Introduction: Several reports have indicated that intravertebral bone mineral density (BMD) of diffuse idiopathic skeletal hyperostosis (DISH) of thoracic spine did not decreased. However, we often encounter thoracic DISH patients with markedly decreased BMD. Furthermore, the differences between patients with and without decreased BMD remains unclear. This study aimed to verify the characteristics of thoracic DISH patients with decreased BMD by measuring the vertebral Hounsfield unit (HU).

Methods: We retrospectively reviewed the medical records of the patients who underwent whole spine CT between January 2017 and July 2020. A total of 117 patients (83 males and 34 females, mean age of 72.1 years) without thoracic spine fracture or treatment of osteoporosis were included in this study. HU values were measured by setting an oval region of interest (ROI) in the vertebral body at lower 1/3 slice. Furthermore, to clarify the characteristics of the difference among each spinal level, the Tx/T1 ratio was calculated by dividing HU value of each vertebra by that of the T1 vertebra. Whole thoracic kyphosis (T1-12), upper thoracic kyphosis (T1-5) and middle and lower thoracic kyphosis (T5-12) were measured. Patients whose HU values were 150 or more at T9 vertebrae were classified as H-BMD group, and patients whose HU values were less than 150 were classified as L-BMD group. Statical analyses were performed using chi-square tests, Student's t-test and multivariate forward selection stepwise logistic regression.  

Results: Multivariate analysis showed that age, obesity (BMI >30 kg/m²), and women were significantly higher rate in L-BMD group (51 patients) than in H-BMD group (66 patients). Furthermore, the Tx/T1 ratio gradually decreased from upper to lower thoracic vertebrae in both groups, and the Tx/T1 ratio of L-BMD group was significantly lower in each vertebra below T5 than H-BMD group (Figure 1). Upper thoracic kyphosis was not different between the two groups, but whole thoracic kyphosis and mid-lower thoracic kyphosis of L-BMD group were significantly greater than those of H-BMD group.

Discussion: Older age, women, and obesity were risk factors for decreased vertebral BMD in patients with thoracic DISH, suggesting that postmenopausal osteoporosis and the duration of exist of DISH are involved. Furthermore, this study suggested that the patients with advanced osteoporosis had a significant progression of thoracic kyphosis and their BMD decreased especially in the mid and lower thoracic spine.