## 4 Discussion  
Since bone-related diseases such as osteoporosis increase the probability of the bone fracture, several methods have been proposed to accurately measure the trabecular bone density [9]. With regard to an orthodontic treatment, the trabecular bone density also affects the speed of the tooth movement. As the trabecular bone density increases, the rate of the tooth movement tends to decrease, which is associated with a greater resistance to the tooth movement in the mandibular molar [10]. As a result, the mandibular molar serves as a better anchor. The cortical bone and the trabecular bone are divided based on the degree of porosity. The cortical bone has a porosity of 0-30%, whereas the trabecular bone has a porosity of 30-90% [11]. For this reason, in the cortical bone, the bone density can be represented to some extent by only the opacity of radiographs, but since the trabecular bone is a mixture of plate-like and columnar structures, it is difficult to accurately measure the density of the trabecular bone without considering three-dimensional structural characteristics [12; 13]. In the past, two-dimensional methods such as histological analysis and serial sectioning methods have been used to measure the three-dimensional structure of bones. This not only shows the limitations of the two-dimensional structure, but also has the disadvantage of being more invasive since tissue specimens should be produced. However, the analysis using CT is not only non-destructive compared to the histological analysis, but also has the advantage of realizing a more accurate three-dimensional image at high resolution [14; 15]. The Hounsfield unit defines the radiation density of distilled water as 0 HU and the radiation density of air as 1000 HU at a standard pressure and temperature, and determines the radiation density of the measurement object based on these two radiation densities. In this study, the effect of trabecular bone density according to age and gender was not significant in both the trabecular morphometry analysis and Hounsfield unit analysis. In the maxilla and mandible, males showed slightly higher Hounsfield unit than females,  
Figure 3: Correlations between the Hounsfield unit measurement and trabecular microstructure parameters: (**a**) Positive correlation between Hounsfield unit (HU) and trabecular thickness (Tb.th); (**b**) positive correlation between HU and trabecular number (Tb.n); (**c**) negative correlation between HU and trabecular seperation (Tb.sp); (**d**) positive correlation between HU and bone volume fraction (BV/TV). * \(p\) < 0.001.  
but were not statistically significant. [PERSON] et al. reported that bones that constantly undergo reconstruction were affected by the gender and dental condition [16]. Some previous studies reported that the bone density reduction in the mandible is apparent in women [17; 18]. [PERSON] and [PERSON] reported that the difference between males and females was the delayed calcification in females, resulting in the less calcified bone [19]. However, [PERSON] et al. reported that the difference between trabecular bones by age and gender was still controversial [20]. [PERSON] et al. reported that the bone mineral density of the mandibular alveolar bone was influenced by the activity of the masticatory muscle than on the difference in gender [21]. In this study, it was judged that there was no difference according to the age or gender since the study subjects were relatively young patients undergoing an orthodontic treatment. [PERSON] reported that the mandible had a higher trabecular bone density than the maxilla, as a result of measuring the trabecular bone density based on Hounsfield unit measurements [4]. He also reported that bone densities tended to decrease from the anterior to posterior in both the maxillary and mandibular trabecular bones when analyzed by the Hounsfield unit. In this study, the Tb.th, BV/TV, and HU showed higher values in the anterior base of the maxilla than in the mandibular body.