm long, the cortical is thicker than 1 mm, providing adequate stability, and the mucosa is thinner than 3.5 mm, reducing the moment when force is applied.
No correlation was found between palatal tissue measurements and the patients' ages.Bernhart 2 previously found no association between age and the volume of bone in the palate.The mucosa thickness showed no correlation with age, disagreeing with Song, 20 who observed an increase in palatal mucosa thickness as age progressed, possibly due to the increase of fat in this tissue.
The selection of cross sections 4, 8, 16 and 24 mm distant from the incisive foramen was due to their reproducibility and use in a previous study. 16n order to make the evaluation more clinical, the tooth or the area between two teeth that appeared in each section was recorded.It was observed that in section 4 the first premolar was the most common tooth; in section 8, the second premolars; in section 16, the first molar; and in section 24, the space between first and second molars.
The main contribution of this study to the literature is the evaluation of the thickness of the palatine hard and soft tissues together.However, it has limitations, such as the small sample size, and lack of homogeneity regarding gender and age of patients.Future studies should focus on improving these issues.

Conclusion
The most suitable areas for receiving miniscrews are located along the suture or in cross section 4, in the paramedian region 3 mm lateral to the suture.

Figure 1 -
Figure 1 -Occlusal view of a palate in CBCT.The horizontal lines represent the sites of coronal cross sections: 4, 8, 16 and 24 mm posterior to the incisive foramen.The vertical lines represent the median and paramedian regions evaluated (3 mm and 6 mm laterally to the suture).Their intersections represent the twenty points evaluated.

Figure 3 -
Figure 3 -Coronal slices reconstructed of 4 (a), 8(b), 16(c) and 24(d) mm posterior to the distal wall of the incisive foramen.These cross sections were used for palatal tissue measurements.The detail shows how measurements were taken in each reconstruction at the median suture, 3 and 6 mm laterally to it.

Figure 2 -
Figure 2 -Sagittal and axial slices generated simultaneously by Dolphin Imaging Software were used to determine the posterior wall of the incisive foramen, indicated by the white arrows.

Table 1 -
Descriptive analysis of palatal bone thickness (cortical and total bone) in the suture and paramedian regions at the four cross sections.

Table 2 -
Descriptive analysis of mucosa thickness in the suture and paramedian regions at the four cross sections.