Fig. 5

The bone quality and related quantitative metrics of mice from different treatment groups, as assessed by Micro-CT. A shows significant bone loss and severe trabecular destruction in the CIA group, while BMP9 treatment (CIA + BMP9 group) partially restored bone quality, with relatively intact trabecular structure. Quantitative analysis revealed that BMP9 significantly increased bone mineral density (BMD), trabecular number (Tb.N), and showed statistical differences compared to the CIA group (*p < 0.05). Additionally, while there were no significant differences (p > 0.05), BMP9 treatment led to an increase in the bone volume-to-tissue volume ratio (BV/TV) and bone surface-to-bone volume ratio (BS/BV) compared to the CIA group (B–E). These results suggest that BMP9 has a significant protective and restorative effect on bone loss in the collagen-induced arthritis model