Association between serum interleukin-17 levels and ectopic bone formation in OPLL patients with DISH

Objective: To investigate the relationship between the severity and morphology of heterotopic ossification in the spinal ligaments including sacroiliac (SI) joints, and serum interleukin-17 (IL-17) levels in patients with ossification of the posterior longitudinal ligament (OPLL) with or without diffuse idiopathic skeletal hyperostosis (DISH), as well as a non-OPLL group. Methods: A total of 103 patients with OPLL [DISH (), n ¼ 50; DISH (þ), n ¼ 53] and 53 age- and gender-matched controls were included. The serum levels of IL-17 were analysed, and the severity of ectopic ossification and the morphology of ectopic bone formation were evaluated. The SI joint morphological variations were categorized into four types. Results: No significant differences were found in serum IL-17 levels between the OPLL and control groups. However, the DISH (þ) group showed higher IL-17 levels than the DISH () group, especially in female patients (P ¼ 0.003). Additionally, IL-17 levels were positively correlated with the number of flat vertebral units, this being one of the characteristic DISH ossification types (R² ¼ 0.199, P ¼ 0.012). IL-17 levels in the type showing bridging osteophyte and bone fusion were significantly higher in the DISH (þ) group than in the DISH () group. Conclusion: The morphological characteristics of paravertebral bone formation in the entire spine, including the SI joint, are likely associated with serum IL-17 levels in OPLL. These findings provide pathological and serological evidence of local inflammation contributing to paravertebral ossification of OPLL patients.