## 4 Discussion  
DISH is diagnosed based on the continuous calcification of four or more vertebrae, a maintained intervertebral disc height, or no intervertebral joint rigidity [1]. DISH restrains spinal range of motion and causes low back pain [14]. DISH may easily cause fracture in an inflexible spine and spinal damage [15]. This trauma must not be overlooked because the rate of bone union is low and the attenuation of neurological symptoms is difficult to achieve with conservative treatment [2]. However, in contrast to other spine injuries, the diagnosis of DISH is delayed [3, 4, 16]. One of the factors that make diagnosis difficult is that pre-existent abnormalities on spine radiographs may mask or obscure minimally displaced fractures [3]. Although previous studies reported good outcomes and minimally invasive surgery [17, 18, 19], the misdiagnosis of fractures and diagnostic delays need to be avoided. Because more than 50% of the patients were neurologically intact immediately after the injury, later neurological deterioration occurred in approximately 30% of the total patients [15]. Some previous studies have reported on the risks of trauma of DISH [3, 4, 16]. However, no report has evaluated the impact of ossification thickness and shape on the spine. Therefore, we considered it important to understand the biomechanics of DISH by analyzing the type of force applied to the vertebral body with different ossification shapes and thicknesses. Although cervical vertebrae are the most frequently injured, we analyzed the thoracolumbar spine transition area because the majority of spinal fractures occur in the thoracolumbar spine [17, 20, 21]. Recent studies conducted a fracture analysis via 3D-FEM of the vertebral region. [PERSON] et al. performed an analysis of lumbar spondylosis [22]. In addition, [PERSON] et al. [23] reported the findings of an analysis of compression fractures, while [PERSON] et al. [24] conducted a 3D-FEM analysis of the effects of balloon kyrophoplasty; however, only a few intervertebral analyses have been performed to date. [PERSON] et al. [25] assessed pressure fractures at different angles of the spine. [PERSON] et al. [26] and [PERSON] et al. [27] conducted a compression fracture analysis of a whole-spine model generated from medical images. However, an analysis and visualization of the stress applied to the vertebral body by changing the ossification pattern around the vertebral body, such as that which occurs in DISH, has not yet been performed. A more detailed understanding of the shapes and discontinuities that are most likely to cause damage may contribute to the prevention of misdiagnoses. The present study is the first to demonstrate that the stress on the vertebral body was greater with ossification of the anterior longitudinal ligament than with a healthy ligament. The results obtained herein showed the effects of reduced flexibility. Calcification or ossification of the ligament was identified as a contributing factor to increased stress, which may induce vertebral fractures. [PERSON] et al. reported that all of the subjects had anterior and middle column fractures [15]. In this analysis, the greatest increase in stress  
Figure 14: In D-5/R-1 (**a**) and D-10/R-1 (**b**), the stress at the center of T12 increased at the discontinuity in flexion. was from the anterior in both flexion and extension. We found that the partial hypertrophy of ossification did not affect vertebral stress in the ossified area during extension and flexion, where it had an impact on intervertebral discs. The thickness of local ossification did not appear to affect the vertebral body. Furthermore, stresses were more likely to increase at the center of vertebrae with greater ossification and discontinuity such as cases with D-5/R-2.5, D-5/R-1, and D-10/R-2.5. Therefore, if external forces are applied to a case with large ossification and discontinuities, the vertebral groove may fracture. However, this hypothesis was not applied to cases with D-10/R-1.