# Aging Alters Cervical Vertebral Bone Density Distribution: A Cross-Sectional Study
## 5 Discussion
### Clinical Application  
Consistent with the current findings, volumetric BMDs of human C2 and C3 showed a strong positive correlation in the previous study using QCT [9]. Furthermore, many QCT based human studies have observed that females have higher BMD and smaller size of cervical and lumbar vertebrae than males [7, 8]. These observations support reliability of the current dental CBCT based BMD and morphological analysis. The bigger cross sectional area and volume of male lumber vertebra has an advantage to bear more axial static loading due to body weight [7, 29]. In contrast, it is indicated that incidence of fall-induced cervical vertebral injury is more in males than females [6,30]. These results provide an insight that higher BMD in female cervical vertebra may provide more resistance to fall-induced impact loading than bigger size of male cervical vertebra. Further studies are needed to clarify the relationships between functional demands and characteristics at different anatomical sites of spine. While both males and females had the similar reducing trend of gray level parameters with advancing age, females showed significant rapid reduction between 50- and 60-age groups, which was not observed in males. These findings suggest that the altered gray level parameters of female 60-age group likely results from rapid decrease in BMD due to menopause while males experience progressive decrease in BMD due to senile osteoporosis as observed in other anatomical sites [1]. The gray level parameters and volume of cervical vertebrae were computed based on 3D images. The volume had significant correlations with other morphological parameters that changed slightly with ages. These findings indicate that the structure of cervical vertebrae was maintained with ages in adult patients examined in the current study. In addition, the SD of gray level, which represents heterogeneity of BMD and changes by adding or losing bone tissues, was also consistent with age, indicating that the absolute values of BMD decreased without bone loss. A limitation of the current study is that cross-sectional analysis was performed using the existing patients' CBCT images. A longitudinal study for individual patients at different time periods of age is expected to provide more significant results for progressively developing osteoporosis with aging. However, multiple radiographic scanning for the same patient is limited due to potential accumulation of radiation dose. Another limitation may arise from that the current CBCT images were investigated retrospectively. The current findings would be more informative if they are compared with those from other anatomical sites using the standard BMD measurement with DEXA and the same methodologies used in the current study. However, it is not available to rescan the same patients. Instead, we found a previous study showing that a radiographic density obtained from a 2D slide of CBCT image for the cervical vertebrae had significant correlations with DEXA based osteoporotic scores at lumbar and femoral neck [31]. Moreover, 3D QCT based BMD of C2 and C3 showed significant correlations with those of thoracic and lumbar vertebrae [9]. These results suggest further studies that can be designed to compare BMD and morphology of cervical vertebrae using dental CBCT and those of other vertebrae using medical CT. In conclusion, this is the first study that examined capability of dental CBCT to assess the integral volume BMD of human cervical vertebrae. The current study provided evidence that CBCT based analysis may suggest additional information to detect the rapid BMD reduction of C2 and C3 at the postmenopausal age.