Palatal bone thickness measured by palatal index method using cone-beam computed tomography in nonorthodontic patients for placement of mini-implants.

INTRODUCTION: The purpose of this study was to compare the bone thickness of the palatal areas in different palatal index (PI) groups.
MATERIALS AND METHODS: Cone-beam computed tomography scans of 10 subjects were selected with ameanage group of 18 years. The measurements of palatal bone thickness were made at 36 sites using CareStream 3D Imaging software. The PIwas measured using Korkhaus ratio (palatal height/palatal width). One-way analysis of variance was used to analyze intergroup differences, as well as the PI difference.
RESULTS: Bone thickness was higher in the anterior region than in the middle and posterior regions P <0.001. Furthermore, significant differences were found among the midline, medial, and lateralareas of the palate.
CONCLUSIONS: These findings might be helpful for clinicians to enhance the successful useof temporary anchorage devices in the palate.

The key for successful orthodontic treatment is based on anchorage control. Graber defined anchorage as the nature and degree of resistance to displacement offered by an anatomic unit when used for the purpose of affecting tooth movement.[123] But as indicated by Newtons third law - “For every action there is an equal and opposite reaction.”Hence, there are limitations to completely control all aspects of tooth movement. Since the type A anchorage is difficult to accomplish with conventional biomechanics, temporary anchorage devices were introduced in orthodontics, enabling more predictable, effective, and efficient tooth movement.

Bone quality and quantity play important roles in the success of mini-implants.[4] Miniscrews in the maxilla have less stability than in the mandible because of its porous nature. However, since the palatal area is composed of dense cortical bone, this site has been determined as the best anchorage site in the maxilla. 5 The hard and soft tissue of the palatal area guarantees biomechanical stability for placement of miniscrews. They require minimal compliance on behalf of the patient and provide a simple, convenient, and relatively low-cost method for providing absolute anchorage.

Previously, lateral cephalograms and dental computed tomography were used to assess the bone volume in different sites. Cone-beam computed tomography (CBCT) introduced in 1971 by Dr. Hounsfield in England had come to replace traditional two-dimensional radiographs in current practice. With the advent of CBCT accurate evaluation of vertical bone volume in a hard palate can be done.

Many studies have been previously conducted to measure the palatal bone thickness according to sex, age, and physical conditions.[6789101112] The preferred area for mini-implant placement in the palate is the midpalatal area between the first premolars. Gracco et al. concluded that the palatebeing a high-density bone structure with sufficient bone height (from the midpalatal suture to the cresta nasalis) is a good location for orthodontic screw placement.[10] Wehrbein et al. studied maximum bone height at the midpalatal suture area for placing orthodontic screws without perforating the nasal cavity and recommended small diameter(3.3-mm), short to medium length (4-6-mm) screws. 2] However, bone thickness was not assessed in different palatal index (PI) using CBCT scanning procedure.

Therefore, this study was conducted to measure the thickness of palatal bone in a permanent dentition and to compare the palatal bone thickness in relation to the PI for placement of mini-implants using CBCT.

Materials and Methods

The sample consisted of CBCT scans of 10 patients with a mean age of 18 years (range: 14–20 years) who visited the Orthodontic Department of Sree Balaji Dental College and Hospital, Bharath University, India. CBCT equipment used in this study is Kodak 9500 (KODAK DENTAL SYSTEMS, PracticeWorks ©PracticeWorks Systems, LLC, 2009, New York. USA). The settings were 90 kVp; 10 mA; field of view 18 cm × 21 cm; exposure time 15 s with a spatial resolution of 10 line pairs/cm and an isotropic 0.2-mm voxel size was used. The exclusion criteria included patients with missing or grossly decayed teeth; dental restorations that altered tooth size, shape; prosthetic crowns or gingival defects; facial asymmetry with unilateral or bilateral crossbite pathological diseases. The institutional review board of Bharath University reviewed and approved the study. Informed consents were obtained from all patients or their parents or guardians.

CareStream 3D Imaging software (Care stream 3D imaging software, Carestream dental LLC, 2014, Rochester, New York,USA) was used to measure bone thickness. The palatal width (PW) was measured as the largest palatal distance between the maxillary first molars at the cemento-enamel junction, and the palatal height (PH) was measured as the distance from the bony cortex of the hard palate at the midline of the palate perpendicular to the line measuring the width. The ratio between the PH/PW is the PI and is expressed in percentage.

The palatal bone thickness is measured at 0, 2, 4, and 6-mm lateral to the midpalatal suture on the coronal plane and from 0 to 24 mm at 4-mm intervals posterior to the level of the posterior margin of the incisive foramen on the sagittal plane. The thickness of the palatal bone was measured perpendicular to the horizontal plane at each designated point [Figure 1].

Figure 1

Sagittal view of reference lines for measuring palatal bone thickness

Statistical analysis

The measured bone thickness values was averaged for the subjects, maintaining their groupings with these three designated mediolateral areas: The midline area at the midpalatal suture, the medial area at the reference lines 2- and 4-mm lateral to the midpalatal suture, and the lateral area at the line 6-mm lateral to the midpalatal suture.

Likewise, there were three anteroposterior areas: The anterior area at lines 4 and 8 mm; the middle area at lines 12 and 16 mm; and the posterior area at 20 and 24 mm posterior to the incisive foramen. One-way analysis of variance was used to test for differences in bonethickness. Between-subjects factors were PI, and within-subjects variables were the three mediolateral areas and the three anteroposterior areas. Statistical significance was determined at P = 0.05. The PI was classified as deep when the PI was > 44% and shallow when it was below 44%

Results

Table 1 shows bone thicknesses at the various palatal areas in the two groups.

Table 1

Comparison with one-way ANOVA of palatal bone thickness among subjects with shallow and deep PI (in millimeters)

Bone thickness is significantly different among the three anteroposterior areas of the palate. It is concluded that bone thickness at anterior position is higher compared to Middle and Posterior positions in shallow palate P < 0.001 [Figure 2]. Similarly, in deep palate, the P < 0.001. The bone thickness at anterior position is higher compared to the other two positions [Figure 3].

Figure 2

Palatal bone thickness in anteroposterior and mediolateral areas in shallow palate

Figure 3

Palatal bone thickness in anteroposterior and mediolateral areas in deep palate

Anteroposteriorly, in the shallow palate, the mean bone thickness in the lateral region is found to be high in the anterior region, average at midline, and decreases in the posterior region. Bone thickness in the middle region is high in anterior parts and remains more or less at the same level both in middle and posterior region. The thickness measured at midline is high at anterior, decreases in the middle region and slightly increases in the posterior region of the palate. In deep palate, the mean bone thickness of in all areas lateral, middle, midline areas is found to be high in anterior, average at midline and decreases in the posterior regions.

The total lateral area demonstrated significantly less bone thickness than did the medial and midline area and the anterior values are high compared to middle and posterior areas [Figure 4]. Thus, there is a significant interaction between the anteroposterior and mediolateral areas (P < 0.001), and among the anteroposterior and mediolateral areas and the two groups [Table 2].

Figure 4

Palatal bone thickness in total sample showing interaction between anteroposterior and mediolateral areas

Table 2

Correlation of palatal bone thickness with different PI

Discussion

Cone-beam computed tomography produces three-dimensional anatomical images, from which slices can be displayed from any angle in any part of the imaged region and archived digitally. It also allows secondary reconstructions, such as sagittal, coronal, and para-axial cuts, and three-dimensional reconstructions of different craniofacial structures that are not magnified nor distorted in size or shape.[6]

Placement of temporary skeletal anchorage devices in the paramedian palatal area has been recommended because of its thin keratinized soft tissue and sufficient cortical bone. It provides sufficient bone thickness for high safety and stability of temporary skeletal anchorage devices in all the cases. According to the classification of Misch, the maxilla is mostly composed of porous bone corresponding to D3 or D4, whereas the midpalatal area has dense cortical bone corresponding to D1 or D2.[13] Bernhart et al. reported that the most suitable area in adults for implant placement in the palate was located 6 –9 mm posterior to the incisive foramen and 3– 6 mm para-median to the suture.[8]

Hence, the bone quantity of several placement site was evaluated to verify whether adequate bone thickness is available for placementof temporary skeletal anchorage device in palate. The results of this study indicated that the area of highest bone thickness in the paramedian area extended 4-mm posteriorly to the incisive foramen[Figure 5].

Figure 5

Comparison of palatal bone thickness according to the mediolateral and anteroposterior areas in the shallow and deep palate groups

Poggio et al. suggested that 1-mm of bone should be around temporary skeletal anchorage devices for safe placement.[14] The significant interaction between the mediolateral and anteroposterior positions indicated that the anterior region was thicker than the middle and posterior ones in both the groups. Furthermore, the midline areas were thicker than medial and lateral areas. This might be due to the difference in the amounts of remodeling growth between the posterior and anterior parts.[15]

King et al. demonstrated sufficient vertical bone depth at 4-mm distal and 3-mm lateral to the incisive foramen to install a 3-mm-long implant in adolescents. 6] Kang et al. reported that bone thickness decreased laterally and posteriorly in the paramedian area in adults.[7] Ryu et al. also stated that bone thickness was higher in the anterior region than in the middle and posterior regions and also significant differences were found among the midline, medial, and lateral areas of the palate in adolescent and adult patients.[12]

Moon et al. also measured the palatal bone density in adult subject. He stated that bone density tended to decrease from anterior to posterior areas and from the middle to lateral areas of the palate.[16] This was in accordance with the study conducted by Han et al. where cortical bone density in the adult group, ranged between 1059 and 573 HU, approximately corresponding to the D2 (850– 1250 HU) and the upper range of the D3 (350– 850 HU) categories inclassification of bone tissue by Misch.[17] This was explained in part by the fact that the palatal midline gap, formed during the prenatal period, becomes reduced by deposition of bone, and by the age 21, fibrous tissue with collagen fibers run parallel to the surface.

Thus, further studies of palatal bone thickness measured by PI method according to gender, age and racial differences is needed, to provide important information for clinicians.

Conclusions

Our findings regarding palatal bone thickness of adults in different PI can be summarized as follows: (1) Bone thicknesses in anterior areas were high compared to the middle and posterior areas. (2) Bone thickness in midline areas is higher compared to the middle and lateral areas. Thus, these findings might be helpful for clinicians to enhance the successful use of temporary anchorage devices in the palate in a permanent dentition.