Posterior ligamentous injury induces progressive ankylosis-like structural remodeling in rhesus macaques

Structural progression in ankylosing spondylitis (AS) is difficult to model in species with human-relevant spinal anatomy and long-term imaging readouts. Here, we established a rhesus macaque model of AS-like structural remodeling by standardized posterior L2-L6 musculoligamentous injury, with sham-operated animals serving as negative controls. Longitudinal CT/MRI assessment was performed for up to 50 months, supplemented by ex vivo micro-CT and whole-section histology. Model animals developed cumulative structural changes that progressed from early focal entheseal/subchondral remodeling to posterior ossification, segmental bridging, sacroiliac joint structural involvement, and late ankylosis-like remodeling, whereas sham animals showed no typical AS-like structural lesions. CT-based semiquantitative assessment demonstrated a progressive increase in structural burden over time and indicated that posterior elements were the dominant anatomical domain contributing to the terminal ankylosis-like phenotype. Whole-section histology further supported a tissue-level continuum from posterior and peri-discal fibrotic remodeling to late fibro-osseous change, consistent with the imaging-defined progression from focal remodeling to bridging and structural fixation. These findings show that posterior musculoligamentous injury in rhesus macaques can drive long-term, quantifiable, AS-like structural remodeling and nominate the posterior musculoligamentous complex as a potential anatomical and histological hub linking local injury repair, fibrosis, fibro-osseous transition, and ankylosis-like progression.