## 4 Discussion
[48] measured the buccal and palatal/lingual internal cortical bone and trabecular bone density at a depth of 2 mm from the alveolar crest. They recorded higher bone density values for the anterior region in contrast to the posterior, for both the maxilla and mandible. The highest bone density value was measured in the anterior mandibular area, and the lowest bone density value was found in the posterior mandible region, followed by the density at the posterior maxillary region. In the present study, we aimed to establish a relationship between the evaluation of cortical bone thickness from a CBCT and from clinical land-marks, in an attempt to not use CBCT imaging for this purpose. This was also attempted by a recent study [49], in which the buccal alveolar bone thickness was evaluated in CBCT and in corresponding digital models, and a strong correlation between them was identified. In the latter study, buccal cortical bone thickness was also measured at 2, 4 and 6 mm distance from the alveolar ridge in the CBCT image, and was found to range from 1.06 to 1.73 mm, while no significant differences between 2, 4, and 6 mm were recorded, which is in disagreement with the presented study. On the contrary, [PERSON] et al. [50], in a meta-analysis study, reported on \"safe-zones\" for mini-implant insertion, regarding cortical bone thickness. They concluded that in the posterior maxilla, the safest site is mesial to the first molar, at 6 mm from the cementoenamel junction. In the present study, the cortical bone thickness was evaluated at the middle of the interradicular space between the first molar and second premolar at three height levels of the attached gingiva, but in a study by [PERSON] et al. [51], in which the cortical bone was evaluated at 4 and 6 mm from the alveolar crest, it was found to be thicker adjacent to roots, than at the midpoint between two adjacent teeth, while the 6 mm level measurement was not significantly higher than the 4 mm level. Utilizing CBCT imaging for measuring bone density is not believed to be reliable by some researchers, because of the influence of parameters such as the device or the positioning of the patient [52]. Other studies [53] have declared that CBCT should not be the examination of choice for the characterization of bone quality on the basis of values compared with predetermined standards and, instead, values for symmetrically positioned areas, inside the field of view (FOV), could be used. On the other hand, there are studies that consider CBCT useful for measuring bone density, and thus provide clinicians with a density scale for the safe placement of implants [54]. In this study, bone density was assessed in CBCT images, using HU, for the buccal cortical bone corresponding to the three studied levels in the attached gingiva (lower, middle, upper), and for the trabecular bone, very close to the inner border of the cortex, at the three studied levels, as well. In the study conducted by [PERSON] et al. [17], it was found that at the buccal region in the maxilla, and distal to the second molar (mean value 1.3 mm), the cortical bone thickness was significantly lower than that in the mesial and distal region of the first molar (mean values 1.8 mm and 1.5 mm, respectively). It was also found that as the trabecular bone density decreased, the values of resonance frequency (RF) analysis (utilized as the measure of the primary stability of the mini-implants) also decreased. In the present study, a correlation between the trabecular bone density very close to the buccal cortex, and the vertical levels of the attached gingiva, was not established. As to the possible correlation between the cortical bone thickness and density, it was found in the present study that there is a positive correlation between them (Table 2). This finding is not in agreement with the study conducted by [PERSON] et al.